Insulation value of drywall?

Old 10-30-04, 07:40 PM
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Insulation value of drywall?

Thinking about going with 1/4" plywood and 3/8" Georgia Pacific plywood with beaded pine facing. Is there any chance I will lose any insulation value going this route versus comparable thickness of drywall?

Old 10-31-04, 09:39 AM
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The R-value is nearly the same between the two. The real difference between the two applications is the air tightness. The seams between sheet rock is taped and spackled and between the plywood they will not be. Though it may appear not to be that important, it probably is. Especially when you consider what occurs inside your home during the winter months.

The primary source of air exchange within a structure during the winter is heating and cooling of the air inside the structure. Even though you cannot see or sense it. In most cases when the heat comes on inside the house, the heating causes the air to expand. This creates positve pressure inside the house about 1 to 2 Pascals. Even though this is quite slight and we cannot sense it, it is significant enough to push air out of the structure.

When this same air begins to lose temperature, the air contracts. Which results in 1 to 2 Pascals negative pressure. This negative pressure inside the house is significant enough to pull air from outside the house into the house.

For your information this is where the controversy over Sick Building Syndrome, Indoor Air Quality, Air Sealing and Energy Conservation comes from. In a house that is not energy efficient, it cost more to heat which implies the heat comes on more frequently, which results in more air exchange. In a house that is energy efficient, it costs less to heat, which implies the heat comes on less frequently, which results in less air exchange.

I am assuming in your application that it is an above grade application and not a basement. In this application the concern is mold that occurs inside wall cavities. Air inflitration occurs when the heat inside the house cools, which brings air from outside that carries the mold spores into the wall cavitiy. Air leakage occurs when the heat is on and expands the air into the wall cavity, which introduces air transported moisture into the wall cavity because all heat has moisture in it. This condition results in a high probability for mold to grow.

Clearly this situation is much different than providing enough air exchange inside the house, yet many people who oppose energy efficient building methods make this part of their argument concerning Indoor Air Quality.

As far as your application is concerned, Air Sealing may be interpretted as an Energy Conservation Measure, however, its primary purpose is to control where the air exchange occurs. It is better for the air exchange to occur at your windows and doors than through your walls. It is okay to use the plywood provided you apply an Air/Vapor Barrier to the wall first. A Vapor Barrier being plastic sheet at least 6 mil thick, an Air Barrier by taping all the seams and penetration in the plastic sheet after it is applied to the wall.

What this does is make the wall more resistant to air infiltration and leakage and your windows and doors far less resistant to air infiltration and leakage. It is good to note that this application in most cases does result in less air exchange inside the house. It does not mean that the reduced amount of air exchange is not sufficient to sustain a Healthy Indoor Environment.
Old 11-02-04, 09:47 PM
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Thanks for the informative reply. I had to read it a few times to relate your information to my circumstances.

One thing I failed to mention in my original post is that the building is not a typical living space. It it a stand alone 24x24 ft. framed woodshop on a raised foundation. The 2x4 walls are insulated with R13 and I am now putting up a 6 mil vapor barrier. The shop has 4 opening windows and an 8 ft. roll up door. My guess is that the door sealing (or lack thereof) will provide ample opportunity for air transport during heating/cooling cycles.

Still, I will probably spend time sealing up the barrier because that is the way I typically do things. Thanks again for the reply.


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