Attic Hatch Insulation: Is Rigid Foam Polyisocyanurate Insulation Best?

Attic Hatch Insulation?

I am thinking of gluing rigid foam polyisocyanurate insulation to the back of our upstairs attic hatch access panels. Any feedback or suggestions?

Bob K.

Polyisocyanurate insulation is eco friendly

First, let’s consider the environmental aspects of the material. Polyisocyanurate (called “polyiso”) is a rigid foam insulation panel that usually has at least one heat-reflective foil face. Although polyiso’s very name sounds as anti-environmental as “clear cutting,” and as toxic as “carcinogen,” it is in fact rather benign.

Virtually all polyiso foam has some recycled content, mostly discarded plastic bottles, and the recycled content of the aluminum foil facing varies between 80 and 100 percent. While the foam was once expanded by blowing ozone-depleting CFC’s through resin, it is now produced with zero-ozone-depleting blowing agents.

With higher R-values (insulating values) than any other readily-available insulation, polyiso’s superior insulating qualities far outweigh its environmental negatives. Both the Environmental Protection Agency and the U.S. Green Building Council, a non-profit industry group that promotes sustainable building practices, have recognized polyiso products as beneficial to the environment.

Polyisocyanurate insulation R value

A 1.5 inch thick polyiso panel is rated R-9 —that’s almost double the rating per inch of typical attic insulations. Ideally, your attic hatch insulation will match roughly the R-value of the rest of the attic insulation, so the “almost double” value provides a handy guide to the thickness of your polyisocyanurate insulation.

For example, if you have standard 6.5 inch-thick fiberglass insulation in your attic (R-19), you would want to install 3 inch-thick polyiso (+ /- R-18) on your attic hatch. Polyiso is available in half-inch-increment thicknesses, so for the example above you could either buy a three-inch thick panel or stack two layers of 1.5-inch thick foam, depending on the panel’s price and availability in your area.

Planning the Installation

Which brings me to polyisocyanurate insulation’s main drawbacks: price and flexibility. Typically available in 4×8 sheets, polyiso is at least twice as expensive as standard insulation, so you don’t want to use it for your (usually small) attic hatch and throw the rest of the sheet away. (That’s an environmental “no-no” anyway.) Try to find a use for the polyiso scraps, or perhaps go in on one sheet with a neighbor who also needs attic hatch insulation.

Polyiso is easy to cut with a hand saw (wear a dust mask), and because of its closed-cell foam structure, it will maintain its structural integrity in a seldom-used opening such as an attic hatch — that is, you shouldn’t have to put a wood frame around it to keep it from disintegrating at the edges. (Wrapping the edges with some reflective metallic tape might be a nice touch, however.)

The foils side should always face toward the living space. You can use contact cement to attach the panel to your hatch. (Be careful to keep the cement away from the polyiso core which might react with it.)

~PSW

3 Responses to “Attic Hatch Insulation: Is Rigid Foam Polyisocyanurate Insulation Best?”

  1. Jim Says:

    Making several layers of foil backed polyiso could be an issue with moisture.

  2. Philip S. Wenz Says:

    I haven’t had that problem, but I’ve only done it a couple of times.
    ~PSW

  3. KM Says:

    Great info here, thanks! First winter in the new house, and have realized that the attic hatch has no weather-stripping or insulation. I don’t like working with the puffy pink stuff, so rigid it will be. Plus I can use the rest of the sheet in the coldframe greenhouse I’m planning for the spring. :)

    Thanks again.

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