By David Braddy LEED GA
We have a tendency to forget about the crawl space in our home, but proper sealing and insulating of a crawl space is essential to prevent mold and rot. And most importantly to live in a healthier home. Fiberglass batts, which have been commonly installed over the years are probably the most inefficient way to insulate a floor and many times cause more harm than good. You should never install moisture barrier or use batt insulation with Kraft paper under the floor joist and if you do use insulation with Kraft paper make sure the paper is not facing down towards the ground or you will trap moisture and create the perfect place for mold and rot.
You do not have to have standing or visible water under a house or building to have these problems. Your home will actually draw moisture into it, through what is known as the stack effect, unless you have an airtight seal to prevent it. There are two very effective methods to accomplish this; either closed cell polyurethane spray foam or crawl space encapsulation. Either way the ground must be covered and properly sealed. There are different methods of encapsulation including positive pressure systems, but they all have one thing in common; no vents
The next thing you must have is some way to condition the air in your crawl space, some people use their HVAC system by installing registers under the house, but this also has some draw backs if not done properly. By far the best method is to install a properly sized dehumidifier that drains to the outside.
Many people have tied to accomplish a vapor seal with poly or other methods, but they end up making the problem worse by trapping moisture. If you have read my other articles you understand that moisture or humidity always moves from higher pressure to lower pressure or hotter air to colder air. Any vapor/humidity will find even the smallest of penetrations to move through. You can never get a permanent or total seal by using other methods and I have seen many attempts; including foam board with taped seams, poly or vapor barrier stapled to joist, Kraft paper facing down and in every case it has failed. Even using the proper methods, it is imperative to make sure there are no penetrations left unsealed.
It is better to use nothing rather than improperly installed materials, at the minimum, all crawl spaces should make sure the ground is covered with a minimum of 6 mil poly with any penetrations properly sealed.
The bigger problem here is that while it may be somewhat cooler under a crawl space, the humidity is usually higher than the outside air, so there is an even greater force to penetrate into the living area above. This is known as the stack effect. The same effect on exterior wall assemblies is called vapor drive, but that is another article. When a crawl space is vented as is (mistakenly) required still in some local areas, the humid air does not come in one side and out the other or flow through, it comes in wherever the vents are and this humid air collects and increases under the house and moves up into the living area through any available penetration or crack. Now add moisture content from the ground and you have a potent recipe for mold and moisture problems.
Keep in mind whatever is in the air under the house will be in the house, if not sealed.
This is where the musty smell comes from in a home that has been closed up for a period of time.
The crawl space under your home is vitally important to your families and your homes health, but is one of the most overlooked areas for improvement. Approximately 50% of the air in the average home comes from below.
Most building scientist, engineers and green builders have now come to the conclusion that venting a crawl space to the outside in a humid environment is just plain wrong. You could never dry your crawl space of moisture by open vents in a humid environment. If your home ever smells musty you already have a mold problem, this can lead to premature failure of the structure and health issues for its occupants, including but not limited to allergies and asthma.
Homes in high humidity areas and around water are the most at risk; make sure you do not overlook this important area of your home. It is imperative that you call a professional to correct moisture problems in your crawl space.