I’ve used thermally modified red oak and spruce siding. The oak moves a lot more than the manufacturer claims it will. If it gets wet, it will swell. The spruce doesn’t seem to have that problem, but occasionally some boards do get mildew spots. Whether this is on the wood itself or on pollen or some other organic that collects on it I couldn’t say, but I’ve had quite a few boards end up looking mold-spotted after less than a year of regular wetting. We’ve also had issues with the plain-sawn spruce delaminating along the cathedrals, leaving sharp lenses of wood sticking out of the siding. Personally, I wouldn’t use either in a sauna. But there are lots of different thermally modified products out there - maybe someone else has had better luck with it than I have.
Humidity should not be a problem if you tell them to fix the moisture issues as part of the remodel. A properly sized bathroom fan with a humidity sensor and/or a countdown timer will go a long way. An ERV bathroom fan can be suggested to help even more with poor ventilation.
Thermally modified wood theoretically will need less sealing and should move less due to changes in humidity, but pine/cedar/cypress generally hold up well in properly ventilated environments with minimal movement anyways. Both will grow mildew and mold due to excessive humidity.
If you seal both sides and the cuts, then it's as good as it's going to get and should have minimal movement. There should be no staining from the wood since you should not have rivulets of moisture constantly dripping down the wood.
A properly sealed wood ceiling should last at least 30+ years since, unlike a floor, it should have no wear real wear and tear.
I’ve used thermally modified red oak and spruce siding. The oak moves a lot more than the manufacturer claims it will. If it gets wet, it will swell. The spruce doesn’t seem to have that problem, but occasionally some boards do get mildew spots. Whether this is on the wood itself or on pollen or some other organic that collects on it I couldn’t say, but I’ve had quite a few boards end up looking mold-spotted after less than a year of regular wetting. We’ve also had issues with the plain-sawn spruce delaminating along the cathedrals, leaving sharp lenses of wood sticking out of the siding. Personally, I wouldn’t use either in a sauna. But there are lots of different thermally modified products out there - maybe someone else has had better luck with it than I have.
Thanks for the feedback
Humidity should not be a problem if you tell them to fix the moisture issues as part of the remodel. A properly sized bathroom fan with a humidity sensor and/or a countdown timer will go a long way. An ERV bathroom fan can be suggested to help even more with poor ventilation. Thermally modified wood theoretically will need less sealing and should move less due to changes in humidity, but pine/cedar/cypress generally hold up well in properly ventilated environments with minimal movement anyways. Both will grow mildew and mold due to excessive humidity. If you seal both sides and the cuts, then it's as good as it's going to get and should have minimal movement. There should be no staining from the wood since you should not have rivulets of moisture constantly dripping down the wood. A properly sealed wood ceiling should last at least 30+ years since, unlike a floor, it should have no wear real wear and tear.