The air is not the problem
A 294 cubic foot sauna holds about 22 pounds of air. Air has a specific heat around 0.24 BTU per pound per degree, so raising all of it by 110 degrees takes roughly 580 BTU. A 7 kW heater produces about 23,900 BTU an hour, so on air alone the room would be at temperature in a minute and a half. Anyone who has waited for a sauna knows that is not what happens.
What takes the time is everything solid. The cladding, the benches and the stones between them have a heat capacity a hundred times the air, and until they are warm the room does not feel or behave like a sauna even if a thermometer near the ceiling says otherwise. A room with hot air and cold benches is an unpleasant room, and it is exactly what you get by measuring warm-up with a thermometer instead of a hand on the timber.
How deep the wood gets hot
Wood conducts heat slowly. Over the course of a one hour warm-up only a surface layer of the cladding reaches anything near room temperature; the timber behind it is still climbing hours later, and the framing behind that never gets there at all. Modelling that properly needs a transient conduction solution. Modelling it usefully needs one number: how deep the layer that comes up with the room is.
A third of an inch is a defensible working figure for an hour long warm-up on ordinary sauna cladding, which is why it is the default. It is an input rather than a constant because the honest answer depends on how long the warm-up takes, and the calculation is circular. If your room takes two hours, the depth is larger and the answer here understates it; if you are looking at a fifteen minute reheat of a room that was hot yesterday, the depth is much smaller and the calculation as it stands overstates it badly.
| Case | Working depth | Why |
|---|---|---|
| Cold start, one hour | 0.3-0.4 in | Surface layer only, the default case |
| Cold start, two hours or more | 0.5-0.7 in | The heat front has had time to travel |
| Reheat of a room used yesterday | 0.1-0.2 in | The mass never fully cooled, so less of it is starting cold |
| Solid timber or log walls | 0.6 in or more | Much more material available to absorb, no insulation stopping it |
Stones, and why heater choice changes the wait
Stone has a specific heat around 0.2 BTU per pound per degree, which is low per pound, but heaters carry a lot of pounds. A traditional heater with a hundred pounds of stones brings 20 BTU per degree to the total, which on a 110 degree rise is 2,200 BTU of warm-up. A large stone capacity heater with three hundred pounds triples that. A compact stone-light unit brings almost none.
That is the trade people are actually making when they choose between heater styles, and it goes both ways. Stone mass slows the warm-up and then stabilises the room, holds temperature through the door opening, and gives the steam behaviour that stone-light heaters cannot. Stone-light heaters come up fast and drop fast. Neither is wrong; they are different rooms to sit in, and the warm-up figure is one side of the ledger.
Where the loss percentage comes from and why it dominates long warm-ups
While the room is coming up, the heater is doing two jobs: putting heat into the mass and replacing heat leaving through the envelope and the vent. The loss share here is a single blunt figure for the second job, and 35 percent is a reasonable default for a well-built insulated room with modest glazing at a moderate outdoor temperature. A glass-fronted room in winter can be far higher; a small, tight, well-insulated one in a heated building lower.
The reason it matters more than it looks is that the same losses continue after the room is at temperature. Once the mass is up, everything the heater produces is going out through the envelope, which is what the holding figure at the bottom of the results represents. That figure is the running cost, and it is the one that accumulates. Getting the envelope right pays twice: a shorter warm-up and a smaller number every hour after it. The heater sizing calculator shows the same envelope effect from the output side, and the general building version of the arithmetic is on the heat loss calculator.
Questions people ask
How long does a sauna take to heat up?
It depends on what you mean by heated. A thermometer near the ceiling reads target in half an hour on a well-matched heater, and manufacturers quote that figure. The benches and the cladding take much longer, and the room only behaves like a sauna once they are up, which for a typical domestic electric room is somewhere between one and two hours from cold. This calculator models the second one, and the spread comes almost entirely from stone weight and envelope quality rather than from room size. The calculation on this page is a heat capacity divided by an effective heater output: total the air, the surface layer of the cladding, the benches and the stones, multiply by the temperature rise, and divide by whatever fraction of the heater output is not going straight out through the walls. Ceiling height barely appears because air is under a tenth of the total mass.
Why does the second session of the day heat so much faster?
Because the mass never went back to cold. The cladding, the benches and especially the stones retain a great deal of heat for hours, so the temperature rise the heater has to deliver is much smaller and the depth of timber starting from cold is much shallower. Set the starting temperature to what the room actually sits at and reduce the timber depth to a tenth or a fifth of an inch, and the calculator will show the same effect. A room reheated a few hours later can be ready in a fraction of the cold start time.
Does a bigger heater heat the room faster?
Yes, close to proportionally, because the mass to be warmed does not change. Doubling the kilowatts roughly halves the warm-up, minus a little because a faster warm-up spends less time losing heat. That is not a reason to buy a much larger heater than the room calls for, though. An oversized heater short-cycles once it reaches temperature and gives an uneven room, and it raises the current the installation has to be built for. Sizing on volume and going up at most one step is the usual convention.
What does one sauna session cost in electricity?
The warm-up and the holding period are two separate figures and this page gives both. The warm-up is the heater running flat out for the time shown, so a 7 kW heater taking 80 minutes uses about 9.3 kilowatt hours. After that the heater is only replacing losses, which on the default assumptions is somewhere around 2.4 kilowatts, so each further hour adds roughly that. Multiply by your own rate. Note that this page gives no view on how long the room should be occupied, and the holding cost is per hour of the room running rather than per anything else.
Is the timber depth figure not a bit arbitrary?
It is an approximation, and an unavoidable one. Heat entering wood is a transient conduction problem whose answer depends on how long the process runs, which is the thing being calculated, so a closed form does not exist without iterating. Exposing the depth as an input keeps the assumption visible instead of burying it in a coefficient. A third of an inch reproduces observed warm-up times for ordinary clad rooms reasonably well; the table in the guide gives figures for the cases where it clearly does not apply.