Spray Foam Myths Debunked: Unvented attics need more insulation and R-value than conventional attics

July 12, 2017 Anthony Naimo

If you’ve read that spray foam insulation is unproven or that spray foam insulation doesn’t perform because it is frequently applied without enough R-value, then it’s time to separate fact from fiction. Noise and myths about building materials, such as spray foam, can lead to confusion and misconceptions for builders, architects, building owners and even, regulators. Confidently bust the myths about spray foam insulation so you can make informed decisions about the materials you specify and use in your residential or commercial buildings.

Spray Foam Myth:  Because the roof line is a larger area than the floor of the attic, I will need more insulation and/or R-value.

This myth seems to come from the perception that surface area means more product, however, larger roof lines do not mean more insulation or additional R-value.  It comes down to the type of attic assembly. Unvented attics (also known as UVAs) typically outperform conventional (vented) attic assemblies where ducts and mechanicals are housed within the attic space. This is because duct leakage is eliminated, ducts and mechanicals are not operating in adverse conditions, and air barrier and insulating details can be made continuous much more easily.

Recent studies from the American Chemistry Council have indicated that an unvented attic with R-28 spray foam insulation installed will outperform a Title 24 (2016) conventional attic with a total R-value of R-51 in all Californian climate zones. Furthermore, in many zones, an unvented attic featuring R-22 spray foam insulation installed will outperform the conventional “High Performance” R-51 attic.

Taking a look at both approaches, a Title 24 prescriptive attic provides higher levels of insulation to reduce the amount of heat loss and gain. Also, insulation is added at the roof deck to moderate attic temperatures. The attic remains vented because duct leakage and many penetrations through the attic floor still exist. Leakage of moist air remains a concern. The graphic below shows that there is additional energy loss from the HVAC and ductwork because the space is leaky and under insulated.

Prescriptive attic approach

Compare the conventional attic approach to that of an unvented attic (UVA) design approach where a plane of air tightness and insulation is applied at the roof line. This application has fewer penetrations and is easier to make air tight. Spray foam insulation adheres to adjacent materials so the air barrier is continuous. Additionally, the requirement for venting the attic space is eliminated and because ducts and mechanicals are within the thermal boundary, direct duct leakage to the exterior is eliminated. The graphic below identifies how the unvented attic with spray foam insulation on the underside of the roof deck is more efficient in minimizing energy loss.

Unvented Attic Approach - Icynene spray foam insulation

For architects and builders in California, the American Chemistry Council – Spray Foam Coalition has released a detailed publication on spray foam insulation and the California Title 24 2016 requirement for high performance attics. The piece provides modelling guidance for both CBECC and Energy Pro software.

For architects and builders in other states, Icynene’s Building Science and Engineering experts can work with you to understand how spray foam insulation can be integrated into your current and upcoming residential and commercial projects. Plus, they can help debunk the myth that you need more insulation or R-value in an attic roof line.  

Be sure to catch the next debunked spray foam myth on Icynene’s Building Genius blog. 

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