Insulation in crawl space and basement

I’ve heard different oppinions about installing insulation with vapor barrier here in central Florida, being different from the way they do it up in the North. What I learned, and I’m a nube in this industry, vapor barrier always against the warm side of the house. Is it incorrect, in Central Florida, if the vapor barrier is facing the basemen or the crawl space?

Probably not, but I’m a northerner. A mild correction: the vapor retarder should be on the **winter **warm side of an assembly.


Send me your e-mail and I will send you a Power Point presentation.


15 years ago, I bought a 5 year old house with 3 vapour barriers throughout the wall system (Oh my god!!!). The best of the 3 is the outermost , just under the cedar clapboard siding and serving as the sheathing membrane in place of tarpaper, tyvek, typar, etc. Here’s the wall system in to out:

  • 1/2" drywall, not sealed

  • 6 mil certified polyethylene, partially sealed

  • 2" x 4" studs with inlet 1" x 4" corner bracing; cavities contain R12 fiberglass batts

  • sheathing is 1" x 4’ x 8’ Purlboard (R6 in the foam itself; not counting the foil- virtually useless for R; gives strength and some protection to the board). This product used to be manufactured locally but is now out of business. The Purlboard is a 1" extruded polyurethane sheet with unperforated foil (the best vapour barrier in terms of permeance) on both sides with seams taped at the exterior (foil and sheathing tape).

  • 8" western red cedar clapboard (a “seconds” variety)

This system is equal to or slightly better on an averaged R value basis than is a 2" x 6" wall with wood sheathing and an R20 batt. The Purlboard covers the framing (15 - 22% of the wall) and reduces “thermal bridging” through the relatively weak R value (1.25R per inch) of the wood.

By theory my house should be on the ground composting itself by now. The only problems I’ve had (being on the edge of the ocean with horizontal rains) is leakage at a few locations on windward sides due to poor installallation/ flashing of windows. Story is: all is not steady state in the world as it in in theory and calculations. One day wind/mechanical/stack pressures may take some moisture into the wall; the next day, conditions change and air is moving exactly the reverse, drying if necessary, any condensation. Another item to account for no rot, etc is that with the insulated sheathing, the wall cavities are warmer and probably moisture doesn’t condense at all.

All is not as it appears!!! Research into these phenomena goes back to the 1930’s but has been slow to get out of the researcher’s hands and into the public’s minds. (a lot of resistance there) Look at recent threads about roof ventilation to:

  • Prevent ice damming
  • Prolong shingle life
  • Prevent condensation, mould, rot of roof sheathing
  • Reduce cooling bills

There are a # of researchers saying we really don’t need vapour barriers (as defined) for our walls until we get quite far up in Canada where the majority of houses aren’t!!

This may help.

In theory, it’s simple: When water vapor from inside or outside a building gets inside wall assemblies, the water vapor condenses on cool surfaces, such as pipes or cavity walls. As most of you know, water in walls equals big trouble, including mold, corrosion and rot, as well as increased heating and cooling costs and uncomfortable living conditions.

Want to avoid all that? Just keep the water vapor out. Do that one thing and you and your client’s buildings are protected.
Doing it, though, is where things get complicated. That’s because products and procedures vary for keeping water vapor out of walls. If you’re trying to make sense of the often conflicting information, the first question you run into might be: Which is best for keeping water vapor out — a vapor barrier or an air barrier? And the fact that correctly installed vapor barriers also act as air barriers doesn’t make the question any easier.
**Vapor Barriers
**Construction professionals have long used impermeable vapor barriers, such as rubberized asphalt sheets, to waterproof below-grade foundations and roof assemblies. So when they found that water vapor moving through walls was a problem, they turned to that same type of product to provide a similar protection.

In a mostly cold climate, an impermeable vapor barrier installed on the warm side of the insulation stops living-space water vapor from diffusing into the wall assembly where it can condense. A phenomenon called vapor drive, which moves water vapor from areas of high density to low density, is what drives the water vapor through the walls. However, in hot, humid climates, the vapor barrier should be installed on the warm exterior side of the insulation to stop vapor from diffusing through the wall from outside.
The problem is that in climates with hot summers and cold winters, the vapor barrier will be on the wrong side of the insulation at least part of the year. In summer, hot humid air from outside may condense on a vapor barrier placed for a cold climate, but cooled by air conditioning. And warm, moist air from living spaces can condense on vapor barriers placed for a hot climate but cooled by winter temperatures. Both conditions contribute to water inside the wall assembly.

Recent studies have shown that comparatively little water actually gets into the walls through diffusion. It is mostly carried in through air leaks in the wall assembly. So a vapor barrier also has to stop those leaks.
Since stopping air movement turns out to be what’s important, some systems have dispensed with the problematic vapor-impermeability of vapor barriers. Air barriers just concentrate on stopping air movement.
**Air Barriers
**As we’ve discovered, much more water gets into walls through air leakage than by diffusion. While diffusion may deposit several ounces of water into the cavity over the course of a year, moisture carried into walls by humid air leaking through small holes, seams and penetrations can deposit several gallons.

Effective air barriers stop air from leaking through the sheathing or CMU backup by swathing it 100 percent, from top to bottom. If air, which always carries water, can leak in or out anywhere, the entire installation is no good.
That’s the key to air barriers: they don’t have to be impermeable. Many will let a little vapor diffuse through them as part of stopping water buildup in the cavity. In fact, letting water vapor diffuse out of a cavity contributes to keeping the cavity dry.
But air barriers must stop air movement — period.
**“Must Have” Air Barrier Features
**Air barriers come in a variety of types — liquid and solid; vapor-permeable and vapor impermeable (vapor barriers); mechanically fastened and adhesive-backed — to name just a few! Some are both air and moisture barriers. They stop liquid water as well as air from leaking into walls. That can be important, in case water gets into the air space between the masonry veneer and the sheathing or CMU backup. One thing they’ve all got in common — they’re all battling to get on your project!
Sort through this crowded field of products by looking for these basic characteristics:
**Continuous and Seamless
**The air barrier must be able to cover all the nooks, crannies, cracks and transitions of the wall assembly, 100 percent, with no gaps, holes or seams.
**It must stick like blue on sky to the sheathing or CMU backup so it won’t come off in wind during construction, or from air pressure differentials after the walls are completed.
**The air barrier must be able to resist weathering unprotected in case of construction delays, with no rips, tears or breaches of any kind or size — for months, if need be.
Some other features to consider in an air and moisture barrier:

  • Is it easy and fast to install, or labor intensive?

  • If fluid-applied, is it water-based or solvent-based? Water-based systems are more likely to comply with VOC (volatile organic compound) regulations.

  • Is the installed cost, including labor, competitive? Some products may be cheap to buy, but difficult and time-consuming to apply correctly.

When you multiply all these different air and vapor barrier products by all the different types of wall assemblies, you get an intimidating array of possibilities. Some will work great. Some will accelerate the very problems they’re meant to stop.
Following the simple guidelines of “continuous, structural and durable” will help you choose the right air barrier. The right air barrier will do that one simple thing — keep uncontrolled air and the destructive water it carries out of walls. That means the ultimate victors in the battle of the barriers are your clients and you.
Gary Henry is a business communication specialist with PROSOCO, a national manufacturer of products for cleaning, protecting and maintaining masonry, concrete and stone. For more information, contact Gary Henry at (785) 830-7343, or e-mail

Marcel :slight_smile: :slight_smile:

My e-mail: Topic: Insulation in the crawl space.

Jorge Mondragon

Prime Home Inspection INc.

Florida Code has created its own unique energy code.

You need to consult your local energy code authorities for Florida applications and exceptions.

You can start here by clicking chapter 13 of the Florida Building Code:$fn=default.htm$up=1$3.0$vid=icc:fl

And ignore anyone who has any opinions north of Florida’s border with Georgia and Alabama…

Look at what is common in your region. In the gulf coast region no vapor barrier is recommended, just insulation.