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Heating a shop

user-716970 | Posted in General Questions on

My neighbor has a 28 X 30 shop/garage that he likes to heat to about 45°F in the winter months. We are in a cold climate (9500 HDD) and the walls are the typical 2 X 6 stud walls, fiberglass insulation, poly on the inside, plywood sheathing inside, OSB outside, then two layers of building paper and stucco. Heat source is a radiant concrete floor.

Windows are double-glazed low-E vinyl. The big problem seems to be the incredible amount of ice that forms on the interior of the glass during colder spells. Air change happens frequently as the garage door opens twice a day, and of course the door seals are typically leaky.

I don’t see any solution to the icing problem as the interior temp is too low to prevent condensation. Any ideas out there?

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    try a dehumidifier with a hose to a floor drain, drys and heats. the humidity is probably comong from workers breathing but if you can't keep it warm at least you can keep it dry.

  2. user-716970 | | #2

    Will a dehumidifier even work at 45*F? The shop/garage is rarely occupied...

  3. TJ Elder | | #3

    FYI, to type a degree symbol in the Arial font, hold down Alt and type 0176. Sadly this does not address whether a dehumifier will work at 45°F.

  4. Riversong | | #4

    Is this a new garage with a concrete slab that hasn't fully cured? Is there insulation and VB under the slab, or could it be wicking up moisture?

    Perhaps if he kept his coffee in a covered travel mug rather than an open cup? Or held his breath while in the shop? Or knocked the snow off his boots before coming in?

    I would put in a woodstove and get the place warm when in use. Let the radiant slab just keep it from freezing.


    if the de-humidifier has a humidity setting low enough it seems to me that it should work, it is clear that you have a humidity problem. I think Robert is right to be wondering where the moisture is coming from.

  6. Riversong | | #6

    type a degree symbol in the Arial font, hold down Alt and type 017

    ALT-248 is the universal degree symbol in the IBM ASCII extended character set. It'll work for any PC-based word processor (American Standard Code for Information Interchange).

    But those who weren't suckled on the original DOS-based PCs (or telegraph machines) probably wouldn't know that.

  7. Richard Fontaine | | #7

    I've got an idea regarding the cause of the moisture. I think Robert was on the right track when he talked about knocking snow off his boots before entering the garage. Are vehicles coming in and out of this garage daily? Winnipeg, Canada averages 10,400 (HDD Fht.) and gets plenty of snow. When I park my car in my uninsulated unheated garage the residual heat from the engine is enough to melt some of the snow off the car and run across the floor at temperatures well below freezing. If you park snow encrusted vehicles in your above-freezing garage that could be a huge source of moisture. An unsealed concrete slab could also absorb a lot of water. Can't help on the solution side of this though. Good Luck!

  8. Riversong | | #8

    It may help to seal the concrete with a penetrating sealer.

  9. homedesign | | #9

    The big problem seems to be the incredible amount of ice that forms on the interior of the glass during colder spells

    I suspect that the BIG problem is the incredible amount of liquid water that occurs before and "after" the ice.
    Maybe the interior of the window should be "detailed" in the same way as the outside.
    With moisture tolerant materials and a "SLOPING" sill that drain the water AWAY from the wall.

  10. user-716970 | | #10

    Thanks All
    I suspect that the slab is the moisture source...I don't think that there was any insulation or poly under the floor.
    If the interior temp is raised to 70°F or so, the problem goes away. Makes me wonder what happens inside the walls of the homes of "snowbirds" . They usually crank down the thermostat while they are gone for several months at a time...

  11. Riversong | | #11

    Set-back thermostats, while saving some energy, also cause changes in the indoor RH.

    For instance, one source suggests that "While a set-back thermometer (70° to 63° from 9pm to 7am) might save 5% of heating costs, it will also increase the RH during set-back and increase window condensation (2%) and drywall MC (3%)."

    If you maintain your ski condo at 70° and 40% RH, and then turn the thermostat down to 50° when you leave, you'll raise the RH to 80% - which puts everything into the mold danger zone.

  12. homedesign | | #12

    Garth:I suspect that the slab is the moisture source

    So... If the Slab is "the source" I wonder if "heating" the concrete is such a good idea?

  13. MICHAEL CHANDLER | | #13

    If he can clear the floor off enough to spray acid on it this may be an ideal place to use the
    A-tech Hydra-Block I mentioned in the "seal a slab" response. It's a sodium silicate acid product and would discolor any wood or metal it came near but since it's a shop you could divide the space in half and do one half in the morning, shift everything to the treated half and do the other half in the afternoon.

    Working with acid concrete treatments is seriously hazardous to your health and to any installed woodwork and metals including wood doors . so be forewarned.

  14. aj builder | | #14

    I take care of several homes that are not occupied in the winter here in upstate ny.

    45*f setting in 2 homes the humidity reading is 50ish. No moisture to be found, anywhere. Wood floors are engineered type and still as installed. Painted trim is as installed. No movement or moisture issues. Unvented open cell insulation, sump pumps run in basements also dehumidifiers set at 55% in basements. Basements walls and floor coated with drylok.

    Everytime I find moisture problems there has been an abnormal condition; boiling gallons of water, hours of nonvented showering, cellars that are letting in swimming pools of water, dehumidifiers running way too much, plant rooms that have more plants than greenhouse stores an on and on.

    That garage has a problem with excess moisture, that you are aware of.

    I concur.

  15. user-716970 | | #15

    But you can't see inside the walls where the dewpoint has moved much closer to the interior of the prepared to be sued...

  16. aj builder | | #16

    The walls are fine. It is YOUR building that has issues. LOL

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