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Would using spray on foam as insulation allow me to avoid the need to use any drywall in my exterior walls?

Would using spray on foam as insulation allow me to avoid the need to use any drywall in my exterior walls if I am using tongue and groove wood for interior walls? Also is there any interior treatment that can reduce thermal transfer through the 2x6 studs?

I would like to use tongue and groove pine, spruce or aspen for most of my interior walls. I am building a new home (it is a tall barn home built around a 170 year old barn frame). The exterior is OSB, tyvek, then shiplap spruce or board and batten spruce or 4 inch thick limestone. We did not install any rigid foam boards outside so we have the problem of the 2 by 6 studs acting as a thermal transmitter at least where we have exterior wood siding.

The ceiling of the structure is made of old barn wood, then tar paper, then thick plastic, then a cavity of about 24 inches filled with fiberglass insulation that is between 6 and 18 inches thick (the Ijoists in the ceiling do not act as a thermal bridge because they do not connect with the interior barnwood ceiling, which rests on the old barn wood frame and rafters).

I would like to avoid using any sheetrock due to the mess it may make of the big old hand hewn barn beams and due to the expense. I can get tongue and groove wood cheap at a local mill and paint it with whitewash as an interior finish.

Can I use spray in foam to create an adequate air barrier and then avoid using any sheetrock? Also is there anything I can do on the interior to reduce thermal transfer through the 2x6 studs?

FYI, the barn sits on a large new cement slab (no basement) and so far even with no insulation it stays cool in the summer. we also have cupolas with automatic window openers to allow for venting if the barn heats up. I am mostly worried about winter. This is in Utah at about 6,000 ft.

Asked by Stan Silver
Posted Jun 30, 2014 1:52 PM ET
Edited Jun 30, 2014 1:59 PM ET


2 Answers

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There are several issues to address here.

First of all, is your roof assembly vented or unvented?

Second, there is a question as to whether your roof assembly includes a tight enough air barrier under the fiberglass insulation. Asphalt felt plus polyethylene isn't necessarily airtight.

Third, interior polyethylene is usually a bad idea, because it will prevent your roof assembly from drying inward.

Fourth, the parts of your roof assembly that have only 6 inches of fiberglass insulation are poorly insulated and do not meet code. This could contribute to ice dams.

You should ask your local building inspector whether you can omit gypsum drywall on your walls. The gypsum layer is usually required for fire protection, so I wouldn't be surprised if your inspector requires the drywall.

Answered by Martin Holladay
Posted Jun 30, 2014 2:04 PM ET


Thank you for responding.

1) The roof assembly is fully vented.
2) I agree that there is probably not a great air barrier above the barnwood but it is already done and I am stuck with it. Right now it seems to be functioning extremely well -- the interior of the structure has stayed fairly cool even on hot days. Maybe the problem will be in the winter?

3) I understand about the interior polyethylene issue now, but it can't be removed at this point. Fortunately it is older barn wood so more porous and it is not tightly installed (there are small gaps between each piece). I hope that this will allow it to not have problems.

4) the six inch pieces of fiberglass sit below the roof joists, against the barnwood ceiling. If they weren't there at all, then the roof joists would meet the barn wood and it would be even worse. So I think those 6 inch porches are well insulated since they create a barrier between where the joists would otherwise act as a thermal transmitter.

We will check with the building inspector on the fire protection issue. I was just wondering if there is a less expensive alternative.

Answered by Stan Silver
Posted Jun 30, 2014 3:33 PM ET

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