Insulating a Crawl Space — My Home Science

Insulating a Crawl Space - My Home Science

Insulating a Crawl Space

What’s right, what’s wrong & what will cause problems

Improving performance & appearance. A vented crawl space (above) often has falling fiberglass insulation and moisture problems.

Builders love crawl space foundations because they’re quick and inexpensive to build. You don’t need heavy equipment to excavate a giant hole for a full basement. Instead, you’re erecting short walls to get the first floor framing up off the ground. If soil at the building site is rocky or wet, a crawl space minimizes the risk involved in digging and building a full basement.

The failure of fluffy stuff

The encapsulation process results in a clean, dry interior space, as shown here.

For many years, the standard way to build a crawl space involved venting the crawl space walls and installing fiberglass batt insulation between joists in the crawl space. The widespread availability of fiberglass insulation, along with its low cost, went along well with the other economies of crawl space construction.

Unfortunately, the same fluffy batt insulation that works well in wood-framed walls performs miserably in crawl spaces. Here’s how energy expert Martin Holladay describes the disastrous results of installing fiberglass batts in a vented crawl space in a recent blog at GreenBuildingAdvisor.com :

“If you’re perverse, and you want to build a damp, moldy, nasty crawl space, just do two things: insulate the crawl space ceiling with fiberglass batts, and vent the crawl space to the exterior. If you live in the Southeast, within a few short years the fiberglass batts will begin to hang down at odd angles like drunken stalactites. Every summer, the open vents will introduce huge amounts of moisture into the crawl space. You’ll end up with a classic moldy crawl space — one that represents a significant source of moisture for the house above.”

The mold that thrives in a damp crawl space won’t just damage wood and other organic materials in the crawl space. Research has shown that up to 40% of the air we breathe upstairs comes from the basement or crawl space, so the potential for hazardous indoor air pollution (from airborne mold spores) is very high. These are serious problems.

Insulating a Crawl Space - My Home Science

The solution: Change the thermal and pressure boundaries

A smarter insulating strategy. Installed against crawl space walls, rigid foam insulation won’t degrade or fall out of place.

OK, I know this heading may seem a little cryptic, but stay with me. In the old-style vented crawl space, a layer of poor-performing fiberglass insulation is supposed to provide a thermal barrier directly beneath the first floor of your living space. The pressure (air) barrier in a vented crawl space is also the floor sheathing, even though it’s got plenty of holes in it to bring wiring, plumbing and ductwork up through the crawl space.

What happens if we change these pressure and thermal boundaries? Seal the crawl space vents so that moist exterior air can’t get into the crawl space. If the crawl space has a dirt floor, seal that with a thick plastic vapor barrier. Install rigid foam insulation against crawl space walls, so that we’re insulating the crawl space instead of the first floor.

The result of this crawl space “encapsulation” process is a clean, dry crawl space that isn’t affected by moisture in the soil or in the outside air. Unlike fiberglass crawl space insulation that can degrade into a pink pile on the crawl space floor, rigid foam insulation can’t absorb moisture or be damaged by it, and will never fall out of place. Any moisture that remains in the wood framing after a crawl space is encapsulated can be effectively removed with a dehumidifier. Once this happens, youve got a controlled environment where mold can’t grow, and a crawl space that’s more of an asset to your house than a liability.


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