In the construction market presents a huge range of artificial insulated materials that retain heat well and have a breathable surface which in turn prevents destruction of the walls. Warm bath any of them, it all depends on your material possibilities.
Fit an outer wall of the bath with insulating material and produce a finish in its sole discretion. For extra warmth use the trim brick or wood. If financial possibilities do not allow to produce additional trim for insulation of walls baths, you can just on top of the insulation to cover the walls with roofing material. It protects the wall, and insulating material from getting wet, which further prolongs the service life of the walls and securely retain the insulation material.
But only produce outer wall insulation is insufficient to protect the walls of the bathhouse from destruction. Additionally you need to produce protection of the walls from the effects of steam and moisture inside the bath. To do this, use vapor barrier material. They upholstered all the interior walls of the bath and the ceiling and then produce the wood trim or, at its discretion.
To the bath was warm, fit an outside ceiling vapor barrier material on it lay a thick layer of insulating material and vapor barrier again.
Be sure to insulate the Foundation of the bath first vapor barrier material and the insulation on top of it.
With this method of insulation and insulation of walls and the Foundation, the walls and ceiling of the bath will not be exposed to wet and condensation due to the change of temperature. Bath will be warm, and to repair it is not necessary.