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How should I insulate my roof?

GBA Editor | Posted in GBA Pro Help on

I am building a log house and am at the point of insulating my roof. It is a metal roof with 2×10 rafters. The guys I am hiring to insulate said I have a couple of options. Condensed fiberglass R-30 batts are out because they are special order and I can’t get them for 2wks and the guys are coming next week. I could put 2×2 furring strips to create the 12″ I need for air flow and do regular R-30. I could do spray foam. They said 1″. and then do the regular R-30. This woul give me about R-37. Or I could do blown in fibergalss insulation. They said with it being condensed I could get R-38. My concern with the blown in is the lack of air space to prevent condensation. They said that with it so densely packed in the air space wouldn’t be necessary. It seems everyone has their own opinion about how things should be done. I am finding this throughout the whole project, 3 years in the making. : ) Thanks for any advise you can give me. We are covering the valted ceiling with T&G. I don’t know if that info is helpful also.

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  1. Riversong | | #1

    The T&G ceiling is essential inforamation, since it won't be an effective air/vapor barrier and then it becomes more important to ventilate the roof - both for moisture elimination and for reduction of solar radiant heat transmission.

    You need to tell us what climate zone (or geographic area) you're in. And I think you meant 2" ventilation cavity, not 12". Is there sheathing under the metal roofing and, if so, what kind?

  2. Riversong | | #2

    Addendum (can't edit these darn posts): some kind of air barrier is essential, however, since building a vented roof above an unsealed ceiling just creates a chimney for air and moisture to exfiltrate.

  3. Amy | | #3

    I am in Eastern Washington, Colville area which is much snowier than Spokane for instance. And much colder and drier than Seatlle. We get usually 2 times in the year where we get below or around zero. Otherwise it stays in the 20-30s in the winter.

    I meant 12" total space, so yes 2" additional space to vent. Under the metal is tar paper and plywood. So from the inside right now I see the plywood with the 2x10 rafters.
    The guys I spoke with talked about using some kind of visquene as a vapor barrier under the T & G. Since we are doing that instead of sheetrock.
    I hope that answers all your questions. I appreciate the help!

  4. Doug McEvers | | #4

    R- 30 or 38 is not near enough ceiling insulation for Colville, WA. Weather Underground has Colville at 8,157 heating degree days (hdd) for the last year. A well insulated building will have walls with an R-value based on hdd divided by 180 and ceiling R-value of hdd divided by 120. In your case this is R-45 walls and R-68 ceilings.

    With a log home you need to beef up ceiling and foundation insulation to make up for the log walls, they can be comfortable but do not scrimp on the areas mentioned.

    I would go with the R- 30 high density batts allowing an airspace above the batts and add at least 2" of polyisocyanurate on the underside of the 2x10 rafters. Tape the seams with a foil tape and this will be the air barrier.

  5. Amy | | #5

    Thank you for your response. Don't I need an air space for venting? I was thinking that if we did 1" of the polyiso underneath that would give 1/4" of air space with the 8 1/4" batts and the boards being approx. 9 1/2". I am not familiar with the hdd, but I checked with the county and code is listed as R-30-R-38. This would give us a total of R-36. But is the 1/4" venting space enough?
    Thank-you again.

  6. Riversong | | #6

    Adequate roof ventilation requires a 1½"-2" air space and it has to be isolated from fiberglass batts to prevent wind-washing which will dramatically reduce the effective R-value. It also requires continuous soffit vents and continuous wind-baffled ridge vents.

    I would recommend nailing 2x2 (actually 1½") nailers on the sides of rafters tight to the roof sheathing, attaching ¼" hardboard baffles to those to create an isolated vent channel, installing R-30 batts (compressing them will reduce the R-value slightly) between joists, adding 1" foil-faced polyiso foam board below the rafters as thermal break and air barrier (tape the seams with foil tape and spray foam edges at framing), and then your T&G (no additional vapor barrier required).

  7. Doug McEvers | | #7

    I'm with Robert on the isolated vent channel, even 1x2 on edge is enough of a nailer.

    Another option for the polyiso sheathing on the room side of the rafters is to attach a layer of 1 1/2" directly to the 2x10 with a few plastic cap nails and tape the seams. Now add 2x2 strapping on the underside of each 2x10 with the 1 1/2" polyiso sandwiched in between, fill in the space between the firring strips with an additional 1 1/2" polyiso sheathing. This will give an another R-20 to the R-30 batts, provide a thermal break, give solid nailing for the T&G ceiling and eliminate some holes in the air barrier.

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