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.

https://ashrae-meteo.info/v2.0/?lat=42.147&lng=-121.724&place=%27%27&wmo=725895&si_ip=IP&ashrae_version=2021

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…?