Sauna Insulation Calculator: Packs, Foil and Tape to Buy

Ordering insulation for a sauna build goes wrong in two directions: people buy by the floor area and run short on the walls, or they forget that the timber studs take up part of every wall and buy packs they never open. The foil vapour barrier is the layer that matters most, and it needs a 75mm overlap at every seam and foil tape along every joint, so a plain area sum always under-orders it. Put your cabin's internal measurements in below and this calculator works out the mineral wool to fill between the studs, the number of packs, the rolls of foil laid in overlapping courses, and the metres of foil tape to seal it.

The pack and roll figures default to example sizes only. Insulation packs in particular vary a lot with thickness, because a 100mm pack holds far fewer square metres than a 50mm one, so copy the coverage printed on the pack you are actually buying before you trust the count.

How the calculator works it out

Mineral wool. It starts with the gross area of the four walls, takes off the door opening and any glass, and adds the ceiling and, if you tick it, the floor. Batts sit between the studs, not over them, so it then removes the share of each surface the timber covers: a 45mm stud at 600mm centres takes 7.5 per cent of the wall, and at 400mm centres it takes just over 11 per cent. A 10 per cent allowance goes back on for offcuts around noggins, the heater wall and the corners, and the total is divided by your pack coverage and rounded up.

Foil vapour barrier. The walls are covered in horizontal courses that run continuously around the room, each course overlapping the one below by 75mm, which is the overlap our guide to building a sauna recommends. If the ceiling is foiled, the top course is carried 75mm onto it so the wall and ceiling sheets lap rather than butt. The ceiling is covered in strips running the length of the room, with the same overlap between them. Where a course is longer than one roll, a 75mm overlap is added at each joint. The foil is not reduced for the door, because you cut the opening out of a full sheet, and 5 per cent is added for trimming before the metres are turned into rolls.

Foil tape. Every place two sheets meet gets taped: each horizontal seam all the way round the room, the vertical joint where each course starts and finishes, any joint where one roll runs out and the next begins, the line where the walls meet the ceiling, every ceiling seam, and the edges of the door opening and any glass. Ten per cent is added for patching staple holes, cable entries and the vents.

Getting the layers in the right order

From the outside in, a sauna wall runs: the outer structure, the stud frame filled with non-combustible mineral wool, the foil with its shiny face towards the room, then thin battens fixed over the foil, then the timber cladding. The battens matter because they leave a small air gap between the foil and the back of the boards, which lets the cladding breathe and dry out between sessions. Do not use foam boards or plastic sheet in place of the mineral wool and foil inside the hot room unless the manufacturer rates them for sauna temperatures.

The floor is the exception. In an indoor sauna it is usually left as a tiled or sealed concrete floor with no foil at all, because water needs to drain off it. In an outdoor cabin raised on bearers, insulating under the floor keeps your feet warmer, which is what the floor option is for; leave the foil off it either way.

Once the shell is planned, the sauna cladding calculator works out the boards to go over the battens, and the heater size calculator tells you how many kilowatts the finished room needs, including the extra allowance for any glass you entered here.