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Unvented Flat roof insulation options

callumshill | Posted in General Questions on

So I had planned on putting dense pack cellulose in my ceiling and walls but one of the only companies in this area seems to be closed. While I can use rockwool batts in the walls the roof has me a bit stumped. It originally had vents which of course did not really work so I’ve sealed them up and had 6″ of EPS added with a torch down roof. The way they built it was to run 1×2 perpendicular to the 2×10 joists. This means that if I try to shove a batt up against the plywood it’s going to end up with an air gap and from I’ve read here and other articles you need to have the insulation in direct contact with the sheeting. I’ve attached a photo of the roof.   If anyone has any ideas I’d love to hear them.

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Replies

  1. mr_reference_Hugh | | #1

    callumshill, this GBA article might convince you to choose closed cell spray foam plus some batt insulation.

    https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work

    I am not a big fan of spray foam but in some cases, there are certain constraints that just push you towards it. With that strapping and apparently an inability to dense pack cellulose (and other types of insulation?) you are left with fewer choices. I personally would not count on the spray foam to be an air barrier. There are many articles on GBA about the how spray foam can separate from the wood and cause air leaks and other install issues. I would work hard to find the best qualified installer and I would add a really good air barrier under the roof joists.

    This is not the full list of options but it gives a pretty good idea.

    You can also look at the https://www.wallassembly.com/ which provides comments on different wall assemblies if you want to do more research. This is a competition that is indirectly related to GBA and provides comments from experts on different wall AND roof assemblies. The competition is apparently running its 4th competition in 2023 so you have a few examples from past ?3? competitions to review and consider if you want to look at more than the options in the GBA article.

    Hope this helps.

  2. Expert Member
    MALCOLM TAYLOR | | #2

    callumshill,

    With batts, I don't think you would end up with enough of a gap to worry about.

    You may find this useful: https://www.greenbuildingadvisor.com/article/insulating-low-slope-residential-roofs

  3. callumshill | | #3

    Thanks for the options guys! I'll go and read those articles.
    I did find out last night that there are two building supply shops here (Victoria,BC) that have Krendel 325 blowers so I'm going to reach out to Krendel to see if that model will be able to do dense pack. I really was hoping to have someone else do that.

    1. Expert Member
      MALCOLM TAYLOR | | #4

      callumshill,

      That's partly why I suggested sticking with batts. It's a more predictable and DIY friendly approach.

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