An Open Cell Foam Install Gone Bad – GreenBuildingAdvisor

By Chuck Bickford | June 30, 2025

First developed the 1930s, both open-cell and closed-cell spray polyurethane foams have been widely used in residential construction for more than thirty years. As the name suggests, open-cell foam is less dense, has an R-value around 4 per in., is less expensive, absorbs sound well, and has a permeance (the ability for water vapor to pass through the material) of about 7.5 per in., making it a class III vapor retarder. Closed-cell foam is denser, more expensive, has an R-value of 7 per in., and a permeance of less than 1 per 2-in., a class II vapor retarder at that thickness.

Although open-cell and closed-cell spray foams are an effective insulation, neither is considered environmentally friendly due to their high-carbon manufacturing processes. Until fairly recently, the blowing agents used had a high global warming potential. (High GWP blowing agents have been banned for several years in a number of states, and there is a nationwide ban on making new material with high GWP blowing agents as of Jan 1, 2025, though existing product can be used until 2028.) Another consideration with regards to both versions of spray foam, correct installation of the products is critical for their performance.

Witness an installation gone wrong

Member “Woodworker99” has posted a question about a spray foam failure in the Q&A forums. Nine years ago, he built his house in climate zone 4A that featured a 30-year metal roof (with no decking or underlayment) and an unvented, unconditioned attic. His contractor sprayed 5 ½-in. of open-cell spray foam directly onto the underside of the roofing. No vapor barrier was installed, as it wasn’t required by code at the time.

Then things went sideways. He writes, “This year I started noticing major leaks. I assumed bad screws or washers, but after pulling panels I found every screw badly rusted, 30-year…