Many, many years ago here in the south we discovered the disastrous results of putting vinyl wallpaper on exterior insulated walls. The science behind this is simple but it took alot of ripping into kitchen and bathroom walls to figure out what was happening.
A little building science fun: Since God created the Earth certain things have always been constant. These are kind of like Laws of Nature. Warm things want to heat up cold things. Wet things want to get dry things wet. Wet stuff travels from warm areas to cool areas. There are a few more truths to be told but for now I think you get the picture. Let's see how all this translates into a blog post.
One really cool way nature wants to make dry things wet is through diffusion. Let's take the lovely Gulf Coast climate in the heat of summer as our laboratory for the day. It's August. It's 101 degrees outside. The humidity outside is a dripping 98%. Geez!! It's a horrible sticky day. Your hair is a mess. You sweat just walking out to the car. Thank God for air conditioning.
So, you love your A/C system so much you crank it down in the house to a chilly 70 degrees inside. Ahhhhh, that's much better. Now you have set up Mother Nature to wreck havoc on your house. You see, that "diffusion" thing has been here since day one. However, man has done crazy things in an attempt to make life comfortable and in the process we have ignored basic science.
That humidity outside is actually water in the air. And guess where it wants to go. It will find it's way into your house (where it's not as humid as the outside - see how that works?) through little cracks and holes and yes, even straight through the wall materials of your house. Yup! Water is going to travel through your brick, your siding, right through your fiberglass insulation, through your sheetrock and into your house. The good news is the A/C unit is capable of dehumidifying a large amount of air so you will stay comfortable.
The bad news is, we do stupid stuff to stop this natural migration. The biggest offender is the use of vinyl wallpaper on those exterior walls. That wallpaper on the inside acts as a vapor barrier stopping those water molecules in their tracks. Then, since we just love our nice chilly air on the inside we make the wallpaper nice and cold too and before you know it we have a condensation problem right there at the back side of the wallpaper. Repeat this over a summer or two and bingo! You have got yourself a petri dish growing all sorts of colorful molds and turning your sheetrock walls into mush.
OK, we have figured this wallpaper stuff out and most people have not been doing this for many years. Fast forward to the 21st century and guess what. We are doing it again!!
Here along the Gulf Coast raised floor construction (houses up on stilts, houses with vented crawl spaces, all homes where the air underneath it is not air conditioned) is very popular. Remember, that water vapor stuff will go through most common building materials. So now the bottoms of these houses look like our walls used to. We will usually put a cladding on the bottom of the raised floor to make it look pretty when painted. Most people will even put fiberglass insulation in the floor cavity itself. Then some sort of plywood subfloor is installed and capped off with a beautiful solid floor material to walk on. Does this sound familiar?
Carpet on the floor would be a great idea in this type of home. You see, carpet allows the vapor to go into the house then get treated by the A/C system. But nooooooooooo, we have got to put down pretty stuff like tile, laminates, woods, all sorts of things that in most applications create a vapor barrier on top of our plywood subfloor. Problems usually show up as cupped wood flooring or other flooring issues. This has been documented many times along the Gulf Coast since Hurricane Ike years ago. We now have wood floors buckling up, mold growing under laminates and tile, plywood subfloors turning to mush because we have put vinyl wallpaper on our floors. It's time to rethink how we do our floors in raised homes in hot-humid climates.
(Pictures above are from a seminar I attended concerning raised floor issues along the Texas Gulf Coast. That's all mold under different flooring materials. The fourth one is the actual floor joist cavity with mold present.)
Join me in a few days as I discuss what I did to avoid this problem on my raised home. I have all the same natural issues (humidity, heat, etc.) but through science we came up with a way to avoid creating a petri dish under my floor. See you on my next post.