Crawl Space Insulation

Batt insulation — sold in rolls or batts — is the most common and widely available type of insulation for crawl spaces, wall cavities, and attics alike. It's made of mineral fiber, typically processed fiberglass, and fiberglass batts themselves are made from molten glass spun or blown into fibers using a mix of recycled and virgin glass.

Batt Insulation

Batt and blanket insulation is used to insulate below floors, above ceilings, and within walls. It's generally the least expensive insulation option, though it requires careful, consistent installation to perform the way it's rated to — gaps and compression both reduce its effectiveness significantly. It's a tried-and-true product that's been field tested for generations.

Why Crawl Space Insulation Matters

Uninsulated or poorly insulated crawl spaces let outside temperatures transfer directly into your floor system, making rooms above harder and more expensive to heat and cool. Insulation installed as part of a sealed, encapsulated crawl space also helps guard against condensation forming on cold surfaces, which is one of the ways moisture problems get started in the first place.

Replacing Fallen or Failed Insulation

If you have looked under your house recently, there is a good chance you saw fiberglass hanging down between the joists, or lying on the ground in sheets. It is the most common thing we find in a Triangle crawl space, and it is not a fixing problem — it is a symptom.

Fiberglass batts fail in a damp crawl space for a specific reason. Fiberglass does not repel moisture; it holds it. A batt that absorbs humidity gets heavy, sags away from the subfloor, and eventually pulls free of whatever was holding it up. Once it is damp and compressed it has lost most of its R-value anyway, and the paper facing becomes something for mold to grow on. Stapling it back up puts a wet sponge against your floor.

So replacement starts with why it fell. If the space is still damp, new batts do the same thing within a few years. Correcting the moisture — a vapor barrier, drainage, or encapsulation — comes first, and the insulation is specified to suit whichever of those the space ends up with.

Insulate the Floor, or Insulate the Walls?

This is the decision that actually matters, and it follows from whether the crawl space is vented or sealed.

• A vented crawl space is outside the building envelope, so the insulation goes in the floor system, between the joists, keeping the conditioned rooms above separated from an unconditioned space below.
• A sealed crawl space brings the space inside the envelope, so the insulation goes on the foundation walls and rim joist instead. The floor above no longer needs it, the ductwork and pipes down there are now in semi-conditioned space, and there is nothing between the joists to absorb moisture and fall down.

Doing both is usually wasted money, and doing the wrong one is why so much crawl space insulation ends up on the ground. Which applies to your home is settled at the inspection, not on a price list.