The cell structure of open cell foam also referred to as 0.5 lb. foam (0.5 lb./cu. ft.) consists of very small cells that are not completely closed, allowing air to fill in the very small voids. The fact that air fills in the very small voids makes the foam softer than closed cell.

The cell structure of closed cell foam also referred to as 2.0 lb. foam (2.0 lb./cu. ft.) consists of very small cells that are closed and packed tightly together. Closed cell foam uses a gas as a blowing agent. This gas becomes trapped in the closed cells, providing a better insulator than air. This same principle is used in the construction of windows where the area between the two panes of glass is filled with argon since it is a better insulator than air.

Closed cell foam has some advantages over open cell foam. Closed cell foam is approximately four times as dense as open cell. Closed cell foam is very rigid, adding structural strength, especially racking strength, to a structure. Closed cell has almost double the R-value per inch as compared to open cell. Open cell's R-value is approximatley 3.5 to 3.8 per inch. Closed cell's R-value is approximately 6 to 7 per inch. Closed cell foam at two or more inches thick has a permeance rating less than 1, qualifying as a vapor retarder. Open cell does very little to stop water vapor. Vapor drive is a very interesting issue that will be discussed more under Air, Vapor & Moisture Barriers. The parts of the country that our Company applies foam are no longer required to use a vapor barrier due to heating part of the year and cooling the other part of the year.

The main disadvantage of closed cell foam is that since it is denser, requiring more material, it is more expensive. Even though the R-value may be almost double, closed cell foam is still more expensive than open cell when both are applied at a thickness to achieve the same R-value

Open cell foam cannot be used in certain applications, such as on the exterior of a structure below grade due to its very soft nature. Inside a structure, especially a residential structure, either foam can be used in crawl spaces, on basement walls, above grade walls, on roof decks in the attic creating an unvented attic, and on the the floor of the attic creating a traditional vented attic. See Foam Insulation in Attics and Foam Insulation in Crawl Spaces for more information on such applications.