Re: Tyvek
interesting question, but I don't think it will work well. Tyvek actually takes advantage of the fact that water in a vapor state is much smaller than in its liquid sate. In its liquid state, surface tension causes the molecules to clump together, forming large "spheres". In its vapor state it doesn't clump. Water in a liquid state will not go through tyvek because of this. However, when the humidity level on one side is lower than the other, the water will migrate through the membrane in vapor form. (Look up water vapor pressure). That is why it won't allow liquid water into a wall, but will allow the liquid to migrate out of the wall.
In theory I think it would work on the deck, in practice you are going to have screw holes wear spots (from walking on it), etc that will create holes for the water to get though....a liquid aplied mebrane, I.E, fiberglass will work much better. Best of both worlds would be if someone created a breathable fiberglass.....which would keep water out, and allow any water that did get in to migrate out instead of being trapped.
