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Roof insulation in Montana

user-7022518 | Posted in Energy Efficiency and Durability on

I wanted to ask a quick question for my sister in Whitefish MT. They are having their insulation re-mediated after a disastrous install last year which led to both ice dams and mold. The roof is a mix of attic and vaulted ceiling. The new proposed solution is to spray 7″ of closed cell foam underneath the roof sheathing to create a conditioned space–which is a big improvement. However, they have had a hellish experience and I wanted to check with this forum that this is all that will be needed. They plan to install drywall with recessed light canisters afterwards.

Lisa

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Replies

  1. Expert Member
    BILL WICHERS | | #1

    Recessed lights are asking for trouble.

    Regarding the rest of your question, you haven’t really provide enough info for us to give you good advice. Can you provide some drawings, or pictures of the existing roof assembly? The best way to insulate depends on the space, and if it’s a vented attic, unvented attic, cathedral ceiling, etc.

    Bill

  2. Expert Member
    Peter Yost | | #2

    HI Lisa -

    Bill is right: to offer more we need more. You will find a wide variety of perspectives on spray foam insulation on this website, which is a good thing all in all, but it can make quick easy straightforward answers/direction maddeningly elusive.

    One thing to keep in mind if you are going to offer more in terms of visual/text details: transitions from cathedral roofs to attic spaces can get complex and tricky, since what is needed is continuity of both the insulation and the air barriers from one roof assembly to the next. Also, how complex your roof(s) are--hips, valleys, dormers, skylights, etc--will be important to offer more guidance.

    Peter

  3. Jon_R | | #3

    Thermal bridging can be an issue with spray foam. Both for conducting heat to roof snow (causing ice dams) and conducting cold to the interior (causing moisture accumulation from sorption and condensation). Internal or external rigid foam would help.

  4. GBA Editor
    Martin Holladay | | #4

    Lisa,
    Assuming that the spray foam is competently installed, the suggested approach will yield an airtight roof assembly with an R-value of about R-45. That's decent, and it should solve your problems.

    Of course, there are always several ways that a job can go wrong, but it sounds like you're on the right track. Make sure that the recessed lights are installed in such a way that they don't take up room where the 7 inches of insulation should be. If the recessed lights intrude on the insulation layer, change your lighting plan. (Perhaps you could use some of the new LED pancake fixtures.)

  5. user-7022518 | | #5

    Thank you for all your replies! I don't have as much information to share as I should because I'm asking for someone else. I noted the possibility of thermal bridging and the protrusion of recessed lights into the area with insulation and will pass that on.This site is a fantastic resource--thank you! Lisa

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