Alder Hill Mobile Sauna Cooperative (Leeds): Our mobile barrel sauna has been built to be highly energy efficient while maintaining a comfortable environment with pleasant air circulation inside. The multiple layers of insulation limit heat losses through conduction and infra-red radiation, this allows the sauna to get up to temperature quickly and stay there with very little energy. The sauna measures 1.8m wide by 3.4m long, enough room to comfortably host 10 people. The generated heat is insulated by 7cm thick walls consisting of three different insulation layers; Aspen softwood cladding provides an attractive fragrant environment on the inside, on the outside is a layer of polycarbonate twin-wall (the same stuff used for greenhouses), sandwiched in-between is a 4cm air gap and a layer of aluminium foil coated bubble wrap. This combination results in an overall thermal resistance of R=3.1 K.m2/W. So it takes about 4kW to maintain an internal temperature of 80oC during winter