Best Insulation for Small Attic

Alexandra Gerhardt asks,

Hi guys! Love the podcast! My husband and I are under contract on a house. During inspection, mold was found in the attic on sheathing and in the insulation. We are getting the mold on the sheathing remediated, insulation replaced and improving the ventilation by rerouting bathroom exhaust duct, adding a new ridge vent and air sealing penetrations like the recessed lighting.

The attic is not an occupied space and really too small and too cumbersome to get into (small scuttle in the linen closet). The quote we got was for cellulose insulation. However, we were wondering if it makes sense to pay more for spray foam or a polyiso insulation? Thanks for any input!

Our Answer

Great job identifying the source of the mold issues in the attic! Improving ventilation (the most important of which is getting that bath fan exhaust duct extended), was absolutely the right thing to do. As for insulation types, cellulose and spray foam are quite different. If you went with cellulose, for one, you’d need all that ventilation. Like fiberglass, cellulose is designed to work in an “unconditioned” space. Heat is held to the ceiling level, and everything above that is at ambient temperature (hot or cold).

Spray foam, on the other hand, is designed to convert that to “conditioned” space, meaning ventilation is no longer an issue. We used this on our 1886 home and it’s never been warmer in the winter or cooler in the summer.

When it’s very difficult to get insulation in a tight space in either case, spray foam insulation might be your best bet. It can fill in tough-to-get-to cavities, has a higher R-value and does not need to be ventilated. A good resource is The Money Pit Guide to Insulation, which discusses pros and cons of spray foam insulation and its types. Best of luck with the project!