I am consulting on an addition to house an indoor swim spa or endless pool. I am getting all kinds of feedback from the different insulation companies about what type of insulation to use. I don't want to recommend something that might be prone to rot in the future, however the spa company has no real guidelines in this area. Their only suggestion is to install a bath fan in the room. If the drywall is sealed with bathroom paint and the drywall is caulked impeccably, is moisture in the insulation cavity even an issue?
I can find no one for whom excessive moisture with one of these things has been an issue. Is blown in cellulose an option here? I hate batt insulation because getting a good install is so difficult. Anyone with some insight would be greatly appreciated.
If building in cold climate in priority sequence:
- As Marshall Minshew says detach the spa area as much as possible form the rest of the house or at the very least contain the spa's mechnicals within the spa's thermal envelope / extension.
- As you and the spa company suggest, the room MUST be depressurised to the outdoors to the tune of a few pascals to keep the moist air from driving into the thermal envelope. You can turn it off if the humidity outside ever becomes greater.
- Avoid skylights (condensation issues in skylight shaft),
- Keep penetrations through the vapour barrier to an absolute minimum,
- Air tightness test the envelope before the drywall goes on and make it as tight as possible,
- Don't run any duct work through the attic space.
- Isolate the spa's attic from the rest of the house.
In the attached picture the ductwork was providing descicated air from a 'Decatron'. Some of that conditioned air was piped into the pool room and some into the flat attic space (see small pipe going up into drywall) so that each skylight shaft could be "washed" with conditioned air (you can faintly see teh grill) to minimise condensation. The result is the drywall was saturated under the domed skylights and the pressurised air ducts running into the flat attic space were likely driving moisture up into the flat roof.
Some of my thoughts as I finish my lunch...out here from Seattle area...maybe similar winter weather to yours, but you're summers are more humid, for sure.
I agree with much of what has been said...detach (if possible, or isolate at the very least from rest of conditioned space of home ), manage to slight negative pressure, mechanical balanced ventilation, basically all comments made about tightness of envelope (with balance mechanical ventilation!) Seems to me your challenge from a vapor drive issue is on those cool winter days when relative humidity outside is 30% or below and you're 70 Fand 60+% RH indoors...monumental vapor drive forces at work from indoors through building envelope to dew point surfaces within wall and roof assemblies. Bad.
As far as insulation, I'd consider the relatively higher cost option of a full cavity fill of expanding polyurethane foam insulation shaved to the studs (polyurethane is key here, no open cell foams!). The full cavity fill will do a great job of preventing air movement through any of those inevitable penetrations through finished wall and roof (maybe even floor) penetration. Upfront cost hurdles yes, but great control of vapor drive through wall and roof assemblies (note roof ventilation, or non-ventilated roof assemblies with foam insulation, may or may not be acceptable to local building officials). In the absence of full cavity spray foam, I would be giving great air sealing attention to junction boxes for switches and outlets and any other penetration through interior finished surfaces like can lights and supply and return register penetrations. These are the areas where a good deal of very warm and very moist air are going to drive into the wall and roof assemblies in search of dew point surfaces to condense back into liquid water to begin those dangerous biological experiments. My assumption is that in summer your indoor environment and outdoor (warm and humid) environments will be more closely matched with a corresponding lower vapor drive potential...right or wrong?
Best of luck,
Given that it is a large hot tub with a cover the moisture load will not be as great as a pool. I would use an interior poly vapor barrier and making sure the roof is vented so it can dry to the outside. I would not have any penetrations in the ceiling drywall. Use paperless drywall. I think dense pack cellulose would work.
I would go to buildingscience.com and look at the air tight drywall approach. For cellulose use only 100% borate.
I think winter is a bigger issue for vapor drive from the inside to out, cold temps and low humdity. Will the room have its own air and thermal barriers?
I would also install a high quality bathroom fan with a humidity sensor.