All Precision Therapy infrared saunas feature high quality lumber and double wall construction. Using seperate tongue and groove panels on the inside and outside provide an insulation gap between the 2 walls, ensuring proper heat retention in your sauna. This same technology is used in building houses.The wood used in these saunas is not the normal 4" wide planks that are found in most saunas. Our saunas use wood that is approx. 2" in width. This helps create a tighter more rigid sauna room and retains heat much better. Using these narrow planks also eliminates the cracking you will find in other saunas. When the saunas heat up, the wood can bow, warp and crack on the 4" planks. The narrow boards that we use will not have any of these issues.