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Moisture control dilemma: DIY walk-in cooler

Flytrapper | Posted in Green Building Techniques on

I’m helping a great non-profit (www.nourishnc.org) create an 8’X8′ DIY walk-in cooler in an unconditioned warehouse. Plans include: 2×6 construction, R23 Roxul insul, wrapping the exterior of cube in Grace Ice and Water Shield, and a really nice, donated commercial refrigerator door. Interior is FRP (fiberglass reinforced panels) glued to 1/2″ OSB. Temperature of 35-40 deg F will be managed with a 12K BTU window a/c controlled by a “CoolBot”. http://storeitcold.com/

Question:
Presumed goal is to eliminate warm, moist air from infiltrating structure (thus, the Grace air barrier). But since interior holds perishable food, it needs the FRP for sanitation. Moisture barriers on both sides of these walls seems a really bad idea! So, do I place some sort of interior vents between the studs, and count on the a/c to dehumidify, or will adding vents just invite moisture into the structural elements?

I’m beginning to see why most commercial coolers are closed-cell foam with a stainless steel skin! In this case, cost was prohibitive. Need your thoughts!

Karl
Wilmington, NC

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Karl,
    In this case, the direction of the vapor drive is from the warm, moist exterior toward the cool, dry interior. So the vapor barrier belongs on the exterior of the assembly. That's a good place to put your Ice & Water Shield. So far, so good.

    If you do that, I wouldn't worry too much about the fiber-reinforced plastic (FRP) panels on the interior. I doubt whether they will cause any problems.

    That said, the best way to build this cooler with be with SIPs or with rigid foam insulation over studs (instead of Roxul insulation between the studs). Foam insulation is immune to moisture problems, and your plan (a stud wall with Roxul insulation) doesn't have a high R-value, and has thermal bridging through the studs.

    You might consider installing a continuous layer of rigid foam insulation on the interior side of the studs before you install the FRP panels.

  2. Flytrapper | | #2

    Thanks Martin. My preference was also for SIPs, but couldn't make them happen for this project. We did go with 24" centers, single top plate, and 2-stud corners. Surprisingly (or not?), R25 is industry standard for coolers, R32 for freezers. Curious to see how this one performs over time.

  3. charlie_sullivan | | #3

    I would be in favor of vents through the FRP to allow the interior of the wall to dry, if you can get away with that from a sanitary/cleanability perspective. But given you that don't have to worry about things like rainwater getting in through poor window flashing, allowing the interior of the wall to dry isn't a high priority.

  4. user-5574861 | | #4

    Wouldn't Polyiso work really well in this application? High r-value to start with, not cold enough to see reduction in r-value, foil-faced exterior would provide VB, relatively low cost. I remember reading an article that Joe L wrote about wine cellar construction (slightly higher optimal temps and humidity) and I recall that he recommended Polyiso.

  5. GBA Editor
    Martin Holladay | | #5

    Jonathan,
    As I wrote in my answer, any type of rigid foam insulation (including polyiso) would make more sense for this application than mineral wool.

  6. user-5574861 | | #6

    Agreed Martin. I was just wondering if it is the best of the foams. It has a higher r-value the XPS and EPS, especially at the temps used in this application, lower GWP than both and easy to get foil faced. I have not priced it out, so I am not sure how it compares on that front.

  7. Flytrapper | | #7

    Our project was constrained by the need to obtain supplies locally when possible, and to minimize cost. We estimated 5" of EPS board in the floor, roof, and walls of our 8X8 cube would cost around $1500- triple the cost of the Roxul. I later found a business in Cleveland that offered me 10 used commercial panels for $1900, enough to build the entire cube. These assemble really fast via a locking mechanism, but my timing was off and the materials had already been purchased.

    I'm continuing to ponder venting the interior walls/ceiling. Assuming I am able to tightly seal the exterior, the only air entrainment should be when the door is opened. In that case, condensation should only occur on the surface of the FRP, and should readily dry as the a/c dehumidifies the room. So maybe interior vents between the studs would be a bad idea after all! Opinions on this?

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