Dedicated ventilation for interior spa room
I’m building a custom home in zone 6B (southern Oregon, Klamath basin) and am planning to have a therapeutic spa in a room in the walkout basement. The room will have one exterior wall on the walkout side of the lower level (slab on grade with radiant heat), with one large exterior door, so I can easily incorporate whatever additional exterior venting is warranted. It will have one interior door leading to a hallway and be otherwise separated from adjacent rooms and the main floor by interior walls and ceiling.
Some home background – Hybrid Timber Frame, ICF below grade/SIPs walls/roof above grade exterior (no unconditioned space in building envelope), goal of 1 ACH50 or less. Will include Warmboard radiant heat, Zehnder ERV and Unico high-vel A/C.
Being in a dry climate, per ASHRAE site, I’ve concluded I need to incorporate 8.3 gpd moisture via humidifier in winter, equipment and method of delivery is TBD. So the air outside this room is generally pretty dry, even in summer.
In addition to looking into proper building materials to withstand the onslaught of water vapor I am seeking advice on how to vent the spa room.
The room’s volume is 1848 ft3, so to exchange 100% of the air in one hour would require ~30 CFM. The spa will be used at most 1 hour per day several days per week, and mostly during a very dry time of year (even in summer the RH max is 50%) . So I see there being 3 different states to consider –
1. Spa is idle and covered, need to vent to control humidity, but spa cover helps…?
2. Spa is in use, maximum dispersion of water vapor, need to aggressively reduce humidity.
3. Spa has recently been used, but again covered, need to continue to aggressively reduce humidity until the room returns to state 1.
My thoughts/questions:
1. Zehnder spec’d a 170 CFM ERV setup for the home, including a 12 CFM exhaust for the spa room, but that seems like an awfully low ventilation rate, even on boost, and directing the moist, chemical-laden air from this room back to the Zehnder unit just seems wrong. I’m wondering if I miscommunicated to Zehnder about this room…
2. I’ve become aware of others using dedicated dehumidifers in spa rooms, but I don’t know how to do the math to determine whether a dehumidifier can “keep up” with a spa, with or without drawing in outside air, how to size such a set up, how to heat the room if drawing in outside air on a cold winter day, etc.
3. Have any of you done the math for such a scenario in the past (or know off the top of your head how to do such math), if so, can you provide a clue or two, so I can figure this out?
4. Have any of you done something similar and had success and/or failure, i.e. actually put in place equipment in similar scenarios? If so, can you please share whatever details you can, including specific equipment choices?
5. Are there any special materials/products you recommend when building out this room? I’m thinking of it like a large, properly sealed shower…?
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Anyone have any thoughts on this? Or perhaps a suggestion for where else I could post such an inquiry to get some feedback?
We're planning something similar in a new custom build. I used a resource at https://www.dehumidifier.ae/resources/how-to/Pool-dehumidifier-calculation.php to figure out what size we need. I also talked with Santa Fe about a dehumidifier and they suggested using a Quest dehumidifier which is part of the same company apparently but it's built to withstand the chlorine that might harm a regular one.
We're also planning a separate Zehnder for that room and making it airtight so the air doesn't mix with the rest of the house. I'm hoping between the cover being on most of the time, the dehumidifier, and the ERV we'll be in good shape but of course I won't know until we finish.
I also just saw a couple products at the International Builders Show that we might use. One is Hydroblok drywall which is a waterproof substitute for drywall. Another is a waterproof trim product by Trimfinity called Ecopoly which looks promising to me for something like a baseboard.
I hope all goes well for your project! Please let me know what you learn.
How about something simple like an RH activated exhaust fan? I would reach out to your ERV manufacturer to see if the pickup there is an issue, my gut feel is that it is ok since it is well diluted by the stale air from the rest of the house.
If you want something better than an exhaust fan, there are small condo HRVs you can get. HRV cores tend to hold up better.
P.S. SIP roofs are trouble. Make sure you know how they have to be built and be diligent with the installer to follow it to the letter.
Same for SIP walls, window details have to be perfect.
The 8.3 GPD humidification sounds high. If it's 70F and 50% RH inside and you're running 170 CFM, yeah, that's 8.3 GPD. But that's assuming zero humidity recovery from the ERV and no contribution from the occupants. Figure on about 5 pints per day per occupant.
For the spa, I'd be thinking sealing it off from the rest of the house and a humidity-activated HRV to vent it.
Thanks, I’ll have to recheck my math on GPD. Any specific brands of humidity-activated HRV you recommend to use and/or stay away from?
We installed a spa room in our walkout basement a few years back, different zone (5A), but similar challenges. I found that treating the space like a giant sealed shower was spot on. We went with marine-grade drywall, epoxy paint, and vapor barriers on every surface.
Thanks for sharing your experience, glad it’s working out for you. The exterior wall in this room is SIP panels and the drywall everywhere else will be fastened directly to the interior OSB of the panels, without furring strips. In your case did you put polyurethane or similar sheeting between furring strips/studs and the drywall?
I need to protect that OSB for sure, but was hoping not to have to furr it out.