Under Floor Insulation over Crawl Spaces

Under Floor Insulation over Crawl Spaces

Under floor insulation over crawl spaces and basements

Millions of homes lack adequate underfloor insulation. Floors over crawl spaces and basements should be properly insulated. That’s very important for comfort and home energy improvements. You may save hundreds of dollars every year, by insulating your floors.

Floor insulation is not as important as attic insulation, or as wall insulation, but do not underestimate its importance. Many builders and homeowners underestimate the importance of floor insulation and end up with cold and end up with homes that are expensive to heat (and cool).

Floors should be properly sealed and insulated, with R-values above those required by building codes. If you want to use your basement as a living space, seal and insulate its exterior walls. In this case you don’t have to insulate the floors. Otherwise its better and typically easier to insulate the floors than the walls.

The recommended insulation levels

Building codes fall short of the optimum in floor insulation. Consider levels of floor insulation of R-30 (Metric system: R-5) in cold climates, and at least R-20 (Metric system: R-3.5) in moderate climates. Super-insulated houses (in cold climates) involve higher values: R-40 (Metric system: R-6.5).

Floor insulation materials


Floor insulation is very important in cold and moderate climates;

Levels. R-30 (metric system: R-5) in cold climates; R-20 (metric system: R-3.5) in cold climates;

Materials for open floors. batts and polystyrene;

Materials for enclosed-cavities. cellulose and blown fibers

Fiberglass and mineral wool batts are good choices in open floors (with exposed joists). But if you live in a cold climate it may be better to use extruded polystyrene to meet the recommended levels

Rigid extruded polystyrene is as easy to install as batts, and provide a better R-value.

Floors can be a major cause of heat loss in cold and moderate climates, during the heating season. And as part of the building boundary they — or the foundation walls — should be properly insulated. In enclosed cavity floors you may use cellulose or other loose-fill or sprayed insulation material. In new construction, foams are a good choice, but sprayed cellulose can also be excellent.


Its very important to air seal the floor carefully, before installing the insulation. Typically, deck floors have many holes, related to plumbing drains, wiring, ducts or supply piping, which makes air sealing crucial.

Most insulation materials, namely cellulose and mineral wool, are poor air-sealing products (thats not their function), and without proper sealing the insulation will not perform properly.

Under Floor Insulation over Crawl Spaces

Installing batts or extrude polystyrene in open floors

Its easy to install batts and rigid foam boards in floors with exposed joists (open floors). You just have to buy the material according to the distance between the joists (15-inch, 23-inch-wide insulation).

The insulation should cover the whole depth of the cavity and contact with the floor. Push the insulation up and fasten it to prevent any space between the floor and the material.

Do not use faced batts (to avoid moisture condensation problems), and do not compress or split the material. You may use push rods, wire, twine or wood strips to support the batts.

When installing extruded polystyrene, use wire insulation supports (every 30 inches or so) or screws and fender washers to hold the material in place. You may stuff small pieces of polystyrene into areas close to obstructions. Some building codes may require covering the foam with drywall or plywood, for fire safety.

Some contractors prefer cellulose to insulate open floors. They install some sort of sheathing (plywood, OBS) on the bottom of the joists — to hold the cellulose in place — and blow the cellulose through holes drilled in that sheathing. The results can be good, but the installation process is a little tricky and more expensive.

Installing cellulose or sprayed insulation in enclosed-Cavity floors

Cellulose and loose-fill and sprayed insulation materials are the best choices for floors with solid sheathing on their underside.

The installation process (in existing homes) is very similar to the one used in cavity wall insulation. You will have to drill holes in each bay, and to patch them after installing the insulation.

Leave a Reply