Figure 1. Air Pressure Imbalance. Reproduced with permission from Builder's Guide: Hot-Humid Climates by Joe Lstiburek, Building Science Corporation

Figure 2. Air Grille with Sound Baffle. Reproduced with permission from Builder's Guide: Hot-Humid Climates by Joe Lstiburek, Building Science Corporation.

Burning creates several combustion pollutants, including carbon monoxide (CO). In high levels, CO is deadly, but low levels are also harmful. Every source of combustion in the home -- space heaters, gas fireplaces, gas cooktops, gas water heaters, etc. -- should be vented to the outdoors. If there is any fuel burning in the home or an attached garage, a carbon monoxide detector with a continual read-out and alarm should be installed.

Vent-free gas fireplaces are not recommended indoors. Although they have safeguards to prevent CO poisoning, burning any fuel still generates unwanted moisture and combustion pollutants.

Garage isolation:  An enclosed garage that shares a wall or attic with living space can be a serious source of CO entry by infiltration of residual car exhaust. A garage should be detached, or shared walls must be fully air sealed and have tightly weather-stripped doors. A garage attic should be completely isolated from the home attic with a tightly sealed kneewall.

Protection from backdrafting:  Turning on exhaust fans or any other cause of negative pressure can create backdrafting (reverse airflow) in chimneys and flues, drawing combustion pollutants into the home. Fuel-burning equipment located inside the conditioned space should be direct-vent, sealed combustion (best) or isolated from conditioned space in a sealed closet or at least forced-draft (with a power vent).

• Direct-vent, sealed combustion appliances have both a fresh-air intake from outdoors to the burner and an exhaust vent to the outdoors. They do not use the surrounding indoor air for combustion, so they eliminate risk of backdrafting. Direct-vent gas furnaces, water heaters and even fireplaces are available. Most direct-vent gas furnaces are also high-efficiency (AFUE >.90), but lower-cost, mid-efficiency models are manufactured.
• Return air options: When a home has only one central air return in a common area, closing the door of a room can create a pressure imbalance. A closed bedroom becomes positive, the living area becomes negative, and a smoldering open fireplace can backdraft into the central air system. Even without backdrafting, pressure imbalances create other problems. Provide each bedroom with an air return, such as:
• through-the-wall (or -door) grilles with a sound-and-light baffle (best)
• a “jump duct” through the ceilings from the bedroom to the common area
• an open transom above the door
• a return duct from every bedroom to the air handler unit
• large enough clearance between finished flooring and door to equal the size of the supply air register (not generally acceptable). (See Figure 1.)
• Laundry room: Since a clothes dryer pulls a great deal of air from the home, it’s best for the laundry room to have its own source of make-up air. Having a fresh air inlet (filtered, passive air inlet or dampered inlet activated by the dryer operation) eliminates the need for the homeowner to remember to open and close a window each time the dryer is used. (See Figure 2.)

Related Article

• Control of Other Indoor Air Pollutants

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