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Zone 5 – Cathedral Ceiling

Paul Szymanski | Posted in Plans Review on

Hello all. Long time lurker building a new house by Candlewood Lake in CT. We are currently finishing up framing and I am getting all kinds of debate on the cathedral ceiling in my master bedroom. Above it is proposed 2 x 12 rafters and we were going to go with a vented dense packed cellulose insulation. Now I am considering adding a 1.5″ layer of PolyIso on the inside.

Is there any potential to not vent? All the articles I see here strongly suggest venting when dense packed cellulose is used in a cathedral. What say you?

Is the 1.5″ of Polyiso worth it?

We are doing 2 x 6 wall construction dense packed cellulose with 1.5″ ZIP-R.

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Replies

  1. Aj Builder, Upstate NY Zone 6a | | #1

    Venting is safer. The cellulose main rep dude says you don't have to vent. Many apparently have been built unvented.

    As to rigid foam, I like any rigid foam vented whether interior or exterior placement.

  2. GBA Editor
    Martin Holladay | | #2

    Paul,
    If you are installing cellulose between your rafters, venting is required by code (and is a good idea). The only way you can avoid venting is if you install an adequate thickness of rigid foam above the roof sheathing (not under the rafters).

    If you have the vent channels as required by code, go ahead and install the cellulose. The polyiso under the rafters is a good idea, because it addresses thermal bridging through the rafters.

  3. GBA Editor
    Martin Holladay | | #3

    Paul,
    For more information on this issue, see How to Build an Insulated Cathedral Ceiling.

  4. Expert Member
    Dana Dorsett | | #4

    In a 2x12 cavity with cellulose and 1" (code minimum, but 2" would be better) vent gap you would get at most R38. That may meet code min (the lousiest insulation that's legal to build) if CT is still based on IRC 2009, but in locations with "stretch codes" based on IRC 2012 you would need R49.

    At 1.5" with 1.5lb density polyiso you would get another R9-R10, so you might still be a bit shy of R49 center-cavity, but since you nearly double the R-value over the fraction bridged by rafters it will outperform an R49-all-between-joists/rafters solution.

    A more resilient solution would be to put 4" of foam (R20-ish) on the exterior, and drop back to 2x10s with R-30-R35 cellulose between the rafters, and no interior side vapor retarders (other than latex paint.) This offers a more reliable drying path than 1" of roof deck gap with soffit to ridge venting would, especially if the pitch of the roof is lower than 8:12. Even though the labeled R-value of 4" of polyiso would be R25-ish, the outer 2" would substantially underperform it's labeled R during the mid-winter season when moisture accumulation in the roof deck would be occurring. In your climate it's better to make the outer 2" EPS, with the layer next to the roof deck polyiso. The labeled-R of that stackup might only be R21-ish (R8-8.4 for the EPS, R12-13 for the polyiso) but during the coldest hours the EPS would be performing north of R9, whereas if it were 2" of polyiso on the coldest edge would be performing at about R6-7. As long as it's fully 4" an all polyiso solution would still work for dew point control at the roof deck if the 2x10 rafter fill is R30, but would be on the edge if the rafter fill hits R35. An R21-ish EPS/polyiso stackup would still work with either. Of course even more foam is better- the higher the ratio of exterior-R to total-R, the warmer the roof deck is, which makes it adsorb less moisture, and release it more quickly.

    In southern New England there are multiple sources of reclaimed roofing foam, which can take substantial sting out of high-R foam-overs. The two nearest me are:

    http://nationwidefoam.com/

    http://www.greeninsulationgroup.com/

    But there are others. Many of the smaller operations advertise on Craigslist- search the materials section for "rigid insulation" or "foam insulation". The cost of the goods is typically 1/4-1/3 the cost of virgin-stock, so that 1.5" of polyiso you were considering can grow to 4.5-6" at low to no additional cost.

    BTW: 1.5" ZIP-R isn't sufficient exterior R for dew point control at the foam/fiber boundary and would be outside the bounds of the IRC chapter 7 prescriptives. ( http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_par025.htm ). But with the moisture buffering capacity of dense-packed cellulose that won't be a problem. (Doing it with rock wool or fiberglass cavity fill might.) An interior side "smart" vapor retarder would enhance resilience, and reduce the moisture cycling of the cellulose though. Going with standard sheathing and a couple of inches of exterior foam (any type) adds an inch to the wall thickness, but brings it within the IRC prescriptive values, and would thus unquestionably meet code without an interior side vapor retarder.

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