Q&A / 

Condensation Problems

DEAR TIM: My husband and I have huge condensation problems in our garage and new home. The garage was unfinished by the builder, but we just added wall and ceiling insulation and vapor barriers on the walls and stapled to the underside of the garage ceiling. We then covered the walls with plywood and the ceiling with drywall. A vent-free propane heater helps keep the garage somewhat warm. We are also getting condensation on windows inside our home that are close to the garage. What’s wrong? What can we do to stop the window condensation as well as the frost on the garage ceiling and the water that’s running down the garage walls? Regina W., Alexandria, MN

DEAR REGINA: Oh my. You’ve got major problems indeed. Preventing condensation is not as easy as one might think, especially where you live in frigid Minnesota.

There are several dynamics happening that are contributing to your massive garage condensation issue. First, you made an enormous mistake installing a vapor barrier on the ceiling of the garage.

You need to immediately crawl up into the attic space above the garage, move the insulation out of the way and cut out the plastic vapor barrier from between each of the trusses. You can replace the insulation once you get the plastic out of the way.

Be sure you have plenty of roof ventilation, I suggest several turbine roof vents, to adequately provide lots of air movement through the attic of the garage. Be sure you have plenty of soffit ventilation so cold, dry air can be brought up into the attic space as the turbines exhaust the water vapor.

The water vapor generated by the melting snow off your cars and that created by the propane heater needs to exit the attic space before it can condense on the underside of the roof sheathing. Turbine vents moving in the wind up in Minnesota will do a fantastic job.

Cars can bring in snow that melts into gallons of water packed onto the undercarriage of the vehicle. This liquid water easily condenses on the cold surfaces of the garage. PHOTO CREDIT: Tim Carter

Cars can bring in snow that melts into gallons of water packed onto the undercarriage of the vehicle. This liquid water easily condenses on the cold surfaces of the garage. PHOTO CREDIT: Tim Carter

Keep in mind that the vent-free propane heater you’re using is one of the culprits in this case. When you burn any fossil fuel like propane, water vapor is a by-product of the combustion process. You’re pumping lots of water vapor into the air inside your garage when the heater is operating. Add that to all the meltwater that undoubtedly comes off your cars, and you can see that this water needs someplace to go.

The vapor barriers you installed on the walls and ceiling are keeping the water vapor trapped inside the garage. You want a vapor barrier on the walls, but not the ceiling. The water vapor will readily pass through the ceiling drywall and the insulation on its way outdoors.

To minimize condensation in your garage, and I suspect some of the water vapor from the garage is getting into your main living area, you need to cut down on the water that’s in the garage. It’s not easy to do, but try to get as much snow off the underside and out of the wheel wells of your car before you park it inside each night.

Limit the use of the propane heater. When it’s on, it’s pumping water into the air of the garage. My guess is that you want to heat the garage just to keep the cars warm. If that’s the case, I can tell you that they don’t mind being cold.

The condensation you’re experiencing inside your home may lessen considerably as soon as you allow the water in the garage to escape through the garage attic and then to the outdoors through roof vents.

If the condensation in the house does not abate, then look for probable sources of water. Lots of houseplants, excessive cooking where you’re boiling water, allowing laundry to dry indoors, a humidifier set too high, etc. are all contributing factors to elevated indoor humidity.

Remember that condensation is a moving target. As the outdoor temperature drops making window surfaces colder, water vapor in the air will condense on the glass that much faster and in greater volumes.

To eliminate condensation on surfaces you have three choices. You can do all of them or just pick one. These all come with a price tag.

First, you need to minimize or eliminate the source of the moisture. That’s not always as easy as it seems.

Second, you can raise the temperature of the surface where the condensation forms above the dew point of the humid air. This prevents condensation from forming on the surface. This option can be very expensive as you’re burning lots of fuel.

Third, you can set up a fan to blow air across the surface where the condensation is forming. This air movement will often prevent the condensation from forming because it just puts the water vapor back into the air until such time as it can’t hold any more.

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One Response to Condensation Problems

  1. I have somewhat of the same issue. I had a new garage door installed and the garage is so sealed, the door gets horrible frost and ice on the inside of it. I too heat with a ventless Nat. gas heater and try to maintain a decent temp. My window would also freeze. I have a dehumidifier now and hope that cuts down on the water in the air. I plan on putting in a vented gas heater sometime in the near future. Like Regina, my garage is finished as well. I did notice black mold. But cure most of it with my pressure washer. Next Spring I plan on pulling everything out of the garage and cleaning it thourghly. I would like some more feedback on vented vs non vented heaters.

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