Rigid Foam Insulation Under Floor Joists

Rigid foam insulation also known as insulating boards provide high r values.

Rigid foam insulation under floor joists.

A semi rigid material is generally best as it can be cut very slightly oversized and squeezed in between the floor joists thereby ensuring no draught causing gaps. Fiberglass insulation and expanding foam seal the open top of hollow concrete blocks. The insulation should cover the whole depth of the cavity and contact with the floor. If it fits proper with no space between on either side of the board pick it up and put a thin layer of glue on.

Installing fiberglass or mineral wool batts or rigid foam boards in floors with exposed joists open floors is straightforward. Instead of installing the foam insulation between floor joists in the crawl space the foam board is installed against the crawl space foundation walls. Next you will need to place the foam board insulation between each joist as a test. Unlike fiberglass rigid foam insulation does not absorb water support mold or allow air to pass through providing that joints between boards are sealed with tape.

Airtight insulation reduces heat loss through the rim joist band joist. Installers should push the insulation up and fasten it to prevent any void or gap between the floor and the material. The insulation should be pushed tight to the underside of the floorboards. That would need 90mm of rigid foam or 150mm mineral wool.

Insulated rim joists place rigid roam against the rim joists then caulk along the insulation. Install it anywhere from the roof to the foundation. You can even place it under the foundation slab.