Crawlspace Moisture Issues and control
If you get three general contractors together and the topic of discussion turns crawlspaces and moisture control, you will probably hear six opinions. Folks, seem to have different and often conflicting opinions about what tack is the best to take. The idea of crawlspace encapsulation (sealing) has been around for a while now. Your Crawl Space, Inc pioneered encapsulation in the southeast and sells innovative crawlspace products throughout North America. Here is our take on the subject.
There has been much research on crawlspace encapsulation during the past decade. The US Environmental Protection Agency, the US Department of Energy, the ZEBRAlliance at Oak Ridge National Laboratories, and a non-profit research organization called Advanced Energy have all done excellent and groundbreaking research in the area of crawlspace encapsulation. Their research findings were instrumental in the last revision of the International Building Code. They all agree in a few, very important areas.
When considering your home, think of it as a single system. Your HVAC system, insulation, windows, attic, living space, and crawl space all work together. All of these building components need to be optimized and balanced to achieve maximum comfort, performance and energy efficiency. Your crawlspace is an important part of this system.
Your house is like a giant chimney. By natural convection, air is drawn in through crawlspace vents and air leaks. Because warm air rises, the outside air is drawn through the living space (along with mold spores, odors, and moisture) and exhausted through the attic. In the summer, your crawlspace is naturally cooler than the ambient outside temperature. So when the warm, humid outside air reaches the cooler crawlspace surface areas, the moisture condenses on framing, plumbing, wiring, insulation and especially HVAC ductwork and “sweats”, just like an iced tea glass sweats on your kitchen table in the summer. In the southeast it is not unheard of for crawlspace humidity to approach 100% and actually rain inside the crawlspace.
The moisture in your crawlspace creates an ideal environment for wood destroying organisms, mold, and mildew. It can saturate and destroy the effectiveness of your insulation and promote wood rot. And because of the chimney effect, the humid crawlspace air, full of mold and mildew spores, eventually finds its way into your living space creating an unhealthy environment and causing your air conditioning to work overtime to dehumidify the air. Just a vapor barrier might help a bit, but most of the moisture is coming from outside, not your dirt, crawlspace floor.
So, the alternative to a vented crawlspace is an encapsulated (sealed) crawlspace. Crawlspace encapsulation involves sealing all outside vents, installing a high-performance vapor retarder on all exposed wall and floor surfaces, insulating the walls and rim joist, and conditioning the air.
The prevailing research claims that by insulating the walls and rim joist, it is unnecessary to insulate the floor. However, if the existing floor insulation is in good condition, leave it in place. We generally recommend spray foam or a rigid foam board insulation that is fire-retardant, low VOC, and offers an R-13 value or greater. Foam board cuts easily and can be used for sealing existing crawlspace vents too. The sill plate should be caulked and spray foam or paper-faced fiberglass insulation is used to insulate the rim joist.
The building codes in most areas require the air in an encapsulated crawlspace to be conditioned. That generally means adding a dehumidifier or using the existing HVAC system to condition the air. The EPA and the Department of Energy recommend using the existing HVAC system at a rate of one cubic foot per minute of conditioned air per fifty square feet of crawlspace area. A qualified HVAC contractor can add one or two 4” or 6” vents to a system for a nominal cost. The vents are usually equipped with a butterfly valve that can be adjusted to get the desired air flow.
We prefer using this method since the installation of the vents is generally less expensive than a crawlspace dehumidifier. And since running a dehumidifier is about the same as running a small refrigerator, the cost of conditioning the air with an HVAC system is generally less than running a dehumidifier. Remember that you’re not trying to heat and cool your crawlspace; you’re just adding a small amount of conditioned air. A relative humidity target of 60% or lower is a good place to start.
Sometimes HVAC ducts are in the attic. In this case, a dehumidifier is the only choice. And sometimes humidity is such a major problem that we have added a dehumidifier and an HVAC baffle vent. A remote humidistat is inexpensive and an invaluable tool for determining the correct amount of dehumidification.
When comparing high performance vapor barriers, look at puncture and tear resistance. Other than being absolutely water tight, a high performance vapor barrier should also be flexible, fire resistant, low VOC, and lightweight. There is a world of difference between the 6 mil vapor barrier material sold in most hardware stores and a high performance specialty product. Most reputable suppliers will provide samples.
All of the experts agree that when you seal a crawlspace or add a high performance vapor barrier, the vapor barrier should be firmly attached and sealed to the crawlspace foundation wall. Most crawlspace contractors use some sort of mechanical fastener to accomplish this. Even after being attached, the vapor barrier needs to be sealed the foundation. Some contractors us flammable and potentially explosive mastic. This approach is dangerous, labor intensive, and can even damage the foundation wall.
YourCrawlSpace has developed a product for sealing the vapor barrier to the foundation wall in a single, easy step. It cuts hours from the installation and uses only safe low VOC materials.
In addition to dramatically increasing indoor air quality and protecting the structure of your home, several research studies from Advanced Energy indicate that crawlspace encapsulation can actually lower energy usage. These studies were done in several parts of the country with varying climates. A link to their study results is below.