Crawlspace Moisture Control – The Case for Encapsulation

If you happen to get three general contractors together and the topic of discussion turns crawlspaces and moisture control, you will probably see six opinions. Even the experts seem to have different and often conflicting opinions about what tack is the best to take.

There is much research on crawlspace encapsulation during the past decade. The US Environmental Protection Agency, the US Department of Energy, this ZEBRAlliance at Oak Ridge National Laboratories, and a nonprofit research organization called Advanced Energy have just about all done excellent and groundbreaking research in the area of crawlspace encapsulation. Their research findings were instrumental within the last revision of the International Building Code. They all agree in a few, very important areas.

When considering your home, think of the idea as a single system. Your HVAC system, insulation, windows, attic, living space, and crawl space all come together. All of these building components need to be optimized and balanced to achieve maximum comfort, performance and energy efficiency. Ones crawlspace is an important part of this system.

Your house is like a giant chimney. By natural convection, air is used through crawlspace vents and air leaks. Because warm air rises, the outside air is drawn through the living area (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 your cooler crawlspace surface areas, the moisture condenses on framing, plumbing, wiring, insulation and especially HEATING AND AIR CONDITIONING 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 inside your crawlspace creates an ideal environment for wood destroying organisms, mold, and mildew. It can saturate and demolish the effectiveness of your insulation and promote wood rot. And because of the chimney effect, the humid crawlspace discuss, full of mold and mildew spores, eventually finds its way into your living space creating an unhealthy environment in addition to causing your air conditioning to work overtime to dehumidify the air. Just a vapor barrier might help a bit, but the vast majority of moisture is coming from outside, not your dirt, crawlspace floor.

So , the alternative to a vented crawlspace can be an encapsulated (sealed) crawlspace. Crawlspace encapsulation involves sealing all outside vents, installing a high-performance vapor retarder on all exposed wall and floor surfaces at or below grade, insulating the walls along with rim joist, and conditioning the air.

Elite Moisture Solutions claims that by the walls and even rim joist, it is unnecessary to insulate the floor. However , if the existing floor insulation is in good condition, give it in place. We generally recommend a rigid foam board insulation that is fire-retardant, low VOC, and an R-13 value or greater. It cuts easily and can be used for sealing existing crawlspace grills too. The sill plate should be caulked and paper-faced fiberglass insulation is used to insulate the side joist.

The building codes in most areas require the air in an encapsulated crawlspace to be conditioned. That typically means adding a dehumidifier or using the existing HVAC system to condition the air. The EPA along with the Department of Energy recommend using the existing HVAC system at a rate of one cubic foot per minute of conditioned environment per fifty square feet of crawlspace area. A qualified HVAC contractor can add one or two 4″ and 6″ vents to a system for a nominal cost. The vents are usually equipped with a butterfly valve that could 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 HEATING system is generally less than running a dehumidifier. Remember that you’re not trying to heat and cool your crawlspace; you’re simply adding a small amount of conditioned air. A relative humidity target of 60% or lower is a good place to start.

When comparing top rated vapor barriers, look at puncture and tear resistance. Other than being absolutely water tight, a high performance vapor filter 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 manufacturers will provide samples.

In addition to dramatically increasing indoor air quality and protecting the structure of your home, several research studies with Advanced Energy indicate that crawlspace encapsulation can actually lower energy usage. These studies were done in just a few parts of the country with varying climates.

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