How to insulate a wood garden shed to house a small 7’x7’ heated above ground therapy pool
Rethink the number of windows. Any window is going to be a condensation nightmare fall, winter and spring with an 85-90 °F pool in the building. If it's a window that came with a &quot;garden shed&quot; it will probably be atrocious in thermal performance, as garden sheds are not typically heated and good windows are expensive, but even a good &quot;high performance&quot; window is a spot of terrible insulation in a well insulated wall, and that means that it will be cold, and with an open warm pool in the building it will drip most of the year.
Given small pool and high temperature differential, I'd rob some interior space to make a very thick wall (another set of studs, offset from the studs of the wall) with lots of insulation, and a roof structure that allows having a lot of insulation and a vented, cold roof on top of that.
This basically means building a room inside the cabin so you have adequate space for a lot of insulation material, since working with the (presumably 3-1/2&quot; thick) spaces provided between studs will be woefully inadequate. Using a well-detailed (all holes get fixed, try not to make too many) vapor barrier on the inside keeps the insulation dry. Cellulose tends to tbe the best cost ratio when you provide the space, rockwool is another good option.
The other approach to this would be to use sheet foam products, filling the wall cavities with insulation, and installing vents in the roof cavities before sheeting them over (leaving them empty for ventilation) with rigid foam on the interior, sealing the joints for vapor-tightness and then finishing with a fire-protective layer such as drywall (foil faced foam MAY qualify in some areas. Exposed foam is a deathtrap in a fire due to the fumes it releases.)