About Air Barriers

About Air Barriers

Difference between Air Barrier & Vapor Barrier & Water Resistive Barrier

Vapor Barriers are not to be confused with an air barrier. A vapor barrier is designed to restrict the flow of water vapor through a material, just the same as a air barrier material restricts the flow of air through a material.

Vapor barriers or vapor retarders are intended to control the rate of diffusion into a building assembly. As a vapor barrier they will control the rate of moisture flow where they are placed. Therefore the vapor barrier does not have to be continuous, does not have to be free of holes, does not have to be lapped, does not have to be sealed, etc. A hole for example in a vapor barrier will simply mean that there will be more vapor diffusion in that area compared to the other areas where the vapor barrier.

Water vapor permenance is measured by the amount of water which will work its way through a material. This is normally reported in ng/(Pa•s• m2) Many areas require a vapor barrier which has a maximum water transmission rate of 60 ng/(Pa•s•2).

Much work is being done and much discussion is being held on whether vapor barrier should be used at all and if they are used should the water transmission rate be. There is discussion on the need to allow buildings to dry. Keep in mind that during this time period where vapor barriers are being discussed you still need to meet the local building code which may demand it even when Building Science would indicate that it should not be used.

Water vapor may be transported by air leakage but you deal with this by installing a proper air barrier.
Vapor barriers are designed to be installed on the warm side of the insulation.

**Water Resistive Barriers **are materials which are primarily designed to be used to keep liquid water from entering the building enclosure. Water resistive barriers are specifically designed not to be a vapor barrier. The minimum water vapor permeance for a water resistive barrier is 300 ng/(Pa•s• m2).
Water resistive barriers are combined with flashing and other materials to ensure that there is a shingled assembly to direct liquid water which passes on the cladding system to be directed to the exterior.
Water resistive barriers are designed to be installed o the cold side of the insulation.

**Combined air barriers, vapor barriers and water resistive barriers **can be provided in a single material. There are also vapor permeable air barriers, and there are water resistive barriers which are not air barriers.
Please understand the three separate functions and then determine whether the material you choose provides more that one function and then you need to decide whether you will design your building so it actually performs more that one function.

As an example, you can have two, three or even four air barrier materials in a wall assembly but it will depend on which material you have chosen and how you have connected the air barrier materials together.

Gypsum wallboard, polyethelyne film, exterior grade drywall, self adhered membranes and spray polyurethane foam insulation are all air barrier materials in a specific wall assembly but you need to choose which one is the air barrier and then ensure that you have the air barrier material chosen has been connected.

More articles can be found here;

Very informative.

Marcel :slight_smile:

It has been said a number of times now by some BS researchers that we really don’t need vapour barriers in houses except in the far north. 98 -99% of the interior moisture that moves into walls and attics is from air leakage. If we stop that amount of moisture by using air barriers or more correctly air barrier systems (different materials sealed/gasketed together to form an integral barrier), the 1-2% that gets into the walls,etc will be so small as to not create prolems and will be absorbed by materials to be released when conditions change by warming, etc.

The need for a vapour barrier has been one of those strongly ingrained concepts that people do not want to give up, no matter what the research evidence says. What we need is an effective air barrier system. This has been stated by Joe Lstiburek and Building Science Corp., the US Corps of Army Engineers, The Canadian National Research Council, and others.

A Canadian Building Research Digest in 1975 stated “Air leakage is now considered to the prime cause of most condensation problems in walls and roof spaces. If, therefore, a building can be made tight against air leakage, it may not need a vapour barrier, as defined. On the other hand, if there are openings that permit air to leak from the warm side to the cold side of the insulation, adding a vapour barrier (even of zero permeance ) that does not seal off the opening will be useless… The control of air leakage is probably the single most important factor in obtaining a problem-free building envelope.”.

From 1960 Digest #9 *Vapour Barriers in Home Construction: “*The most important general principle to be followed in both design and installation is to reduce to a minimum the number of openings in the barrier. Where such openings are necessary, special care should be taken to seal the barrier so as to approach complete continuity.” (This last statement is from a mentor to Joe Ltsiburek!!! so you know where he gets it from!!)

As you can see, the air leakage info is not new!!! A lot of moisture damage in walls and attics could have been prevented if people read this stuff as it was produced. Now even 47 years later I find construction industry folks fighting air tightness as bad…they got to go back to school because they don’t see the whole picturel!!!

Thanks Brian, that was usefull information.

It kind of sparked some memories of how we were building in the 60’s.

My father was a strong believer in saturated felts on the exterior under siding.

And to this day in all my building experience, yet have I seen any problems with the #15 felt when properly installed.

I just removed a brick facade’ on a building 8 months ago and saw for myself that the saturated felt installed behind the cavity and brick ties were like the day it was installed. 25 years old.

I have removed siding with felt paper as well under sidings that had failed, and looked like the day it was built under the paper.

So all the science is trying to do is understanding how to prevent air infiltration to the warm side of the envelope.

Proper installation I believe would be the key word here to prevent air leakage and infiltration within the assembly.

Again thank you for your informative post.

Marcel :slight_smile:

This short history of NRC’s division of Building Research (DBR)will interest you. Most of their research and *Building Digests *are free online now. This was my greatest find in building science way back in 1977…I only paid $15 or $20 for the results millions of dollars of research!!!

From a post I made on the roof ventilation thread a few months back:

*"Consequences of an Ineffective Air Barrier System *

*The concept of the building envelope as an environmental separator was first promoted by Dr. Neil Hutcheon, former Director of the Division of Building Research, in a talk to the Engineering Institute of Canada in 1953. Dr. Hutcheon listed the principal requirements of the building envelope, so that each could be addressed separately and in conjunction with its counterparts. He noted that building envelops had to be designed to control air flow and the moisture and energy flow associated with it. To quote from his talk: **“The flow of heat, moisture and air in walls have implications not only by themselves, but for all the other considerations listed. **Air merits major consideration mainly because of its influence on heat and moisture flow.”***Remember, this is 1953 (hell, I was only 3 then), more than a 1/2 century ago! How smart are we really to not listen to or respect those that have much deeper insights into what is really happening in buildings???

Things are changing though and we’re beginning to listen to our elders!! From a course offered at Minnesota State U. recently:

[FONT=Verdana]"*In 1950s, Neil Hutcheon from the National Research Council of Canada openly challenged the theory of diffusion as the only moisture transport contributor to moisture induced problems. He put the theory of diffusion to the test using the cold climates in Canada, showing that the measured rate of condensation was 10 times greater than that offered by Fick’s theory. **Prescriptive measures using simple formulas were found to be imprecise, misleading, or even detrimental to the envelope design. That was the shifting ***point at least in Canada for consideration of air leakage effects.

*According to (Bill) Rose, it was not until 1980 that US practice began shifting back toward a **concern for convective air transport of moisture." *

Here’s a great front page article from Energy Design Update (EDU) …been reading this periodical for almost 25 years now…keeps you ahead of the curve. It’s a bit pricey at US $385/year (CAN $621/year **{GASP} **at the low of our former US$0.62 Canadian “peso” …that’s now worth US$1.00+…Hey, I can afford to go south again!!!). I particularly like the last paragraph that describes what happened to building science as design won out over function in modern architecture:


Thanks Brian, that was very interesting, and enough that I might post a few excerpts from the article. I can relate very well with the Article due working with Architects and Engineers for over 40 years.

I have learned to recognize the systems of building science as described and designed by working with many Architects.

Again thank you for the information.

Marcel :slight_smile: :smiley:

Thanks Guys.