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Suggestions for Dehumidifying and Ventilating an Unconditioned Storage Shed

jameshowison | Posted in General Questions on

Hot Humid Climate Zone 2. Austin TX.

I’m building a shed.  Well, it’s a shed that will likely eventually be a conditioned workshop, so I’m building it as airtight as I can. I’m using a low-perm air-tight wrap (DA from 475 building products), 1.43 perms (with a vapor open diffusion port at the ridge). Small building, just 196 sq ft.

While it’s a shed, I’d like to dehumidify and ventilate, just enough.  One of the things it’ll store will be bikes, and they get pretty wet. As does yard stuff etc.

I’d love to use something like a ventilating dehumidifier but those are all $$$ and sized for big stuff!  Are there any really small ones? Spring for

I have a plug-in dehu that could run (drained outside via a drain with a u-bend, I guess).  Or a dessciant dehumidifier?

But how to do ventilation? Perhaps a bath fan with a passive inlet (ie balanced ventili?  The Panasonic balanced things (WhisperGreen Select + WhisperFresh Select?) or WhisperComfort FV-04VE1?  I doubt an ERV will help inside humidity levels much (but at least it won’t hurt).

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  1. Jon_R | | #1

    As long as it's an unoccupied shed, don't ventilate at all. But do dehumidify with a portable dehumidifier.

  2. jameshowison | | #2

    Makes sense, Jon :) I do wonder about things smelling, though as paints, oils, plastics etc, evaporate (off-gas). Seems useful to have some ventilation, doesn't it?

    1. Expert Member
      NICK KEENAN | | #4

      Ventilation is the enemy of dehumidification. That humidity mostly gets in there through the air. If you have an air-tight building envelope, a vapor barrier under the floor and a weatherproof exterior there shouldn't be any humidity getting in except when you open the doors, and the dehumidifier shouldn't have to work very hard to get rid of it.

      Things that are evaporating like paint and oil will reach equilibrium pretty quickly. Unless you do a lot of ventilation -- several air changes per hour -- it won't have much effect.

      1. maine_tyler | | #7

        DC, what do you mean by paint and oil will reach equilibrium?

        1. Expert Member
          NICK KEENAN | | #8

          Imagine you take a can and fill it half way up with a liquid, and then put a cap on it. Some of the liquid will evaporate into the air in the can. But very quickly as much as is going to evaporate will have done so, and no more evaporation will happen, the air is saturated. Now imagine you put that same can in a sealed room. The same thing will happen, except that the room air will get saturated instead of the can air. Now imagine you have a vent fan. The vent fan will keep the room air from getting saturated, so evaporation will never stop, until all of the liquid is evaporated.

          1. maine_tyler | | #9

            Ok, that's what I thought but wanted to be sure. Same principle as with water.

            I would be surprised to find that EVP would be reached under most circumstances, though I've never really given that much thought.
            Why do you think things like paint, oil (etc.?) would reach EVP while a pan of water would not? Or do you think a small pan of water would also? (without dehumidification of course)

            Obviously there are several factors at play such as size of room, how tight the room is, how much liquid there is, temperature, vapor pressure of the liquid at that temperature, etc. I never considered the possibility that something off-gassing (evaporating) would actually saturate a room (a tight container is another story). And if it DID saturate, wow, that seems like a lot of vapor.

            My house has plenty of half empty (or are they half full?) glasses of water sitting around and I've never detected 100% RH.

            Regardless of this, I agree that the need to ventilate may not be present given 'unoccupied'. On the other hand, if one were going to spend a lot of time in this structure and there were legitimate concerns with 'fumes,' maybe venting (or cracking a window) would be wise. (I realize the original post here is over a year old).

  3. The_Wagonista | | #3

    I'm planning on building a tiny house on wheels, with a small (4x7) storage area for my motorcycle. This space will be insulated but not conditioned. One wall and the ceiling will be abut conditioned space. These will be insulated as well. Does this space need to be ventilated at all? By the sounds of this thread, a dessicant dehumidifier should be enough? There will be 2 doors and a french casement window. I'm not going to be in there except to drive the bike in and drive it out... I just dont want the motorcycle to get wet (or have mold problems/rot in my house structure).

  4. Expert Member
    NICK KEENAN | | #5

    The amount of moisture that might come in on a bike or a tool might be a few ounces. A smallish dehumidifier might be rated at 50 pints per day -- that's over 6 gallons a day. The amount of water in the air is immensely larger than what will come in as liquid.

    1. The_Wagonista | | #6

      I'm going to have the house parked mostly in climate zone 5, sometimes cz 4. Since it's less humid here, I should be ok with the dessicant dehumidifier?

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