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Vapor barrier on DIY walk-in cooler?

Joe__Z | Posted in GBA Pro Help on

Hello, I need someone to answer a simple question for. I have searched the web and gotten conflicting answers.

I’m building a 15X20 walk-cooler that will be kept between 35-40 degrees F. This is for storing poultry eggs on a egg production farm in Milton Freewater, Oregon. (NE Oregon)

Three walls will be inside the larger barn structure, which will be kept between 60-80 degrees F, the fourth cooler wall is formed by the exterior barn wall, facing west.

The design completed so far is as follows;

1. 4″ Concrete floor pour on top of 4″ of rigid foam insulation, and isolated from the larger barn floor on the sides with 2″ of foam insulation.
2. The walls are two 2×4 walls separated in the middle by two inches, straddling the 2″ of rigid foam that isolates the concrete cooler floor from the larger barn floor, for a total wall width of 9″.

My idea was to spray closed cell foam 2″ thick up the wall and across the ceiling to create an unbroken foam barrier from below the floor, up the walls, and across the ceiling. i would then fill the rest of the wall cavity and attic with batts or blown in insulation.

Finally, we had planned to line both the outside and inside of the cooler walls with standard white 29 gauge corrugated metal siding.

My question is whether I should spray the foam (vapor barrier) to the interior (cold side) of the wall assembly or spray the foam (vapor barrier) to the exterior (warm side) of the wall assembly?

My sense tells me to create the vapor barrier on the exterior, warm side of the wall assembly, as the vapor drive will be from warm towards cold. But will that create a problem in that the wall will never be able to dry to the outside, to the warm side?

Also, in installing corrugated metal siding inside the cooler, am I trapping moisture inside the wall? I know that metal siding is impermeable, similar to a vapor barrier, but am not sure if metal siding is sealed enough to create a virtual vapor barrier or not.

In summary, my basic question is;

1. Do I want to create my spray foam / vapor barrier on the cold side or the warm side of my wall assembly?
2. Am I trapping moisture inside the wall assembly in using corrugated metal siding both inside and outside the cooler walls?

Thanks! Let me know if something isn’t clear….


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  1. Dana1 | | #1

    The walk-in will be kept below the dew point of the outdoor air, so the vapor barrier should be at a layer that's above the outdoor dew point. As long as the cooler is maintained at 35-40F, there is no way it will be able to dry toward the exterior, which has an average dew point above 40F. The mean summertime dew point in Milton Freewater is about 45F, and only averages below 35F from November through March. Pull up a dew point graph, and scroll out to see a full year, and use the cursors to eyeball the averages:!dashboard;a=USA/OR/Milton-freewater

    A cheaper vapor barrier would be polyethylene sheeting on the exterior side of the EPS, and you can seal it with a full-cavity fill of 3.5" of open cell foam, which would be cheaper, higher performance and much greener than 2" of closed cell foam.

    A stackup where R8+ rigid EPS foam that has R13-R15 closed cell on the warm side of the EPS will have a temperature at the EPS warm side of the EPS layer of about 1/3 the delta-T above the cool interior and the warm exterior. In a 60F barn with a 35F cooler the temp at the EPS/open cell boundary (where the vapor barrier is) would be about 43F, which is below the ~45F summertime average dew point, but in summertime it's probably safe to assume that you won't be air-conditioning the barn to 60F- the barn temp will be much warmer, raising the temp at the polyethylene sheeting. When it's 80F in the barn and 35F in the cooler the temp at the poly would be about 50F,

    This all assumes that the barn is sufficiently ventilated that the indoor dew point tracks the outdoor dew point reasonably closely.

    But it might be cheaper overall to go with 4" of EPS and leave the stud bay cavities open, which would be about the same performance as 3.5" of o.c. foam between studs with a continuous 2" of EPS. EPS is the right foam to use for the rigid foam, since it's performance increases significantly as the average temp drops, and it's ~10cents or less per R per square foot, compared to 12-13 cents/R-ft^2 for open cell foam. EPS slightly less green that open cell foam since it takes about 2x the polymer per R, and uses pentane instead of water as the blowing agent. But it's far greener than XPS, which uses HFC134a as the blowing agent, with a global warming potential ~ 1400x CO2, or closed cell polyurethane, which uses HFC245fa for a blowing agent, at about 1000x CO2. Pentane runs about 7X CO2.

  2. Joe__Z | | #2

    Thanks for you comments. I kinda think I was able to follow most of it. :) I'm pretty much a layman in terms of building science.

    One clarification if you don't mind. You calculated that the temp at the vapor barrier would be as high as 50 degrees F in summer, which is above the dew point of 45 degrees F. Does that translate into a problem of having condensation in my wall, leading to rot? How would that moisture ever be disposed of?

    Also, do you have any comment on the relevance or advisability of metal siding inside and outside?

    Thanks again for your advice!


  3. GBA Editor
    Martin Holladay | | #3

    Dana Dorsett's answer is somewhat confusing, especially his advice that "A cheaper vapor barrier would be polyethylene sheeting on the exterior side of the EPS." The way I read that sentence, it sounds like Dana assumed that you would be using EPS -- but I don't think you mentioned EPS.

    You are proposing a type of assembly called "flash and batt" or "flash and fill." The usual way to install this type of insulation is with the spray foam on the cold side of the assembly. In your case, that would be on the interior of your walk-in cooler. Closed-cell spray foam is preferred to open-cell spray foam for flash-and-batt assemblies, because a 1.5-inch or 2-inch layer of closed-cell spray foam is a more effective air barrier than a thin layer of open-cell spray foam.

    For more information on this insulation system, see Flash-and-Batt Insulation.

  4. charlie_sullivan | | #4

    If the temperature at the vapor barrier is 50, and the dew point in the barn or outdoors is 45, that means that any moisture at outside surface of the vapor barrier can slowly dry towards the barn or outdoors, and it also means that you won't get condensation on the vapor barrier. So that's all good. If the vapor barrier was at 43 with a dew point of 45, that would be where you'd have potential trouble. So Dana was arguing that the numbers indicate it will be OK.

    I personally doubt the generally accepted idea that 1.5 to 2 inches of closed cell foam applied to an air barrier to seal crack in it is better than the same thickness of open cell foam, but you can read my arguments about that in the comments on the page Martin links to.

  5. Joe__Z | | #5

    Thanks for the input, this is helpful. I did read the flash and batt article, and it did clarify some things. However, I'm getting confused about whether to place the foam on the cold side or the warm side of the wall assembly. Martin is saying inside, but if I understood Dana right he is saying outside. I've also read elsewhere to put it outside, or the warm side.

    I understand that even experts don't always agree, but can someone explain the arguments for me?

    The one thing I've read that seems to make sense to me is that moisture wants to go from the warm air to the cold air (vapor drive?) so I would want to foam or otherwise install a vapor barrier on the exterior of the wall to prevent it from entering the wall cavity. It seems logical to me that with the foam to the inside of the wall, the vapor would enter the wall and condense on the inside of the cold foam if it was on the inside of the cooler wall, creating condensation problems inside my wall.

    Isn't this the same logic that discourages the use of poly sheeting under the sheet rock in hot climates? My exterior temp will be higher than my interior temp pretty much year round.

    I'm sorry to be obtuse… just don't want to be remodeling my cooler in a few years!

  6. GBA Editor
    Martin Holladay | | #6

    You can turn the flash-and-batt wall around, and put the spray foam on the hot side of the wall if you want. The lead photo in my flash-and-batt article shows an example of this "doing it backwards" approach. Either way will work.

    For more information on putting the foam layer on the hot side of the assembly, see Walls With Interior Rigid Foam.

  7. Joe__Z | | #7


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