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Closed Cell vs. Open Cell Foam

By Steven O’Donnell Senior Trainer for AHIT

There are many different types of Spray Foams available in the marketplace that you will find on during a home inspection.  You will hear terms like open cell (½ lb) and closed cell (2 lb) foam.  We will discuss these terms to give you a better understanding as a home inspector on what to look for in each type.

In a closed-cell foam, the gas forms discrete pockets, each completely surrounded by the solid material. In an open-cell foam, the gas pockets connect with each other.  An example of open-cell foam is a bath sponge.

Open-cell or ½ lb foams have an R-value of about 3.7 per inch and require a vapor retarder as dictated by local building codes and climate.  Open-cell foams are easily identified by their soft spongy surface.  Closed-cell or 2 lb foams have an R-value in the mid 6 per inch and generally do not require a vapor retarder.  Closed-cell foams typically have a hard “crust” on their surface.

There are also many “Green” types of foam on the market, soy foam, corn foam, sugar foam, etc.  They make it seem as though the product is completely Green.  In real life the Green component is about 5-8% of the Component B while all of them still have the nasty petroleum based Component A, MDI (methylene diphenyl diisocyanate).

Open-cell is less durable and must be used inside only.  Closed-cell is more durable, has a greater R-value per inch, and adds structural rigidity in most applications.  In either case when used in wall and ceiling cavities virtually all applications must be covered with ½ inch drywall or an intumescent coating as a thermal / ignition barrier.  When used in an attic application (vented or unvented) many closed-cell products can be left uncovered.

With all of the differences and benefits of closed-cell foam you can imagine that it costs more than open-cell foam and it generally does.

One common application currently being used is “flash and batt” which involves spraying about 1-2 inches of foam on the walls (backside of sheathing) then insulating with standard fiberglass batts.  There is a potential problem with this procedure in cold climates with too thin of foam.  Condensation may form in the wall cavity on the surface of the foam since the warm humid air within the home will move right through the fiberglass until it meets the cool surface of the foam.  Be sure to check manufacturer’s recommendations for proper installation.


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