1/2 lb or 2 lb Spray Foam – What’s the Difference?

You might know that spray foam insulation is the most effective insulation for reducing air leaks in building materials.  You may also be aware that it comes in two different types.  We’ll get to the types a bit later on.

Spray foam is a result of the blending of two components together.  Industry insiders refer to these parts as A and B for simplicity.  These two parts come in large drums from the manufacturer.  A spray foam installer has specialized equipment that travels to your project and is used to mix parts A and B through a series of hoses to a spray nozzle.  These hoses heat the materials as they route from the drums in a truck to the job site, blend in the spray gun nozzle, which disperses the liquid to the substrate.  These two components vary in formula, but always produce a chemical reaction that foams, then expands, and then cures into the hardened substance you see as foam insulation.

1/2 lb foam is also referred to as open cell spray foam.  Open cell foam blocks air but are moisture permeable and open cell foam is suitable for areas that are not subject to moisture intrusion.  The density of this foam is 1/2 lb per cubic foot, which is why it is so named.  Open cell refers to the fact that the cells are not completely closed.  This type of foam is less expensive because it uses less material.  Other features of 1/2 lb foam include an r-value of 3.5 per inch of thickness and can be made from bio materials such as soy. 

Closed cell foam is also known as 2 lb foam with a density of 2 lbs per cubic foot.  This greater density produces an r-value of 6.0 per inch or more.  This formulation costs more than open cell foam and provides a barrier against both moisture and air infiltration.  The closed cells offer a complete barrier against water vapor infiltration.  This foam option is dense enough and provides enough adhesive power that it also adds structural strength to building materials such as sheathing and roof decking.

In order for the mix of parts A and B to be sprayed, there needs to be a blowing agent.  The agents used to blow 1/2 lb foam are water and carbon dioxide.  The agent typically used for closed cell foam is HFC.  Recent developments and improvements by some foam manufacturers have reduced the amount of HFCs needed to dispense closed cell foam resulting in a more “green” application method.


Sustainability Qualities of Spray Foam Insulation
Spray Foam and New Energy Code Requirements