The Cost of Re-installing Spray Foam Insulation

Above is the Heat Flow Reduction chart that illustrates why there is no need to install any more than 3” closed cell spray foam. This amount provides a heat flow reduction of 96%. If we double that thickness, the heat flow only goes up a little over 1% which equated to an almost immeasurable/insignificant amount of energy savings. This is the diminishing rate of return of foam and why we only recommend 2” closed cell spray foam in the walls (94% heat flow reduction) and 3” closed cell spray foam under the roof.

While you will receive a 94% heat flow reduction with only 2” of closed cell spray foam insulation, we recommend a minimum of 3” under and roof deck or in any attic assembly.

This is due to 2 factors:

1. A typical roof assembly is subject to a significant increase in temperature due to the radiant heat gain from the solar exposure of the roofing material. The 3” of Closed Cell Spray Foam will guarantee the 96% heat flow reduction which is needed for this increased heat gain.
2. Large temperature swings in a roof assembly could cause an increase in moisture or vapor drive and the 3” of Closed Cell Spray Foam will almost eliminate the thermal drive which carries the moisture. (Vapor drive=latent moisture driven through an assembly by a thermal drive). This Vapor drive will vary from area to area based on the climate zones and the average heat days per year and is generally calculated by a WUFI scale for condensation.

Ken Ashcom
EnergyModelingAgency.com