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EPS for attic insulation

I guess it's a "cut and cobble" question. How can EPS be used in attic insulation application for a new construction?

BTW I am in zone 6.

Asked by asim majeed
Posted Jun 14, 2014 9:29 PM ET


3 Answers

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EPS is not the best choice for attic insulation. Depending on the thickness of EPS you plan to use, you could either:

(a) design a high ceiling for your top floor, attach the EPS to the underside of your ceiling joists with cap nails, install furring strips (strapping) under the EPS with long screws, and then install your ceiling drywall; or

(b) Install a layer of OSB on top of your attic floor joists, followed by EPS on top of the OSB. This method may or may not require that the EPS be protected by a thermal barrier for fire safety; you'll have to check with your local building department on this matter.

Either method will require you to pay attention to air sealing and the continuity of your air barrier and your thermal layer at the perimeter of the attic.

I don't recommend cutting EPS into thin strips and inserting it between your joists.

In almost all cases, cellulose insulation will be cheaper, easier to install, and will perform better.

Answered by Martin Holladay, GBA Advisor
Posted Jun 15, 2014 5:43 AM ET
Edited Jun 15, 2014 5:46 AM ET.


Labor needs to be considered just as much as the installed building products.
EPS is not labor-efficient to install in a regular stick-frame attic.
Even if the EPS you have free.

The only place where i would see EPS is as the top layer screwed on the framing after it has been filled with another loose or bat insulation ... Even then you'd probably have to cut/trim/fit alot of parts to fill up to the edges.

Still only worth it if you have the EPS for free or extremely low price.

Have a few pro quote you on loose insulation installtiong before pushing this one .

Answered by Jin Kazama
Posted Jun 16, 2014 7:31 AM ET


In NEW construction the best use of EPS in attics would be to put in on the exterior of the roof deck, making it a conditioned attic.

To do that without creating soggy roof sheathing from wintertime moisture accumulation in US Climate Zone 6 you need a minimum of half the total R value to be on the exterior (R25 in exterior foam, out of a total code-min R49) :


At R4.2/inch you can get there with 6.5" of EPS, and R23 rock wool batts (or 5.5" of cut'n'cobbled EPS) between the rafters.

Answered by Dana Dorsett
Posted Jun 16, 2014 10:34 AM ET

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