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Community and Q&A

Firestone polyiso insulation

Btrunner | Posted in General Questions on

I think I would like to use 3 1/4″ Polyisocyanurate in the 2×4 exterior wall stud cavities in a bathroom. As far as I know Firestone is the only company currently producing such a thick piece. This product is intended for roofing applications. This would provide a LTTR of 20.1. Much better than I could get with Roxul or fiberglass. I’m not sure what problems might arise from using this product. I think the main worry would be a moisture issue of one type or another. As a second choice several layers of the DOW Tuff R could perform the same function. I have called DOW and asked if this installation would cause moisture problems and they feel it would be OK. This house has shingles. I would probably use redgard as a waterproofing. Can someone with experience in the matter please chime in to provide some guidance?

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  1. GBA Editor
    Martin Holladay | | #1

    If you want to install foam insulation in your stud cavities, the easiest way to proceed is to use spray polyurethane foam.

    If you want to install a thick layer of rigid foam -- which is a good idea -- the best way to install the rigid foam is in a continuous layer (either on the exterior side of the studs -- the best approach -- or on the interior side of the studs, if you can't put the foam on the exterior) rather than to cut up the foam into small rectangles. If you put the foam between the studs instead of over the studs, you end up with a thermal bridging problem.

    When all is said and done, however, a lot of homeowners end up doing what you propose. Many people call this approach "cut and cobble." It is fussy work, but you can do it if you want. You won't have any moisture problems with this approach, as long as you pay careful attention to air sealing. That means that each piece of rigid foam needs to be sealed at the perimeter with caulk, canned spray foam, or high-quality tape.

  2. Btrunner | | #2

    Cut and cobble is my only viable option. Compounding the matter is the fact the stud cavities are not of any standard width. There's an old window rough in frame which bastardizes things further. It was closed off many years ago. I can cut the foam on my table saw and as you suggested seal the perimeter with spray foam. So then, what are the implications of thermal bridging?

  3. GBA Editor
    Martin Holladay | | #3

    Thermal bridging through the wall framing reduces the thermal performance of your wall by lowering the whole-wall R-value. More information:

    Thermal Bridging

    The Thermal Bridge to Nowhere

    Insulation Overview

    Thermal Bridging

  4. Expert Member
    Dana Dorsett | | #4

    Almost all roofing-iso vendors make both 3.25" and 3.5" as well as 3" goods, not just Firestone. It's typically either 2lb or 3lb per cubic foot density (compared to 1.5lb density for most foil-faced iso products). The fiber facers are usually class-II vapor retarders (< 1 US perm), which is sufficiently vapor-tight for use in a wall cavity in most climates (provided you meticulously air-seal it, as Martin points out.)

    Cutting the iso with a loose enough fit to the stud bay to allow a can-foam nozzle to fit between the framing & foam, then filling the gaps with spray foam is a more reliable air-seal than trying to caulk/foam a tighter-fit to the1/4" of exposed stud.

    In many areas there are sources for reclaimed roofing foam from commercial roofing demolition/re-roofing. I have several vendors in my area, but you can also call around to commercial roofing contractors, who will often have a stack of reclaimed or damaged goods. Since you're cutting it all up into pieces, you don't get much benefit from using virgin-stock, and your scrap rates are likely to be high. In my area reclaimed goods run about 25-35% the price of virgin stock, which is a substantial savings on projects of any size. (3.25" iso runs $60-65 for a 4' x 8' sheet f.o.b. the distributor's yard in my area, but only $15-20 /sheet for near-perfect reclaimed goods, less if it's broken or has majorly dinged-up corners & facers, etc.)

    At 3.25" 2lb polyiso runs about ~R18.5, when properly derated for cold climates (it's R-value slowly drops with the average temperature through the material), and in a 2x4 wall with a 25% framing fraction, adding in the siding & gypsum you're at about R9.5-R10 for a "whole wall" R. That's about where you'd end up for whole-wall R with R15 Roxul too, but if you have plank sheathing or sealing the gypsum is difficult, the cut'n'cobble iso would be SUBSTANTIALLY more air-tight, and thus less prone to moisture issues.

  5. Btrunner | | #5

    Martin & Dana thanks for all the information.

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