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Solar Vapor Drive Season is almost here

by David Braddy LEED GA

There is probably not a week that goes by that I do not see the results of vapor drive in the form of mold or rot in building cavities. This is a very misunderstood problem and usually results from misapplied building materials. International Building codes were changed in 2004 to address this problem, but since codes are enforced locally and many local codes do not reflect these changes, it is sometimes the code itself that causes the problems.

I have said this before and I will say it again; do not use a vapor barrier, especially poly on your interior walls. Now let me attempt to give you a simple explanation as to why.

First let’s look at a couple of established facts; air carrying water vapor always goes from higher areas of pressure to lower or from more to less, in other words hot humid air always tries to force its way to cooler dryer air.

Example: imagine two tanks of water sitting side by side, one full and one empty. What happens if you connect them with a line half way up so water can move freely between them? It is pretty obvious; the water will run out of the full tank until the water level has equalized into the empty one.  Let’s now pack that line with insulation, wood & drywall. Will it stop the water? No it will slow it down, but the water will still pass to the other side due to the pressure behind it, because those materials are not meant to stop water.

This is a little over simplified, but the fact is more humid always tries to get to less humid. Now when you have a wall assembly separating two different environments and air can find its way through you create vapor drive. Have you ever felt air move through an electrical outlet?

For decades the only concern was winter driven vapor drive, but as many builders found out the hard way, if a home has air conditioning, which most do, vapor diffusion from outdoors is a much bigger problem. In many areas the air in a home is actually a lot dryer in the winter and humidifiers must be used, like this area.

The second fact is that Inward Driven Solar Vapor Drive, from the outside in, is much more powerful and destructive in creating rot and mold than winter vapor drive ever was. Vapor drive will occur in any area with higher humidity on one side of a wall, but the sun is a very powerful engine that will drive moisture through most wall components like a knife through butter.

If the components of a wall are poorly chosen, there is no quicker way to destroy a home than inward vapor solar drive. You must worry about vapor diffusion from the outside in; not the inside out.

There are many things you can do to minimize vapor drive, but if an exterior wall is not completely sealed to prevent vapor or air from entering, which is not an easy task, never use a material on the interior that will trap moisture in the wall, like poly or vinyl wallpaper.

The best option is to consult a professional before building or remodeling.


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