DIY walk-in cooler in basement
I would like to build a walk-in cooler in the basement of my home. I plan to insulate to >=R25 on walls and ceiling and I thought I had a good plan for insulation until I considered vapor diffusion. The space will be in the basement which is generally 65-75 degrees year-round and has a RH of 40-50% (active dehumidification in the summer and fresh air ventilation in the winter to maintain humidity).
The main problem is in the ceiling. I had planned on using Roxul in the joist bay to get ~R30 and using a moisture-resistant sheathing on the ceiling & walls of the cooler, which is on the cold side of the assembly year-round. However, it occurred to me that the dew point of the basement (outside of the walk-in cooler) would always be greater than the dew point inside the cooler, leading to constant vapor diffusion from the basement living space towards the interior of the cooler. My thought is that it would make sense then to place a vapor barrier on the exterior portion of the joists to block vapor movement and prevent condensation from accumulating within the Roxul or in the wood of the joists. However, that side of the joists is the main floor subfloor and and pulling that up is not a feasible option.
Any suggestions on how to safely prevent condensation issues in the ceiling assembly, or is this idea scratched? Without knowing where the dew point will be reached within the ceiling assembly, I am hesitant to put in a vapor barrier. As a last option, I could place 2 inches of XPS on the bottom of the joists and 2 inches of polyiso below that, but that would require losing several inches of head space, which is less than ideal, and still not quite reach R25.Thanks for any input and thoughts.
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