Reducing condensation requires a careful plan for home and garage

By Tim Carter
Friday, February 4, 2011; 10:29 AM

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 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. Inside our home, we are also getting condensation on the windows closest to the garage. What's wrong? What can we do to stop the window condensation, the frost on the garage ceiling and the water that's running down the garage walls? - Regina W., Alexandria, Minn.

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

There are several things contributing to your 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, which will provide lots of air movement through the attic of the garage. Also, 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 snow melting off your cars and by the propane heater needs to exit the attic space before it can condense on the underside of the roof sheathing. Turbine vents will do a great job at that.

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 to that all the melted snow from your cars and you can see that you've got a lot of water that needs to go someplace.

The vapor barriers you installed on the walls and ceiling keep 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 each night.

You also need to limit the use of that propane heater. 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 cars 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 and a humidifier set too high 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.

Tim Carter is a columnist for Tribune Media Services. He can be contacted through his Web site, www.askthebuilder.com.

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