Clearing the Air for a Healthy Home

Claudette Reichel  |  6/25/2008 1:21:29 AM

Figure 1, Duct Leaks and Negative Pressure

Figure 2, Leaky Return Air Plenum

Exhaust fans: Separate spot exhaust fans in the bathrooms and a kitchen hood exhaust fan are important to remove wet air from bathing and cooking activities; even with a supplemental dehumidification system. A central, constant-run exhaust system is not recommended in this climate since it would create a constant negative pressure in the home. Spot exhausts cause only temporary negative pressures when and where needed to remove excessive moisture. Install quiet fans to encourage use when needed, with a timer or humidistat, so they don’t stay on when not needed.

Duct leaks and IAQ: Unless they are within the conditioned space, supply duct leaks (in vented attics or crawl spaces) cause a negative pressure imbalance in the home. Ducts and their connections to air registers and equipment should be sealed with mastic, NOT duct tape. A leaky return plenum (path from the filter to the air handler) draws dirty, hot, humid air from the attic or wall cavity into the HVAC system. This is a recipe for growing mold. Returns should be ducted or thoroughly sealed air tight.  (Figure 1, Duct Leaks and Negative Pressure)  (Figure 2, Leaky Return Air Plenum)

No attic power vent: Powered attic vents are not recommended. They can remove conditioned air from a home through ceiling leaks and bypasses (increasing air infiltration), pull pollutants from the crawlspace into a home, and cause exhaust gases from fireplaces and combustion appliances to enter the home. In addition, the cost of electricity to run them can exceed the savings from reduced attic temperatures. Soffit and ridge vent attic ventilation systems are preferred. With any attic venting system, the ceiling should be tightly air sealed, ventilation space above insulation ensure (with baffles, if needed) and ample soffit vents should be installed under overhangs and maintained to fully accommodate the air flow induced by the exit vents.

HVAC filter: Pleated filters provide more filter surface area and generally capture smaller particles than standard filters. There are several filter efficiency rating systems. To reduce small dust particles in the air, look for a MERV 7 or higher (Minimum Efficiency Reporting Value ranges from 1-20) or a Dust Spot Efficiency of 25% or higher. To affect allergies, choose a filter rated MERV 10-12. However, keep in mind that filters clean only the air that goes through them, so source control is first priority in maintaining good indoor air quality.

Furnishings and vacuum cleaners: Dust mites thrive in carpeting and upholstered chairs because they collect food (human skin cells) and moisture and are difficult to clean thoroughly. Selecting smooth, cleanable floorings, furniture and washable rugs is recommended for people with severe allergies. Although more research is needed, a study at Virginia Tech University found lower dust mite populations in treated wool (for moth resistance) and olefin (often used in Berber) carpeting compared to other fibers. Olefin fibers are virtually nonabsorbent, so may maintain lower humidity levels less hospitable to dust mites.

A central vacuum system and portable vacuum cleaners with an HEPA (high efficiency particulate air) filter capture small particles and allergens more effectively than standard vacuum cleaners. A central vacuum system collects duct and exhausts outdoors, but temporarily causes a high negative pressure in the home. Providing make-up air (such as opening a window) is a good idea when operating the central vacuum.

Source Control: Be sure nothing in your home will hold standing water; specify sloped drain pans and clog resistant drains; avoid indoor water features. A well-sealed house (with mechanical fresh air) has the added benefit of eliminating insect and rodent entry gaps and reduces the entry of outdoor pollen.

Design your home so all entries have space for a large outdoor doormat and a large washable rug just inside the doorway. This helps trap both biological and other pollutants from being tracked in. Also plan safe, outdoor space for pets, especially if anyone in the household has pet dander allergies; or, at least locate bedrooms so a pet gate can be used to keep pets out of the bedroom area.

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