First, build a small drain on the floor. Next, build a standard sauna doorway that is two feet by six feet. Build upper and lower benches eighteen or twenty-four inches deep. Insulate the walls with R13 class insulation and the ceilings with R22-R26, then bind with sauna foil moisture barrier.
Choose not to be too small, the average size for a two- or three-person sauna is 4' x 6' to 5' x 7'. Depending on the number of users and settings, dimensions can go up to 12' x 12', while a single 3' x 3' sauna can still be used. Public saunas are usually at least 100 square feet in size.
The sauna wood can be selected from Canadian red cedar. The cedar is colorful and varied in shades and is stable, yet soft, and will not deform under the heat and humidity changes that are prevalent in saunas. Cedar also has a low density, making it a good insulator, heating up and cooling quickly.
Many sauna heaters require 220 volts and must be hardwired to the electrical box. For a sauna with a 4.5kw or 6.0kw heater, a 30 amp breaker and 10/2 wire is required, unless it is more than 30 feet from the breaker, in which case 8/2 wire is required.