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My question is on the suitability and application of foil-faced rigid foam to exterior walls with our climate conditions

I am a homeowner, little sun from Nov to March, average daytime winter temp -8 C, night -15 C, with intermittent -20 C days lasting no more than a week, summer daytime temp, average mid 30 C, night 20 C, with intermittent weeks of high 30 C., average annual rainfall 20 inches, average annual snowfall 60 inches, generally calm, however, can get an occasional rare storm with winds of 20-40 mph and generally low humidity particularly in the winter.

In our area, XPS and faced polyiso (one matte and one foil-faced side) rigid foam insulation is almost the same price. Given our climate, I am thinking it would be beneficial to capitalize on the higher R value of polyiso and face the foil on the exterior wall to the outside, matte on the inside, as foil to the inside against the sheathing would act as a conductor directing heat away from the interior.

If I understand correctly, the foil-faced outward could possibly have somewhat of a benefit in winter, minimally heating the 1" air space between foam and siding, believing it would at least not have a detrimental effect. The main benefit though would be in the summer deflecting heat away. Would this assumption be valid?

Because of cold temperature shrinkage, is there sufficient benefit to overlapping two 1" sheets at a 20% cost increase over one 2" sheet? I could not find any information quantifying shrinkage relative to colder temperature.

I am also thinking foil to the exterior deflecting heat away may result is less temperature increase of the foam and therefore less expansion of the foam in the summer.

Wall construction from the interior would be paint, gypsum board, 2x4 16" OC with unfaced fiberglass insulation, 3/4" TG sheathing, Tyvek housewrap, 2" exterior foam either single or staggered double 1", 1x4 strapping, and vinyl siding.

My understanding is the foil face would be the drainage plane however, not 100% because of shrinkage. Should joints be left untaped, as shrinkage/expansion could cause the tape to fail and potentially obstruct water movement out?

I would like to put windows flush with the exterior and read that in those instances the WRB should be to the exterior of the foam, however, would this negate any positive effects of the foil-faced outward?

Any guidance you could give would be greatly appreciated. I have enjoyed reading the wealth of information that you provide on this site.
Thank you.
Marilyn Limbert

Asked by Marilyn Limbert
Posted Jul 16, 2014 6:30 PM ET
Edited Jul 17, 2014 4:31 AM ET


1 Answer

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You have lots of questions. Before I address a few of them, I'm going to direct you to a few articles on wall construction. If you read the articles, you'll find the answers to many of your questions.

How to Install Rigid Foam Sheathing

Using Rigid Foam As a Water-Resistive Barrier

All About Rainscreens

Calculating the Minimum Thickness of Rigid Foam Sheathing

Polyisocyanurate has an R-value of about R-6 or R-6.5 per inch, although its thermal performance drops in cold temperatures.

The foil facing on some types of polyiso will provide no thermal benefit unless it is facing an air space. If you can come up with a way to install the polyiso so that the foil faces an air space, the resulting wall assembly will have a total R-value that is about R-1 or R-2 higher than it would have been if the facing on the polyiso were not foil.

The thermal performance of foil-faced polyiso in summer or winter does not depend on whether the foil faces inward or outward. It only depends on the R-value of the wall assembly.

There have been some reports of problems with dimensional instability (shrinking) with both XPS and polyiso. For more information on these problems, see "Shrinking Insulation Boards Plague Roofers" and the sections on foam shrinkage at the end of this article: Using Rigid Foam As a Water-Resistive Barrier.

Answered by Martin Holladay
Posted Jul 17, 2014 4:45 AM ET
Edited Jul 17, 2014 4:46 AM ET.

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