Swim spa addition (new construction) — PERSIST / REMOTE vs. closed-cell high-density spray foam
Hi GBA Community,
I’m putting together plans to build an addition in central Maryland (zone 4 mixed/humid environment) to be used to house a swim spa and a small hot tub and I’m trying to decide between building using a PERSIST/REMOTE building technique (exterior rigid foam insulation, continuous envelope air/vapor membranes and classic vented roof – basically following the recommendations from Joseph Lstiburek’s building science article “In the Deep End”) or using traditional construction techniques and insulating the walls and underside of the roof deck with several inches of high density closed cell spray foam. I’m of course concerned with energy performance, but my top priority (probably closely correlated anyway) is building it safely to avoid potential rot and mold issues. I’ve done a lot of research and I understand that the former is the preferred method, all else being equal, however, we’re considering using the closed cell foam as a second best option for the following reasons:
1) Possible Overkill – This is not a traditional “indoor pool”. It’s a residential swim spa and a small hot tub, both of which will have well sealed covers on 22-23 hours per day. The water surface area is small and the air temp will be kept pretty close to the water temp (in the swim spa…The hot tub water will be warmer). We’ve done the projected moisture load calculations and it shouldn’t be too bad – projecting between 4-6 lbs per hour, but only for an hour or two per day. The room will have negative pressure and mechanical dehumidification to control the indoor humidity to keep it at or below 55-60% RH (when the covers are off…should be lower the rest of the time, allowing for significant interior drying). All that being said, I’m hoping that although PERSIST/REMOTE would probably be safer, closed cell foam would be “good enough” to deal with what should be a lower outward winter vapor drive than what otherwise might be experienced with a true indoor pool (I’m of course also concerned about inward solar vapor drive in the summer, however, my understanding is that the “indoor pool” aspects of this shouldn’t make a huge difference (if anything, the indoor humidity might reduce inward drive in the summer because the interior and exterior should be closer to being equalized) and that closed cell foam should perform just fine in the summer months…please let me know if you think I’m missing something or should be more concerned about summer time issues as well).
2) Experienced Contractor Availability – The build will be done in central Maryland and we have been unable to find any residential contractors available in the area with any meaningful experience using the PERSIST/REMOTE methods. The architect can spec it properly but we’re concerned about proper field execution, especially the details around doors, windows and wall/ceiling transitions. On the other hand, there are plenty of reputable spray foam installers readily available and the rest of the build will be standard construction and therefore much easier to execute properly. In theory the PERSIST method may be better, but I wonder if a properly built closed cell foam structure would be safer (and better performing from an energy perspective) than a PERSIST building that wasn’t done completely correctly the way it might be done by builders in Alaska with a good amount of experience using these methods.
3) Cost (of course) – Spray Foam isn’t cheap, but neither are rigid foam and adhering vapor barrier membranes. In addition to material costs, the labor for the PERSIST building will be higher overall, especially given that every detail will be the first time for each contractor, so there will inevitably be mistakes, do-overs and just generally take longer because of the more complicated construction (especially door and window details) and because it’s their first time doing it all. Based on the conversations I’ve had so far, the total costs would be significantly less if we just go the route of traditional construction with closed cell foam insulation.
4) Code/Permits/Inspections – Similar to the issue of experienced contractor availability, nobody at the permits offices here and none if of the local county inspectors will have any experience with PERSIST/REMOTE so it will inevitably lead to some issues with permits and inspections. Hopefully none of the issues would be insurmountable, but it’s another complication to factor in to the analysis.
5) Limited Space – We’re dealing with a very limited amount of space on the property between the existing home and the building restriction line 10 feet from the edge of the property. If we do closed cell foam we’ll probably use 2 x 6 construction so the walls won’t exactly be “thin”, but they will be significantly thinner than the PERSIST walls. It can be done with PERSIST, but the extra foot of interior space in what will already be a very narrow building would certainly be nice.
I would greatly appreciate any thoughts on whether you think we’d be okay for this particular project just using closed cell foam or if you think it would be foolish and that we should just deal with the issues above and build using the PERSIST/REMOTE method.
Thank you in advance for your time and thoughts.
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