Far Infrared Sauna Design: Panels, Size, Circuit

A far infrared cabin is laid out around one fact: radiant heat only warms what it can reach. There is no mass of hot air doing the work, no kiuas — the stone-topped sauna stove — and no löyly, the steam and spirit thrown from those stones (both in the glossary). What that difference means for your body and your expectations is worked through in the honest take on infrared versus traditional saunas. This page is about the box itself.

Where do the panels have to go?

Facing you, close, on as many surfaces as the cabin can carry. A panel warms the part of you in its line of sight and nothing else, and intensity falls off with distance. That one rule decides the whole interior.

Panel position	What it reaches	Why it earns its place
Back wall, behind the bench	Spine, shoulders	Harvia designs its radiators around the spine — the most useful target in the cabin
Front wall or door	Chest, stomach, thighs	Without it, your front stays cool while your back cooks
Side walls	Ribs, upper arms	Fills the gap between front and back
Calf, under-bench, floor	Shins, ankles, feet	The omission people notice first

The lower panels are the ones cheap cabins leave out. A Finnish review of several commercial installations found exactly that: feet not resting where a panel could see them stayed cool, however long the session ran. In one cabin the panels stopped at bench height, so lying down beat sitting up — it kept the whole body at the same short distance.

Ask any seller for a panel layout drawing before you commit. Count the panels, note which surfaces they sit on, and check that something is aimed below knee height. Harvia’s Onni Small carries two 400 W full-spectrum radiators on the back plus four more panels at the sides, calves, and floor. Two panels behind a bench and nothing else is a back warmer with a door on it.

How big should the cabin be?

Smaller than people expect, and bigger is not better. A traditional room is sized by volume, because you are heating air. A far infrared cabin is sized by your shoulders and knees, because you are heating a body — extra floor area only moves you further from the panels.

Capacity	Footprint	Height
One person	900 × 900 mm (about 35 × 35 in)	1.91 m (75 in)
Two people	1,300 × 1,043 mm (about 51 × 41 in)	2.0 m (79 in)

Those are Harvia’s own external dimensions for a one-person Radiant cabin and a two-person Onni Small. “Two people” means shoulder to shoulder, sitting up, with a towel each. Check bench depth against the tallest user — under about 18 inches you sit on an edge rather than in a seat.

Measure the route, not just the room. Cabins ship as flat wall panels, and the widest panel is roughly the width of the finished cabin. A basement stairwell with a tight turn has ended more infrared purchases than any spec sheet.

What circuit does it need?

Its own. A 120-volt, 15-amp circuit carries 1,800 watts on paper, and a typical one- or two-person cabin draws most of that. Share the run with a hair dryer or a space heater and the breaker settles the argument for you. Larger cabins are commonly specified for a dedicated 20-amp circuit, and some want two separate outlets on separate circuits.

This is the one place infrared is genuinely easier. A traditional electric heater needs a dedicated 240-volt feeder and a permit-grade install, laid out in the electric sauna heater guide. Infrared asks for one household circuit — but a real one, run by an electrician, not an extension cord.

Building panels into an existing room is a different sum. Finnish practice for a built-in installation is roughly 1 kW per cubic metre, plus more where the room is finished in tile, glass, or concrete, because hard surfaces soak up energy that never reaches you. One documented Finnish bathroom conversion put 2.7 kW into a 6.6 m³ tiled room and watched the air rise less than 10 °C in an hour.

What is the cabin made of?

Ideally, solid wood and nothing clever. The interior is a small enclosure held at 50–65 °C with you inside it, and heat is the variable that matters for indoor emissions. The U.S. Consumer Product Safety Commission’s summary is blunt: formaldehyde emissions increase with increased temperature, and increase again with increased humidity. The EPA names pressed wood made with urea-formaldehyde resin as the most significant household source, with MDF the highest-emitting of the group.

That does not make cabins dangerous, and we will not pretend to know a number we have not seen measured — published third-party VOC data from inside consumer cabins is thin, and most “low-VOC” claims come from the people selling the cabin. What it does give you is a clean specification to hold a seller to:

• Solid wood interior surfaces — basswood, cedar, hemlock, poplar — with no composite panel where the heat lives.
• No stain, varnish, or paint on any interior face.
• If plywood or MDF appears anywhere, ask for its TSCA Title VI or CARB Phase 2 certification. That labelling has been mandatory on composite wood sold in the United States since 2019, so a seller who cannot produce it is telling you something.
• Air it first. EPA’s standing advice when a new emission source enters a home is more ventilation — run the cabin hot and empty with the door open a few times, in a room you can vent.

What should you do before buying one?

Sit in one for a full session, not two minutes in a showroom. Bring a tape measure for bench depth, interior height, and the delivery route. Get the panel layout and the circuit requirement in writing. Then compare the quote against what a home sauna actually costs across every other route, because an infrared cabin competes with a converted closet and a pre-cut kit, not only with other cabins.

A well-designed infrared cabin gives you gentle radiant heat in a small footprint on a household plug. It does not give you the steam cycle, and the reasons that matters — including which health research belongs to which appliance — are set out in our comparison of infrared and traditional saunas. Design the box for what it can actually do, and it does that part well.