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Insulating a 14-inch I-joist vented cathedral roof in Climate Zone 5

user-5594283 | Posted in Energy Efficiency and Durability on

Hello and thank you for taking the time to answer my question.

I have a Cathedral Roof constructed with 14″ i-Joists in SE BC, Canada.
The Roof Construction from Exterior to Interior follows:

1) Standing Seam 12″ , 24 Gauge Metal Roof
2) Titanium UDL 50 Underlayment
3) PSU 30 Underlayment on along ridge, eaves and rakes
4) 5/8 T&G Plywood
5) Entire roof strapped with 2×4″‘s – Roof is Fully Vented with a 3″ Airspace along ALL rafter bays.
6) 14″ I-joist Rafters
7) Amvic 1/2″ SiverBoard Foam Board – Glued, Taped and Foamed to INSIDE I-Joist Flange. R 2.1- this is bottom of the ventilation channel

My question:

I would like to fill the remainder of the cavity with Roxul Comfort Batt to R-44 and then
slip in a 1″ Dow Corning Foamular Pink Board R 3.0 on the inside of the bottom flange to create a Foam-Roxul-Foam Sandwich – Total R Value approx: R-46. This would of course be taped and foamed along all seams. Would this provide an adequate Foam/Soft Insulation Ratio or will I have to increase the amount of foam under the Amvic SilverBoard?

If this is OK? I would then proceed to seal the ceiling with 6mil poly using Acoustical Caulk and Builder’s Tape. Note: There will NO pot lights installed anywhere in the ceiling. We would finish off the ceiling with 1×6′ T&G Knotty Pine. My goal is to make this ceiling as airtight as possible. Spraying foam is out of the question as I’m in a rural area – the cost will be exorbitant and I would like to do the insulation myself to keep our costs down.

Thank you for your help and advice.

Mark Kozlowski
Vancouver, BC
Canada

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Mark,
    The product you are talking about -- Amvic 1/2" SilverBoard -- is foam-faced EPS. The foam facings are perforated, so the product has a permeance of 3.48 perms. So it isn't a total vapor barrier.

    Your plan to use XPS on the interior isn't particularly worrisome from a water vapor perspective. The worry is that you won't have a durable interior air barrier.

    The 1x6 pine boards are obviously not an air barrier. Over time, XPS rigid foam can shrink, so it's not a particularly robust air barrier. If you want to use interior rigid foam on your ceiling, you can -- but I would also install either taped drywall or a high-quality European air barrier material, carefully taped, to be sure that you have a good interior air barrier.

  2. charlie_sullivan | | #2

    Another option would be foil-faced polyiso, with taped seams. The foil provides a good surface for foil tape or other tape to grip.

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