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Community and Q&A

DIY walk-in cooler in basement

Ken French | Posted in Energy Efficiency and Durability on

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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  1. Brad Hardie | | #1


    You can calculate where the dew point will be. There are a few posts about doing such calculations here on GBA. Just search for it.

    I think you'd be better to put the Iso above the XPS (to the warmer side, so it doesn't thermally degrade at lower temps.)

    Is your concern over space within the walk-in cooler, because you are planning to store brontosaurus burgers in there?

    I like the idea and interested to hear more....

  2. GBA Editor
    Martin Holladay | | #2

    The vapor pressure difference is not going to be significant enough to cause problems, as long as you follow the usual rule about paying attention to airtightness. If I were you, I would install a layer of foil-faced polyiso on the underside of the joists.

    It doesn't sound as if you have come up with a plan to insulate the slab. Unfortunately, the slab is a thermal bridge. If you want to get all energy-nerdy, you can install a layer of rigid foam on top of the slab, followed by a layer of plywood.

  3. Ken French | | #3

    Brad, No brontosaurus burgers. :-) But the more inches I add to the underside of the joists, and to the floor, the more enclosed the space becomes. Thanks for the advice about searching for calculations. I'm still trying to understand completely, but I get the sense that I can avoid condensation as long as I pay attention to the airtightness details and maintain appropriate humidity levels in the basement.

    Martin, That is really refreshing, glad to hear the vapor pressure difference is not significant enough to cause problems. Just to make sure I am understanding correctly, you are suggesting I should place the rockwool in the cavities and use foil-faced polyiso on the interior of the joists, tape the seams to maintain a vapor barrier inside the cooler? Is the thickness of the rockwool or the polyiso a factor if the R-value is >=25?

    Yes, I actually do plan on putting 1" of XPS on the floor and cover it with a layer of plywood, exactly as you stated. Do you think 1" is enough? Getting "all energy-nerdy" has become a habit of an example, I have my Kill-A-Watt plugged into our LG refrigerator right now, gathering baseline data to compare to the walk-in cooler when the project is completed.

  4. GBA Editor
    Martin Holladay | | #4

    Q. "You are suggesting I should place the rockwool in the cavities and use foil-faced polyiso on the interior of the joists, tape the seams to maintain a vapor barrier inside the cooler?"

    A. More or less. The reason you are taping the seams, however, is to create an air barrier, not a vapor barrier.

    Q. "I plan on putting 1 inch of XPS on the floor and cover it with a layer of plywood, exactly as you stated. Do you think 1 inch is enough?"

    A. It will work (in the sense that your brontosaurus burger meat won't spoil). However, 2 inches of rigid foam would have twice the R-value of 1 inch. The effect of added insulation is a lower energy bill.

    By the way, green builders try to avoid using XPS because XPS is manufactured with a blowing agent that has a high global warming potential. It's better to use EPS.

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