How do I insulate the attic stairs to R-38?
According to 2015 IRC Section N1102.2.4:
“Access doors from conditioned spaces to unconditioned spaces such as attics and crawl spaces shall be weatherstripped and insulated to a level equivalent to the insulation on the surrounding surfaces.”
Our Habitat for Humanity affiliate insulates the attic to R-38 with blow insulation, so we are trying to figure out how to insulate the attic stairs to the same value. Currently, we build a box that we fit the stairs into, place a lid on top, and attach two pieces of 1/2″ XPS to the lid to make an R-5 attic hatch. I have looked into products such as the Battic Door and Energy Guardian’s products, but we would like to use rigid foam if possible, since we receive it as a donated product.
Would it be possible to use four layers of 2″ XPS to achieve R-40? If so, how would you attach that to the lid?
Thanks,
Dylan
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Dylan,
The best solution is NOT to have pull-down attic stairs. Instead, use a well-insulated hatch.
In some homes -- those with an attached garage and a common attic over the conditioned house and the unconditioned garage -- you can put the pull-down attic stairs in the garage ceiling, so you don't have to penetrate a conditioned ceiling.
Well-built code-compliant insulated caps for pull-down attic stairs are rare. The problem is weight. I have seen photos of caps that are connected to a hinge and a rope through a pulley, with the rope attached to a counterweight to make it easier to lift the heavy cap. It's complicated to build.
For more information, see these two articles:
Insulating Attic Stairs
How to Insulate and Air-Seal Pull-Down Attic Stairs