How to build a sauna in your basement

First figure out where you can put a sauna heater and floor drain.

The sauna floor drain will be the most limiting factor if you decide to install one. Do you need a floor drain in a basement sauna? It depends on how steamy you like your sauna, how frequently you will use the sauna, how dry your basement is in the wettest season. The sauna floor drain will have to connect to your existing plumbing. If you have a bathroom in the basement, it may be relatively easy to install a floor drain in a sauna built next to the bathroom. You may need a plumber to help you figure out where the in-floor PVC pipes are buried and connect a floor drain without causing issues with backflow and code compliance.

Next thing to think about is the electric requirements for the basement sauna. Indoor basement saunas almost always use electric sauna heaters so you'll need to place the stove where you can run power to it. Luckily, if you have a full basement in your house it's likely that your electrical panel is located there. It will probably be relatively easy to run cable to your desired location but come up with a plan or discuss with an electrician. Your sauna heater will most likely require 220v service and you won't be able to use existing outlets. Also make sure you have room for a double pole breaker in your electric box. Also think about lights and other power requirements during the planning stage, although you may be able to use existing circuits for those.

Remember that basements have special building codes that often require GFIs or metal conduits below grade.

Now that you've determined the electrical and plumbing restrictions, you can consider the sauna size. A basement sauna can be anywhere from 4x4 to 12x16 feet. The ceiling for a basement sauna should be 7 feet or less. If it's under 7 feet you probably won't have the height for a top bench, which is OK in a small sauna. Remember the smaller the sauna the faster it will heater up, the less energy it will use, and the cheaper it will be to build.

Now the fun part: planning the layout of your sauna.

We recommend starting your basement sauna design with a bench across the longest wall. We take a deep dive into sauna bench placement here, but the important design principles are to keep the top bench as high as possible without having anyone hit their head. If you want the best heat and steam without wasting energy, you want your head almost touching the ceiling.

Depending on your sauna dimensions, determine if you can have a lower bench, or if you will need a step to get to the bench. The lower bench is often 16-18" below the top bench. The benches can be 16-24" deep, and it's nice to have the bench wide enough to lay down.

Now plan where the sauna door and heater will go. Start looking at sauna heaters and find out how far they need to be from flammable surfaces, like your benches. Think about the door swing, for a small sauna it's best to have the door swing out into the basement if possible.

Draw it, model it if you can, and lay out the design with painters tape in the basement. Put some chairs or a makeshift bench in there and sit with it for awhile and make adjustments.

Before framing out your walls or anything else, you need to install a floor drain (if you choose to). This involves some serious demolition and tearing up some concrete in your basement. You can also damage your plumbing lines so use caution, know where your PVC lines are, or hire this part out.

It's important that the floor is slopped to the drain. Use water or a ball to see where the natural slope of the basement floor is, which will tell you which corner to install the drain.

If you have a choice, put the drain next to the sauna heater (where you'll be splashing water).

Follow standard wall framing techniques with 2x4s and a header over the door to frame out your sauna walls and a "false ceiling," ideally about one foot below the basement ceiling. this will give you room to run electrical cables and insulate your sauna. You can find 2x4s, screws, etc any building supply store. Use treated 2x4s for any lumber that touches the floor.

Try to maintain 16" on center framing for both the walls and ceilings, it will make the next step much easier.

For interior walls you should be able to build the wall on the floor and lift it into place. Check out some wall framing videos on YouTube if you've never done it before. It's pretty easy and our walls are not going to be structural to the house. It's more like building a fort within your house.

If you have a shared wall with the building foundation, we recommend putting 2" rigid foam against the foundation and build your wall (with mineral wood insulation in the cavities) outside of the rigid foam.

Use construction adhesive and concrete screws or anchors to attach the bottom plate of your sauna walls to the ground. For indoor saunas you can usually use 2x4s for the ceiling, every 16" for the shortest possible span. Example: a 12x8 sauna would have 8' rafters. The ceiling is build just like the walls with top and bottom plates.

If you are planning on having a floating bench, now is the time to notch out a space in your studs to have an inset 2x4 that goes horizontally through the wall that will be the back 2x4 of your sauna bench. That will give your bench a nice built in look and a clean design. If you're not doing floating sauna benches you'll want to add blocking horizontally where the benches will be attached.

Run your electrical cables to lighting, outlets, mechanical ventilation, and sauna heater locations. You should have your sauna heater narrowed down by now, and your electrician will be able to determine a cable size that will work for them all. It's a good idea to "size up" the cable in case you ever want to install a larger heater.

Select "wet rated" ceiling lights and run power to those locations. Small, black casing recessed lights look really cool. These are our favorites because they're dimmable and you can adjust the color temperature.

Electrical note: make sure any outlets in or near the sauna have vapor proof boxes and are GFCI protected. The sauna heater probably won't need GFCI protection but check specs.

Insulate the wall and ceiling cavities with mineral wool insulation, also known as rockwool. Never use fiberglass insulation for a sauna (or a basement) because it's not good with moisture and degrades over time. We also don't like spray foam near a basement sauna for the potential to offgas. It also is more prone to fire than mineral wool.

Cut the mineral wool bats to fit each cavity without crunching them too much. Gaps should be filled with insulation. If you have enough of a drop ceiling to access it from above, two layers of alternating insulation bats will be the most effective.

After insulating each cavity and feeding wires through, it's time to apply a foil faced vapor barrier, unless the sauna shares a wall that already has one.

A foil faced vapor barrier is important for two reasons. First, it serves as a vapor barrier that will keep your walls from getting moldy. Second, the foil face will stop radiant heat from escaping your sauna. The wall insulation is great for stopping most heat transfer but without the radiant barrier it's still easy for heat to escape.

Trying to save money by using a regular plastic vapor barrier will cost you more in the long run. Radiant barriers can save 30-50% on energy bills for basement saunas!

Starting at the bottom of the interior walls, staple the radiant barrier tightly to the walls. This can be easier with two people. Tape the seams of the foil, which should create a continuous air barrier, overlapping from the top down so moisture always runs down to the next sheet until it hits the ground.

Using either cedar or treated wood, use a tablesaw to cut 2x2" boards that slope inward where the foil meets the floor. Screw the drip edge to the bottom plate of your framed walls with construction adhesive on the bottom. This "starter strip" for your wood paneling will catch any moisture on the wall and send it back toward the drain.

Now's the time to make sure the floor is waterproof and sloped towards the drain. Depending on the condition of your basement concrete, you may be able to leave it be and install raised wood flooring over it. This is the easiest option and also keeps your feet the warmest.

However, it's more waterproof to to install tiles or a layer of cement over durock. This will give you more control of the slope to the drain and makes it easier to keep clean. It's important to use a tile drip edge where the tile meets the wall to make sure any moisture behind the cedar wall panels lands on the tiles.

If you want to REALLY have a waterproof sauna, look up instructions for tiling a shower floor and integrate that into your wall system.

Now the fun part, cut and attach your tongue and groove cedar, or other appropriate hot room cladding. If you are doing a floating bench, you'll want to install that into the framing and then rip down your cedar to go right up to the bench.

If you're using a floor-supported sauna bench go ahead and install all your wall cedar now and then build your benches.

We recommend mechanical ventilation that takes hot humid air outside when the sauna is over. Even without a sauna, basements often have mold and humidity problems in the spring and summer. You may be temped to keep the free heat in the house, and on occasion that's OK. But you are going to want the mechanical ventilation to keep the walls from getting moldy in the long run.

If you decide not to use a powered exhaust vent to the outdoors, make sure you have a large gap under the sauna door, and preferably a vent or two into adjacent rooms.

Install your heater and lighting as specified in the manual and fire it up! Get a few sauna sessions in and decide if you want to make any changes before doing some more of the finish details. Congratulations, you built a sauna in your basement!