Garage Moisture Problem

[Keith]

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 appreciated. Thanks

[m Ransley]

Any area below grade may always be a moisture problem.

[Joseph Meehan]

About all you can do is ventilate.

[Edwin Pawlowski]

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.

[RICARDO AQUINO]

Install a good de-humidifier.

Rich

[Norminn]

[nicksanspam]

Which probably slows water vapor or liquid wicking up from below.

Sounds like that will make it worse.

Edw>The warm air hits the cold surfaces and the moisture condenses on the surface.

For instance, the cold slab. But where does the warm air come from?

IMO, it's a vapor barrier problem. You might fix it by making the garage more airtight with lots of caulk and poly film under the new drywall, carefully taped with Tyvek tape at the film joints.

Nick

[Joseph Meehan]

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.

[AutoTracer]

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 problem.

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 controlled fan).

Also avoid venting a clothes drier into the area on the cold days and nights.

[nicksanspam]

That will make it worse, IMO. Cold slab + damp outdoor air = condensation. With no damp air flowing in, there would be no condensation, beyond a tiny amount from the unchanging air in the room.

Nick

[Joseph Meehan]

That will depend on the situation. I would hazard to guess that ventilation will help in more cases than it will hurt.

[nicksanspam]

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...

Nick

[Norminn]

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.

[AutoTracer]

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.