Crawl Space Insulation in Kansas City: What Works and What Doesn't

If you have looked under a KC home, you have probably seen it: sagging, blackened fiberglass batts hanging out of the floor joists, half of them already on the ground. Crawl space insulation is one of the most commonly done and commonly failed jobs in our area. Here is what goes wrong and what actually works.

Why fiberglass between the joists fails here

The old standard was fiberglass batts stuffed up between the floor joists. In a humid, vented Kansas City crawl space, that approach fights a losing battle. Fiberglass absorbs moisture from the damp air, gets heavy, loses its insulating value, and eventually sags and drops. Once it is wet it also becomes a home for mold and a nesting spot for rodents. You end up insulating almost nothing while feeding a moisture problem.

The real issue is moisture, not R-value

The reason batts fail is that the crawl space is open to the ground and to outside air. As long as humid air and soil moisture are moving through the space, any insulation in there is working against the weather. That is why the modern approach flips the strategy: instead of trying to insulate a wet, vented space, you seal the space off from moisture first.

What actually works: seal, then insulate the walls

The approach that holds up in our climate is to encapsulate the crawl space, sealing the floor and walls with a heavy vapor barrier and closing the vents, then insulate the perimeter walls rather than the ceiling. This brings the crawl space inside the home's thermal envelope, keeps it dry, and the insulation stays put because it is no longer sitting in damp, moving air. Rigid foam board on the walls is common because it does not soak up water the way fiberglass does.

What it does for the house

A sealed and properly insulated crawl space makes the floors above it noticeably warmer in winter, cuts the humidity that makes summers feel muggy, and takes load off the HVAC system. It also stops the cycle of insulation falling down and being replaced every few years. It is the difference between doing it once and doing it over and over.

If the insulation under your house is drooping or on the ground, that is the signal the moisture needs handling first. Insulating over it just resets the clock.

Frequently asked questions