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Community and Q&A

Hot roof with 2 lb. spray foam on a 3/12 pitch roof

canada01 | Posted in Green Building Techniques on

I’m in Toronto And I’ve built a house with a roof pitch of 3/12. I’ve been told by every insulation contractor that I would have to do a hot roof 2lb sprayfoam application. Reason for this is that the city of Toronto requires r60 in the Roof/attic. If I tried to put in loosefill or batts I’m told there will be no room for air circulation and I’ll get mold. Im going to put 5inch of 2lb sprayfoam ( Honeywell Solstice LBA or Insulthane Extreme In Canada) that will give me r31 or r32 on the underside of my roof . The city is saying I would have to spray to make it r60. Sprayfoam people saying that it’s a waste of money to add that much foam and the r31 is more then enough and will do a better job then r60 batts/fill. If the city won’t let me get away with the r31 hotroof can I stick some roxul r30 batts to the underside of the roof and under the sprayfoam so I can pass my insulation inspection ? I’m not sure if I can afford the extra cost of the sprayfoam for the Roof and even the sprayfoam people are saying it’s not necessary. The rest of the house exterior walls from the basement to the roof will be sprayed as well. Any suggestions or where I might be able to point the city in the right direction would be a great help.


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  1. GBA Editor
    Martin Holladay | | #1

    First of all, the city is right, because R-31 is not R-60. (Maybe the city official reads GBA.) A roof insulated to R-31 will leak heat at almost twice the rate of an R-60 roof. For more information on this issue, see It’s OK to Skimp On Insulation, Icynene Says.

    I'm surprised that the minimum code requirement for roofs is Toronto is as high as R-60 -- not just surprised, but impressed. That's a good code.

    The solution you are suggesting is called a "flash-and-batt" job. Briefly, the answer to your question is, "Yes, you can improve the R-value of this roof assembly by adding mineral wool insulation to the interior." Note that the mineral wool insulation has to be in direct contact with the cured spray foam layer.

    In your climate zone (Zone 6), the spray foam layer needs to provide at least 51% of the total R-value of the roof assembly. Here is a link to an article that explains what you need to know about the foam-to-fluffy ratio issue: Combining Exterior Rigid Foam With Fluffy Insulation. (While the article discusses rigid foam, not spray foam, the foam-to-fluffy ratio discussion applies to either type of foam.) You'll just squeak by with your R-31 spray foam layer.

    For more information on flash-and-batt, see Flash-and-Batt Insulation.

  2. canada01 | | #2

    Thanks Martin for the information. See the three sprayfoam companies that I got quotes from all said the same thing that anything over the r31 is overkill and they are not pushing me to spend another 5k to get r60. They said they can do it but it’s a waste of money and the benefit is not worth it. If I was to attach the Roxul comfortbatts r30 or r28 to the underside of the roof how could I attach it? I don’t have much room to add this extra insulation but I guess I’m going to need to try. Roxul seems like the best bet to do this. Sorry for the questions I just want to do my house correctly and not get mold in my non existent attic. Thanks for your time.

  3. GBA Editor
    Martin Holladay | | #3

    I don't know what your attic looks like. But you need to come up with a way to keep the mineral wool batts in contact with the cured spray foam. This might involve steel wires or some type of membrane (as long as the membrane can be left exposed without violating fire codes).

  4. Expert Member
    Dana Dorsett | | #4

    Toronto's climate roughly the boundary between a US zone 5A and 6A climate. In zone 5A more than 40% of the total R needs to be foam in this type of stackup for dew point control at the foam/fiber boundary, in zone 6A it has to be 50%. So the risk of moisture buildup is low if you install only R30 under your R31 foam. Whether the city accepts that stackup is a question for the local code enforcement.

    To be sure the foam installers are correct that the additional $5K is not "worth it" in terms of lifecycle payback in financial terms- indeed anything beyond the first 3" is probably in the "pays never" category on a lifecycle financial basis. But that's due to the high $/R that comes with spray foam solution, and has nothing to do with code compliance.

  5. AlanB4 | | #5

    How did you get your plans approved for a building permit and built without solving this already?

    The spray foam installers are quoting the "R value myth" it conflates air leakage with R value, what they are really saying is R30 will be as good as R60 with high air leakage. The code does not make such a distinction, its a ploy to sell their product because its the most expensive per R so they say its as good as higher Rs so you can install less and you will give them the job instead of telling them to take a hike when you see the monster quote.
    If this is a cathedral ceiling how about rigid foam above the sheathing or is the roof already finished?

    @Martin "Maybe the city official reads GBA" I wish they did :)
    The code allows fiberglass/vapour barrier for basements, getting rigid foam or spray foam passed is at their discretion.

  6. Yupster | | #6

    In your first reply you wrote "in my non-existent attic". If your attic truly is non-existent, then SB-12 (the energy efficiency part of the Ontario building code) permits r31 in a "ceiling without attic space".

  7. canada01 | | #7

    We tried to get away with the r31 and use that building code but it was rejected because there is one area that would permit adding say two bundles of r 30 Roxul but it would fill the whole space and would not allow any airflow inside the attic. There is no way that someone will be able to put their body into this space once my drywall is up. Maybe just your head poking through the attic opening in a closet. Once it’s sprayed and closed that’s it. Wish I could show you the lack of space . The roof and shingles are on already. When the plans were approved the code was only r50 and it switched as I was building it and now I have to do r60. I did not know it was a issues with insulation or loose fill till I called Insulation people and they all told me to call spray foam because batts would not work in the roof and everyone said I would get mold. City of Toronto is saying I need to get r60 so I guess that’s what I’ll need to do. With the trusses maybe no one realized that there was not enough space . I don’t know . I’m going to bring the building inspector and insulation guy together and maybe they can figure out if my roof will count as a vaulted ceiling.

  8. AlanB4 | | #8

    They insist you meet a new code that took effect after your building permit was issued and the building was erected?

  9. jberks | | #9

    @ Canada,

    I'm also in Toronto. To my current understanding, the inspector has to inspect and make you comply to whatever is on the permit drawing, I don't think they can bring in new codes after the fact unless they're not specifically stated. From my experience, usually the plans examiner will make sure R values are stated on the drawing before issuing the permit, or they'll put their own remarks of R Value directly on the drawings. I'd be gunning for whatever assembly that says (which I assume is your easier option)

    Which new code came into effect this year? What is the inspector referencing for the R-60? (I'd like to to know for my own roofs as well) As far as I know, the City still references OBC 2012, where SB-12 says R 50 at the ceiling of a vented roof , and R-31 at the roof for an unvented application.

    I don't know exactly whats going on, but it sounds like your spray foam guys are saying to opt for a conditioned attic space so you can get away with R31 of insulation at the roof rafters which would meet SB-12. Otherwise, if you lay out 15" thick worth of Roxul or Knauf (R60), at your ceiling, and leave your rafters uninsulated, creating an unconditioned attic, make sure you create channels or ducts or something along your rafters down to your soffits so you can let your (hopefully vented) soffits breath. which is what the concern about mold is.

    Martin, do I have all this correct?

  10. Expert Member
    MALCOLM TAYLOR | | #10


    Yes that's the way I understand it too. That's why there is a flurry of activity just before the code changes as people pull their permits under the old regime. Where there is a grey area is when you take longer than the prescribed time limit to complete construction. They then might have the discretionary power to make you follow the new amendments.

    I worked on a house that had neglected to get final occupancy for 15 years. They made us replace the bedroom windows to meet current egress requirements.

  11. Yupster | | #11

    SB-12 has 6 different prescriptive packages for energy efficiency, two of which permit R50 for a ceiling with attic space, the other 4 are R60. Your code officials are correct though, if you have attic space it should be R50-60. An alternative would be to have someone run a energy simulation using HOT 2000 and prove that your building won't use any more energy than a building built to the prescriptive standard through compensation somewhere else, whether it be higher R-value windows or more basement insulation, higher efficiency furnace, etc. Your energy model could tell you what is required. If you are savvy enough, you could probably run the simulation yourself, HOT 2000 is a free software. Because your permit was approved under a prescriptive package, you would have to get the inspector to agree to the change to a performance based solution.

  12. jberks | | #12

    @ Yupster

    That's some very good information you just wrote. I'll keep that in my back pocket.

    Thank you

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