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I need assistance in choosing a spray foam insulation for my roof rafters

Ron Farmer | Posted in Energy Efficiency and Durability on

I have a house that was built 1904. The upstairs has no insulation between the room & the shingles. The roof rafters are true 2″x 4″‘s with plywood & shingles above that. The height of the room at the peak is 6 foot, so no room to add any insulation below the rafters.

Spray foam appears to be my only choice, 4″ of foam will be better than none at all. We live in the NE corner of Alberta, in the Village of Vilna, 60 miles NE of the City of Edmonton. Winters get down to -40 degrees.

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Replies

  1. David Meiland | | #1

    How old is the roof? Any chance you need to replace the shingles soon?

  2. Ron Farmer | | #2

    Roof looks to be in reasonable shape, maybe 10 years old. I know that another option is to put another roof on top of the one that I already have & build it using 2x8"s construction. Is that what you were thinking of?
    Thanks for the reply
    Ron

  3. David Meiland | | #3

    Ron, you have a couple of options if replacing the roofing, but probably the most effective is to install rigid foam over your roof deck, then another layer of plywood and new roofing. There are variations on this theme, but the basic idea is to put the insulation above the framing.

    If you have to work from below, I would use rigid foam cut to fit into the rafter spaces, and then if at all possible put another 1" - 2" of continuous insulation under the rafters, in spite of the loss of headroom.

  4. Mark Fredericks | | #4

    Ron, closed cell spray foam is a good bet if you only have 4 inches to work with and you're in the cold of northern Alberta! Closed cell is typically R6.5 per inch so you might get R26 which is probably not going to meet building code. However I've found here in Nova Scotia the code allows a sloped roof to have a lower R value than required for a flat ceiling with attic space above. This may be the case in AB too, but this flexibility only reduces the R50 requirement to R31 so if you have building inspectors involved or if you want more thermal performance, you could run furring across the rafters to deepen the cavities - at a loss of precious head room. Another option as David suggests is to insulate on top of the roof deck.

    This article is really helpful, and many FAQ's are answered in the comments section
    https://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling

  5. Ron Farmer | | #5

    Thank you both

  6. GBA Editor
    Martin Holladay | | #6

    Ron,
    If the height of the room at the peak is only 6 feet, it is barely usable. But I suppose you know that.

    The best way to proceed is with exterior rigid foam, followed by a second layer of roof sheathing (plywood or OSB) and new roofing. You could also use nailbase if you wanted (rigid foam adhered to a layer of OSB).

  7. Ron Farmer | | #7

    Thank you Martin, Mark & David. I agree I have to put it on the outside & put on a new roof. Always better to get more than one opinion. Thank you ALL.
    Ron

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