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Indoor hot tub concerns

HDGFHkB5cP | Posted in General Questions on

I’m proposing a 360 sf, one story addition to be constructed in southern NH using 4.5″ polyiso SIPs for the walls, and high density cellulose blown into the scissor trussed roof/ceiling. Inboard of the SIPs we’ll be framing a 2×4 wall for additional insulation as well as a place for the wiring. The addition will be open space that will include a 4 person hot tub. My concern is the potential for excessive water vapor from the hot tub especially in the superinsulated and air tight shell that we are proposing. The owner assures me that the lid on the hot tub will be in place the majority of the time.

I know there are a number of other variables to consider, but I’m wondering if we should just move ahead with the project and monitor the humidity levels after construction is complete (being prepared to react accordingly), or should we be planning to install a HRV system.

I’ve never done an indoor hot tub, and from those I’ve seen, excessive humidity doesn’t seem to be an issue.

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  1. Riversong | | #1

    I would advise using a vapor barrier under the ceiling, venting the roof, and not installing any additional insulation in the inboard framing as this will create a condensation plane at the inside surface of the SIPs. Also, no vapor barrier at the inside wall surface as this will create a double vapor barrier, though vapor retarder primer would help. Make sure all drywall is paperless.

    Additionally, I would install an oversized HRV for that volume, and make sure that there is a boost switch that gets used when the hot tub is open, to force 1 ACH when the tub is in use.

  2. J99aAMQzYo | | #2

    Adding to Robert's recommendations, the boost switch for the HRV should probably also be tied into a humidistat so that even if the owner's forget to turn it on or even if the cover isn't as tight as they think, then the fan will still kick in to high-speed when needed.

    We just finished removing an indoor hot tub for a client last month due to the humidity issues. The interior was (is) all cedar and it had all turned black due to the excessive moisture. The structure was only moderately insulated and there was only a single passive vent (read: hole in the ceiling with a fancy brass register). If the original designer had taken two seconds to think about ventilation the space could have been much nicer. We could have retrofitted ventilation, but at this point, our client just wanted the unit gone and to have us convert it into a floored gazebo.

    Another big concern to plan for is galvanic reaction. All of the sliding door tracks had corroded and the light and plug devices all needed to be replaced due to excess corrosion as well. In addition to the poor ventilation, the owner’s excessive use of chlorinated pool chemicals were attacking everything metal in the building. The copper electrical wiring was fairing “okay” according to my electrician, but that probably wouldn’t have been the case in another few years. But everything else from floor joist hangers to electrical devices to floor and ceiling nails were corroding badly.

    So from your standpoint, you should plan to use stainless steel fasteners for everything and/or marine-grade hardware. Simpson Strong-tie has a whole line of hardware designed for saltwater/harsh ocean environments And either Swan Secure or McFeely’s both offer a wide range of stainless screws, nails, etc. These are WAY more expensive than traditional fasteners, so you’ll need to factor that into your estimate. But you can assure the client that it’s ultimately way less expensive than having to replace everything in a few years time.

    Lastly, you should also advise them to use non-chlorine based water additives. Aside from the personal health issues with immersing yourself in that stuff, there’s the building health issues as mentioned above. There are some salt alternatives on the market, but they are just as corrosive in an enclosed structure, so they’re no better. Here are three possible alternatives which were recommended to me by a local aqua-culture company when I was looking into options for our project. Pristine Blue, Baquacil, and EcoSmartePool

    Lastly, you may also want to think about drafting some kind of waiver into your contract regarding release of design liability (or something to that effect) for an indoor water environment. Our client said that they would have gone after the original contractor for not using stainless, active ventilation, etc. but that the company was no longer around. If you client was spending plenty of $$ for an engineered ventilation solution (like they do for health clubs) then that would be one thing, but since this is on you, you may want to cover yourself.

  3. user-659915 | | #3

    We try and dissuade our clients from indoor hot tubs or lap pools for all the reasons that Andy mentions and find that most can be persuaded by the superior experience of an outdoor tub. Even if you use all the non-chlorine treatments for routine maintenance you will still be advised to run regular chlorine 'shocks' for water quality reasons. If your client is stuck on the indoor tub, in addition to the precautions Andy recommends we would suggest a separate HVAC system (e.g. ductless mini-split) rather than bringing the space into the whole-house system. I have no experience of using a HRV in these circumstances but it sounds like trouble and more trouble on the corrosion front.

  4. Riversong | | #4

    I was going to suggest an outdoor hot tub, but figured that this client probably would not be interested and it is highly wasteful to heat an outdoor tub with electricity.

    I have used a wood-fire Snorkel hot tub for more than 10 years (it is now my only bath, since I have no indoor plumbing). It is a red cedar barrel with a submerged aluminum top-feeding woodstove. The combination of the cedar extractives (still bleeding into the water after all these years) and an occasional box of baking soda keep the water clean and odor-free, and I have to change the water only 2 or 3 times a year, mostly because of leaf and grit accumulation.

    There is nothing more relaxing than soaking under the night sky, even in a rain or snow storm, and I use this tub at least once a week all year long here in Vermont. No moisture issues at all!

  5. HDGFHkB5cP | | #5

    The client has had an outdoor hot tub for ~20 yrs. They're both south of 60, but not by much, which is why they are looking to relocate indoors. In fact, the patio where the hot tub is currently located is the footprint of the addition. A side benefit of the addition is it will provide an interior chase for the solar hot water piping that comes down off the existing roof. It's great to be working with a client that "gets it".

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