Venting Sauna Roof
I’m building a 10 x 12 outdoor sauna with a wood stove as the heating source. The interior will have a foil vapor barrier. The walls and ceiling will have mineral wool insulation, then plywood sheathing. It will have a low slope 1:12 roof with 2×6 rafters as the finished ceiling. It will be used a maximum of 1 hour per day, and I’m wondering if I need to vent the roof.
If so, I could put in rockwool R23 and use 2×8 rafters to have a 2 inch space above the insulation for venting. Or 2×6 with rockwool r15. Or I could not worry about it and do rockwool R23 with 2×6 rafters and no vent and not worry about it. How much of a concern is roof wetting/rot/mold in this type of building?
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Interesting question. This is mostly a bump, though I am interested in someday making a sauna of my own. Assuming you are planning a mostly-dry sauna, the very high heat generated (140-200 degrees, if I recall correctly) indoors and the low temperatures outdoors will push most moisture outward through any assemblies--including around any imperfections in an attempted vapor barrier.
What type of roofing are you planning to use? When moisture reaches the underside of impervious roofing it won't be able to get through and will accumulate, leading to rot. I would prioritize a vented roof, even if it means having to use less insulation than ideal. A roof with a 1:12 slope is technically too low for a vented roof to work well, but I imagine your sauna will be a small building which is more forgiving of a small vent space. If you can get 2" of vent space, that would be a lot better than only 1".
Thanks for the reply. I was planning on doing asphalt shingles but I understood a low slope roof necessitates ice and water shield over plywood which would be impermeable. Would it be better to use roofing felt?
With a 1:12 slope your options are pretty limited, if you care about meeting manufacturers' requirements. I would use a monolithic membrane such as EPDM. Alternatively, some types of metal roofing can be applied on such low slopes, such as flat-seam soldered copper. I recently used asphalt roll roofing on a family member's shed with about a 1.5:12 slope but it's not warrantied for that slope. You can try shingles over ice and water shield but it's not a warrantied assembly. I definitely wouldn't rely on roofing felt; that is mainly a slip sheet, it's not waterproof.
I'm not really concerned with manufacturer requirements at all in this scenario, just what is best. It seems like a roofing membrane is impermeable so could cause sheathing rot, but a permeable membrane won't be waterproof and so won't work with such a low slow proof. So this seems like a between a rock and a hard place scenario. Am I interpreting that correctly?
Manufacture requirements typically ARE what is best; they are what have the the least chance of failure. When you take actions that void a warranty, in most cases you're saying that you understand you are not building what is best. My stock answer is to follow building codes and manufacturer instructions if you want a reliably safe assembly. But I understand that this is a somewhat unique situation so I'm suggesting some alternatives based on the conditions you have stated.
Almost all roofing is impermeable to moisture, aside from options like wood shingles/shakes or slate tiles with gaps. So a roofing membrane like EPDM is no better or worse in that regard than asphalt shingles or metal. None of them will directly cause the roof to rot, but all will prevent drying that will eventually lead to rot.
You have three options:
1. Prevent any moisture from getting into the roof assembly, which will be next to impossible with the high temperatures you will be using.
2. Allow some moisture to get into the assembly and to dry through the roof surface. 3. Allow some moisture to get into the assembly and allow it to dry via venting.
There is a fourth option for some projects, involving continuous exterior insulation, but I'm sure it's more complicated than you would want to deal with for a sauna.
How far along are you with design or construction of your sauna?
I've just framed the walls and I'm going to put up the roof and sheathing this weekend. It does sound like trying to keep a 2-in channel above the rockwool open and vent it might be the best I can do in this situation so I can get some 2x8s instead of the 2x6s I have. In that situation I could also use R15 to have a 4 inch gap instead. A wood stove in what is essentially a 7x7 hot room feels like it should be able to heat up nicely with even minimal insulation up top.
Reply to #6: that sounds like a good plan. I agree that in such a small space I don't think you need much insulation. I would put some attention into air sealing, which will have a similar effect on keeping the space warm.
Reddit forum on Sauna has enormous amounts of detailed info and resources on sauna builds, ventilation dos and don'ts and many many people who built theirs, popular topics are ventilation, specifics of insulation and bench position and where to place the fan etc. Much of the info is gathered from extremely detail obsessed sauna people and I have found it enormously helpful place to start.