 | | Duct Leaks and Negative Pressure (Figure 1). |
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 | | Leaky Return Air Plenum (Figure 2). |
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Biological air pollutants affect the health of more people than any other indoor pollutant. Molds can lead to health effects ranging from allergic reactions and asthma to suppression of immunity and neurological effects, depending on the type of mold, the level of exposure (to mold and/or toxins) and the sensitivity of the person affected. Dust mites can cause and worsen asthma and allergies. Insect and rodent residue, pollen and pet animal dander are allergens.
Effective control of biological pollutants requires both moisture and air infiltration control. In addition to keeping building assemblies from staying wet, indoor relative humidity (RH) must be controlled. Maintaining an indoor RH below 60% prevents surface mold. Dust mites cannot reproduce if RH is below 50%. For health and comfort, maintain an indoor RH between 40% and 60%, or below 50% if family members suffer from asthma or dust mite allergies.
In a humid climate, this requires a means of dehumidification. It is best achieved with a system that includes both sufficient dehumidification, even during mild temperature seasons, and controlled, filtered fresh air (ventilation) to maintain good indoor air quality and a slight positive air pressure in the home. A negative pressure (suction) increases unwanted infiltration of humid air through the building assembly which increases risk of moisture and hidden mold problems. A positive air pressure reduces infiltration.
Ventilation and Dehumidification: Fresh air ventilation in a building is essential. Humidity and air quality are much easier to control in a tight house with controlled, filtered ventilation. Leaky homes tend to have inconsistent and inappropriate levels of “natural” ventilation and humidity.
Mechanical ventilation systems are recommended for all houses. The amount of ventilation needed depends on the number of occupants, their lifestyle and the type of home. ASHRAE Standard 62.2 (Ventilation for Acceptable Indoor Air Quality in Low-Rise Residential Buildings) recommends a ventilation rate of 7.5 cfm/person + 0.01 cfm/ft2 of conditioned floor area. For a typical single-family detached house (3 bedrooms; 2,000 sq.ft.) this works out to 50 cfm of outdoor air (cfm is cubic feet of air per minute, a measure of air flow).
Right Sized A/C: An air conditioner both cools and removes moisture from the air, but it cannot dehumidify sufficiently if it is oversized (too big), has too little latent capacity (ability to remove moisture) or doesn’t run because no cooling is needed. Make sure that your air conditioner is correctly sized with a Manual J calculation that takes into account the details of your house design (NOT by a square footage rule of thumb). Check that the system has an SHF of 0.70 or less, meaning that at least 30% of its capacity is for dehumidification. Note that many high-SEER systems have less dehumidification capacity (higher SHF) – an unwise trade-off in this climate.
IMPORTANT FACT: Energy-efficient homes in a humid climate, especially when ductwork is well sealed or within the conditioned space, need little or no cooling during periods of mild temperature, resulting in not enough dehumidification and higher-than-desired indoor humidity. The best solution is not to sacrifice energy efficiency but to add dehumidification. See Reducing Humidity in an Energy-Efficient Home.
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 and 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. (See Figure 1.) 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. (See Figure 2.) This is a recipe for growing mold. Returns should be ducted or thoroughly sealed air tight.
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, 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 leaners: 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 non-absorbent, so they may maintain lower humidity levels that are less hospitable to dust mites.
A central vacuum system and portable vacuum cleaners with a 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 reducing 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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