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  • High Humidity / Moisture in garage

    I have a 28x32 attached garage with in-floor heat. The humidity level in the garage has been upwards of 60% this winter. I have run a furnace fan periodically to circulate the air and that seems to help a little. Today I put in a dehumidifier before I left for work, but I think it will be ineffective and freeze up as the temperature is only set at 55 degrees.

    What would be a good humidity control? I am thinking of an air exchanger, but would installing a vent through the wall connected to a humistat work the same? Or would I be better off installing a fan type heater to move the air without losing heat?

    Thanks for your help

  • #2
    Re: High Humidity / Moisture in garage

    First you need to figure out where the moisture is coming from. If you're in MN and keeping the garage at 55 degrees, it should be bone dry. If it's damp, there's a moisture source somewhere.

    Some possibilities jump immediately to mind:
    1. water leaks from the walls/roof (of course)
    2. subslab - is there insulation and/or a VB under the slab? Are there any big holes in the slab/exposed earth?
    3. What are you doing/storing in the garage? wet lumber? finishing? wet vehicles coming & going?

    Once you've identified the moisture source(s) and determined that they're not causing immediate damage, you can start looking at solutions. Identifying the sources might suggest the solution itself (fixing leaks).

    If the garage is well insulated, use an HRV to exchange the air with dryer air from outside. If the garage is not well insulated, the HRV will be a waste of money. Spend money on insulation first, then the HRV. Even without an HRV, bringing in some outside air will certainly dry the place in winter.
    All complex problems have a simple solution. That solution is invariably wrong.

    Peter Engle, PE
    Almost Home, Inc.
    www.almosthome.com

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    • #3
      Re: High Humidity / Moisture in garage

      Hi Peter, Thanks for your quick reply. The moisture is most definitely coming from the vehicles. The garage is 2x4 construction with r-11 fiberglas and 10 inches of cellulose in the attic.

      I try to "squeegee" most of the water and snow that falls from the vehicles, but the floor is still always wet from them.

      With this info what would you suggest? Air exchanger / open a window?

      Thanks again,

      Chris

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      • #4
        Re: High Humidity / Moisture in garage

        Originally posted by chsfink View Post
        I have a 28x32 attached garage with in-floor heat. The humidity level in the garage has been upwards of 60% this winter...the temperature is only set at 55 degrees.

        What would be a good humidity control?
        If you're not experiencing severe condensation or other obvious moisture problems, then you don't have a problem. That RH is perfect.

        Air at 55° and 60% RH has the same dewpoint temperature (41.4°) as air at 68° and 38% RH, which is nearly perfect for a conditioned space in winter. In other words, if you pushed up the garage thermostat to 68°, you would find the RH to be 38%. The only reason that it's reading 60% is because you're keeping the air temperature lower. The amount of moisture in the air remains the same (0.000421 lbs water/cf air, 0.27" HG partial pressure).
        Last edited by Riversong; 01-12-2009, 04:22 PM.
        Robert Riversong
        Master HouseWright

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        • #5
          Re: High Humidity / Moisture in garage

          Agree with Robert. Try heating the space to 68 degrees and see what the RH is then, it should be perfect. It is very common to see folks posting RH readings on these forums without mentioning the air temp.

          My shop is currently 53 degrees and 58% RH. If I go in there to work and turn on the electric heater, the temp will be at 68 in about half an hour, and the RH will be around 40% I would guess. No moisture will have come or gone.
          Bailer Hill Construction, Inc. - Friday Harbor, WA
          Website - Facebook

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          • #6
            Re: High Humidity / Moisture in garage

            That's still a dewpoint of about 42 degrees. In MN, that's too high, especially for a garage where none of the equipment would suffer from lower humidity.

            The good news is that if the humidity is that high, it means he's got the place sealed up pretty tight. That begs another question - if the place is that tight, do you have problems with the vehicle exhaust? Or is that pipes outside well enough to keep the air clean? Have you got CO monitors inside? If there's chance of vehicle exhaust leakage, a little bit more outside air wouldn't be such a bad idea.

            I think it would be a good idea to reduce the humidity. There are some high-performance dehumidifiers on the market that operate as low as 35 degrees. Try here: http://www.thermastor.com/Commercial-Dehumidification/

            If air quality is not a problem, dehumidification would be a better solution than ventilation, because you're not wasting heat. All of the energy you put into the dehumidifier will remain in the conditioned space as additional heat, and in MN that's not a bad thing. Dropping the indoor humidity to 40% or so will give you 10-20 degrees of additional dew point protection. That can make a big difference in the amount of water building up in your walls and roof.

            Then again, what's your construction like? If you've got materials that are pretty insensitive to moisture, then don't bother with any of this. There's nothing necessarily wrong with 60% RH, it just increases the risk of condensation within the envelope. If your envelope doesn't care about moisture, then neither do you.
            All complex problems have a simple solution. That solution is invariably wrong.

            Peter Engle, PE
            Almost Home, Inc.
            www.almosthome.com

            Comment


            • #7
              Re: High Humidity / Moisture in garage

              One thing that's not mentioned is how old the garage is. At his latitude his slab will be giving off moisture for quite a few years. If his slab is sitting on poorly draining fill such as sand, and has no barrier underneath, it may be giving off moisture permanently.
              Bailer Hill Construction, Inc. - Friday Harbor, WA
              Website - Facebook

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              • #8
                Re: High Humidity / Moisture in garage

                He's wetting the slab every day. It will certainly be giving off moisture forever.
                All complex problems have a simple solution. That solution is invariably wrong.

                Peter Engle, PE
                Almost Home, Inc.
                www.almosthome.com

                Comment


                • #9
                  Re: High Humidity / Moisture in garage

                  Ah, I missed the part about water and snow coming from the vehicles. Still don't see how that in itself is a problem. Your concern is that the moisture brought in is migrating to the back of the sheathing? I agree with your point about the dehumidifier not wasting heat but is it needed in the first place? The ones I have run are not cheap to operate.
                  Bailer Hill Construction, Inc. - Friday Harbor, WA
                  Website - Facebook

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                  • #10
                    Re: High Humidity / Moisture in garage

                    Thanks for all the info! Just a few more details: the garage is attached, built in 2001 on 5 feet of taconite tailings with drain tile. The walls are 2x4 R-11 with a 6 mil vapor barrier and 1/2 drywall. The ceiling is 5/8 type X with 10-12 inches of blown in cellulose. I have it sealed pretty tight, but have no problems with exhaust.

                    My main concern was condensation behind the drywall because the windows have been so wet, but after reading the replies I think I will dial the heat up to 60-70 degrees and see what happens......except not for a few days; We have high temps of -2 forecast until the weekend.

                    Thanks again for all the help!

                    Chris

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                    • #11
                      Re: High Humidity / Moisture in garage

                      Just curious, what are taconite tailings like? Is that crushed rock?
                      Bailer Hill Construction, Inc. - Friday Harbor, WA
                      Website - Facebook

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                      • #12
                        Re: High Humidity / Moisture in garage

                        He is using in floor heat so this has got to exacerbate the problem in that it will speed the drying process of the slab through temperature assisted evaporation, much the same as a pan of water warming on the stove.
                        What if one were to seal the slab to eliminate this absorption and allow the moisture to be mopped up or squeegeed out of the heated environment.
                        Then this added moisture would be able to be controlled
                        Mark Parlee
                        BESI(building envelope science institute) Envelope Inspector
                        EDI Certified EIFS Inspector/Moisture Analyst/Quality Control/Building Envelope II
                        EDI Seminar Instructor
                        Level one thermographer (Snell)
                        www.thebuildingconsultant.com
                        www.parleebuilders.com
                        You build to code, code is the minimum to pass this test. Congratulations your grade is a D-

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                        • #13
                          Re: High Humidity / Moisture in garage

                          Taconite tailings are the byproducts of mining the ore in our area. It is common to use this in place of sand for backfill when building. It compacts so well after watering it and running over it with a wacker a few times it does not move...at all.

                          Attached is a link to a major project that used the tailings this past year. These are coarse tailings. I used fine in my garage and coarse in my driveway

                          http://www.nrri.umn.edu/egg/tacagg/




                          Chris

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                          • #14
                            Re: High Humidity / Moisture in garage

                            Mark,

                            Sealing the top of the slab is a good idea. It would certainly make squeegieing (how can you possibly spell that?) and mopping more effective.

                            Chris,

                            Don't bother turning up the heat. It will just drive the moisture into the walls harder. Relative Humidity isn't really the issue, it's just a convenient way to measure moisture. The real issue is the amount of water in the air, by weight. once you know RH and temp, you can use a psychrometric chart to calculate everything else you need to know. But you really don't need to know anything else, except the dew point.

                            For your air, 55 degrees and 60% RH, the dew point is around 45 degrees. If you warm the air up to 68 degrees, the RH will go down to about 38%, but the dewpoint will remain the same. However, the warmer the air is, the higher the vapor drive will be, so you could end up moving more vapor into the walls.

                            If your poly film was well detailed, it will prevent vapor drive into the walls and stop the problem of condensation behind the sheathing. If your electric outlets and other penetrations of the exterior walls are not foamed, panned or otherwise sealed, go ahead and do that. That and sealing the floor might be all that you need.
                            All complex problems have a simple solution. That solution is invariably wrong.

                            Peter Engle, PE
                            Almost Home, Inc.
                            www.almosthome.com

                            Comment


                            • #15
                              Re: High Humidity / Moisture in garage

                              Originally posted by chsfink View Post
                              Taconite tailings are the byproducts of mining the ore in our area. It is common to use this in place of sand for backfill when building.
                              And there's concern that MN taconite tailings contain respirable asbestos.
                              Robert Riversong
                              Master HouseWright

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