Understanding Closed Cell Foam Insulation Versus Open Cell Foam Insulation
Polyurethane spray foam is often categorized as either “open-cell” or “closed-cell”. There are several major differences between the two types, leading to advantages and disadvantages for both, depending on the desired application requirements. In addition, polyurethane spray foam is an extremely versatile material that is available in a variety of final physical properties and densities, making it necessary for the end-user to have an understanding of these differences in order to choose the spray foam system that is best suited for the particular application requirements.

Closed Cell Spray Foam
Closed cell spray foam is among the most efficient insulating materials residentially and commercially available, with an R-value of 6.8 per inch. Common foam density for closed-cell polyurethane foam is approximately 1.7 to 2.0 pounds per cubic feet. This foam expands 20 to 30 times its initial liquid size, and rarely requires trimming which provides little to no waste. Closed-cell polyurethane foams are usually characterized by their rigidity and strength, in addition to the superior R-Value. These heavier and denser foams have what is known as a closed-cell structure, which means that the gas (EPA approved Honeywell 245fa) bubbles that form during the application process remain permanently locked into the cured foam. These millions of microscopic bubbles created during the manufacturing stage remain closed and intact, thus trapping the reaction gases. Therefore the R-Value of Closed cell foam is close to that of the reaction gas, or 6.8 per inch.  Since there are no open interconnections between individual bubbles, the foam can not absorb water and also effectively diffuses (retards) the passage of water vapor. According to most building codes, a vapor retarder must have a perm rating of less than 1.0; Closed cell foam meet this standard. And, very importantly, due to the closed cell structure and higher density range, it provides the optimum strength increasing shear and racking strength by up to 300%.  Closed cell foam, unlike open cell, stops convection (air-flow) both through and within the wall cavity. One additional note concerning closed cell foam is that some companies market and install closed cell foams without this 245fa (or comparable inert gas) blowing agent. These products, by definition, are no longer true closed cell polyurethane foams. Without this blowing agent the foam loses a significant amount of vapor diffusion and/or vapor barrier qualities and should therefore create concern when being used for these reasons, such as under floors or in the exterior walls of old wood-sided homes without code approved vapor retarders for example.

Open Cell Spray Foam
Open cell spray foam is also referred to as ½ pound or low density foam. This foam expands to over 100 times its initial liquid size, and may require trimming and disposal of the excess waste material after application. The physical characteristics of Open-cell polyurethane foam usually include a softer, “spongier” appearance, as well as lower strength and rigidity than closed-cell foams.

Open cell foam cures soft while the irregular bubbles which form during the expansion-reaction process remain open and interconnected. Open cell foam derives its R value of 3.8 per inch from the dormant air within the open bubble pockets which is about half that of closed-cell foam. The R-Value of open cell foam is quoted close to that of conventional fiberglass and cellulose insulation because they all use the same principle of trapping dormant air.

However, these similarly quoted insulation values are very misleading since open cell foam greatly reduces air-flow and significantly slows moisture transfer relative to these competing conventional insulations. Please refer to the “R Fairy Tale” for more information about the inaccuracies of R-values. Open cell foam does not stop, but rather, significantly slows down convection through and within the wall cavity. Open cell spray foam is more permeable to moisture vapor, with perm ratings of approximately 16 per 3 inches thickness, therefore is not a code approved vapor barrier. Open cell foam should not be used in exterior applications, such as underneath a raised house, due to its inability to retard moisture flow. However, Open cell foams are incredibly effective as a sound barrier, having about twice the sound resistance in normal frequency ranges as closed-cell foam.

Applications that typically use open-celled foam include residential construction insulation, under roof trusses for “conditioned attics”, and for sound deadening in media rooms, etc. Closed-cell foam, while a superior insulating product used in residential construction, is also used in a variety of commercial and industrial applications because of the excellent strength and insulating properties and vapor barrier qualities which may be required by building codes.