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Thermal Conductivity and Rigid Foam

I've read this discussion of point thermal bridges from a few years ago:

Has there been more recent research or "industry thinking" as to the impact of thermal conductivity through the fasteners in rigid foam.

I'm planning to do 1.5" rigid foam outside of plywood sheathing, with fiber cement lap siding (no rainscreen) and fiberglass batts in the 2x6 cavities. Am trying to assess how much better this is than dropping the rigid foam and just doing a better job in the cavity (spray foam).


Asked by Dave Frank
Posted Fri, 12/20/2013 - 15:09


2 Answers

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Spray foam insulation between the stud cavities will never make a wall assembly that performs as well as your planned assembly using 1 1/2 inch of exterior rigid foam. Thermal bridging through the studs greatly compromises the performance of the spray foam insulation.

Although the James Hardie Co. allows their fiber-cement lap siding to be installed directly over rigid foam up to 1.5 inch thick, my own preference is for an installation with a rainscreen gap.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 12/20/2013 - 15:49

Helpful? 0

Even with R-infinity cavity fill (made of pure unobtainium) the best whole-wall R you'd get out of 2x6 construction with fiber cement would be about R27.5 at a typical 25% framing fraction. With 5" of R7/inch closed cell foam cavity fill you'd be burning up the planet with the high global-warming potential of the HFC blowing agents just to squeak an R17 out of it.

If you long-nailed 10,000 nails to hang the siding through 1.5" of exterior foam and put cellulose or open cell foam in the cavities you'd take some hit from the nails- how much depends on the material and actual diameters of the nails, but it would still likely be slightly ahead of the closed cell foam cavity fill solution. With 1.5" of Type-II EPS with furring timber-screwed 24" o.c. on which to hang the fiber cement you'd be a hair over R21 whole-wall (the additional two air films in the cavity more than offsetting the thermal bridging of the timber screws.)

Whether the R6.3 foam would be sufficient for dew point control to allow you to build without an interior side vapor retarder is climate dependent, but with a rainscreen gap and semi-permeable EPS (or VERY permeable rigid rock wool) you'd be able to "cheat" the IRC minimums by R1 or so, and you'd be fine with just R6-ish in a US zone 5 climate, despite the code prescriptive R7.5.

Answered by Dana Dorsett
Posted Thu, 01/02/2014 - 12:22

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