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Choice of rigid exterior roof insulation

The February 2014 issue of the Journal of Light Construction has an article starting on page 41 titled "Insulating Cathedral Ceilings." On page 46, the second paragraph under "Rigid Foam Above Sheathing," there first is mention of polyisocyanurate and XPS rigid board as options for this purpose.

Then there is a statement that appears to be contrary to what has been said here and elsewhere about the use of EPS board: "Expanded polystyrene (bead board), which has a high water absorption rate, is not recommended on roofs." This seems to be at odds with recommendations here on GBA and elsewhere (eg. If EPS were inappropriate for a roof for reasons of water retention, why is it OK for use as sub-slab insulation, and why would it be OK to use polyiso on the roof, a material which is NOT recommended for buried applications for the same reason? Am I missing something?

Asked by Dick Russell
Posted Sat, 03/01/2014 - 17:31
Edited Sat, 03/01/2014 - 17:32


2 Answers

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To put it bluntly, the author of that article, Clayton DeKorne, is wrong.

EPS manufacturers have been supplying EPS to roofers for years. Here are some links:

GBA has published several photos showing roofers installing EPS insulation on top of roof sheathing; for example, I used the photo shown below to illustrate my article, Insulating Low-Slope Residential Roofs.


Low-slope roof insulation being installed 2.jpg
Answered by Martin Holladay, GBA Advisor
Posted Sun, 03/02/2014 - 05:45

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Some of the very low-density EPS used in shipping & packing industry has large fractions of open cells, and it IS possible to waterlog some (but not all) of that stuff. But EPS at densities that low never ends up on roofs, and isn't usually sold as sheet-insulation goods.

The lowest density sheet goods used in roofing applications is Type-I (1.0lbs/cubic foot nominal density) . That's the minimum density that has sufficient compressive strength to make it a minimally "walkable" roof so that it can be installed with out damaging it. Type-I EPS just isn't the sponge that the article implies, far from it. Type I & Type II (1.5lb nominal density) EPS is probably THE most common roofing insulation out there under torch-down roofs. Type-II EPS is also widely used as dock floats, boat & surfboard cores, lobster & crab pot buoys- if it's water-loggable all of these applications are living on borrowed time! Yet somehow it seems to survive decades in those applications. Roofing applications are pretty light-duty water exposures compared to the myriad marine applications for the material.

Polyiso is hygroscopic, and needs the water proof vapor-retardent facers to work in roofing applications. The facers come in a variety of types, fiberglass/asphalted paper/composite, and most of which run between 0.5-1 perm, and are thus class-II vapor retarders. That allows SOME vapor-diffusion drying, while being waterproof to liquid water, and it'll be fine under roofing, but not with soil on both sides. It's best used where it can dry to at least one side or the other to avoid becoming waterlogged. If applied over a vapor-tight membrane on a roofing deck it's best to have at least a tiny drying channel between the iso and the roofing. But with EPS it doesn't much matter- even with all interstitial space filled with H2O from persistent immersion, it's still delivering something like 90% of it's bone-dry thermal performance.

Answered by Dana Dorsett
Posted Tue, 03/04/2014 - 17:15

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