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Insulation Board vs. Drywall at Vaulted/Cathedral Ceiling

bennytheclam | Posted in Energy Efficiency and Durability on

Hi and sorry in advance if this is a repeat question.

We are renovating a Cape Cod style cabin in upstate NY with the bedrooms located on the lofted second floor.  The previous owner had run the ceilings to the ridge and installed foil-faced insulation that filled the rafter bays, which left us with some moldy spots in the drywall and some dry rot in the roof deck.

We took down the ceiling, pulled out the insulation, and had the roof replaced this past summer and are currently working to close up the ceiling in hopes of cutting down on the heating bills.

To date, we’ve furred down the rafters (2×6 + 2×4 furring for 9” total depth) installed baffles in each bay, and installed 8”thick Kraft faced HD insulation.

We are planning to install salvaged 1×6 tongue and groove paneling on the ceiling.

I know we need an air barrier at the bottom of the rafters but I have a few questions:

1. Is faced insulation board with foil tape at the seams a suitable air barrier? If so, which way should the reflective foil face?

2. Can we install wood panel directly over the insulation board?

3. If the answer to 1 or 2 is “No,” is there value to installing housewrap inside out (i.e. trapping air inside the house) or air barrier (e.g. Membrain) between the insulation and the drywall? 

Thanks for any help.

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  1. Expert Member
    BILL WICHERS | | #1

    Yes, you can installed "faced insulation board" and tape the seams to have it act as an air barrier. I did exactly that behind a T+G ceiling in my own home in fact. I would use polyiso here, since it gets you the most R per inch, it tends to be stiffer than EPS or XPS, and that stiffness helps to make the install a little easier in this case. Install the polyiso using a few construction or drywall screws through the big plastic insulation washers, then put up your T+G over the polyiso. Tape the seams with a good tape, I like the Nashua 324A foil tape that the orange store carries.

    Since you have a vented ceiling assembly (I'm assuming you also have soffit and ridge vents so that the vent channels you installed will be able to allow for airflow under the roof sheathing, which is important), you're OK using batts here, especially with a good air barrier underneath. I would put a bead of sealant around the perimeter too, to seal the polyiso to the framing (or wall materials), to make sure there are no sneaky air leaks.

    You can install the T+G ceiling directly over the polyiso. There is no benefit to installing Tyvek/housewrap, since the taped foil facer of the polyiso is already an air barrier. There is no benefit to installing MemBrain (a smart vapor retarder) here either, for the same reason -- and that foil facer is a vapor barrier too.

    BTW, I'm assuming your "8" thick HD batts" are R30. That's not a lot for a roof. You can make up for that with thicker polyiso on the interior, 1.5" polyiso for example would bring your assembly up to R39. I would use at least 1" polyiso here, which would bring your assembly up to R36, but thicker polyiso = higher R value = better overal thermal performance for the assembly.


    1. bennytheclam | | #2

      Thank you so much, Bill. You're right about the total R-value hitting 30. This is a weekend house that used to be someone's 3-season hunting cabin, so we're trying to do as much as we can without breaking the bank or shrinking the space, so we stuck with salvaged 2x4's for the furring. It won't get us as much as we should have, but it will certainly be a step up from the R19 with untaped drywall that it replaced. As for venting, I'd heard some stories about the roof system we installed not venting well at the ridge, so we sealed it and installed a gable fan with a thermostat and humidistat to keep things under control in the "attic."

      At this point, it feels like our biggest adversary is the air that leaks through the floor and roof, so being able to seal it up with the insulation board will be a big help. We're also installing new plywood underlay over the T&G subfloor to hopefully trap some of our heat on the first floor instead of letting it waft through.

      Thanks again for your help.

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