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Roof/ceiling insulation for new home in climate zone 2a

RandomLengths | Posted in General Questions on

Designing house to be built, general contractors and insulation salesmen all telling me they all do this and it’s fine.

5.5 inches of open cell foam at 0.5 pound density in under-roof rafter cavities.

Depends on one’s definition of “fine.”

But the code says to insulate to R30, and 5.5 inches of a foam such as that is only to deliver R16 to R20.

The contractors and the insulation salesmen say not to worry, because R does not mean as much as air sealing, and that the water-blown open-cell product they use has “superior” air sealing capacity.

Are they wrong? What should be done?

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  1. GBA Editor
    Martin Holladay | | #1

    The salesmen are wrong. Read all about this long-running but misleading sales pitch here: It’s OK to Skimp On Insulation, Icynene Says.

  2. Expert Member
    Dana Dorsett | | #2

    This is a common refrain amongst foam installers, but dead wrong.

    Also, in zone 2A you'd be wise to put at least an inch of EPS on the exterior (2" is better) as both a thermal break and moisture control at the roof deck when using anything as vapor-open as half-pound foam on the underside of the roof deck. With 2" of exterior EPS and 5.5" of half-pound foam under the roof deck you'd only be at ~R28 or so, but it would perform at a level comparable to~R36 between joists/rafters due to the R8 thermal break over the rafters, better than a full R30 of open cell foam between rafters.

    Another reason foam installers balk is that o.c. foam can't be applied at more than 5-6" at a time without creating a short term fire hazard, out of spec densities, and a longer term issue of shrinkage gaps at the edges. Some manufacturers (eg BASF see: ) only specify the density & R-value if sprayed in 4" lifts. It takes about 9" to hit the R30 code min, and while many installers will do it all in one shot, that is against the manufacturer's explicit standards- it has to be done right it takes at least 2 lifts, with a specified cooling/curing time interval between lifts, which is a PITA for the installers.

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