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Does it make sense to insulate a roof when walls have no insulation?

PNWHappyCamper | Posted in General Questions on

We need to replace the cathedral roof on a 1930s addition to our home that we use as a family room. The room has exterior walls on three sides. The wood panel walls have no insulation and there is less than 2″ between the exterior and interior. We have been told that there is no possibility of insulating these walls without replacing the siding, which we are not planning to do. The windows are original and single pane. We are not planning on replacing the windows – which take up about 70% of the east facing wall – but plan to purchase mini-blinds that add insulation. Below the floor, there is spray foam insulation that looks pretty deep and thorough. The ceiling in the family room is 22 feet tall at its greatest height.
I include all this detail about what we are not replacing and what we have to see if it makes sense to add insulation to our roof when the rest of the room is probably very leaky and the walls have no insulation. Like most folks, we have a limited budget, but want to do what we can to improve the energy efficiency of our house.
I have been told that our current roof is simply plywood applied directly on top of the ceiling and then the asphalt shingles are on top of the plywood. So there is not an easy way to add insulation. To replace the roof on the whole house it would be about $25,000. To add insulation to the roof above the family room and a flat section above our bedroom it costs an additional $25,000. The insulation the contractor proposed is 2.5″ of Polyisocyanuarte rigid foam with an R value of 14.4. We had a proposal for 3″ of closed cell spray foam, but it was even more expensive. We live in Seattle, so it very rarely gets below freezing. Currently, we keep the family room at 58 degrees in the winter (use blankets to keep warm) and roast in the summer.
My questions are (1) will adding the 2.5″ rigid foam roof insulation reduce energy usage by much when the walls are not insulated and will it keep it cooler in the summer so it is more comfortable to use. (2) will it make much of a difference if we added more rigid foam, e.g. if I asked the roofer to add two layers of 2.5″ rigid foam insulation?
Replacing our roof is our chance to improve the comfort energy efficiency But I don’t want to double the cost of a new roof, if it will not have much of an impact. We have other green projects, like those insulating mini-blinds, that we could spend our money on if the roof does not make sense.
Thanks for any advice!


  1. Expert Member
    Dana Dorsett | | #1

    Energy use is a surface area x U-factor thing. Add up the square feet of each surface type and do the math.

    Hollow walls with wood siding on one side and plaster or wallboard on the other have a U-factor of about 0.30 BTU/hr per square foot per degree-F temperature difference. That's pretty crummy, but still better than:

    A wood-sash single pane has a U-factor of about 1.0 BTU/degree-ft-hr.

    An uninsulated cathedalized ceiling with maybe 3/4" of plank and 3/4" of plywood, without cavities enclosed by ceiling gypsum has a U-factor of about 0.4 BTU/degree-ft-hr.

    In western WA you can get by with as little as 1.5" of polyiso on the roof deck and then put up to R40 of fiber insulation under the roof deck without increasing moisture risk. There's no good reason to go with 2.5" polyiso as your only ceiling insulation. Whatever your rafter depths are, fill them with fluff and put ceiling gypsum on the interior, which would far outperform the "extra" inch of rigid polyiso, probably at a lower cost. If you dropped to an inch of polyiso they could just long-nail the shingles in place (most likely) which should drop the price considerably, but that would limit you to about R30 total. Assuming it's 2x4 rafters and you can sculpt R15 fiberglass or rock wool to fit, with 1"/R6 polyiso above it you'd have a U-factor of about U0.05. That's an 88% reduction in heat loss through the roof. If it's 2x6 rafters and you can get R20s in there it would run about U0.039, a 90% reduction in heat loss through the ceilings, and you'd still have plenty of dew point margin even with only an inch of polyiso up top, and it would dramatically outperform 2.5" of polyiso.

    I did this on a 2x6 raftered room in zone 4A NY ~25 years ago, with R6 polyiso up top and R19 cavity fill. It's continuing to work just fine, despite having less than the code-prescribed 30% R-ratio for that climate. In Zone 4C /Seattle it only needs ~20% of the total R above the deck to be moisture safe, so with R6 polyiso you'd be good on the prescribed ratio (but not total R) even with R23 rock wool in 2x6 cavities.

    Insulated mini-blinds are more likely to cause window condensation and rot than reducing the window losses by a big fraction. With tight low-E exterior storm windows covering the antique windows it would bring the effective U-factor down to U0.32-U0.35. That's a 65% or better reduction in heat loss, and provides quite a bit more comfort when it's sub-40F outside. At about $70-90 per window (box store pricing for Larson low-E storms) and a DIY installation it could be dirt-cheap. Professionally installed budget $150-200 per window, but get quotes. Since windows are lossier than the rest (by far), this might be the best bang/buck, but doing at least SOMETHING at the roof makes sense. Low-E storms are somewhat more expensive than un-coated storm windows, but will in fact pay back quicker due to the measurably higher performance (~U0.33 instead of ~U0.50).

    Air sealing the walls makes sense, even if you can't safely insulate them. I'm guessing that the walls are full-dimension 2x4 studs turned 90 degrees to the typical/normal, with siding nailed directly onto the studs, without benefit of sheathing, maybe with or maybe without #15 felt or similar between the siding & studs. Am I close? If there is a sheathing & #15 felt layer, those walls probably CAN be safely insulated without gutting them, but we'd need more information on the stackup and roof overhangs to make that call.

  2. GBA Editor
    Martin Holladay | | #2

    Da Kin,
    I would strongly urge you to install at least 2.5 inches of polyiso above your roof deck in conjunction with your re-roofing job. You may not replace your roofing for another 15 or 18 years, so this is an opportunity that you don't want to pass up.

    I think that your contractor's estimate that adding polyiso to the job will cost an additional $25,000 sounds a little high. You might want to seek competitive bids on this work.

    I'd like to echo Dana Doresett's suggestion about improving the performance of your single-glazed windows: installing exterior storm windows (ideally, storms with low-e glazing) will give you the best improvement in performance per dollar invested.

    Good luck with your energy retrofit work. Remember, this work will not only lower your energy bills -- it will improve your comfort, so that you can enjoy your room without wrapping yourself up in a blanket.

  3. PNWHappyCamper | | #3

    Martin and Dana - Thank you for the solid advice. After reading on this site about the importance of sealing, I wasn't sure the insulation would help much with a leaky room. We will get more bids to insulate the roof. I think the cost is high because the roof has no rafters. The roof is simply the ceiling (tongue and groove fir), plywood, and shingles applied directly on top of each other. Maybe that is why the roofer proposed rigid foam. I will see if there is much cost difference to apply more than 2.5" of rigid foam.
    There is no drywall or plaster anywhere in the family room - just wood. I assume that is what was affordable in Seattle in the 30's. The walls are simply fir on the inside and rough cedar on the exterior. No sheathing. I've had 4 different insulation companies come look at it and say there is nothing we can do without replacing the siding. It might be 2 x4 turned 90 degrees to the typical. In other parts of the house, a contractor described the walls as "flat framed" but they were able to add some insulation. The problem in the family room might be not only the lack of depth in the wall, but the fact that the walls are fir - not drywall. We will definitely start looking at storm windows once I get this re-roofing project underway.
    With much appreciation, Dawn

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