In brief
When somebody tells me they want a spa at home, my first question is what exactly they have in mind. A sauna, a steam room and a hot tub are three different rooms: in a sauna the air is around 90 degrees and dry, in a hammam it is forty-something and saturated with steam, and hot water with an uncovered surface evaporates around the clock. Each of them needs different walls, a different extract and different materials – the humidity that is the whole point of a steam room would destroy the timber of a sauna. The hardest of the three is the hammam, because there everything is wet, the ceiling included. All of this has to be designed together with the architecture, before the heights and the space for ductwork are fixed; it cannot be fitted into finished interiors afterwards. And before anybody calculates volumes, there is one more question: how many of these rooms will you actually use.
Three climates under one roof
People mean very different things by the word “spa”. For some it is a sauna and a shower; for others it is half a floor with a steam room, a tub and a room for lying down. In building terms these are completely different rooms, even though they sit next to each other in the catalogues.
A Finnish sauna is 80 to 90 degrees at 10 to 20 per cent humidity. A mild sauna – the name varies with the seller – is 50 to 60 degrees at roughly half humidity. A hammam, or steam room, is forty-something degrees at close to 100 per cent. A hydro-massage tub is water at 38 degrees in a room that is in the low twenties. The relaxation room is an ordinary room, only quieter. (There is also the infrared cabin and the cold plunge, but these are not separate climates in the sense I am writing about here – the first heats the body rather than the air, and the second is simply cold water.)
Sauna people have a simple rule for this: temperature plus humidity should add up to about 110. Ninety degrees and 20 per cent; 60 and 50. Above that it gets stifling. It is a rule of thumb, not a standard, but it shows what we are dealing with. Each of these rooms is a different pair of numbers. The walls, the air and the materials are chosen for that pair, so there is no such thing as a “spa finish” – there is a finish for a sauna, another for a hammam and yet another for a room with hot water in it.
That is why a spa zone in a residence is harder than a single sauna in an ordinary house. A ready-made cabin can be fitted into a house fairly simply; somebody still has to tie in the heater, the electrics and the extract, but it is one room. The trouble starts when next to it there is to be a room at 100 per cent humidity, next to that hot water, and above all of it the bedrooms.
The sauna: dry only for the person inside
There is one thing about saunas that almost everybody gets wrong: they confuse the humidity on the hygrometer with the amount of water in the air. The hygrometer in the cabin reads 10 or 15 per cent, so the sauna seems dry. For the person sitting in it, it is. For the house, it is not.
Air at 90 degrees holds several times more water than the air in a living room (I worked this out assuming 10 to 15 per cent humidity in the cabin between ladles of water on the stones). That water is invisible as long as the air stays hot. When it meets anything cooler than forty-odd degrees, it gives the water back as droplets. Anybody who wears glasses knows this from every winter: you walk in from the frost into a warm shop and for a moment you cannot see a thing. In a sauna, that warm shop is the glass in the door, the extract duct and, above all, the wall behind the timber lining – if the vapour has any way of getting to it.
Hence the construction of the cabin, which looks simple from the inside and is in fact a set of layers. The timber lining. Behind it a gap, so the timber can dry out after every session. Behind the gap an aluminium foil vapour barrier, which stops the vapour on the warm side and reflects heat back into the room. Only behind the foil come the insulation and the wall.
Of these layers the foil matters most, because its mistakes cannot be corrected once the lining is screwed on. It usually fails where the cable passes through to the heater, at the light fitting or in the corner of the ceiling. For a few seasons the vapour travels into the wall and condenses where it is cold, and the first sign you get is the timber darkening from the back, or the room next door smelling of cellar. The foil is laid by a human being, so somewhere there will be a gap nobody saw. That is why I prefer layers that forgive: a proper foil and proper tapes instead of the bargain kind, and stone wool rather than glass wool, because after a wetting it dries out more easily and recovers most of its properties. Although wet insulation is in any case a sign that the foil has failed somewhere, not a condition you live with.
The second thing a sauna must have is its own air. A supply by the heater, an extract on the opposite side, a few air changes an hour – too few and it gets stuffy within ten minutes, too many and the heater cannot keep up. Where exactly the extract sits, high or low, depends on whether the draught is natural or mechanical; that is for the ventilation designer to decide, together with the heater manufacturer’s instructions, not the joiner building the cabin. You do not simply connect the cabin to the duct that serves the rest of the house – if at all, then on an arrangement the designer has provided for that temperature. The room around the cabin has its own extract, because after a session hot, wet air comes out of the open door and has to have somewhere to go. How long the sauna lasts depends largely on drying it after use: door open, ventilation on, sometimes the heater on low for a while. A sauna closed up wet ages much faster.
The third thing is the heater and the timber. The heater is sized to the volume – roughly a kilowatt per cubic metre for a well-insulated cabin – and every window and every heavy wall adds its share. An undersized heater does not usually break. It will heat for an hour, reach 70 degrees and stay there. The heater is also its own circuit, and an installation done to the rules for sauna cabins, with their temperature zones – not a “better cable” added at handover. The glass in the door is the coolest surface in the cabin, tens of degrees below the air, so that is where the water collects and where the heat escapes; a large glazed wall in a sauna can be done, you just have to know that you pay for it in heat.
And the timber in a sauna should be boring. For the benches, where bare skin sits, you use light species without resin or knots that do not heat up to the touch: aspen, alder, abachi, thermo-treated aspen. Resin at 90 degrees runs and burns. For the walls and ceiling, selected Nordic spruce is common, because nobody touches it there. No varnish and no preservative anywhere, because at that temperature they evaporate and you breathe them. A handsome oak bench ends in a burn.
The hammam: everything is wet, the ceiling included
With a hammam it is the other way round. The humidity is close to 100 per cent, so the dew point equals the air temperature, which in plain words means: every surface even slightly cooler than the air is wet. Walls, benches, door, ceiling. For the whole session and for a long time afterwards. In an ordinary bathroom the tanking goes over the whole floor, up the walls as a skirting and, in the shower area, all the way to the ceiling. A hammam has to be treated as a shower on every surface, ceiling included – a shower that nobody ever turns off. An ordinary bathroom liquid membrane is not enough here; what you need is a system made for steam rooms, in which the boards, the membrane, the tapes, the adhesives and the grout all come from one kit.
The most common mistake in domestic steam rooms concerns the ceiling. Anybody who has slept in a tent knows that in the morning the canvas is wet on the inside although it did not rain overnight: the breath of a few people condensed on the cold fabric and now drips on the sleeping bag. In a hammam exactly the same thing happens, only without pause. The steam condenses on the ceiling, because the ceiling is the coolest surface, and on a flat ceiling it gathers into drops that fall on the back of your neck. Cold ones. So a steam room ceiling is built with a fall, or as a vault – so that the water runs down the walls instead of dripping – and it is insulated, so that it is no colder than it has to be. How much fall it needs, I will not tell you off the top of my head; the designer settles that with the manufacturer of the system the room is built from. A flat, cold ceiling above a steam room, on the other hand, is a mistake that shows in the very first sessions.
A traditional hammam does not have this problem, because its vault is stone, massive and heated for hours before use. At home, heated walls and benches take its place. Here is something I know from every bathroom: the water in a typical underfloor system is around 30 degrees and will not even get a towel rail warm to the touch, and the heat source runs in bursts, because the concrete floor weighs tonnes and holds its temperature on its own. A hammam bench has to be noticeably warmer than a floor in a house, so it gets its own circuit at a higher temperature, or electric heating – it is not hung “while we are at it” off the house underfloor system. Under the finish of the benches and walls there are water-resistant boards, on them the tanking with tapes in every corner, on that mosaic or porcelain tile; epoxy grout or another grout specified in the steam-room system, because cement grout soaks up water and turns black, and flexible movement joints in the corners and at the benches instead of rigid grout.
The steam comes from a generator that stands outside the cabin – in a services cupboard or the room next door – and reaches the nozzle through a pipe. The pipe should be short, without any low points where condensed water can sit, and laid with the fall the manufacturer asks for; otherwise the nozzle spits hot drops instead of giving even steam. The generator boils water, so hard water leaves scale in it, and scale on the heating element is the commonest failure these units have. In Poland mains water is often hard (a test kit or a word with the neighbours will tell you), so the generator gets water treated to the manufacturer’s requirements – usually softened, although not every generator likes completely soft water – and an automatic drain that empties the tank after each session. And service access from outside. It is the same principle as a concealed shower valve in a bathroom: the biggest risk is a failure that means breaking out the tiles. Anything with a heating element, a valve or a filter in it has to be reachable without touching the mosaic.
Finally the drain. In the floor, with a fall, preferably a linear one with a sealing collar – like under any shower. With one difference: a steam room may be used once every few weeks, and the trap in an unused drain dries out, and then the air from the drains comes into the cabin. The owner’s first thought is that the steam room has “started to smell”. It has not; the trap has dried out. There are drains with a valve that closes without water, and devices that top up rarely used traps automatically. With me, that question comes up before the design.
Hot water in a closed room
A hydro-massage tub looks innocent and is the most evaporative thing in the whole zone. I mean a tub that stands filled and warm all the time; a bath filled for one soak and drained afterwards is a lighter problem – then it is mainly about extraction during the bath and the hygiene of the pipework. Water at 38 to 40 degrees evaporates almost twice as hard as pool water at twenty-eight – for the same surface area – and the jets break it into droplets and bubbles on top of that, which increases the surface it evaporates from. The principle is the same as for an indoor pool, only the water is warmer, so everything happens faster; I wrote about the dew point, the glazing and dehumidification in the piece on the indoor pool climate. A cover on the tub when nobody is in it is not an accessory; without it the room evaporates around the clock.
There are two more things you cannot see from the bath. A filled tub with two people in it weighs as much as a car, only it stands on 2 to 3 square metres rather than on four wheels. On the ground that is a small problem; on a floor slab it is a conversation with the structural engineer before the slab exists. And the pumps. The massage pump and the blower are loud, and a tub built into a masonry surround passes their vibration into the slab, which works like a membrane – in the bedroom above the spa you hear it more clearly than in the bathroom itself. Anti-vibration mounts, flexible pipe connections, an enclosure and an access hatch to the pumps have to be in the design, because once the stone is on, nobody goes back to it. There is also water standing in the hydro-massage pipework, so disinfection and flushing of the system have to be provided for; without them the smell will come from the jets, not from the air.
Ventilating a hammam at home: separate air for the whole zone
Ventilation is a subject of its own. And the single most important thing here: I would not hang a spa zone off the heat-recovery unit that serves the house. A domestic unit is not a dehumidifier and is not built for constant humidity, and here two things come into play that a house does not have. Out of a sauna comes air tens of degrees above anything the exchanger in a domestic unit is built for. Out of a hammam comes air saturated with steam, which turns to water in the first cooler stretch of duct. Let both into a shared duct network and the condensate will appear in places nobody looks at.
So the zone gets its own separately calculated air system: ventilation, and next to a tub of hot water usually dehumidification as well, because air changes alone cannot keep up with the moisture. The ducts from the sauna and from the steam room are separate, made of a material that tolerates heat and moisture, and insulated where they pass through cooler spaces. It is the same physics as an intake duct run through a warm room: a cold pipe in warm air sweats on the outside, a warm wet pipe in a cold space sweats on the inside. The whole zone runs at a slight negative pressure to the house, so that through the gaps around the doors the steam goes to the extract rather than into the corridor – with planned make-up air and a check whether the house has a fireplace that draws its air from indoors. And it has at least three modes – full power while somebody is using it, drying after a session until the humidity drops, and standby for the rest of the week. The automation switches between them on the humidity in the room.
There is one trap in this that I know from heat-recovery systems in ordinary houses: a device that annoys people gets switched off. I have met people who turned their heat-recovery unit off altogether because it was noisy at night. If the drying cycle of the spa zone can be heard in the bedroom, somebody will switch it off within a fortnight, and from then on the steam room dries on its own, which is to say not at all. Hence silencers, anti-vibration bases and a plant room away from the bedrooms, with spare space, because one day the unit gets replaced. Heat recovery from this extract is possible and sensible, because a lot of energy leaves through it – but only with a unit built for wet air, with a condensate drain; that is the designer’s calculation.
I do not calculate the moisture balance of the whole zone myself. That is a separate trade – the ventilation designer – and the cabin supplier will not do it as a side job during installation. What the designer gets from me is real data: which rooms, how often they are used, whether the tub will be covered, where the bedrooms are. And space in the design for the ducts and the plant room, before the architect fixes the heights. After that my job is only to make sure nobody slims those assumptions down along the way.
A home spa: what to plan and where to put it
The zone has its own sequence: heat, cool down, rest – and so on two or three times. The layout of the rooms should follow that sequence, so that nobody walks through the kitchen in a bathrobe. Between the sauna and the steam room a shower or a lobby, because those two climates must not share a door; a relaxation room with normal, dry air; a route outside, if there is to be a cold plunge in the garden.
Where in the house is a decision with consequences either way. A basement is quiet, stable and easy to give a plant room, but it has cold external walls, and a cold wall next to a wet room is a ready-made dew point. Such a wall needs its own analysis – the waterproofing against the ground, the insulation and the vapour barrier chosen together so as not to trap moisture inside the wall; a foil on the inside alone does not settle it. A basement also has no daylight, and in a relaxation room without a window people stop relaxing. The ground floor by the garden gives light and a way outside after the sauna, but the noise goes into the living areas. Next to the pool is a natural fit – a shared plant room and shared wet air – as long as both systems are calculated separately. One thing holds for every location: a bedroom above the pumps and the heater is a mistake that shows on the first night.
There are also a few decisions I put forward for you to choose from, not to research, because they cannot be read out of a catalogue: a dry sauna, a mild one or both; a hammam, or is a steam shower enough; timber or stone in the relaxation room; a window in the sauna, with its price in heat; whether daylight should reach the spa at all. I wrote about how such choices work in practice in the piece on aesthetic decisions in a residence. Here I will add one thing from my own bathroom: in a spa zone I put in a towel rail with an electric element, because the underfloor heating will not get it warm, and a bathrobe after the sauna should be warm. Mine runs even in summer.
How you can tell it has drifted from the physics
Mistakes in a spa zone have one thing in common: at handover everything looks fine, and the symptoms arrive over the seasons. Cold drops from the steam room ceiling. A steam nozzle that spits droplets. Grout in the corners cracking after the first seasons, because nobody left a movement joint at a bench that heats up and cools down every day. Sauna timber darkening and going soft from the back. White scale on the mosaic from hard water. A smell when you open a steam room that has stood closed for a fortnight. Rust bloom on the fittings of the glass door. A damp patch on the wall of the room behind the sauna.
The cause is almost always the same: the room was designed like an ordinary bathroom with a more expensive finish. Every one of these things can be seen in the design, if somebody looks at it before the finishes go on. After the finishes, what is left is repair.
When a spa zone is not worth it
A full zone with a hammam means equipment that needs servicing, a generator that needs looking after, ventilation that runs after every session, and rooms that have to be used to stay healthy. A steam room used twice a year usually gives more trouble than one used twice a week, because the traps dry out, water stands in the equipment and nobody runs the drying cycle. So before we calculate any volumes, I talk about how you will really use it – the whole family, not only the person who had the idea of the sauna. I do not know that on your behalf, and it is not a rhetorical question – the answer changes the size of the whole zone.
Not everybody needs a hammam. It sometimes turns out, after that conversation, that a sauna with a good shower and a door to the garden gives most of the pleasure for a fraction of the upkeep, and the hammam is a memory of a hotel you stayed in once. When that is the case I say so, and sometimes I advise against the steam room – and I show both routes with what each of them means after a year of use. I do not push either. The choice is yours.
What I do not promise: that a spa zone will look after itself, because it will not, even with good automation. That it will be cheap to run, because hot air and hot water do not allow it. Nor will you hear a figure from me before the design – without the list of rooms, the climates and how often they will be used, any number would be invented. What I can tell you straight away is what sets that figure.
What I ask before I say yes
Before a job like this I do not start with tiles or timber. I start with a few questions, which will come up anyway – better that they come up before the architect closes the floor plans:
- Which rooms will really be part of the zone, and how often will they be used? Everything below depends on this.
- Is the climate of each room – the walls, the extract, the drying cycle – designed separately by a ventilation designer, or “selected” by the cabin supplier?
- Where are the plant room and the steam generator cupboard, do they have service access from outside, and is there spare space for replacing the equipment?
- What is above the zone and next to it? Bedrooms, cold external walls, a slab over a basement – each of those neighbours changes something.
- Does the tanking of the hammam include the ceiling, and does the ceiling have a fall and insulation?
- Do the concealed pipe joints and the tanking get a pressure or flood test before they are covered, with a record?
- What is the water hardness, and does the steam generator have treatment to the manufacturer’s requirements and an automatic drain?
- Has anybody set the total load of the heater, the generator, the pumps and the ventilation against the house’s supply capacity? (It is on the scale of a kitchen with several ovens on at once.)
- Is the electrical installation in the cabin done to the standard for sauna cabins, which divides the cabin into temperature zones – and does the electrician know it?
- Do the cabin doors open outwards without a lock, and is the steam nozzle out of reach of people’s legs?
- Does remote pre-heating of the sauna have its safeguards – a door sensor, heater monitoring, automatic switch-off after a set time – and who teaches the household to use it after handover?
Not all the answers have to be there at once. But before the architect closes the floor plans, all of them must be. How working with me looks, from budget to settlement, is in the Questions section.
Frequently asked questions
Does a hammam at home need its own ventilation?
Yes. A steam room gets its own extract, through a separate moisture-resistant duct, with a drying cycle after every session, and the whole spa zone runs at a slight negative pressure to the house. A domestic heat-recovery unit is not built for this – the air from a hammam is saturated with steam and turns to water in the first cooler stretch of a shared duct.
Sauna or steam room – which one for a house?
It depends on what you are looking for; there is no “better” one. A sauna is dry, hot air and a simpler construction; a steam room is a milder temperature at full humidity, and in building terms the hardest room in the house. If you will use it rarely, the sauna forgives more. Both at once make sense when both will actually be used.
Is a sauna in the basement a good idea?
It can be, on two conditions. The cold external walls of a basement next to a hot, wet room are a ready-made place for condensation, so the walls need their own analysis – waterproofing, insulation and vapour barrier chosen together, not a foil on the inside alone. And daylight – the sauna itself does not need it, but a relaxation room without a window stops being a place to rest.
What should be planned first for a home spa?
The programme: which rooms, and how often they will be used. Then, still on the floor plans, the space for ducts above the ceiling and for a plant room alongside, because without those the rest has nowhere to stand. Only after that come the tanking, the generator, the electrics and the servicing. In terms of area, a sauna with a shower fits into a dozen or so square metres; a full zone with a steam room, a tub and a relaxation room is several dozen – plus the plant room, which everybody forgets.
If your house is to have a sauna, a hammam or a whole spa zone, say so before the floor plans are drawn. At that stage the ducts, the falls, the plant room and the servicing can still be provided for; after the finishes, what is left is breaking things out. Let’s talk about your project.