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Community and Q&A

Unvented flat roof with SPF underside

flatrooftrouble | Posted in General Questions on

Hi everyone, I live in climate zone 6. I had done allot of research (clearly not enough) when I decided to spray the underside of my flat roof with 5 inch of closed cell SPF. My understanding was this would create an air and vapour barrier and something like R28. However after the job, I noticed that it was in NO WAY a vapour barrier. They sprayed inside the joists only. There were places I could actually see the roofing membrane. I filled the small gaps in the SPF with great stuff. However there are many placed where the framing wood create small air leaks. Joists that are sistered for instance. or where the joists meet the wall framing. Any place where one piece of wood is nailed to another creates a place where air can leak up and touch the membrane. After watching this video:¬† i’ve been using silicon caulk to try to close up all the air leaks, but the likelyhood that i’ll be completely successful is slim at best. It’s an old building. I will also try to create a complete air barrier with the gyprock but again being perfect is hard and hard to determine. It only takes a small leak to allow allot of moisture though. I believe I’ve created a recipe for mould and rot, any advice on how to fix this now, short of ripping up the roof and putting a proper slanted roof on.

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  1. Expert Member
    Dana Dorsett | | #1

    An actual location (or the base 18C annual heating degree-day data for the location) would be necessary to come up with a reasonable suggestion here.

  2. flatrooftrouble | | #2

    Montreal Quebec.

  3. GBA Editor
    Martin Holladay | | #3

    Flat Roof Trouble,
    First of all, Montreal is in Climate Zone 6. Here is a link to a climate zone map that includes Canada: "Climate Zone Map Including Canada."

    Second, planning for R-28 roof insulation in your climate zone isn't very wise. I'm not sure of the local code requirements in Montreal, but here in nearby Vermont, most people aim for R-49 -- that's the minimum code requirement.

    Third, it sounds like you have been victimized by an incompetent spray foam contractor. If your insulation has cracks and gaps that allow you to see the roofing membrane, something is wrong. A properly installed application of closed-cell spray foam shouldn't result in shrinkage or gaps. You need to call back your spray foam contractor and insist that the contractor fix the problem (presumably by installing additional spray foam to seal the gaps).

  4. flatrooftrouble | | #4

    Ok sorry. Their remedy was to tell me that they did a great job and threaten a law suite for my bad Google review so I'm pretty sure I'm on my own here.

    Yes, you are correct on all accounts and owner was very aggressive. Where I could see the membrane was between 2 peices of wood that where to close together to spray.

    Thank you for your time.

  5. Expert Member
    Peter Engle | | #5

    This thread highlights one of my biggest frustrations with contractors in general, and spray foam contractors specifically. It seems like Canadian contractors are no different. The two big things that our local spray foam contractors will never tell you are:
    1. They won't spray any gap less than 1/4" wide (or pick your number, up to 1"). They know that they're not going to do it, they know that the detail work should be done first, and they see that it hasn't been done before they start, but they don't tell anyone about it and you end up right where the OP is now.
    2. They're not responsible for masking the work area, even though they know that there will be foam snot everywhere.
    If challenged after the fact, they simply say that wasn't their job. A homeowner never knows what unique features of any given trade are considered "not their job," but they end up paying for the problems.

    Enough whining. End of rant.

  6. flatrooftrouble | | #6

    Yes peter, this is exactly what they did. And exactly what I didn't understand. I asked them 4 times, 4 different people if it was a airtight vapour barrier because this space was going to be high humidity, and the response each time was YES, that SPF is a air tight barrier at 1.5 inch.
    But I agree, enough whining. I am where I am and i'm sure there are more people to be in my position. Hopefully they will read this and recognize the issue before they just drywall over it (as I was told to do for fire code) and proceed to mould and rot out their roof.
    Now, how to proceed given that I can't go back in time. My latest plan is to
    1. dig the SPF out of the joints up to where it's level to 5"
    2. use contega polyurethane caulk and maybe contega solido tape
    3. spray on top another 1.5" over the joints (assuming it will stick to the back of contega tape) to make sure air doesn't get out from between the wood and seal where I was not able to remove all the SPF (its not going to be possible to completely remove it.)

    If I do this I will have 1.5 inch of uneven SPF on the joist making it, I believe, impossible to install any more non-SPF insulation like boards or even drywall. Not what i wanted, but if that''s my only remaining option, i'll take it.

    If anyone has had this issue and successfully remedied it or any ideas for me, I'm open to all ideas even non-traditional.

  7. Jon_R | | #7

    > being perfect is hard and hard to determine.

    I'd say impossible. But a blower door will make it much easier to determine.

    While it's accessible, you might install some moisture probe points to catch any problem early. While air sealing is best, there are alternative solutions to ex-filtration driven moisture.

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