Which vapor barrier or vapor retarder should be installed in the bathroom?

Hello,

I want to completely renovate my guest bathroom and would like to know which vapor barrier or vapor retarder I should best use for heat regulation. Since I will be tearing everything up and different regulations apply in wet rooms than in the living room regarding renovation, I do not want to use the cheapest of the cheap but rather install reasonable, durable material. Unfortunately, I am completely overwhelmed because there are so many specifications regarding material selection (sd value, variable vapor permeability, diffusion resistance, etc.) that I am now seeking help. An additional question would be whether I can also use this foil for wall construction, since the guest bathroom wall is on the exterior façade and is not very well insulated (thermal bridge).

Canadian wood prefab house built in 1977
No roof insulation/attic insulation (guest bathroom is directly under the attic)
Bathroom window for shock ventilation is available
Standard heating will be dismantled - underfloor heating will be newly installed
Planned installation: battens/foil/counter battens/drywall wet room panels

Thank you & best regards

hi, with a vapor barrier sd-value 2.0 you are correct there. ( Würth, isover, ampack etc.) Question: why don’t you want to install insulation if you’re tearing everything out anyway? if there is no insulation present, you usually don’t even need a vapor barrier...