Invisible Peril

Good Venting

Is that the answer? Should we all be rolling our cars out of our garages? Steve Klossner has tested three other ideas: passive ventilation up high — installing roof vents in the garage attic; passive ventilation down low — replacing two panels in the garage door with louvers; and mechanical ventilation — placing a simple 150-cfm tube fan so it pushes air from the garage ceiling out a vent in an exterior wall, just like a clothes dryer or powder room would be vented.

Klossner concluded louvers didn't work. When he compared the other options on the test house with 85 ppm in a child's bedroom, the $100 roof-vent job and the $250 tube-fan fix worked equally well. Both brought the house's carbon monoxide reading below 8 ppm. But roof vents are an option only on houses with a garage open clear to the rafters; houses with a ceiling in the garage need a fan. The fan can be connected to a switch and a timer, or left running (at a cost of about 12 cents a day where electric rates are 6 cents per kilowatt hour).

To see how such a fan might be installed, This Old House magazine asked Klossner and Greiner to fix the Gubbels residence too. There, ironically, the job came to more than $2,000 — complicated by the fact that because Jim has worked so hard to insulate, weather-strip, caulk and replace windows, the house is very tight. Greiner had measured 0.17 air changes per hour, whereas most houses undergo nearly half an air change each hour.

Greiner and Klossner worried that installing just a roof vent or tube fan in the garage might rob the house of the makeup air it needs. When a house can't get enough makeup air, bad things happen: A fireplace or clothes dryer might suck air from the furnace or water heater flue, causing a backdraft of combustion fumes (including carbon monoxide) into the house. Nine of the 50 homes in the Minnegasco study had backdraft problems, and in most of those nine the backdrafting produced a burst of carbon monoxide exceeding 4,000 ppm at the source.

So, much to Jim's initial dismay, a six-inch hole was cut in the side of his house to provide makeup air for both the furnace and water heater. A powered vent was installed on the water heater. In the garage, two ceiling vents were ducted to a fan that pushes air through a roof vent.

When all the holes, ducts and fans were in place, Klossner and Greiner again filled the garage with theatrical smoke. It was soon billowing above the roof — but this time none came into the house. The fan, set at 300 cfm, provided just enough pull to change the air dynamics of the house. The garage is still attached to the house, but in terms of air flow, it might as well be halfway down the block. Jim even cracked the door to the house several inches; the smoke stayed in place. "It was really impressive," he says. "The smoke was like a wall."

Most homes do not require such an elaborate solution. Regardless, every home should have a carbon monoxide detector with a digital display — ideally, one on each level. If you see readings well above 10 or 20 parts per million, call a heating contractor or your local gas company to have every combustible appliance checked out. If all other causes are ruled out but high readings persist, try starting the car out of the garage. Or vent your garage by opening a window. Still having a problem? Then call in a specialist in building diagnostics.

Another winter is on its way — and as usual, hundreds of Americans will die from carbon monoxide poisoning. Thousands more will suffer from its effects, but not those who understand the danger that lurks in their garages.

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