Control Humidity to Control Mold Dust Mites

Claudette Reichel  |  6/25/2008 1:25:38 AM

Figure 1, Dehumidifying Ventilation System

Figure 2, Fresh Air Supply Integrated with HVAC System

Figure 3, Whole House Dehumidifier Schematic and Portable Dehumidifier under Duct

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 that increases risk of moisture and hidden mold problems. A positive air pressure reduces infiltration. (See Building Science – Air Basics section).

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 (cubic feet of air per minute) of outdoor air.

Right Size 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) and when it doesn’t run because cooling is not 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 SHR 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 SHR) – 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 temperatures, resulting in not enough dehumidification and higher-than-desired indoor humidity. Research has shown the best solution is not to sacrifice energy efficiency, but to add dehumidification [1]. Alternative solutions include:

Dehumidifying ventilation systems: Spot exhaust fans and a separate ventilation system supply filtered and dehumidified fresh air to an airtight home at a controlled rate is the optimal solution for air quality and comfort. It does it all — brings in clean, fresh, dry air through a duct instead of gaps, dehumidifies during mild temperature seasons when the A/C doesn’t run enough and maintains a slight positive air pressure. An energy-efficient home ventilating dehumidifier may cost $1,500-$2,000 but can be a good investment in health, comfort and moisture control, particularly for households with allergies or high moisture loads (30-50 liters per day).  (Figure 1, Dehumidifying Ventilation System)

An enthalpy energy recovery ventilator: An ERV is a fresh air supply and exhaust system that exchanges heat and moisture between the incoming and outgoing air. The heat exchange has energy-saving benefits, but the level of dehumidification provided is less, so may not be sufficient and need supplementation. Its cost is similar or slightly lower than a dehumidifying ventilation system

Fresh air supply to AH + Dehumidifier: A dampered and filtered fresh air duct can be added inexpensively to the return side of the central HVAC air handler (AH) with a specialized flow controller. The controller activates the AH blower a few minutes periodically to draw in and mix fresh air. To prevent moisture problems, it’s important that the flow controller have a time-delay feature that does not run the blower for a set time after a cooling cycle, to allow time for the coils to drain and duct warming. (Figure 2, Fresh Air Supply Integrated with HVAC system)

Since some of the fresh air drawn in is not conditioned, this system should be combined with supplemental dehumidification:

A separate whole-house, high capacity dehumidifier unit may be installed near the air handler, yet with a humidistat in the living space. It can be installed to draw and dehumidify house air when it reaches your set RH level, then distribute the dryer air through the supply duct system. This option is comparable in effectiveness and energy efficiency and similar or slightly lower cost than the dehumidifying ventilation system described above. (Figure 3, Whole House Dehumidifier Schematic and Portable Dehumidifier under Duct)

When dehumidification needs are modest (10-25 liters per day), a standard capacity dehumidifier placed in a closet with louvered doors or beneath the air return plenum (with a drain line) is an inexpensive ($200-$300), practical solution for many. Although a low-cost solution, portable standard dehumidifiers are less energy efficient (so cost more to operate) and produce noise and heat when operating. Choose an ENERGY STAR labeled model.

Variable capacity air conditioner systems operate at different speeds and capacities to more closely match the home’s cooling load. As a result, they run longer and dehumidify more effectively than single speed units. They are typically energy efficient with high SEER ratings, but may cost up to 30% more than single speed units.

[1] Building America Systems Engineering Approach to Development of Advanced Residential Buildings, 10. Case Study Material, Betsy Pettit, AIA, November 15, 2002

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