910 Crawl Space Pros provides crawl space insulation across Wilmington and the Cape Fear region. Crawl space insulation is the practice of adding thermal resistance either to the underside of the floor above a vented crawl space, or to the perimeter walls of a sealed one. Which of those two is correct is decided entirely by whether the crawl space is open to outside air, and installing the wrong one traps moisture against wood.
This is the most commonly botched item in a crawl space, because the two approaches look interchangeable and are not.
What we do on the first visit
We establish first whether the space is vented or closed, because that decides everything else. Then relative humidity, moisture content in the joists and subfloor, and the condition of any existing insulation — fallen batts and rodent-damaged fiberglass are usual.
Subfloor batts vs wall insulation: which one your crawl space needs
In a vented crawl space, the thermal boundary is the floor above. Insulation belongs between the floor joists, and the crawl space itself stays at outdoor temperature. In a sealed crawl space, the thermal boundary moves to the perimeter walls, the space becomes semi-conditioned, and subfloor insulation becomes unnecessary — and sometimes harmful.
The harm comes from vapor. Fiberglass batts with a kraft facing installed face-down in a humid vented crawl space put a vapor retarder on the cold side of the assembly in summer, which is exactly where condensation forms on the Cape Fear coast. Wet fiberglass loses most of its R-value and holds moisture against the subfloor.
- Vented crawl space → insulate the subfloor, unfaced, supported so it cannot sag
- Sealed crawl space → insulate the perimeter walls, leave the subfloor open
- Never both at once — that sandwiches the framing between two vapor barriers
Why fallen batts are so common here
Fiberglass batts held up by friction alone will eventually come down in a humid crawl space, because the moisture adds weight and the fibers relax. Once a batt drops it sits on the ground, wicks moisture and becomes a rodent nest. Roughly speaking, if the insulation in your crawl space is hanging in loops, it stopped doing anything years ago.
Properly supported batts use wire rods or straps at regular spacing, not staples through the facing. In a sealed crawl space we generally move the insulation to the walls with rigid foam board, which cannot sag and does not absorb water.
Insulation and the fire code
Rigid foam board used on crawl space walls has to be covered or be of a type rated for exposed use, because unprotected foam is a flame-spread issue. This is the detail cheap installs skip. Where foam is left exposed, it must be an approved product with a tested thermal barrier or ignition barrier for that application.
What's included
- Assessment of whether the space is vented or sealed before any material is chosen
- Removal and disposal of fallen, wet or rodent-contaminated existing insulation
- Unfaced batts with mechanical support in vented spaces — never friction-fit alone
- Rigid board to the perimeter walls in sealed spaces, with the rim joist detailed
- Rim and band joist insulated and air-sealed, the biggest single leak in most homes
- Code-compliant covering or an approved exposed-use product for any foam
- No vapor-facing installed on the cold side of the assembly
- Photographs of the completed install