Building Science- research from respected sources

I may have posted this as a link in the past but just came across it while cleaning up my computer this AM. I feel we need a full section on Building Science but maybe a thread will do.


Press Release - For Immediate Release
Date: Feb. 18, 2004 Contact: George Couch Phone: (608) 231-9295 E-mail:](

Many Home Repairs Fail to Solve Moisture Problems; Researcher Presents 'Five Myths’
MADISON, WI – Each year American homeowners spend millions of dollars attempting to fix or prevent moisture-related problems. Too often, their efforts don’t fix the problem. In some cases, these efforts actually make matters worse. So says Anton TenWolde, a physicist and researcher who has been studying moisture in buildings for more than 20 years. According to TenWolde, many generally accepted moisture-control practices in the United States are based on limited or no research but mostly on tradition among home builders and others.
“We spend very little on housing research in the United States. Several countries, including Canada and even smaller nations like Denmark, the Netherlands and Sweden, invest more than the United States in research into home-building technology,” he says. TenWolde, a native of the Netherlands, holds degrees in physics and engineering from the University of Delft in the Netherlands and the University of Wisconsin at Madison.
He has been a physicist at the USDA Forest Service Forest Products Laboratory (FPL) in Madison, Wis., since 1980. He is currently project leader of the Building Moisture and Durability research unit there. Since 1999, the FPL has been home to the Advanced Housing Research Center, through which much housing-related research is managed and coordinated.
Several ongoing projects at FPL study moisture-related issues. For example, FPL’s research-demonstration house, a full-size four-bedroom house built in 2000, is equipped with scores of sensors embedded in the walls and elsewhere to track the movement of moisture and water vapor in the walls and foundation. Other projects look at sealants and wood-preservatives as well as adhesives.
The problem of basing home construction or repairs on unproven building practices is amplified because some of those traditional practices have become part of building codes around the country, TenWolde says.
According to TenWolde, building codes historically deal with safety issues such as fire prevention, electrical safety, or structural standards. Building codes sometimes go beyond safety when they try to deal with moisture. “And moisture is one area where current building codes get it wrong, especially when they apply standards that might make some sense in northern Maine or Minnesota to Florida or Texas,” TenWolde says.
TenWolde identifies five main ideas about home construction and maintenance that are widely misunderstood or downright incorrect. He calls them “The Five Myths of Moisture.”
Myth One concerns so-called vapor barriers, or vapor retarders.
" A vapor retarder, normally installed only on exterior walls, is intended to slow the diffusion of moisture from an area of higher humidity to one of lower humidity. Such barriers are ineffective if there is any air movement, which is almost always the case in wood-frame construction. An air barrier, to be effective, needs to envelop the entire house——ceiling and floor as well as walls," TenWolde says. “Problems caused by diffusion are very rare; moisture problems caused by moving air are much more common.”
In warm, humid climates, a vapor barrier can do harm. Nonetheless, practically all building codes require vapor barriers or retarders. (Vapor barriers originated in the 1930s, partly based on research conducted at FPL.) A more effective approach to controlling moisture intrusion would be to make the house as air-tight as possible and provide good drainage around the house, according to TenWolde.
Myth Two is that attics need to have lots of ventilation. Again, venting requirements are not based on rigorous scientific research.
TenWolde explains that attic venting originally arose as a moisture-control strategy for cold climates. Other purported benefits, such as longevity of the shingles, arose later. It is widely believed that increased attic venting will prolong the life of roofing shingles by cooling them. But research shows that venting has very little, if any, effect on shingle temperature. The most important issue in shingle temperature appears to be the color of the shingles. Light-colored shingles reflect sunlight and don’t get as hot as dark shingles.
One possible real benefit of attic venting in climates with large snowfalls is to reduce snow melt on the roof to avoid the formation of ice dams. But according to TenWolde, a more effective——and energy-efficient——way to control snow melt in almost all climates in the United States would be to use air barriers and insulation to prevent heat from entering the attic.
Myth Three is that new homes are built “too tightly” and that walls have to “breathe.” That is the reason often given for the presence of mold in newly built houses. TenWolde cites recent research in Canada that revealed that houses that leaked air had as much, or in some cases more, mold than tight houses.
“It takes very little air movement to accomplish drying, and even a house with good air barriers usually will permit enough movement to permit moisture to escape, unless there is massive water entry. Uncontrolled air movement may actually cause moisture problems, and certainly can cost money in air conditioning and heating,” he says.
Myth Four: Crawl spaces need to be vented. To TenWolde, venting crawl spaces is just as dubious a practice as venting attics. Venting crawl spaces is marginally effective in dry climates but can be harmful in wet or warm humid climates. The best way to control moisture in crawl spaces is to use site grading, downspouts and soil covers to prevent water from entering the crawl space.
TenWolde’s Myth Five is the belief that building codes actually address residential moisture problems.
“Building codes address only vapor “barriers” and venting attic and crawl spaces. These are only two ways of controlling moisture and not very effective ones at that,” TenWolde says. " Most real moisture damage in homes is caused by water entering the home through leaks or poor flashing details. The most effective practices for controlling moisture are related to proper installation of windows, flashing, site grading, foundations, rain absorption, roof overhangs, and whole-house ventilation and humidity control.""
TenWolde’s “Five Myths” reflect the fact that there is considerable confusion and misunderstanding around moisture problems. In an attempt to resolve some of those differences and publicize the latest research findings, the FPL has joined with industry-related organizations to establish a Residential Moisture-Management Network. The network will evaluate existing research and develop uniform recommendations for dealing with moisture.
The USDA Forest Service Forest Products Laboratory was established in 1910 with the mission of conserving and extending the country’’s wood resources. Today, FPL’’s research scientists explore ways to promote healthy forests and clean water, and improve papermaking and recycling processes. Through FPL’’s Advanced Housing Research Center, researchers also work to improve homebuilding technologies and materials. Additional information about FPL and its research activities is available at](Ý°Ô)/.

Thanks Brian much good information ,Unfortunately many do not agree with this and we need as much of this type of information as we can get .

Thanks … Cookie

Been swimmin’ upstream against the flow in this stuff since 1977. Now seeing more support info from Canada Mortgage and Housing Corporation (CHMC), Forest Products Laboratory, Joe Lstiburek (Building Science Corp), Bill Rose at University of Illinois & Anton Ten Wolde , Energy Design Update, Canadian NRC Division of Building Research & Institute for research in Construction…and others. Will think of others later…have to run to an inspection.

in myth #1… which part is the myth?

…I’m thinking***, the myth is the belief,*** that a vapor barrier is needed on the exterior walls of every house.

Edit for clarity…

Only in the coldest climates like Alaska and maybe north of the sixtieth parallel. Or in very high moisture environments like pools or museums or other facilities with constant high RH.

98 or 99% of moisture that gets into walls/attics from the interior of a home is by air leakage not vapour diffusion through permeable materials (which a vapour retarder is designed to reduce/stop.) So, if you put in an unsealed vapour retarder behind the drywall, you’ve missed 98-99% of the potential problem. On the other hand, if you install airtight drywall and leave out the vapour retarder, you’ve stopped 98-99% of the problem and the other 1-2% is so little that no problems will occur!!

More BS - (Building Science)

I use references such as this in my Online Building science course taught here in Canada. Source: THE NRC - National Research Council.

Thought this article from *Home Energy *magazine (Nov/Dec 2000) should be in the BS section:


The world of the roofer is changing quickly. Only a year or so ago, customers blamed the roofer for ice damming and wet-attic problems. The solution was simply to add attic ventilation, in many cases beyond building code requirements. Why? Because it was the roofer’s only choice. In order to provide the homeowner with a full manufacturer’s warranty, ventilation had to be installed according to the requirements of the local building code: typically 1 ft2 of ventilation for every 300 ft2 of insulated attic space, and twice that amount for low-slope roofs. Unknowingly, the roofing industry was making the wet-attic situation worse. How? They were following the correct procedure for ventilation, but solving only part of the problem instead of the whole problem. Ventilating a previously unventilated attic has the effect of making the attic colder. If nothing is done to stop warm, moist air from entering the attic space from the living space, condensation on the now-cooler surfaces is a certainty. Mold, mildew, and eventually leakage into the living space will probably follow.

Insulation contractors, armed with the same lack of information about attics as the roofers, have caused similar problems. Insulating the attic floor makes the attic colder in the same way as adding ventilation. But, if contractors don’t seal as well as insulate, they don’t stop warm, moist air from entering the attic and causing big problems.

Now, thanks to public debate, reeducation, and the publication of Attic Venting, Moisture and Ice Dams, a report by Canada Mortgage and Housing Corporation (CMHC), roofers know much more about what’s happening, why, and what to do about it.

For the rest of the article, see:…00/001110.html

For the CMHC article, see: roductID=00000000260000000011

Roof Ventilation Update

The construction industry’s leading researcher explains why what we think is true often isn’t, and how some of our best hunches, based on observation of field performance, have paid off with problem-free attic assemblies

by William B. Rose

I’ve gotten many calls over the years about attics and attic ventilation; almost invariably the caller is confused, having heard different things from different people. In this article, I’ll discuss the performance of attic assemblies and try to shed light on why there are so many points of view about roof ventilation.

Research Findings
The temperature of a northern-climate roof we monitored throughout the 1990s is shown below (Figure 1). Here is a summary of the study: The roof gets cold at night and is hot during the day. It gets hotter on a sunny day than on a cloudy day. Attic assemblies with openings to the outdoors (“vented” attics) stay a bit cooler during the daytime than unvented assemblies. They also stay slightly warmer at night.

*Figure 1. Sheathing temperatures are affected somewhat by roof ventilation, but many other factors play a bigger role. *

Many factors influence the temperature on the roof. A prioritized list might include hour of day, outdoor air temperature, cloud cover, color of the roof, roof orientation, where the measurement is taken (sheathing or shingles, top or bottom), latitude, wind speed, rain or snow on the roof, heat conduction across attic insulation, roof framing type (truss or cathedral), and attic ventilation to the outdoors. As you can see, ventilation falls pretty far down the list.

To better understand how wind affects roof ventilation, Canadian researchers T.W. Forest and I.S. Walker measured the air exchange rate in attic assemblies using tracer gases. The graph below (Figure 2) gives us a feel for what they found. That is, air-change rates in the attic tended to increase with wind speed, but the amount of air change at a given wind speed was unpredictable. In fact, even with specific information about climate, construction type, and wind speed and direction, the resulting air-change rates may vary by a factor of 10 or more. Whether air flows out through a roof opening or in through that opening, and whether this airflow induces flow from indoors into the attic or helps dilute and remove moist air from the attic, can never be pinned down very well, except to say that wind is a more powerful factor than buoyancy (the “stack effect”).

Figure 2. While higher wind speeds tend to increase attic ventilation, the relationship is a weak one: Ventilation rates at a given wind speed can vary by a factor of 10.

For the most part, roof assemblies behave like any wood structure — they are wetter when cold and drier when warm. Roof assemblies tend to be hot, thanks to the sun, so they tend to be dry. Of course, if the roof leaks, that becomes the biggest source of wetness. High moisture levels indoors or in basements or crawlspaces can also increase moisture levels in the roof. Roof members can become particularly wet or covered with frost near holes in the ceiling or leaks in attic ductwork, where humid air enters the attic. It was the formation of local frost “walnuts” like those shown on the next page (Figure 3) that led researchers in the late 1930s to recommend attic ventilation. (If only they had offered to seal up the ceiling instead!)

Figure 3. Moist interior air leaking through a hole in the ceiling can produce moldy sheathing or frost on a roof truss. This photo by the author shows results from the Attic Performance Project.

Many attic assemblies are built with vents to the outdoors on the presumption that outdoor air will enter the attic and dilute moisture coming from indoors or from the foundation. The further presumption is that indoor air is wet and outdoor air is dry. Both of these assumptions are often false. If there are openings in the ceiling, then air movement in the attic can induce airflow from below, or dilute air from below, or do nothing, in ways that are just plain unpredictable no matter how much research is done. Attic air movement can also induce flow into the living space below, which is a nasty problem when the air conditioning is running.

Observations in the Field
Suppose that the picture of attic ventilation provided by physics, described above, doesn’t quite cut it. Too many qualifications; nothing pinned down. Then we can go to our own observations and experiences, subjective and incomplete as they may be. Here’s my main finding: Attic assemblies built over the last 15 years or so are pretty good. They may be a crapshoot in building-physics terms, but the crapshoot is heavily biased toward good performance.

Let’s look at attic assemblies by component:

Truss construction seems to do quite well. There are disasters that occur during construction. Truss uplift continues to be a problem requiring cosmetic fixes. The industry has, for the most part, discontinued the use of fire-retardant treatment of truss members, thereby avoiding what was a serious concern for several years. The truss heels in many cases still fail to provide the height necessary for good insulation. Attics have become a forest of truss webs, and thus are less usable for attic storage space. But the overall picture is good (at least by my observations).

Gypsum wallboard ceilings have shown improvement. The message seems to have gotten out that ceilings must be airtight — there is no justification, summer or winter, for allowing indoor air or foundation air to pass into attic cavities. The common culprits, such as framed soffits over kitchen cabinets, open oversized plumbing or mechanical chases, and leaky can lights, are going away in most construction where the word has gotten out. Weatherization of existing buildings has kept the focus on closing off any ceiling bypasses. In my experience, most truss-framed attics do fine without special vapor-barrier membranes in the ceiling, but in cold locations, cathedral ceilings may need vapor protection just as walls do.

Insulation. Regarding insulation, most areas of the country have healthy amounts in the attic — R-30 in general and R-38 in northern areas. Cellulose provides good insulation and helps block airflow. Fiberglass, in sufficient density and with good installation, also provides good thermal insulation. Foam insulation is being used more commonly, and has become the material of choice for residential air-sealing. Structural insulated panels (SIPs) work fine, as long as the airflow problem at joints is addressed. Foam insulation has been sprayed on the underside of board and wood-panel sheathing with great success. Insulated panels (often polyisocyanurate) make for good roof-deck assemblies, as we know from commercial low-slope construction, where the foam insulation is often sandwiched between the structural roof deck and the roofing membrane. (All foam needs fire protection, of course.) Open-cell foams such as Icynene may need more vapor protection than closed-cell foams, which have greater resistance to vapor flow.

Vapor barriers still cause squabbling, but most builders know that moisture flow from below comes mostly through holes in the ceiling. Cathedral ceilings require special care in insulation placement and vapor protection. But the new code provisions should encourage insulated sheathing materials or insulated “sandwich” assemblies that resist moisture transport and heat flow as a package. With these roof assemblies (I call them “insulated vapor retarders” or “fat vapor retarders”), the inside surface stays close to indoor conditions, the outside surface stays close to outdoor conditions, and nothing bad happens in the middle. Our laboratory has had such an assembly in place for more than 15 years, with one inch of foil-faced polyisocyanurate insulation directly beneath the OSB decking; the sheathing gets hot during the day, but the OSB above the foam insulation is the driest sheathing of all. Remember: Hot means dry.

Ductwork in unconditioned attic assemblies is not ideal. It’s best to place all ductwork in conditioned spaces.

OSB has become the universal sheathing material, by economic and environmental necessity. But we still know too little about the moisture performance of this material, such as under what conditions it will begin to fail. In my laboratory, we have seen the material swell by 50 percent or more under extreme conditions. Will it begin to show signs of sagging between trusses, or will workers be putting their feet through it at the time of reroofing? I don’t know, but the absence of signs of product failure in the field — at least to my drive-by observations — is reassuring. Nevertheless, I look forward to the day when the marketplace provides a product with more clearly established performance characteristics. I’ll be a strong supporter.

Shingles. I’m reviewing the condition of the shingles installed on our research laboratory in 1989; after 18 years, signs of aging are appearing. We hope to conduct laboratory tests to pin down and better quantify the shingle performance and the factors that influence it. The aging we see shows some temperature effect: The white shingles are in better shape than the dark, and a few of the most aged-looking shingles are found on the hottest bay, the one with foam directly on the underside of the sheathing. Without the numbers to go by, we must rely on observation, and our observations suggest that performance depends on other factors besides the presence or absence of ventilation and whether the assembly is truss-framed or cathedral ceiling.

Of course, natural weathering tests that began 18 years ago say little about shingles that are made today. I sense that the shingle industry is currently producing dimension shingles that seem to lie quite flat, resist wind uplift, and hang on to their UV-protecting granules. I don’t know how to reroof over dimension shingles, and it does seem unfriendly to the landfill to have that much more mass in the shingle. Nevertheless, my drive-by observations show a lot of good-performing shingles going on roofs over the last couple of decades, and that is very reassuring.

Roof vents. Many years ago, we measured the “net free area” of about a dozen ridge-vent materials. (We used an apparatus that measures the pressure drop across a vent device with great accuracy.) We found that ridge vents with large openings (minimum opening dimension around 1/4 inch) had an equivalent net free area very close to their rated capacity. Vent devices with small openings — or with filter fabrics, or scrims — performed much worse, as much as 75 percent less than their rated area. (If you want to know how restrictive a vent device is, use your imagination — if it looks like air would have a hard time moving through, it probably does.)

This discrepancy would be a big deal, I suppose, for someone who felt that vent regulations were critical to attic performance. I don’t, so for me, having vent devices with less airflow than advertised is not a cause for concern.

Building Codes
You — and your building code inspectors — may be unaware that the 2006 version of the IRC for one- and two-family dwellings permits attic construction with no ventilation of the attic cavity. This new provision, R806.4, is largely due to the efforts of Joseph Lstiburek, Armin Rudd, and their colleagues. In brief, unvented conditioned attic assemblies are permitted when an air-impermeable insulation such as rigid foam is applied in direct contact to the underside/interior of the structural roof deck, with sufficient thickness — given the climate — to prevent condensation on the underside (see “Insulating Unvented Attics With Spray Foam,” 3/07).

This new provision is a direct challenge to the rule of thumb that has been in place for 50 years, which says that you have to vent a steep-roof attic so the ratio of net free vent area to the projected roof area is 1-to-300 (or 1-to-150 when using “cross ventilation” rather than soffit and ridge vents). This ratio arose from observations of frost on protruding nail points in Wisconsin homes by researchers at the Forest Products Laboratory in 1937, and frost on aluminum plates in research “doghouses” at the University of Minnesota in 1938, under “outdoor” conditions of -13°F.

The Federal Housing Authority turned these findings into the famous 1-300 ratio in 1942, to be applied as a minimum building requirement for the small homes in its financing program. The requirements were picked up by model codes and others following World War II, and the rest, as they say, is history. Shingle manufacturers did not begin piggybacking their warranties on venting regulations until reports of shingle problems began piling up following the change in asphalt sources in the early 1980s.

To Vent or Not
Every designer and builder should be able to produce good attic and roof assemblies, both with and without ventilation — or anything in between — with just part of a conventional ventilation system. For example, from our studies, roof assemblies that have holes but not necessarily straight airflow paths (one gable end vent, or soffit-only) should also be candidates for good performance. And although unvented roof assemblies can perform well, there are still good reasons to vent: The truss-framed, steep-roof attic with an insulated ceiling has been the workhorse of single-family construction, and ventilation works well with this construction, at least in the northern United States.

In some cases, there are also good reasons not to vent: in wildfire areas, in complex cathedral ceiling assemblies, in existing and historic buildings that have never had ventilation, in shed roofs beneath clerestory windows, with foam insulation (foam and ventilation do not go together — think fire), and in complex roof assemblies that combine steep and low-slope construction. I’ve also heard persuasive arguments against venting in hurricane-prone regions, but I’m not an expert in that area. In short, since critical performance doesn’t hinge on ventilation, then either vent, no-vent, or an in-between “kinda”-vent can be taken as the starting point. Whether the choice works or not depends mostly on other factors.

So you should vent where venting is appropriate and not vent where it is not appropriate. As it turns out, the worst-performing, most mold-ridden attics I have seen were vented — with a flooded crawlspace and a direct path for air movement from the crawlspace to the attic. You can mess up a vented attic by allowing such airflow. You can mess up an unvented attic as well, usually by not providing vapor protection appropriate to the climate and indoor moisture levels. Tight ceilings would be a great first step toward moisture control, summer and winter.

The father of a colleague of mine says that when the word “ventilation” comes out, people stop using their heads. Vented assemblies often perform well, but not always. Sometimes roofs appear to be vented but actually aren’t. Still, we can take comfort in the observation, based on years of experience, that our attic assemblies are pretty darn good, and — in my opinion — they’re getting better. We need to constantly be on the lookout for new conditions and new problems, as they crop up.

Those of you working in the trenches should continue to build in a way that complies with code and that you know works for your climate. For more information about ice damming, summer cooling load, shingle service life, and moisture issues, visit (TenWolde and Rose, “Issues Related to Venting of Attics and Cathedral Ceilings”). For all four of these concerns, ventilation makes a contribution that is generally more positive than negative, but it hardly ever makes the difference between success and failure.

For the most part, the focus of codes, researchers, designers, and builders on roof ventilation is misplaced. Instead, the focus should be on building an airtight ceiling, which is far more important than roof ventilation in all climates and all seasons. The major causes of moisture problems in attics and roofs are holes in the ceiling and paths for unwanted airflow from basements and crawlspaces. People should focus first on preventing air and moisture from leaking into the attic. Once this is accomplished, roof ventilation becomes pretty much a nonissue.

*William B. Rose is a research architect with the Building Research Council at the University of Illinois at Urbana-Champaign, and the author of Water in Buildings: An Architect’s Guide to Moisture and Mold. This article was adapted from The JLC Guide to Moisture Control. *


Roy: Great find!

In the first insulation rebate program (CHIP: Canadian Housing Insulation Program;1978-85), my insulation company stopped adding ventilation to our retrofitted attics as we had airsealed each attic and usually installed a vented bathroom fan. Also our area had high winds that could blow snow in through attic/roof/soffit vents…sometimes up through vented soffits and vented cathedral slopes 12-13 feet to end up in an attic. So venting became a negative for us.

But we were listed/certified/inspected by CGSB (Canadian General Standards Board; they certify polyethylene vapour barriers, caulking, etc.). They threatened de-listing us 2-3 times after inspections due to attic ventilation “not to code”. De-listing meant our customers not being able to receive the gov’t grants/rebates!!! This money was about 33% of our revenue at the time.

After a full afternoon meeting with the inspector at which we brought NRC/CMHC reasearch to his attention, he wrote his superiors that he felt his training was inadequate due to fact that air leakage and attic moisture had not been included. We were then allowed to treat attics as we saw fit until they had a complaint about attic moisture problems related to our work…never happened.

Funny episode happened the day of the meeting: The inspector had to inspect some of our work, selected at random, before our meeting. One house he chose was a bungalow with a hip roof. The north facing slope already had a triangular gable style vent directly above the attic hatch. It had just snowed and blowed about 2 days before…when he opened the hatch, snow fell on his head. Started to clear the misunderstanding, right on the spot.

Here are a couple of future commercial Building Envelope Conferences for those so inclined:

[FONT=Verdana]From the Canadian Roofing Contractors Association (CRCA) with my bold and some comments in blue (more may follow later):



Many houses and small buildings are constructed with a roof or attic space
between the upper floor ceiling and the roof deck. Whenever a roof space exists over an occupied interior, measures must be taken to properly ventilate the space. Adequate ventilation can reduce (but not fully stop it) the potential for the occurrence of harmful condensation and the formation of ice dams during the heating season, and excessive heat build-up during the summer months. This bulletin reviews the requirements and means of ventilating roof spaces.


In the distant past, most buildings and homes were not built to conserve energy. Few storm doors and windows were installed, and for the most part they leaked air and heat. Under these conditions, the moisture inside also leaked out with the air and through cracks and openings to the outside. Sometimes, an owner of a wood frame house might have experienced the
peeling of paint on the cladding, but for the most part, with the exception of the difficulty in keeping the interior warm and free of drafts, few problems were encountered.

The oil crisis of the early 1970’s led to the need for the conservation of fuel and improvements in the thermal efficiency of buildings. Two energy saving techniques were to reduce air leakage in the building envelope, and the other was to increase the amount of insulation in walls and roofs. This resulted in higher interior humidity and cooler attic temperatures. Both of these, in
turn, increased the potential for condensation in roof spaces in cold weather.
To reduce condensation, these spaces must be properly ventilated and ceilings must be made sufficiently airtight.

Condensation, if uncontrolled, can have serious consequences. It can lead to the premature deterioration of the roofing material, decay, or corrosion of the
structural elements; a loss in the thermal insulating value of the insulation and damage to interior finishes. Wet attics can provide an environment conducive to the growth of fungi and moulds that may be hazardous to the health of the occupants.

Moisture is transferred from the living areas into the roof space by diffusion due to a difference in water vapour pressure and by air leakage, resulting from an air pressure difference due to wind action, stack action and mechanical ventilation. The latter may be particularly significant in tall buildings where pressurization by substantial excess of supply over exhaust air is required to overcome the pressure differences from chimney action at the lower floors or in houses with supply only ventilation systems.
Pressurization magnifies condensation problems that result form the ex-filtration of moist indoor air. When warm air flows into a cold roof space it is cooled and its ability to hold water is reduced. If the temperature of this air falls below the dew point (the temperature at which air has a relative humidity of 100%, the water saturation temperature) condensation will occur.

Human activity produces a lot of moisture. The main sources of water vapour include cooking, baths, showers, laundry, houseplants and non-vented combustion appliances. Occupants of houses generate a significant quantity of moisture through breathing and perspiration. Although most new homes are equipped with fans in bathrooms and kitchens, they are not capable of removing all the excess moisture produced (as they are not well designed and installed !!!). In addition, human comfort depends on a defined range of relative humidity. The American Society of Heating Refrigerating & Air Conditioning Engineers’ (ASHRAE) guidelines suggest that 50% RH is the most desirable level of relative humidity in order to maintain airborne infection at a minimum. However, this level of RH may be too high for most houses in Canada under winter conditions and lead to condensation problems. During winter, the type of windows in a house will significantly affect the maximum allowable RH. For example, to avoid damage to sills and to control fungal growth, the indoor relative humidity generally should not exceed 35% if the windows are double glazed. With triple glazed windows the RH may be increased slightly to around 40%. The maximum allowable RH will vary with the geographical location, wind and outside air temperature.

The flow of water vapour through a material by diffusion varies in proportion to difference in vapour pressure across the material and the permeability of the material. If the vapour pressure is higher in the living area than the adjacent roof space, moisture will diffuse through the ceiling into the space. In most occupied buildings we install vapour barriers to slow down the rate
of diffusion, however, diffusion cannot be prevented entirely.

Diffusion alone accounts for only a small fraction of the moisture that finds its way into the attic or roof space. Most moisture enters a roof space as a result of air leakage. Air from the living area can leak into adjacent roof spaces through many paths. For example, chimneys, vent pipes, electrical wiring and other services, gaps between wall finishes and the framing, and cracks caused by deflection, shrinkage or settlement. Dryer, bathroom and kitchen fans should never be exhausted into an attic. Exhaust outlets should not be located directly beneath the soffits as warm humid air will be drawn into the attic.

Ice dams

Ice dams form at the eaves of sloping roofs as the result of a combination of snow on roofs, outside temperatures below freezing, heat loss form the building or solar radiation that causes the snow to melt, and an open end of snow where ice can form. As the snow melts form the roof due to heat loss or solar radiation it runs down the slope until it reaches the eaves. As the eaves of most sloped roofs extend past the heated interior space the surface temperature at the eave is generally close to the ambient air temperature. When the ambient air temperature is below freezing the melt water will freeze upon contact. Progressive melting and freezing will cause the ice to build-up at the eaves, preventing the water on a roof from running off. On roofs with
roof covers composed of overlapping watershedding units, water may back up at the ice dams and leak into the roof and wall construction causing considerable damage.

The source of this water is normally melting snow. Since snow is a good insulator, the temperatures in a poorly ventilated (warm) attic are sufficient to melt snow that forms on the roof even in relatively cold weather.
Although the primary contributor to snow melting is heat loss from the building interior, solar radiation can also provide sufficient heat to melt snow on a roof. **For example, at Ottawa, enough sunlight can be transmitted through 150 mm of snow cover on a clear and sunny day to cause melting at the roof surface when the outside temperature is -10°C with an attic temperature of -5°C. **(and the attic is below freezing!!!)

The total heat loss will depend on the inside temperature conditions, the amount of insulation in the roof system, the amount of air leakage from the inside spaces, and the ventilation between the insulation and the roofing. It is practically impossible to eliminate the formation of ice in sloping roofs entirely. The amount of build-up can be greatly reduced by having sufficient
insulation and adequate ventilation (what about reducing air leakage?). The continuity of the insulation, to eliminate hot spots, is as important as the amount of insulation. The potential for icing can be further reduced by increasing the slope, making the surface slippery so that snow slides off, and by not installing gutters. In a reasonably well-insulated building, the minimum ventilation requirements asprescribed in the building codes are usually adequate to minimize ice damming

Most building codes recognize the potential for ice dam formation and contain
requirements for eave protection. Some designers and contractors have gone the extra step to apply waterproof membranes over the entire roof surface prior to the application of the roofing cover to prevent water ingression from ice dams or wind driven rain. However, this may exacerbate the problems associated with harmful condensation. These membranes are often vapour impermeable creating the potential of building a vapour trap within in the roof
space. Careful attention to the need for adequate ventilation is necessary when water and vapour impermeable roof covers are applied over attic and roof spaces.

*Heat build-up

The Canadian Asphalt Shingle Manufacturers Association has issued a bulletin titled “Proper Ventilation for Asphalt Shingle Covered Roofs”. In it they state that both heat and moisture build-up in attics is the primary cause of many roof problems including blistering, distortion and curling of the shingles. It has been theorized that poor ventilation of attics can cause excessive heat build-up and high deck temperatures. Since heat is the major contributor to the aging of materials, this heat build-up is said to contribute to the deterioration of many roof-covering materials. Although a recent study (and studies by others) by Simpson, Gumpertz & Heger Inc., demonstrated that geographical location, building orientation, and roof colour have far greater influence on roof surface temperatures than the ventilation below, inadequate ventilation is often (still) cited as the reason for roof performance problems. Material manufacturers may not honour warranties where it can be shown that there is insufficient ventilation of the space below the roof.

Ventilation of roof spaces

Roof spaces can be vented by either natural or mechanical means. In northerly climates natural ventilation is the most common method of venting roof spaces. Wind causes positive pressure on the windward side and
negative pressure on the leeward side of a building. As a result, outside air is drawn into the roof space in zones of positive pressure, and expelled in zones of negative pressure. The rate of air change in the roof spaces depends on the velocity and direction of the wind. Natural ventilation can also result from stack action. As air becomes warmer, its density decreases, and it becomes buoyant and rises. On a sunny day (in winter), the temperature inside the roof space is normally higher than the outside temperature (due to solar
radiation and interior heat loss). The heat will tend to rise towards the upper part of the attic. Air exchanged by this means is facilitated by air intake openings at the lower part of the attic, and exhaust openings at the upper part. Unlike ventilation by wind action, ventilation through stack action
is more constant depending only on air temperature differences. Natural ventilation can be optimized by making use of both stack and wind action.

*Types of roof spaces

If the framing members supporting the roof deck do not also support the ceiling, the roof space or attic is usually fairly large and can be easily ventilated. Air movement is a function of the geometry and volume of the
roof space. If the framing members also support the ceiling, as with flat roofs and cathedral ceilings, the resulting roof space is relatively small and more difficult to ventilate, particularly when it is well insulated with thick layers of insulation. These types of roofs, therefore, require more ventilation than common attic type roofs.

Where the roof space is fairly larger, for example cold, uninhabited attic spaces with roof slopes of greater than 1:6, the total net vent area should be at least 1/300th of the area of insulated ceiling. These vents can be
located in the soffits, in gable walls, in the roof surface or at several of these locations, but should be distributed so as to provide effective cross ventilation.

For flat roofs, or low sloped roofs (with pitches of less than 1:6) such as cathedral ceilings, the total net vent area should be at least twice as large, or 1:150th of the area of the insulated ceiling. A low slope roof decreases the amount of ventilation that can be achieved through stack action, and the presence of large quantities of insulation often restricts the ventilation space between the insulation and the roof sheathing. In such roofs, moisture will tend to condense on the cold surface of the sheathing and framing members because it cannot dissipate over a large area as it can in attic roofs. Ventilation in this type of roof can be improved by installing purlins of at least 38 mm by 38 mm at right angles to the roof joists. This makes it possible for wind coming from any direction to ventilate the roof space, provided that roof vents are installed on all exposed sides.

Purlins may not be necessary if the roof slope is 1:6 or more, provided that the roof framing members run in the same direction as the roof slope, each joist space is separately vented and that a minimum clearance of 63 mm is maintained between the roof sheathing and the top of the insulation. In highly insulated flat and cathedral type roofs, the depth of conventional roof framing may not provide sufficient ventilation space between the insulation and the roof sheathing. Using parallel chord trusses can provide additional
depth, and their open webbing facilitates air movement through the roof space.

Because the small roof space in flat and cathedral ceiling type roofs makes it
practically impossible to inspect the roof space, it is important to keep the air leakage through the ceiling at a minimum, and to provide adequate ventilation.

*Types of vents

Soffit vents, roof vents, ridge vents, gable vents, or any combination thereof may ventilate roof spaces. All vents must be designed to prevent the entry of rain, snow and insects. Soffit vents along the underside of the roof
overhang may be continuous, or consist of individual spaced vents. Soffit vents allow the air to flow easily through the roof space from all wind directions.

Combining soffit vents with vents at the upper part of the roof will further increase ventilation. The passage of air from the soffit should not be restricted by insulation.

Deflectors or baffles should be installed to maintain a space between the insulation and the underside of the deck. Vents should be located at or near the ridge when used in combination with soffit vents.

If roof vents alone are used, only a small volume of air within the immediate area of the vents will be displaced. The rule of thumb is that half the venting should be provided at the eaves, and the other half at or near the ridge.

Turbine vents are sometimes installed near the upper part of the roof. They consist of a rotating assembly of wind driven helicoidal blades that draw air from the roof space. However, if suction is created inside the roof space because of insufficient air supply from the outside, moisture may be drawn through the ceiling from the living space aggravating the condensation problem.

In northerly climates using motorized fans to improve ventilation may help cool the living space in hot weather. However, they may lead to problems under winter conditions. By blowing air out of the attic they can
lower the air pressure within the attic space relative to the air pressure inside the house. The negative pressure produced increases the amount of humid air leaking through the ceiling. In cold weather, the increased
leakage will, in turn, lead to increased condensation. In addition, some of these fans create air currents strong enough to disturb loose fill insulation in the attic.

The most efficient venting system uses continuous ridge vents in combination with soffit vents. This system enables even and continuous ventilation of the attic by combining wind action and stack action. Ridge vents have continuous openings along the ridge. Their location in a zone of negative pressure allows them to act as air exhaust openings for all wind directions.
When the wind is at right angles to the ridge, the air enters the attic through the soffit vents on the upwind side and leaves the attic either through the openings at the downwind soffit or at the ridge. When there is little or no wind, stack action will maintain some air movement inside the attic. Rising warm air escaping through the ridge vents will be replaced by cooler air entering through the soffits.

Whichever ventilation devices are chosen, it is imperative that they be properly located. Placing intake and exhaust vents too close together, or not achieving a proper balance, may create areas of stagnant still air within the attic or roof space effectively short circuiting the ventilation. If the system
cannot be balanced equally, it is recommended that the intake exceed the exhaust as most problems result from insufficient intake.


The question often arises as to whether additional vents are required when adding insulation to the attic of an existing house. Retrofitting provides a good opportunity to inspect the attic for signs of moisture problems due to condensation. If the existing ventilation of the roof spaces conforms to the applicable building codes, no signs of moisture problems are evident and interior occupancy conditions remain the same, it may be wise to leave the
ventilating system unchanged.** If, on the other hand, signs of excessive moisture were evident, it would be preferable to correct the problem by improving the air tightness of the ceiling rather than by increasing the total vent area.** There is a law of diminishing returns. Adding more vents risks creating negative pressure in the attic and increasing air leakage through the ceiling.


It is not unusual to see a thin layer of condensation or frost on the underside of the deck in very cold weather. This is not indicative of ventilation related problems. During very cold weather, the outside air can hold very little moisture. Most of the moisture that finds its way into the roof space will condense out of the air when it comes in contact with the cold sheathing.

Cold air in the attic is not effective in removing condensation because of its low moisture storage capacity. Frost will form, therefore, on the underside of the deck. In most cases, this should not be of concern. When the air gets warmer, as in the late winter, its capacity to absorb moisture increases and more moisture will be removed as drying conditions improve.

Many homeowners misinterpret a dripping or water stained ceiling as signs that the roof cover is defective or has a roof leak. In many instances, the real culprit is not a breech in the roof, but the consequence of ineffective ventilation of the roof space combined with excessive air leakage. Providing adequate attic ventilation can help reduce the potential for condensation, ice dam formation and many other roofing problems.


[FONT=Verdana]Forgues, Y.E. “The Ventilation of Insulated Roofs”, Building Practice Note 57, National Research Council of Canada, Division of Building Research (Ottawa, 1985)

CASMA, “Proper Ventilation for Asphalt Shingle Covered Roofs” Technical Bulletin No. 1 (1992)

Strother, E.F. and Turner, W.C. Thermal Insulation Building Guide. Robert E. Krieger Publishing Company (Malabar, Florida, 1990)

Cash, C.G. and Lyon, E.G. “What’s the Valve of Ventilation?” Professional Roofing (March, 2002)

Baker, M.C. Roofs, Multiscience Publications Limited (Montreal, 1980)


Some great insulating products being advertised here…NOT!!!


(2) **“The P2000 Insulation System reflects up to 97% of radiant heat energy.” **

Yes, but radiant heat is not a large part of heat loss from a regular home. This misleads the consumer away from conduction/convection/air exchange as the main mechanisms of heat loss from buildings.


The website appears to have responded to a complaint to the federal government “anti-competition” dept. as they used to claim R27/inch!!!

In May/06, I was asked to attend a meeting by my former colleagues at the NS Dept. of Energy with them and the P2000 eastern Canada distributor plus his “brought-in” licensed engineer (not an employee of the company). Following is a letter I wrote to the engineer after the meeting with them:
Note: I did all this pro bono!!

Dear ---------:

At our last Friday morning meeting at the NS Department of Energy, I was very concerned about your statement that 70% of heat loss from buildings is radiant! My training in the energy field began in 1977 and has been continuous until present through working specifically in the field, attending energy-related conferences and seminars, and reading periodicals such as “Energy Design Update”, “Home Energy”, “The Journal of Light Construction” and “Solplan Review”. I must admit that I have never heard or read this fact before!! After a fairly extensive search for figures of radiant heat loss from homes, I found that numbers are not readily available (except from many foil promoters) but offer the following quotations regarding this form of heat loss:

(1) In the “The Residential Energy Audit Manual” written by the US Department of Energy with the Oak Ridge National Laboratory, The University of Massachusetts Energy Education Center and The Solar Energy Research Institute, as contributors, Chapter 4 , “The Basics”, states: ***
*** " Although all three mechanisms of heat transfer (conduction, convection and radiation) occur within and through the envelope of a home, in most cases, the effects of radiation occur primarily at the outside surfaces of the outside walls. Heat radiated from these surfaces must first have been conducted out through the walls. This means that heat losses or gains from radiation can be ignored since the initial conductive losses or gains will be carefully calculated. Thus, for the purposes of energy audits, no attempt to calculate radiation losses or gains is made"

(2) From the May 2006 newsletter “Energy Design Update’s” Readers Forum: “One really basic concept I’ve used to explain this to people recently is that radiant heat transfer is largely between surfaces at differing temperatures. Once you have a reasonable (and relatively small) amount of conventional insulation (R10-R15), you will drop (MY NOTE- “have”) nearly all the temperature difference across that insulation.The surfaces on each side of the insulation will be very close to the temperature of the air (or other building materials) on either side- and at that point, you really have no radiant transfer any more.” Bruce Harley, Conservation Services Group, Stamford, Vermont

(3) From the New York State Weatherization Directors Manual, the following example and comments about about radiant heat loss:
*** “[size=2]Example of Radiant Heat Loss - In a home with a wood stove in front of a wall with a lot of glass, a good portion of the radiant heat from the stove (not the house- my edit- MY NOTE) would pass to the outdoors.This form of heat loss is small in most homes and the weatherization program does not address this type of loss.”***[/size]
(MY NOTE: The surface temperature of the stove would in the range of 500 to 1000 Deg F and as such would be a huge radiator in the infrared range. Radiative heat loss through the windows near the stove could be measurable but would still only be fairly small as other sides of the stove would be radiating into the room!!.)

***(4) From the Cincinnati Business Courier - May 19, 2006:[FONT=Arial][size=2] “Take comprehensive approaches to energy savings, including taking advantage of natural daylight entering buildings through windows to reduce heating requirements up to 10 percent. At night, shading windows reduces radiant heat loss from buildings.” (MY NOTE: Office towers and commercial buildings usually have large expanses of windows leading to a fair amount of radiant heat loss due to the transparent glass!!)***[/size][/FONT]

(5) From the NRC-IRC publication “Construction Innovation” Winter 1999 issue, a CCMC submitted article states: “Most of the heat transfer through this assembly of low emmissivity sheet material and furred air spaces is conductive; radiation contributes only about 10%.” (MY NOTE: The only reason that radiation was 10% in this air spaced part of the assembly is that the air spaces were created in the first place!! No air spaces means very little or no radiative heat transfer.

***(6) Notes in a Building Products Evaluation (March 31, 2006) from the State of Winconsin state “Perka P2000 reflective insulation board will only be alotted an R5 per inch as indicated in the ASHRAE Book of Fundamentals-1997” and “However, the State of Winconsin does not recognize the reflective foil facing on the product. Heat transfer in Wisconsin is based solely on conduction and convection.” (available on the web: ***

***It is instructive that these documents basically regard radiant heat loss at wall surfaces as neglible when dealing with energy balances in homes. Due to the number of “heat reflective” products from reflective paint to foil bubble packs to foil faced foam boards being foisted on the public in the last decade, I also call my list of popular energy misconceptions the “Smoke and Mirrors” list. To quote John Straube, PEng., Phd. (building scientist with dual appointments to the Faculties of Architecture and Engineering at the University of Waterloo): “…but I have looked into several products that are based on controlling radiative heat loss. They all seem to be the snake oil of the 90’s. Why? I think it is because no one understands heat transfer, let alone radiative transfer. Just when people start figuring out that R20 is good, people come along to screw up the meaning of R-value again. It all keeps me ungainfully employed.” ***

The above statements and quotes re-affirm my understanding of radiative heat loss in homes and most other buildings. With so little radiant heat loss through walls with well installed regular insulation materials, I cannot see where the foil based products will give an economic benefit to owners of these buildings.

***As a professional engineer, you are held in high regard by the general public and as such owe the public the professionalism to be accurate in your statements. [FONT=Arial][size=2]The field of energy efficiency and conservation in buildings has now become a separate “niche” discipline as would be a civil engineer’s specialization in, for example, suspension bridges. In the past 6 months, I have occasionally posted in the web forum of Fine Home Building magazine in which a few engineers participate. I have been quite surprised with some of the statements made in the section on energy/heating related topics. When presented with information/facts from respected research agencies or government, they usually recant or change opinions. ***[/size][/FONT]
Brian MacNeish
Atlantic Home Inspections

I only received a short response from the engineer in July/06 saying he would fully respond quoting “his own experts”. No response yet!!

Still have not yet received any response from this professional engineer


On the subject of radiant heat barriers…I had a real estate salesman ask me to recommend a product that he had invested in to my clients. Silver attic paint…$57 per gallon…that would “reduce energy bills by 25%” and would “extend the life of an air conditioning unit by 50%”.

When I asked for science data in the form of lab reports (knowing that there could not be any such data) I was referred to a broken link to Reader’s Digest.

I admit that I could probably have been less rude in my rejection of this “business proposal” but it insulted me to have someone suspect that I could be stupid enough to fall for crap like that.

Yet…he did…and he sells it to others.

P.T. Barnum was right.

This may be getting worse in North America…See the scientific literacy study:

BUMP…A thread for Robert!!


Still no letter back from the P2000 engineer!!

New web address for P2000 insulation:

Here’s another similar: 3HT