Part 3 of my DIY backyard sauna build. If you’re just joining, start with Part 1 (site prep and framing) and Part 2 (roof, sheathing, and weatherproofing)
With the structure framed, roofed, and wrapped, it was time to start turning the sauna from “box that looks like a shed” into something that actually functions as a sauna. Part 3 is where that shift starts — I got the exterior siding on, installed the custom glass window, worked through the ventilation system, and started prepping for the electrical.
Ventilation ended up being the piece I researched the most, and honestly it’s the part of a sauna build I see people mess up the most online. So I’ll spend some time on it here.
Cutting the Door Opening and Installing the Front Siding
Before siding went on, I had to cut the rough opening for the door through the sheathing and house wrap. Pretty straightforward — measure twice, cut once, and keep the cut as clean as possible because the trim will cover small mistakes but not big ones.
From there, the front siding went up. I worked off the top, keeping a consistent reveal between boards.
Ventilation Theory (The Part Most DIY Saunas Get Wrong)
A sauna isn’t just a hot box. For it to actually work — meaning the heat stays where bodies are, humidity moves correctly, and you don’t suffocate — you need air moving in a deliberate way.
The short version of what I landed on:
- Intake goes low, near the heater, so fresh air gets pulled up through the stones and into the room at temperature.
- Exhaust goes on the opposite wall, ideally placed to encourage a slow convection loop rather than dumping hot air out the top.
- You want air turnover without killing the heat — maybe 4–6 air changes per hour depending on the source.
Like I mentioned in the videos, I based my setup largely on Trumpkin’s Notes on Building a Sauna, which is the most useful free resource I found on this topic. If you’re planning a build, read it before you frame anything.
Exterior Trim and Caulking
Trim went on after the siding — corners, around the window, around the door opening. Not complicated but the kind of detail work that either looks great or looks off, depending on how patient you are with it.
Then came caulking every seam that would see weather. Use an exterior-grade product rated for the temperature swings in your area. Cheap caulk cracks in a year or two and then you’re chasing leaks.
The Custom Glass Window
This was the moment I was most nervous about in the whole build. I had a custom tempered glass panel made at a local glass shop based on the rough opening dimensions I’d framed in Part 1. Tempered is non-negotiable here — regular glass near sauna temperatures is dangerous.
When it arrived, I dry-fit it first without any sealant just to make sure the opening was right. It was — barely. A snug fit on the right and left side and about a 2 inch gap on top that required additional framing to close that opening.
Then I framed the opening with cedar stops, applied a bead of high-temp silicone (the red stuff — rated for 500°F+), set the glass, and locked it in with the interior stops. High-temp silicone is the right call here. Standard silicone will off-gas or degrade at sauna temperatures.
Prepping for Electrical
With the exterior buttoned up, I started prepping for the electrical run. I’ll cover the actual wiring in Part 4, but at this stage it was mostly about locating the control panel, thinking through the conduit path, and making sure everything I’d need was on hand before I opened anything up.
What I’d Do Differently
A couple of lessons from this stage:
- Dry-fit the glass panel the moment it arrives. If something’s off, you want to know before you’ve committed to sealants and trim.
- Don’t skimp on high-temp silicone. It’s $10 more than the regular tube and it’s the difference between a window that lasts and one that fails in a year.
- Plan your ventilation before you frame, not after. I got lucky that my placements worked out, but if I were doing it again I’d mark the exact intake and exhaust locations during framing.
What’s Next
Part 4 is a short video covering the lighting electrical, vapor barrier, and installing the furring and the first part of the actual interior cedar. Part 5 is finishing the sauna, and using it!
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