The garage-to-house issue: controlling the dangers of carbon monoxide

To fear the worst oft cures the worst.
- Shakespeare. Troilus and Cressida

It wasn't raining when Noah built the ark.
- Howard Ruff

There's bad stuff in the air inside most garages. That air pollution comes from cars, lawn mowers, paints and solvents. and many other things we store in garages to isolate related pollutants from the house. Unfortunately, that air pollution is prone to being drawn into the adjacent hone.

The perfect cure is to build detached garages. a solution that adds cost, doesn't win votes from buyers, and isn't likely in our lifetimes.

Fortunately, it's not that hard to substantially reduce the risk of this problem with attached garages. But first, it helps to know a little bit more about the problem.

THE CARBON MONOXIDE

The carbon monoxide big culprit is our cars.. Plus differences in air pressure between the house and garage. Problems are worst during cold weather. Here's how that works.

On winter mornings, homeowners start their cars and often let them warm up in the garage before backing out. At startup. cars generate massive amounts of CO. Once a car is backed out and the garage door closed. CO concentrations of several hundred parts per million (ppm) arc often trapped within the garage. (The EPA doesn't publish a standard for indoor air, but the U.S. National Ambient Air Quality Standards states that 9 ppm in outdoor air over an eight-hour period is the accepted limit.)

CO in the garage moves indoors in two ways. First, by stack effect: warm air leaking out high holes (e.g., into the attic) is replaced by air drawn in through the lowest holes in the home (holes in the garage-to-house foundation connection). Second. negative pressure in basements, caused by leaky return-air ducts in the basement, can draw garage air indoors whenever the air-handler is operating. Additionally, any return-air ducts in contact with the garage can draw air in wherever they are located.

How commonplace is this problem? Studies indicate it's fairly widespread.

MINNESOTA STUDY

During the 1996-97 heating season. Minnegasco, Minneapolis' gas utility. was hit with more than 16,000 calls from carbon monoxide detectors going off. Because the utility couldn't identify the source of the problem in most homes. they hired a consultant to solve their mystery. The consultant's answer surprised most people: in 74 percent of the homes with unexplained carbon monoxide problems, the source was garage air leaking into homes after the daily car startup.

While cars were generating carbon monoxide, the study firmly established that holes between the house and garage were the ultimate culprits. The testing team found that, on average, 24 percent of each home's total air leakage to the outdoors was entering the home through building surfaces (walls and floors) separating the house and garage.

After homeowners started their cars. backed out and closed their garage doors. concentrations of carbon monoxide within the garage varied between 150 and 1,500 ppm. Use of data loggers in several homes showed that it took an average of five hours for the CO level within a garage to return to zero. During that time, much of the carbon monoxide in the garage air leaked through holes in the adjacent walls and ceilings. CO levels inside the home typically peaked 90 minutes after a car was started in and pulled out of the garage.

OTHER STUDIES

During the late 1990s Iowa's Department of Public Health, working with a large utility, conducted a residential carbon monoxide study. That study determined that out of 5,017 homes checked for carbon monoxide. 1,012 of them (20 percent) had CO levels greater than 20 ppm. Where did the CO come from? Garages were a prime suspect.

The Energy Design Update newsletter cited another late 1990s study in Alaska. Again, attached garages were fingered as the source of unhealthy levels of carbon monoxide plus other pollutants. Though their in-depth study included just a small number of houses, study author John Freeman found that CO from garages entered 1 1 out of 12 houses. Thee houses varied in style: one-story and two-story, large and small, with surface areas shared with the house ranging from 164 to 772 ft=. The most disturbing findings in the Alaskan Study:

• In all but one case, the house was exerting negative pressure on the garage, meaning that it was drawing in polluted air through the connecting building elements. A particularly relevant quote from Freeman: "We also found that CO transfer was particularly high where furnace ductwork was located in the garage, because leaks in the ducts provide a ready way for CO to get into the house."
• In 10 of the 12 homes, the amount of air in the garage that was drawn into the adjacent home varied between 12 percent and 73 percent.
• More CO moved from garages to houses when there were livings spaces above the garages.

HERE IN COLORADO

Is this just a problem found in other states? Unfortunately, the data gathered over the last five years by Rob deKieffer in Fort Collins and E-Star statewide indicate that the garage-to-house connection is just as much of a concern here as in the states cited above.

Consider two houses tested by this writer last month in Pueblo one ranch and one two-story (no living space above the garage). The best way to describe the technical pressure measurements might be to use an analogy: the garages were "22 percent and 24 percent indoors" and "78 percent and 76 percent outdoors." (A garage completely sealed off from a house would be "100 percent outdoors.") And in the worst home tested by deKieffer. the garage tested "half indoors" - a really unfortunate situation.

Garages in Colorado need a cure. The good news is that some builders have already moved in this direction. Most of the details they use come from the following list.

RECOMMENDATIONS

Beyond the best - but impractical - option of detaching garages, consider some package of these items.

1. Aerodynamically. decouple the house from the garage. In other words, seal the hell out of the connecting building elements. Focus on the obvious:
   • seal the joints between the mud sill and rim joist at the foundation line of the garage-to-house wall (Engle Homes found a large peal-and-stick product, much like window flashing, that does the trick here), and seal the bottom of the drywall where it meets framing or foundation elements:
   • seal the bottoms of any cantilevered elements that project into the garage;
   • seal any plumbing, wiring or other penetrations in drywall between the house and garage: and
   • make sure the weather-stripping and the adjustable threshold on the door between the house and garage fit tightly.
2. Build ductwork approximately 10 times tighter than average current practice. Focus on reducing return-air duct leaks by eliminating use of building cavities for return-air ducts. Then seal all ductwork in the home with mastic. particularly any ducts running through walls or floors separating house from garage. (This has many other benefits: increasing comfort for homeowners, reducing callbacks, improving building durability, etc.)
3. To speed up the exhausting of pollutants to the outdoors, ventilate garages with either passive or active vents. A passive vent must he open to the outdoors 24/7. An active vent consists of a fan on a controller, usually linked to operation of the garage door. that runs 15 to 30 minutes for each opening of the garage door.
4. Equip every home with a carbon monoxide sensor. While this technology is still evolving, a rash of carbon monoxide poisonings lead the U.S. Consumer Products Safety Commission to recommend that every home should have at least one CO alarm, and that it should meet the requirements of the most recent Underwriters Laboratories 2034 standard or International Approval Services 6-96 standard.
5. Have someone test at least some of your houses to make sure your efforts to isolate garage pollutants from the house are working. This is typically done when a home is blower-door tested: with the blower door operating, a pressure gauge and a tube can tell you in one minute whether you have a problem.
6. Inform homeowners. They need to know why letting their cars idle in their attached garages is a no-no. And urge them to keep pollutant sources out of their garages, not just their homes.

Once this problem is understood, some of these details, like the air-sealing to isolate the garage, don't cost that much or take that long to do them properly.

Steve Andrews consults with builders for E-Star Colorado and writes on energy issues (sbandrews@att.net). E-Star (www.e-star.com), is a nonprofit home energy rating system that works with both new and existing homes statewide.

2008 Built Green Colorado

Home Builders Association of Metro Denver, 9033 E. Easter Place, Suite 200, Centennial, CO 80112
(303) 778-1400 fax: (303) 733-9440  info@builtgreen.org

Last Updated: 10/05/2007