Tuesday, December 02, 2008

Keeping your walls dry, a tutorial

Keeping your walls dry on the inside is important for all buildings, including our shop. I noticed from Christina's last post, that there is a slight inaccuracy, which leads to this brief and important tutorial.

Most residential structures are built using wooden framing. A few use steel studs. Neither one like moisture inside the wall cavity and both can be totally ruined due to moisture problems. Here's how to avoid that.

In the old days, they just built houses that were really leaky to air infiltration. In an old farm house on a windy cold winter day, it is entirely possible to replace all the air in the house ten times an hour. These houses are about impossible to heat in any affordable fashion, but they were very resistant to moisture damage. Any moisture that got inside the wall got dried out by the huge volumes of air passing through.

These days, for energy and comfort reasons, most houses are considerably tighter than the drafty old farm house. The good news is that they use far less energy and are more comfortable and far more affordable. The bad news is that if moisture does get inside the wall, it will takes weeks or months to dry out. This can cause the wooden framing to rot in a year or two, or cause scary amounts of mold to grow, which can make a house unlivable and is very expensive to fix. The moisture also usually damages the insulation so it never works right again, raising energy use and cost forever. So, what to do?

In new construction, we use two different sheet plastic products. On the inside (warm side in winter) we install 6 mil (that's 0.006", or six thousands of an inch thick) polyethylene plastic sheeting. It's installed on the studs right before you do the drywall, and right after you do the wiring and plumbing inside the wall. Polyethylene is, for all practical purposes, impermeable to moisture, either liquid water or gaseous water vapor. It is commonly called a vapor barrier for just that reason. It prevents warm moist air from inside your house from migrating out through the nooks and cranies in your wall. If we do not prevent that moist air from getting inside the wall, the warm moist air eventually gets cooled off as it gets closer to the outside cold part of the wall.

When it cools off sufficiently, it suddenly can't hold all that moisture and water will change from a gas or vapor into a liquid. Now we have cold liquid water inside our wall where it can do its damage. A good vapor barrier prevents it from ever getting inside the wall in the first place.

A good installation will have every seam/overlap sealed with caulk and stapled. Every penetration (electrical wires, plumbing) sealed with caulk, or mastic or something. This is where good workmanship really pays off.

But, in this world, hardly anything is perfect. There are always little holes and tears and gaps, and perforations from the staples. So we must assume that small amounts of moisture can still get in the wall. That brings us to Tyvek or house wrap. That's the plastic sheet material on the outside, installed on top of the plywood or osb, just before the siding or bricks go on. House wrap is a peculiar polymer. They have discovered ways of manufacturing it so it has pores. The pores are generally big enough for gaseous water molecules (water vapor) to go right through it. Like heat, water vapor tends to go from areas of high concentration to areas of low concentration. In the winter time, the area of high concentration would be the errant water vapor inside your wall and the area of low concentration would be the great dry cold outdoors.

Tyvek allows any trapped water vapor to escape to the out doors. But the really neat trick is that the pores are generally too small for liquid water drops to penetrate much. So, liquid water can't get it, but water vapor CAN get out. Tyvek also serves as an air intrusion barrier, to prevent cold air from entering the wall, and discouraging warm conditioned air from leaving the house.

So, Polyethylene on the inside (which is what we are doing now, in prep. for drywall) and Tyvek goes on the outside, which we will do right after we get the big garage door installed.

If you have an old house, there is no good way to fix this without tearing into the walls. You can vent moisture pro-actively from the kitchen and the bathrooms. You can buy special "vapor barrier" paint. You can caulk every crack and hole you can find, inside and out. That will all help, but will not be a guarantee. You could also do an air-to-air heat exchanger in either the old remodeled house or the new-construction house. Both help control humidity levels to (help) prevent moisture damage.

I am very happy to have the roof done and hope to never do another in my life. We'll see, I am generally cautious about using the "never" word, as you never know what God will send your way. I'm totally convinced He has a sense of humor.

Finest regards,


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