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Another Cathedral Ceiling 1.5″ Vent, 5 inches of Closed Cell, then…..

Tom Wheeler | Posted in General Questions on

I am getting close to finishing my site built baffles going from the soffit to the ridge.  Then I get an insulation quote about 2lb Closed cell +/-5 inches in the ceiling for almost the same price as BIBS.

Can I have dense packed cellulose placed after the 5 inches of foam (~10.5 inches total depth currently)?  I had planned on a vented roof for fluffy stuff, but what do you think?  

With the fluffy, I was going to go with 16″ O/C perpendicular 2×4 to deepen the cavity, and use polyiso board to air seal it under the final finish.

If I do Spray foam, would I still need to worry as much about the air sealing under the final finish?

Thanks

Replies

  1. Expert Member
    Dana Dorsett | | #1

    The amount of fiber insulation that's "safe" depends on your climate zone, but with 5" of closed cell (R30-R35) and 5.5" of cellulose (R20-ish), something more than 60% of the total R is air-impermeable foam, which is completely safe in IECC climate zones 1-7, and probably most of zone 8.

    Depending on your climate zone you may want to dial back on the expensive foam and go with more fiber-R, especially if it's theclimate-damaging HFC blown 2lb foam, not that even HFO blown foam is green:

    https://materialspalette.org/wp-content/uploads/2018/08/CSMP-Insulation_090919-01.png

    But you DON'T want foil faced polyiso on the interior side since that creates a moisture trap. The vapor permeances of the 5" of 2lb foam is about 0.2 perms, the tighter end of Class II vapor retardency, and foil faced foam is less than 0.05 perms. It takes a very long time for any moisture in the fiber layer to make it's way back out into the vent channel through 0.2 perm foam, and basically nothing gets out the foil faced foam side.

    The urgency of air tightness on the interior side depends on the ratio of the exterior foam R to total R, and your climate zone, so...

    ... what climate zone?

  2. Tom Wheeler | | #2

    How embarrassing.....Zone 7, Northern Wisconsin.

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