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Tlynn,
We should probably start with the issue of codes. There is a common misapprehension that in areas of the province where there are no inspections that as you say there are "no building codes". That is untrue. The BCBC is enacted and applies province wide. The house you are building is required to meet the BCBC, and all other applicable codes and standards.
It looks like you have a few options, but before suggesting one can you clarify what is going on with the venting?
- You have vents at the peak and the eaves, but the path appears to be interrupted by the beam mid-span?
- Why is there a space in the roof sheathing near the beam?
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Tlynn,
I guess he is relying on that 5/8" gap where the sheathing is missing to allow air to move from the lower half of the roof into the upper, and exhaust at the peak.
Let's leave aside code again as that ventilation channel is required to be 2 1/2" deep the whole way. Practically it might work alright as long as there are vents at the eaves. Are there perhaps vents where the metal roof meets the metal siding at the low end?
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Tlynn,
That's too bad. The ventilation air in cathedral ceilings moves from low to high. No lower vents, no air movement. I think what he is relying on is the high vents removing any moisture that may accumulate there. With a really good ceiling air barrier would that work? It might and it might not - which probably isn't something you want in your new build.
I hate to say it because I'm not a fan of spray foam either, but in the absence of an effective ventilation channel which would allow you to use permeable insulation (like batts), it is looking like your only option.
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Tlynn,
Both would work.
Probably the easiest would be to retrofit venting at the lower end without extending the overhang. It looks to me like that might be as simple as cutting holes in the top of the wall panels under the drip-edge and adding screened vents. Something like I have attached.
If you leave a 2 1/2" deep airspace, you will only have 3" for batts though. So if you go that route I would suggest furring down the 2"x8"s to get a more reasonable amount, or adding a layer of rigid foam to the underside of the rafters.
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Unfortunately you have built yourself into a corner with the roof.
The vent retrofit Malcom is suggesting can work. I would check first how high the wall metal panels go, you might be able to drill some holes above them for vent intake. These can be covered with perf or ridge vent material. Less than code vent gap does work if your are diligent with air sealing. You would only have to strap out your rafters with 2x3 on edge to fit and R32 mineral wool batt in there and have a reasonable vent gap.
As for spray foam, the suggested flash and batt will also work and won't need any venting. Not a fan of spray foam but I think it is the easier approach in this case.
There is no "better" spray foam. They all use pretty much the same chemistry and all would have the same issues if installed incorrectly. Unfortunately there is no silver bullet to a good spray foam install, about the only thing you can do is a heir and experienced crew. From your end, you can ask them to show you the date code on the material they are using, make sure it is the right stuff for the time of year (winter foam formulation in the winter time) and ask to see the test samples before they start and along the way. If it is the right material, the mix is right and it is not installed in too thick of a layer, it generally works.
P.S. Not seeing closure strips on the roof panels there. You want something to keep critters/bugs out.
Tlynn,
The vents are available on Amazon. Search "Circular vent cover". You only need one per rafter bay, so not 28.
If you go with spray foam and rockwool (flash & batt) it's important the foam is thick enough to stop condensation. For climate zone #5 that would be R-20 of foam. See assembly #5 in this link: https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
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Tlynn,
Ideally that gap would be the same height as the rest of the vent space - around 2" - but that 5/8" should still allow adequate air movement. As Akos said, these assemblies only work with really good air-sealing.
Code required vent area is 1/150 of the roof area, half of that is intake vent and half is exhaust vent area. Looking at the picture, looks like 24' of roof length, so each rafter bay area is
16"x24'=4600 in^2, so you intake vent area is 4600/150/2=15 square inches.
That 5/8" gap in a 16"oc rafter bay is = 5/8"*14.5"=9 square inchs. If you look at the two numbers, that is not that much less than the required net intake area, so it works but not great.
This is why the air tight ceiling is important. That beam actually makes this hard as it cuts through your ceiling air barrier (typically taped poly in Canada).
What I have done in similar situations is put 1"x2" metal framing angle (standard part at the box store) along the beam and under the rafters with a bead of caulk under the 1" leg to seal it to the beam. The 2" leg over the rafters now gives you a nice flat surface to seal the poly against for air barrier continuity.
The plys of the beam also need to be sealed to each other, might not look like it but 30' times a small gap makes for a big hole. So make sure to run a bead of caulk along the bottom or tape over it with quality tape such as ZIP or 3m8067. Blue tuck tape is better than nothing in a pinch.
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If you went that route, it would be Assembly #3 in the "Five Cathedral Ceilings that Work" article linked above. There is a specified ratio of rigid foam to permeable insulation according to your climate zone.