DIY Backyard Sauna Build – Part 4: Lighting, Vapor Barrier & Interior Cedar

Part 4 of my DIY backyard sauna build. If you’re just joining: Part 1 (site prep & framing), Part 2 (roof & weatherproofing), Part 3 (siding, window, ventilation).

This is the episode where the inside of the sauna stops looking like a construction site and starts looking like a sauna. The sequence — lighting wiring, vapor barrier, furring strips, cedar. This has to happen in this order, because once the cedar goes up, anything you forgot is staying forgotten.

It’s also one of the shorter episodes in the series, but the steps in it matter a lot for whether your sauna lasts 30 years or starts rotting from the inside out in 5.

Running the Lighting Electrical

Sauna lighting has to handle high temperatures and high humidity, which rules out a lot of standard fixtures. I went with a setup designed for sauna use, ran the wiring through the wall cavities before any insulation or vapor barrier went on, and left a little slack at each fixture location so I had room to work later.

A few things I’d flag for anyone doing this themselves:

• Run the wire before the vapor barrier. Once the vapor barrier is up, you don’t want to be poking holes in it.
• Use temperature-rated wire. Standard wire jacketing can degrade in sauna conditions. Check the rating on whatever you’re running.
• Plan your switch location outside the hot room. You don’t want a standard switch on the inside of a sauna.

Ran a quick test once it was wired in to make sure everything worked before sealing it all up behind the wall.

Vapor Barrier

This is the part most non-sauna DIY guides skip or get wrong, and it’s arguably the most important step in the whole interior build.

A sauna gets hot and humid in cycles: heat up, cool down, repeat. That moisture wants to migrate out through the walls. If it gets into the insulation and stays there, you get mold, rot, and a sauna that smells wrong forever.

The fix is a foil-faced vapor barrier on the warm side of the insulation (the inside of the sauna). The foil reflects radiant heat back into the room and blocks moisture from getting into the wall cavity. Tape every seam with foil tape, not regular duct tape, which will fail.

Two rules I followed:

1. Continuous coverage. Walls and ceiling, every cavity, every seam taped.
2. Seal around penetrations. Lighting wires, ventilation openings, anything that punctures the barrier needs to be sealed back up.

This step is boring and slow. Do it right anyway.

Installing the Furring Strips

Furring strips go up over the vapor barrier, running perpendicular to the direction the cedar will run. Do not skip furring strips. It does very important things:

1. Creates an air gap between the vapor barrier and the cedar paneling. This gap lets any incidental moisture dry out instead of sitting against the wood and also allows the cedar to dry on both sides.
2. Gives you something to nail the cedar to without puncturing the vapor barrier in random places.
3. My colleague Tyler who also built a sauna pointed out a third benefit I hadn’t fully understood. Without an air gap, the cedar is in direct contact with the framing, so the heat in the wood conducts straight through into the studs and gets pulled into the insulation. With the air gap, the cedar can’t dump its heat into the wall instead, it re-emits the heat as infrared radiation, which the foil-faced vapor barrier reflects back into the sauna. As a result: your cedar walls stay hot, but your insulation stays cool.

I used 1×2 furring strips over each stud, fastening them with 3 inch construction screws since they would hold the whole weight of the cedar.

Installing the Cedar

This is the satisfying part. Tongue and groove cedar going up, board by board.

A few things that helped:

• Start with a level reference line. If your first board isn’t level, every board after it is fighting that mistake.
• Hidden-nail through the tongue. No exposed fasteners on the face of the wood.
• Acclimate the cedar. Let the wood sit in the build space for a few days before installing so it adjusts to ambient humidity. Cedar that’s installed too dry or too damp can move on you.
• Stagger your seams. Same as flooring, staggered seams look way better than aligned seams.

The cedar smell hit immediately. That alone made all the tedious work of the vapor barrier feel worth it.

What’s Next

Part 5 is the finale: exterior ventilation finished out, custom door built and installed, benches designed and built, electrical finished, heater installed, and the first sauna. It’s the longest episode in the series and easily the most satisfying to film.