Sauna in Superinsulated House
Rocky67
| Posted in Green Building Techniques on
Hi All:
Does anyone have experience including a sauna in a super-insulated/high-performance home, within the building envelope (double stud wall, with DPC insulation, plywood with self-adhesive WRB on exterior and Intello vapor barrier on the interior). Is that even possible/advisable? If so, what extra precautions need to be taken to avoid over heating and moisture issues? Thanks in advance for any insights you can provide.
Mike
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I'd be interested in any experience or insight out there on this as well. We're going to be building in the next year or so, and in the concept that we just sent to a drafter to get started on there's a spare section of the basement that is screaming "future sauna" to me. Must be the Scandinavian part of my heritage coming out.
My initial instinct is that HRV exhaust in the bath/change room it would open into will deal with moisture while capturing some heat if we want it. For when the heat and/or moisture is too much - regardless of whether that's warmer seasons or all the time - I'd expect to have to fall back on a conventional bathroom exhaust fan to toss it out before it can get to the rest of the house.
If there are additional problems to watch out for other than standard sauna stuff, or if there are ideas for a more elegant solution than just bolting on another fan, then I'm all ears.
I am intrigued also. Again some Scandinavian heritage involved, but who doesn’t live in a Northern climate and appreciate a sauna? It seems like a CERV2 could accommodate venting to the outside in summer and helping to capture heat in winter. It bears further investigation though.
Andy
Dry sauna or steam sauna? If you mean steam, you're looking at potentially extreme vapor pressures, and a totally sealed zone that doesn't communicate with the rest of the house via HVAC.
The safest course is a separate structure entirely. Think Joe's "perfect wall" on all sides; true vapor barrier and non-moisture sensitive insulation. Intello would open up at 70 or 80% RH and saturate the insulation and whatever sheathing and everything on the cold side. It *could* be done within a home but eventually that steam and heat gets to the main house. In a tight well-insulated one you'll be cracking windows.
Hire a professional. Or buy/build a standalone one for the yard.
I don't think I would worry all that much. Saunas are used intermittently and don't have all that much thermal or moisture mass. Of course everything will be dependent on how often it's used, how it's used, its size relative to the overall house, etc. If the sauna is somewhat well insulated, there's not going to be much heat loss. If you use steam, in most saunas that's just a cup or so of water tossed on the hot rocks at the end. Compared to the moisture loads of living, that's literally a drop in the bucket. Sure, it all adds to the heat&moisture budgets, but probably not all that much.
I'd go with the best insulation package you can get, or build from scratch with good insulation. Foil-faced polyiso would be perfect. Don't build the sauna adjacent to an exterior wall. Include an ERV exhaust vent in the lobby area as suggested above with a boost switch just like a bathroom.
Some good info on building sauna's: http://localmile.org/trumpkins-notes-on-building-a-sauna/
Ideally you want to bring fresh outside air directly in to the sauna (above the heater for ventilation and if needed behind the heater for heater cooling) and exhaust sauna air directly outside. The fresh air is the most critical as it is needed to control CO2 levels. Exhausting in to an adjacent room with good ventilation can work even though it's not ideal.
The above link from @tundracycle (http://localmile.org/trumpkins-notes-on-building-a-sauna/) pulls construction details from this BSC article, which is mainly about wine cellars, but deviates briefly to discuss saunas: https://buildingscience.com/documents/insights/bsi-010-wine-cellars
Things not addressed or curious in "Trumpkin's Notes": 1) I suspect the content regarding wood stoves needs expansion to cover the broad variety of air tightness and heat recovery you can now see in woodstoves. Traditionally they don't recover most of the heat, and pull a lot of the conditioned air outside. But that is not always the case, especially with European and some precocious North American designs. 2) He doesn't discuss floor sealing in detail. If you watch the linked construction video in Trumpkin's notes, you will see several steps in membranes, edge stripping, and liquid sealants are used. Depending on how much gusto is used with the steam, or if the sauna has an adjoining shower, these details may be important for some saunas. 3) There is no discussion about sauna heat recovery via a HRV. I have not researched this yet, but it is on my mind. I am curious if an HRV can handle the air temperature and moisture generated by a sauna, and if it is inefficient to funnel this air through an HRV in summer. I am curious if adding a sauna would change the specs of the HRV system for the home. Assuming the HRV is not hampered by the sauna air in winter, I imagine the ultimate solution might be having a lever to redirect the air to an exterior vent in summer. Theoretically you would flip this lever twice a year, although in reality the temperatures might bouce around a little. I don't know if it could be automated, or what the reliability or ROI would be on such a mechanism. 4) When I was in Finland, I stayed in a cabin with heated floor and a sauna. The uninsulated horizontal hot water tank was below the sauna benches. I am curious about the pros and cons of this application. It likely would depend on the insulation included in the hot water tank, the insulation and size of the sauna... There are a fair amount of variables. If anyone has practical experience or educated guesses on these topics, I would be very happy to hear about it!
Good points. I've looked at a little of this.
Standard HRV/ERV core's do not like the temps and humidity of sauna air and according to an engineer at CERV, if you built one specifically for sauna it'd likely not be very efficient. Given the very limited time of use it's probably not worth it.
I think perhaps the best you could do might be a pipe in pipe scenario or a radiator similar to a furnace (and perhaps add some finns?) that helps to transfer some of the heat to the incoming air?
Energy is much more expensive in Finland than North America so every little bit of conservation helps people's wallets. The sauna is the warmest space in the house so having the hot water tank in the sauna helps to reduce energy use. From a sauna perspective they've learned to be cautious though as this tank encroaches on the 'cold zone' so the benches need to be a bit higher to account for this.
An interesting note on these tanks. Many communities have a central heating system that distributes hot (very hot) water to homes for general heating and DHW. Most tanks like this are actually hot water to hot water heat exchangers similar to indirect heaters used in North America with a boiler.
I am learning and spitballing right now, but I think it MIGHT actually work out. I am not 100% settled on wood vs electric for sauna, mostly because of the labyrinth of regulatory and health concerns (which are not the same at all) This is my thinking regarding the HRV though:
I calculated my sauna air volume, multiplied by 6 for 6ACH and divided by 60 for the expected CFM load via mechanical downdraft to an HRV. ASHRAE 62.2 standard for fresh air to a home is (((bedroom count + 1) * 7.5cfm) + 3% of total cu ft volume). For my house and sauna, the sauna CFM will be 7.45% of the total house CFM, which is about 1:12. In practice, I very much doubt that is constant, but it is all the numbers and knowledge I have to play with at the moment.
I know you refer to Trumpkin often, so you are probably familiar with his diagrams showing temps of 90-115F in the bottom 12" of the sauna where a mechanical dowdraft is often placed (electric heater)... General guidance has HRV systems oversized by 30-50% so they can be boosted for kitchens, bathrooms, parties, illness, etc.
After this it gets into thermodynamics with the RH and temp, but I am guessing that one part of humid ~115F air into 12 parts ambient ~72F air is probably not going to fry a whole house HRV over the course of a few hours. I need to run my thinking by a trustworthy HRV person with an engineers mind!
You need more than 6 ACH in most saunas unless it's rather large for the number of people. Shoot for 20-25 CFM per person.
You generally DO NOT want to mix sauna and house ventilation. Primarily because you can't control the airflow in the sauna so can have too little or too much. You also want as fresh of air as possible in the sauna so ideally only air from outside, not from or mixed with house air that can often be quite bad. You ideally want to exhaust sauna air (moist, high CO2, sweaty, etc.) to outside, not in to the house.
HOWEVER, if you can solve for these then you'd be OK.
Wood stoves provide forced exhaust for sauna ventilation which is critical so you want them to pull some of the conditioned air out. In an electrically heated sauna you need a powered blower to perform this function.
Heat recovery is good though. I think sauna stoves run the hot flue exhaust through the rocks to help with this. The EU clean air requirements are strict and getting stricter so these stoves have to meet those. Do you know how the EU clean air requirements for wood stoves compare to US/Canada?
EU Air quality standards for wood burning appliances: I am assuming they are good, but it probably varies quite a bit by country. I know a friend in Denmark, (in Copenhagen) gave up on getting any sort of wood burning device approved. I am assuming it is different in the countryside though.
There is a lot of discussion about woodstoves in the US and UL/EPA/NFPA standards right now. I need to read up on it. This is actually a concerning area when building a wood burning sauna in a home. Some homeowners insurance policies require UL for woodstoves. You can always shop around for a new insurance company, but it isn't so easy to get around code enforcement if they start looking for the same. Right now wood burning sauna stoves fall into a gray area that is undefined. I suspect the Scandinavian sauna stoves are reasonably efficient and safe, but the EU CE mark has no reciprocity stateside, regardless of how fair it is not. Sigh!
Something to keep in mind is that a good electric sauna provides the exact same sauna experience as a good wood sauna. The only difference is that fires are enjoyable to look at, hear and smell (though you shouldn't smell much of it in a sauna or you've got a problem).
Wow. Ignorance is bliss. I just learned how ERVs work. Elegant in their simplicity but I didn’t realize that the systems are basically passive except where entering and exiting the home, which means if you started monitoring air quality per room, there are probably all kinds of issues. Also ASHRAE 62.2 (or something like that) is not that great (thanks for the tip). We are all just breathing a lot of bad air. 😳
The situations are not completely analogous, but we've had indoor hot tubs for decades, usually in the basement but once in the master bedroom and once in the kitchen (it was a rental situation...). Two points: if it's indoors, the temperature differentials are low, so it's easy to use electricity, just needs a dedicated 120V circuit. No gas and no wood. We only use the hot tub during heating season, so any heat lost just goes into heating the house.
Since we only use it during the winter, a bit of extra humidity is actually a good thing. If your house has excellent air sealing, this may be more of an issue, but I doubt it. In theory I should have a bath exhaust fan in the near vicinity of the tub, but it's never been an issue for humidity or smells.
We have two different friends that have stand-alone dry saunas in bedrooms with no external connections other than an electrical cord. Not exactly like the 'way back days in da UP, eh?, but pretty good.