Molds are our friends because they play a fundamental role on earth as they scavenge and renew. Molds can sometimes become enemies when they trigger adverse heath effects. Reported potential health effects range from troublesome allergies, to infections, to frightful sounding toxicity. Allergic reactions from mold are well documented. Infectious mold is clearly documented but seldom found. However, documentation of toxicity problems falls short of proof.

It is unavoidable that residential air contains some moisture, and moist air is essential for health and comfort. However, there are several problems associated with excess moisture, and one is mold. Mold can grow when there is excess moisture; if moisture is controlled, mold is controlled.

The four practical steps to avoidance and control of residential mold are:

  1.  Design and build the house properly – as a system. Design and build to keep water out, to avoid condensation, and for easy maintenance. Don’t oversize the air conditioner. Moisture control must dominate design.

  2. Operate the house properly. Operate the heating and air-conditioning system for best dehumidification. Check the refrigerator, washer, dishwasher and other sources.

  3. Design ventilation for good indoor air quality for health, comfort, and moisture control. Choose dependable ventilation equipment and install it with quality ducting and fittings. Vent-free equipment does not ventilate properly. Provide interior circulation paths and appropriate controls.

  4. Operate residential ventilation properly for good indoor air quality and moisture control. Operate the continuous ventilation with weather sensitivity. Use kitchen ventilation while cooking and bathroom ventilation 30 minutes after a shower.



Ventilation Design and Installation

Ventilation can control excess moisture. Good residential ventilation has two interdependent and essential components. First, strong sources of moisture, especially the bathroom and kitchen, must be intermittently ventilated. Second, fresh air for breathing must be provided continuously by mechanical ventilation. The two are interdependent – for the continuous ventilation to be effective at a low, energy conserving, mold preventing rate, intermittent strong sources must be mitigated at the source by higher ventilation rates before they spread throughout the entire house.

Quality installation of ventilation requires that ducting be carefully designed, selected, and installed – lower duct velocity and high quality fittings helps performance and minimizes noise. Terminations (i.e., wall and roof caps) must be of good quality if ventilation equipment is to realize its potential. Duct leakage is almost always a problem, so it is imperative ducting be installed properly.

Fresh air should be provided mechanically to be sure it is in sufficient quantity to provide good indoor air quality. Depending on infiltration or open window to provide ‘accidental’ fresh air will hardly ever provide the right quantity; one day too much, the next too little.

There is a variety of ventilation strategies to choose from – each one is more effective and less costly than natural ventilation because extremes caused by temperatures and wind are avoided. The mechanical ventilation strategies are all based on supplying just the right amount of air and avoiding the accidental high moisture loads inevitable with natural ventilation in humid areas. Excess and “accidental” ventilation can increase the risk of mold.

When the mechanical fresh air system is operating at the proper rate, it optimizes the quality of the indoor air and it requires that intermittent strong sources be controlled locally.

Bathroom ventilation must intermittently mitigate shower moisture, the single largest strong source of contaminating moisture in the home. The bathroom exhaust must be located so that air from the source (e.g., diffuser or undercut door) sweeps across and over the shower and into the exhaust fan. Exhaust capacity must be adequate for the application; the Home Ventilating Institute (HVI) recommends eight air changes per hour. An easy approximation is to provide one cubic foot per minute (cfm) per square foot of bathroom. Extra large bathrooms and split bathrooms may be adequately served with slightly less if properly located. Although ventilation in a separated toilet room can effectively control odor, control of moisture is the important function in the bathroom.

Controls are important for bathroom ventilation. Timers and off-delay switches make a bathroom exhaust much more effective; if the bathroom door is opened immediately after a shower the huge moisture load is simply dumped into the rest of the house. Fans with sensors that detect a rapid rise in humidity can sense operation of a shower, and run the fan until the moisture has been expelled. Controls that sense a rapid rise in humidity are available and can even prevent shower stall mildew.

Kitchen ventilation must control the strong sources of excess moisture associated with cooking. Whether boiling, frying or baking, kitchen ventilation mitigates the moisture load that otherwise would disturb the home’s environment, enabling the full-time ventilation system to maintain air quality and optimize its effectiveness against mold. Range hoods capture and exhaust most efficiently; other types of kitchen ventilation are also effective.

Correct Operation of Residential Ventilation

The full-time low-level ventilation for breathing should be operated continuously, especially when the dwelling is occupied. In the very humid coastal south, where there may be a concern that ventilation is bringing in excess moisture, the continuous ventilation can be shut down during periods when the dew point of incoming air is above a certain point, say 65º F, or lower. The dew point follows a more or less regular pattern from day to day during a season. The dew point can easily be obtained from television and the internet; once the local pattern is understood a daily operating procedure can be followed.

Operation of the bathroom exhaust fan must start at the beginning of every shower and continue afterwards for 20 minutes.

Kitchen ventilation must be used whenever cooking, to control heat and moisture. Although boiling water obviously sends moisture to the air, it is also released when we fry meat and sauté vegetables.

Kitchen ventilation also plays a role in mold prevention by limiting the amount of bio-nutrients contaminating the surfaces of the house.