Foam insulation, usually urethane, can be injected into wall cavities or sprayed onto roof or floors. Once applied, it expands and sets in about a minute. After it sets, it shrinks slowly for several weeks. Properly applied, shrinkage is less than 5 percent. Like rigid boards, foam insulation has the advantage of high R-Value per inch. Similarly, it shares the disadvantage of requiring a fireproof covering material when installed inside the home.

The use of spray foams in new construction has been rising in recent years. Foam is produced when you mix Isocyanate with a resin and it comes together at the tip of the spray gun. There are essential two types of foam used today: open-cell foam and closed-cell foam. When mixed with an "open cell" resin, it forms a less dense product that is mainly used in attics and basement bands. When mixed with a "closed cell" resin, the spray foam is much denser and is generally used in crawlspaces. After being sprayed, it expands to roughly 100 times its original volume. As a result, it is able to fill vacant air gaps, and will expand and contract in relation to the building.