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1/2 lb spray foam, light density

In most residential and commercial cavity wall/ceiling applications we can offer the same high performance envelope solution at up to 50% less cost than 2 Lb foam systems. We can explain the differences and give you options that will give you the best combination of value and performance to meet your budget.


Here are some of the details that you need to know about the main physical properties of light density foams ½ lb spray foams.

-1/2 lb spray foam is not a vapour barrier; it is considered a breathable foam system which is able to provide drying potential to a wall, ceiling or floor assembly. Drying potential is being recognized more and more by the building science community as an important performance characteristic of building envelopes. Where a vapour barrier is required it can be provided by vapour barrier coatings or paints, conventional polyethylene, or in many instances by other components of the assembly. For example in a floor assembly the plywood subfloor is recognized as a vapour retarder since it is on the warm side of the assembly.

½ lb foam and 2 lb spray foam systems are air barriers and meet the performance requirements as set out by the National Research Council. To be considered an air barrier a spray foam must have an air permanence of less than 0.02 L/s.m2 @ 75 Pa as per ASTM test 2178. Many contractors assert that open celled foam insulations are not air barriers but this is not true. Not all ½ lb spray foams meet this requirement. Ask for a data sheet to substantiate this claim.

2lb Spray Foam Insulation -Medium Density/Closed Cell

2 lb spray foam, when applied at a thickness of 50mm or more, can act as a vapour barrier. They offer the highest R value per inch of all spray foam systems (typically R5.5-6 per inch depending on the thickness).

They also contribute to the structural strength of an assembly in addition to helping to provide a high performance building envelope.

The high compressive strength makes 2 lb density foams ideal for below grade foundation insulations providing a waterproof insulating blanket on the exterior of the concrete.


2 lb spray foams rely on a captive blowing agent contained in the closed cell structure to give them a higher R value. Over time some of the blowing agent in the cells near the surface of the foam escapes into the environment and is replaced by air. This exchange reduces the R value of 2 lb foams over time. Initial R values for most spray foams are over R 7 per inch at time of installation but drop quickly over time because of the release of the refrigerant gas (HFC 245fa) contained in the cells. (See our page on Light Density, ½ lb spray foam)

Many contractors blatantly misrepresent the R value of 2 lb foams. In Canada there is a far more stringent test protocol than in the US. Spray foam contractors and manufacturers in Canada are required to state their R value test results using the CAN/ULC S 770 test method. This is known as the Long Term Thermal Resistance (LTTR) test. Using this protocol, no medium density spray foam has tested to an R value greater than R6 per inch.


Some less scrupulous contractors will quote US test data using “aged R value” as per an ASTM C518 test which will show R values of up to R7 per inch. The Canadian Urethane Foam Contractors Association (CUFCA) has mandated that their contractors use the LTTR R values when promoting their products.

Think about it. Are you getting the true story from your contractor? If they give you 3” of spray foam and tell you it is R20 or R21, we are quoting 3.5” to give you the same R value. The unscrupulous contractor is using 15% less foam!! There’s a reason they are cheaper.


Would you want to do business with a contractor like that?

Do your homework and ask what the LTTR is for the product you’re being quoted on.

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