Vented Roof Assembly with Spray Foam
Every time I think I’ve come to some conclusion, I read a forum, and I fall back into a rabbit hole.
I have a 1911 “transitional” – mostly an American Foursquare Prairie – in Zone 6… but Detroit, so wet and can get 5-ish sometimes. I have a full attic space which I want to make into living space. It has 3 gable windows. Based on as much as I could process, I had decided on a roof assembly which maximized both internal and external insulation. I am having the roof completely redone, stripped to the old 1x decking. I had planned to do rigid poly, sheathing, shingle on the outside and closed foam with baffle on the inside.
However, MI is ironically not too forward thinking with its trades, and so I finally found a roofer who would do it, but the added costs of both labor and materials now puts it out of reach. Plus, it will apparently void the shingle warranty.
The shingle issue, I already knew about. Apparently, using closed foam directly on the decking can cause issues which make the shingles peel and buckle, because the temp delta is so high both in both winter and summer here. But the 2 layer rigid over the decking, under the sheathing seemed to eliminate that concern.
Now that’s scrapped. So, somewhere I read (though now can’t find the reference) that another solution is to fur the sheathing, creating ventilation channels on the roof, instead of inside. This is particularly pertinent to cathedral ceillings, which is what I will have in the end with the conversion. But then we have the spray foam inside, shingle curling on the outside issue.
Based on info I think I have collected, my idea is now this. Put a full layer of rubber water and ice over the original decking, then fur, then sheathing and the rest. Spray closed on the underside, then baffle, then rigid under my drywall to break the rafter thermal bridge. Cost is a factor, but the internal stuff I can do myself. The additional rubber layer will add a cost, as will labor for the furring. But it seems that sort of addresses all caveats while getting max R value in my budget.
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If you are putting shingles and water and ice on the exterior and closed cell foam on the interior, I suspect you won't have much drying potential in either direction. That might put your deck sheathing at risk.
Have you looked at creating a ventilated roof? It would require open joists from soffit to peak and probably furring down the roof, but it would be safer and more affordable than your current plan. For more info, see "Are My Rafters Deep Enough" in this article: https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling.
Well, to your point, that is why I am proposing to fur out the roof, which would create a 1" air gap between the rubberized decking and sheathing with shingles.
Take a look at what roofer Mike Guertin had to say about a similar project with asphalt shingles and roofing underlayment:
"One approach to fixing these problems is to insulate the underside of the roof with spray foam, which makes the attic semi-conditioned space and brings it into the building envelope. But there are several challenges to this approach, including the high cost of installing spray foam. I decided to use a more cost-effective method and installed a balanced attic-vent system, which uses intake vents (typically installed in the soffits) and exhaust vents (typically installed at the ridge). The system creates steady airflow that helps to keep the attic cooler; carries away excess moisture vapor, reducing the chance for condensation and mold growth; and reduces the likelihood of ice damming."
Here's the full article: Venting a Tricky Old Roof
And this article is a critical resource when considering a vented cathedral ceiling assembly: How to Build an Insulated Cathedral Ceiling.
Unfortunately, I can't get past the pay wall for the cathedral ceiling article, but another article on them is where I got the furring idea. If finis tspace, there will be no attic, although I can square off the peak for a channel, if needed.
Most older houses with complicated roofs around me (Toronto) are insulated with closed cell SPF between the rafters. This is not a new idea, something that has been done for a long time here, there are no issues with shingles. SPF is really only way to seal up a complicated roof profile, just make sure they carry the SPF all the way down to the top plate of the walls bellow. This means pulling the floor boards up around the perimeter of the attic, but it is must if you want the place well sealed.
If you want extra performance, cross strap the rafters with horizontal 2x3 or 2x4 on edge and cover the rafter with at least 1" of SPF (2" is better). This greatly reduce the thermal bridging of the rafters and gives your assembly a significant R value boost. A side benefit is you can strap at 16" OC and shim to flat which makes drywall install much easier if you have uneven rafter spacing.
If you are worried about shingle life, best is to not use it. See if you can find somebody that does metal tile roofs. Part of my home is covered with stone coated metal tiles, was reasonable to buy and pretty easy to install.
I would LOVE a metal roof. But not only can I way not afford it, my historic district would never allow it. As it is, I am in court fighting over using square columns on a Prairie Four Square porch.
A metal roof would cost you less than the foam over-roof your are initially looking at.
Most hysterical association rules pertain to style, usually material is up to you, there are some metal tile roofs that look very close to shingles (roof after the standing seam roof, the third house is standard shingles as reference).
P.S. There is nothing wrong with ice and water over the deck and SPF under it. As long as the deck is dry when the foam is sprayed to the underside you'll have no issues. Close cell SPF prevents any moisture or interior air making its way to the sheathing so there is no need for any drying capacity.
Re cost of metal - not here. Metal roofs are rare, thus expensive: foam is also rare, but as I say, I've already decided that I can't afford that, either, because the addition of labor was going to bring it to $2/sf, instead of about $1 - in addition to the closed cell inside.
Mine is not an association, but a historic district. Both material and style matter. I'm not saying what makes sense - just what is. They are so trifling that we are fighting over whether I can stain my beautiful woodwork (my preference) or have to paint it (theirs).
But what do you think about the furring? Necessary, or no? I think you are saying that since the closed cell will keep moisture from the inside in, the question of shingle-curl is moot. Is that right?
The furring and top venting is only needed in very heavy snow country. Around me, SPF under the deck with no venting above works well enough to prevent ice dams, but we don't generally get very large snow falls.
Furring bellow by cross strapping bellow and covering the rafters with SPF does make a big difference on the assembly R value, so if you have shallow rafters such as 2x4 it is definately worth the extra work. Plus it gives you the chance to shim the ceiling flat.
Shingles typically curl near end of life. There is nothing much you can do to prevent this. Venting bellow the shingles reduces the shingle temperature slightly but not enough to matter much. Getting lighter shingles or ones with better SRI will make a much bigger difference for temperature.
Moisture problems happen with unvented roofs when interior air leaks and moisture make it through the insulation which can rot the roof deck. Closed cell foam completely blocks this so it is a non issue.