Ontario Farm House Roof Retrofit
Hi all!
I’m looking for some help settling on a retrofit roof assembly for my new-to-me farmhouse in Ontario, Canada (Climate Zone 6). The tricky part is that there are so many different existing roof assemblies, and I’d like to ensure that my proposed solution won’t cause any problems. The main part of the addition that I have attached the section for was built in the ’70s, and then was modified in the late ’90s. As you can see in the attached, there is an 18′ x 18′ section of flat roof, with a heat sealed vinyl membrane on it currently, in addition to piles of dormers and bump-outs and valleys to make my life difficult.
We have a few problems we are trying to address in this retrofit: significant ice damming, loads of heat loss, and leaking during rain (in addition to leaking during heavy rain, especially in roof #4 and #50).
My hope is to convert all the roof types to unvented cathedral ceilings, with 3″ of outsulation on top of the roof deck, and a double air space with perpendicular and vertical strapping to help the metal roofing to stay cold in the winter. The kicker is all walls and ceilings have poly vapour barrier currently installed behind the drywall . . . and I’d like not to replace all the drywall if I don’t have to.
My main questions are:
A) how important is it to remove the 6 mil poly in all parts of a roof, if I intend to add relatively impermeable rigid foam on top of the roof deck? Specifically, if I am able to remove the poly in roof assembly #3 (see attached section drawing) and #5, but not #4, would this be a risk worth taking? I assume it helps that the roof deck is solid wood in that assembly?
B) how important is it to get my total insulation level up to code level of R-49? Am I right that my proposed assembly should be ok for dealing with dew point issues — R 12 in foam board on the exterior side of the roof deck, and R-20 in batt insulation on the interior side? I could probably add another 1.5″ of foam board if that was going to be significantly helpful.
B) if existing poly is not a risk worth taking, should I consider swapping out the rigid foam board for rockwool comfortboard above the roof deck? My concern with this is cluster flies and lady beetles . . . we have an annual invasion, and I’d really like to not tempt them to burrow into the Rockwool . . . could I get away with putting some kind of waterproof membrane on top of the Rockwool to seal it up from bugs? Additionally, do I have to install sheathing on top of the Rockwool? I was picturing just installing the strapping on top of the insulation using long screws . . . which I can picture working with rigid foam, but I’m worried about the roof looking wavy if the Rockwool compresses as the strapping is applied.
C) my proposed plan is to seal up the soffits and attics completely. I don’t think they will be 100% air tight or anything but I’m wondering if there is a solution if I want to encapsulate the attic spaces, but don’t want them to be overtly open to the rest of the house. Again, I understand that the ideal would likely be to have some kind of air movement up there; but I guess I’m looking for an explanation of what risks I should be looking to avoid, if I can’t realistically get duct work up there.
Thank you for your help! And, happy to hear any other ideas / thoughts people might have when they see what I’m up against . . . and yes, this house is a giant energy suck, and no, I didn’t do my homework properly before I bought it . . . but I’m hoping to significantly improve its performance so it has a greener life from here on out!
Thanks again!
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Replies
As far as I know, the OBC doesn't provides any prescriptive guidelines for unvented roofs, so I'll just refer to the 2024 IRC.
Under the IRC, an unvented roof cannot have a class I vapor barrier (ex. poly) on the ceiling side. The roof sheathing absorbs moisture over winter and has to be able to dry out in the spring. Although the poly will limit vapor diffusion into the rafter spaces, old houses usually have lots of cracks and gaps that will allow moist air to rise up into the roof, and that moisture can only dry out by diffusion.
It's hard to say how risky it is to leave the poly in place. There would be more risk with a humid space such as a bathroom below. You could reduce the risk a bit if you kept the roof sheathing even warmer by exceeding the minimum ratio of exterior R-value to total R-value.
However, your proposed R-12 exterior to R-32 total only gives you are ratio of 37%, far below the 51% required in climate zone 6. You would need a minimum of R-21 exterior, and that assumes no poly.
Your alternative of Rockwool with a vent space above should work fine, provided you also use a vapor-open roofing underlayment. In that case, the ratio of exterior/total R-value isn't nearly as critical. If you use 2x4s on the flat for the vertical strapping, in won't compress as much compared to narrower strapping, and the horizontal strapping will even out any slight irregularities.
This is super helpful, thank you! Any recommendations / details on a roof worthy, waterproof VO membrane that could be applied over top of the Rockwool but under the strapping? Trying to avoid giving the cluster flies a favourite winter destination in the insulation!
If you want to put the membrane on top then you'll probably need a layer of cover board on top of the Rockwool. I think I would just screen the vent openings and also seal up any gaps between the metal panels and strapping at the ridge and eaves.