I have a 1940s Cape Cod in northern Ohio that has no eaves. What is the best way to insulate the finished attic space?

Asked by Brian
Hudson, OH

I have a 1940s Cape Cod in northern Ohio that has no eaves (so no soffit vents). I'd like to apply closed-cell foam insulation right to the underside of the roof deck and then drywall (the ceiling) over it. The first-story walls are already foam-filled, so this would seal the envelope of the house. Should I completely fill the cavities of the rafters before finishing over them? Not doing this seems like it would create a space for stagnant moist air to cause mold, but the contractor I'm dealing with says it's not necessary because of the "condensation point" being in the middle of the foam...? Any insights while I check with the local building dept. would be greatly appreciated!


Elliot Seibert

Answered by Elliot Seibert

Washington, DC

Everyday Green

November 25, 2010

Hi Brian,

This is a good strategy for an unvented Cape Cod attic such as yours. Your contractor is correct; it is not necessary to fill the cavity with insulation all the way.

Here’s how it works:

  • The exterior side of the insulation is essentially the same as the outdoor temperature (let’s say 20 F on a bad day).
  • Similarly, the interior side of the insulation is essentially the same temperature as the home (probably 70 F).
  • Within the insulation the temperature changes linearly, so on that cold day the very middle of the insulation is about 45 F.
  • It just so happens that 45 F is the point at which normal indoor air will condense. In order for there to be condensation, moisture would have to permeate halfway through the insulation, which simply cannot happen because spray foam at these depths is an air and moisture barrier.

Caveat: this logic only applies to impermeable insulation such as spray foam. Insulating the roof deck with a permeable product like fiberglass batts could very well lead to condensation on the roof sheathing surface.

Bottom line: you don’t need to fill the cavity all the way, although you should put in at least R-38 for energy efficiency.


For more information:

Check our home foam insulation Q&A to see what other homeowners and contractors are saying about foam insulation, condensation, etc.

Tagged In: spray foam, attic insulation

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