I have a 2 car garage where the slab is actually below grade. It has 2 -
8inch high block foundation where the floor would be actually one block
below grade. The floor has been painted and a few walls are drywalled. The
problem is when the temp outside is considerably warmer than the floor temp
moisture appears on the floor. On those strange days which we have here in
Illinois where the temp will be 30* one day and 70* the next the whole
garage is damp with moisture on everything. I want to insulate and drywall
and somehow fix the moisture problem. Do I need a vapor barrier and if so
where does it go and any suggestions for the floor moisture would be
Insulation and a vapor barrier will be of some help, but you have to
realize where the moisture is coming from.
The warm air hits the cold surfaces and the moisture condenses on the
surface. It has nothing to do with insulation or vapor barriers. They only
help if it keeps the temperature up higher so the differential is less than
it would be otherwise. The slab would have had to be insulated before the
concrete was poured.
Ventilation helps. Heat helps as long as the source of the heat does not add
tot he problem. Burning gas gives off water as a product of combustion and
makes the condensation problem worse.
The way to determine if the problem is moisture coming in from the
floor, or through the wall of from the air is to tape some plastic to the
floor and wall. Moisture UNDER the plastic means the moisture is coming in
that way, you may find both floor and wall. Moisture on the side towards
the room means the air is too moist.
How is drainage around the perimeter? Downspouts spill right next to
wall? Ground slopes away from wall? Correcting those issues and
ventillating the garage may fix the problem. Open it up on warm days to
help forestall the condensation forming because the stuff in garage is cold.
Assuming your description is accurate then the moisture is condensation.
Ventalation will help by allowing the garage to warm up faster when the day
does but insulating the walls will probably be a better solution because it
will keep the slab from getting so darn cold in the first place. It will
also make the space more comfortable on cold days and make it more
appropriate for storing things such as paint etc.. You can also use a space
heater to keep the garage from getting too cold (dosen't need to be warm) I
have seen many with anti-freeze settings that come on at 32F or below. The
trick is to try to keep the slab as warm as or warmer than the dew point on
that particular day.
Being below grade does not necessarily play into the moisture problem unless
(as joseph suggested with the plastic sheet experiment) you can prove that
it is coming from below. I doubt it because extranious water should be
frozen during this time of year and not too abundant for causing this
Being below grade probably acts as a blanket helping to keep the slab cold
longer than is typical. It also increases the thermal mass of the slab
making it slower to change temperature.
A combination of insulating to prevent the coldest temps from entering the
garage and ventalation when the temp begins to rise (get a thermostatically
Also avoid venting a clothes drier into the area on the cold days and
You may be right, if it's large and deliberate, so the slab condenses
water one day and evaporates the next (concrete acts as a hygrostat),
but a fan with a "differential humidistat" automatically waiting for
a dry day would use a lot less energy than a continuous fan. And the OP
mentioned insulating the garage. How would you heat or cool a garage
with a continuous fan? :-)
I measured a new efficient dehumidifier which consumed 0.5 kWh per pound
of water, eg 15 kWh for 30 pints/day, ie $1.50/day at 10 cents/kWh.
If a 70 F basement has 60% RH and humidity ratio w = 0.00947 pounds of water
per pound of dry air and outdoor air with a 50 F dewpoint has w = 0.00787,
a 90 W 2470 cfm window fan running 30/(24x60x2470x0.075(0.00947-0.00787))
= 1.7 hours per day could remove 30 pints/day at a cost of 10x1.7x90/1000
= 1.5 cents/day, 100 times cheaper than the dehumidifier. It would be nice
to have an automatic way to control the fan...
There are a number of issues that might cause problems, but we do not
know because we are not there. A perimeter of concrete block with a lot
of water draining along it would almost have to leak water into the
garage. Whether rain or snow melt, if it drains right against the black
foundation it would almost have to allow more moisture into the garage
than correct drainage would cause. If not properly ventillated or used
often, the moisture would remain. A concrete slab will always have
moisture unless it has a membrane beneath it, but snow piles or the
melting runoff would likely accumulate along the block perimeter. The
family car, if loaded with snow, would be another source.
The OP mentioned that the moisture was on everything further supporting the
condensation on a cold thermal mass theory. Ventalation dosen't make sense
continuously. I suggest it mainly as a way to warm up the garage interior
when the outside temp is rising. Insulating and lightly heating the space
would relieve the need for ventalation but you don't want a garage too
tight, got to let the exhaust and other smells out.
Furthermore, there may be an air leak from the house to the garage which
lets excess warm moist air to enter the space. He may only need to insulate
the hopuse wall and caulk.
If the problem is worst when there is dew outside it supports simple
condensation. If there is no dew outside but the garage is wet, it supports
an air leak from the house as the source of water vapor.
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