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Community and Q&A

Roofing and insulation retrofit

Erik Addy | Posted in Energy Efficiency and Durability on

I have a 1921 1.5 story bungalow in Minneapolis (climate zone 6). There are three separate accessible attic areas with knee walls. Two out of the three have HVAC ducts in them, along with some plumbing runs and access doors. They are anything BUT air sealed, in other words. When I purchase the house, the knee walls were insulated with fiberglass bat with the craft paper on the attic side, and someone had also added craft paper faced fiberglass to the roof deck. The spaces were vented with one apx 8×8 vent and an electrically powered exhaust fan each.
I removed the power vents and had the accessible areas of the roof deck (and exterior walls within the attic spaces) sprayed with open cell foam, blocking off the vents. This resulted in noticeable improvements in comfort and energy use, and reduced ice dams. However, I still get some significant ice dams, particularly in the valleys on the south side of the house. A few more details: I have exposed rafter tails, 2×4 roof framing, and the foam was not shaved down, so I have an average depth of apx 5” ( I would say it varies from 4-8”), and the framing is covered with at least 1” of foam. My asphalt shingle roof has about had it and need replacement. The existing roof deck is original (planks with some spaces between them, some knotholes, etc). I am going to get a new roof this summer, so I have an opportunity to make some improvements to help with both energy use and ice dams. It’s a nice house, but I probably wont be here forever, and it’s by no means a high end home or in a top price market area, so expenditure must be considered. Oh yeah, I’m not exactly swimming in cash either! ïŠ
Here are my concerns and questions:
1) I don’t have a vapor barrier. If I was doing this now (about 5 years later, and many FHB articles later), I would probably go with closed cell to get a vapor barrier and more r-value. I would have asked for a thicker application as well.

2) I have thought about adding a few inches of rigid foam over the existing roof deck and then putting new sheathing on top of that. Questions I have about that are what is preferred, XPS or Poly-Iso? What would the min thickness be not to have condensation beneath the existing roof decking? What if I was able to put some sort of VB over the currently installed open cell foam? Could I then go with something thinner for the rigid foam? I ask because I have exposed rafter tails, and I’m not sure how this whole thing would look if I made the roof deck 5”+ thick.

3) I have a few thoughts about how to add a VB and/or more r-value to the existing foam job. Could I have closed cell foam sprayed on top of the open cell? Or, I could shave it down flush with the rafters and add a layer of foil faced poly-iso. Or, I could just spay a VB coating on it, but that would be difficult given how “bumpy” it is.

4) Lastly, I am considering a steel roof, although I suspect it won’t be cost competitive. Is steel usually applied over ridgid foam? How thick? Can you put it right over foam, or does it require a plywood deck between them?


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  1. Chris Koehn | | #1

    Hi Erik,
    I'll take a swing at a couple of these, as I'm from Wisconsin and have built in MSP.
    I wouldn't add insulation to the roof without adding a vapor barrier from below. If I were to add insulation I'd use XPS board for R 5 per inch and 2X (4 or 6) material on edge on 2' centers, with ply over that. Metal should go over felt and preferably UDL underlayment-type.
    Alternatively you might consider nail base- EPS foam with board (usually OSB) applied to the outside and fixed to the existing roof deck with special screws and washers. Same UDL with metal over.
    These are not cheap and they build the thickness of the roof significantly, which may not be acceptable.
    Adding insulation from below might be the most cost-effective and least visually obtrusive.
    Another option might be to build a cold roof on top of your existing deck: an air space below the new roofing to allow the bottom side of the roofing to acclimatize to the outdoor air temp quicker. Again- not a super cheap proposition.
    A consideration about the ice damming: on the south side it may be somewhat inevitable if the slope of the roof puts it in a high solar gain environment and shading may occur. That is to say, it might not all be due to heat loss.
    Hope that helps!

  2. Erik Addy | | #2

    Thanks for the reply (I know I asked a LOT of questions!). Like you mentioned, I suspect building up the roof deck with foam will be both too expensive and not look quite right. I'm leaning towards adding a few inches of foil faced poly-iso on the inside. Of course that doesn't help in the areas that are not accessible where the ceiling is right on the roof framing, but that's just the way it is (there is some cellulous in that area now). Another thought that just occurred to me is if the old roof decking needs to be replaced, I could have some closed cell foam sprayed in this area while the roof deck is off.
    I am still curious about steel roofing, both for how it may help with the snow, and for the possibility of keeping the upstairs cooler in summer. I’ll get a quote or two on that when I put the roof out for bid. Something else I am curious about is the possibility of using those energy star asphalt shingles to reduce the heat gain upstairs in summer. Don’t seem to be easily available in MN though. Thanks.

  3. J Chesnut | | #3

    You are correct that open cell foam applied directly under the roof sheathing is problematic, particularly without a vapor barrier.
    Condensation of vapor that makes it through the foam unto the cold sheathing may be an issue, but a more pressing issue is warm air escaping through leaks or breaks in your thermal/ air layer.

    Because of the framing of your home and the after the fact insulation of the roof plane from the inside there may be several conditions where air is managing to leak out the roof - where the foam application is interrupted by the knee walls is one likely place, dormer framing is another.

    Another thing that happens with thick open cell applications over roof framing is when the foam cures voids form that can be in essence holes where moisture laden warm interior air comes into contact with the roof sheathing. I found two such conditions in my own home.

    I think your first step is have a blower door test with infrared camera inspection of your current conditions. With those results you will want to put priority on establishing as best you can a continuous air barrier at your insulation application.

    Next in priority in conjunction with re-roofing is to provide ventilation of the roof sheathing if possible. Your dormer may interfere enough where this is not feasible.

    Let me know if you are looking for energy auditor/ contractor recommendations. I'm in Minneapolis as well.

  4. Erik Addy | | #4

    I have had a blower door test and IR scan. Came out pretty well, but there are a couple of leaks (at the top of the knee wall, as you suggested) that I need to fix. Will need another blower door test and scan to verify results. By venting of the sheathing, I suppose you are suggesting building a second layer of sheeting spaced above the existing with sleepers? That could be difficult, as the roof is more complicated than just a dormer. I am interested in recomendations for both good energy contractors and roofers in Minneapolis (specifically, roofers who know how to do more than just re-shingle). Thanks. my email address is [email protected] where first is my first name and last is my last name.

  5. GBA Editor
    Martin Holladay | | #5

    It's perfectly fine to add rigid insulation on top of your roof sheathing even if you don't have an interior vapor barrier. In fact, the added rigid insulation on top of the sheathing will keep your roof sheathing warm and lower the danger of condensation or moisture accumulation in your sheathing.

    Following the concepts used to calculate foam thickness for walls (see Calculating the Minimum Thickness of Rigid Foam Sheathing), I would aim for at least R-11.25 of rigid foam -- that is, 2 inches of polyiso.

    That would be the best solution to the complicated list of issues you are facing.

    You may find some of the information in this article helpful: Creating a Conditioned Attic.

  6. Erik Addy | | #6

    Thanks Martin. Rigid foam on top is certainly my prefered solution, but I have a feeling I won't be able to afford it. My next choice is going to be add some foil faced poly-iso to the botom of the framing that has the open cell foam in it. Will get some estimates on the roof this spring and find out.

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