Is low density foam under the sheathing the right type of insulation for attics?
Joe samson
| Posted in GBA Pro Help on
I live in zone 4B and have a house with 3 tab shingles. Recently I was given a proposal from a company that offers energy performance and one of the recommendations was to apply 8″ of low density foam under the sheathing with no mention of creating any type of ventilation in between the sheathing and insulation.
The second recommendation that they proposed was to put 2″ high density foam on the rim joist.
Is the low density foam the right type of foam to apply under the sheathing and is ventilation needed between the foam and the under sheathing?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Joseph,
It would be helpful to know your climate zone.
Either open-cell spray foam or closed-cell spray foam can be used to insulate the underside of roof sheathing. However, in Climate Zone 5 (or colder zones), open-cell spray foam needs an interior-side vapor barrier to prevent moisture accumulation in the roof sheathing.
For more information on these issues, see:
Creating a Conditioned Attic
Open-Cell Spray Foam and Damp Roof Sheathing
Closed cell spray foam is expensive and has high environmental impact (the bubbles are blown with gasses that have 1000X the global warming impact of CO2, or worse). So you might consider other options before going that route.