Controlling Moisture, Preventing Mold

Mold intrusion caused by open window and moist draperies

Moisture is a major cause for mold growth. When moisture is controlled, mold can usually be prevented.

How to Identify the Cause of a Mold and Mildew Problem

Mold and mildew are commonly found on the exterior wall surfaces of corner rooms in heating climate locations. 

An exposed corner room is likely to be significantly colder than adjoining rooms, so that it has a higher relative humidity (RH) than other rooms at the same water vapor pressure. 

If mold and mildew growth are found in a corner room, then relative humidity next to the room surfaces is above 70%. 

However, is the RH above 70% at the surfaces because the room is too cold or because there is too much moisture present (high water vapor pressure)?

There are many ways to control moisture in your home:

Fix leaks and seepage.
If water is entering the house from the outside, your options range from simple landscaping to extensive excavation and waterproofing. (The ground should slope away from the house.)
Water in the basement can result from the lack of gutters or a water flow toward the house.
Water leaks in pipes or around tubs and sinks can provide a place for biological pollutants to grow.
Put a plastic cover over dirt in crawlspaces to prevent moisture from coming in from the ground.

EPA guidelines to Mold Prevention and Treatment

Moisture control is the key to mold control, the Moisture Control Section from Biological Pollutants in Your Home follows:

Moisture Control

> Water in your home can come from many sources.
> Water can enter your home by leaking or by seeping through basement floors.
> Showers or even cooking can add moisture to the air in your home.
> The amount of moisture that the air in your home can hold depends on the temperature of the air.

Moisture On Windows

Your humidistat is set too high if excessive moisture collects on windows and other cold surfaces.

Excess humidity for a prolonged time can damage walls especially when outdoor air temperatures are very low.

Excess moisture condenses on window glass because the glass is cold.

Other sources of excess moisture besides overuse of a humidifier may be long showers, running water for other uses, boiling or steaming in cooking, plants, and drying clothes indoors.

A tight, energy efficient house holds more moisture inside;

you may need to run a kitchen or bath ventilating fan sometimes, or open a window briefly.

Storm windows and caulking around windows keep the interior glass warmer and reduce condensation of moisture there.

Humidifiers are not recommended for use in buildings without proper vapor barriers because of potential damage from moisture buildup.

Consult a building contractor to determine the adequacy of the vapor barrier in your house.

Use a humidity indicator to measure the relative humidity in your house.

The American Society of Heating and Air Conditioning Engineers (ASHRAE) recommends these maximum indoor humidity levels.

Outdoor Recommended Indoor Temperature Relative Humidity

Source:  Anne Field, Extension Specialist, Emeritus, with reference from the Association for Home Appliance Manufacturers (www.aham.org).

In the second case, assume that the RH is 50% and the temperature is 70oF in the middle of the room. 

The higher RH at that temperature indicates that the water vapor pressure is high and there is a relatively large amount of moisture in the air. 

The high surface RH is probably due to air that is "too moist." 

Humidity is the dominating factor, and control strategies should involve decreasing the moisture content of the indoor air.

EPA guidelines to Mold Prevention and Treatment

The amount of moisture in the room can be estimated by measuring both temperature and RH at the same location and at the same time. 

Suppose there are two cases.  In the first case, assume that the RH is 30% and the temperature is 70oF in the middle of the room. 

The low RH at that temperature indicates that the water vapor pressure (or absolute humidity) is low. 

The high surface RH is probably due to room surfaces that are "too cold." 

Temperature is the dominating factor, and control strategies should involve increasing the temperature at cold room surfaces.

Moisture and Mold Prevention and Control Tips

Keep indoor humidity low; if possible, below 60 percent (ideally between 30 and 50 percent) relative humidity. 

Relative humidity can be measured with a moisture or humidity meter, a small, inexpensive ($10-$50) instrument available at many hardware stores.  
If you see condensation or moisture collecting on windows, walls or pipes ACT QUICKLY to dry the wet surface and reduce the moisture/water source. 

Condensation can be a sign of high humidity.

See the following action steps on how to reduce humidity.

Moisture and Mold Prevention Suggestions

Actions that will help to reduce humidity

• Vent appliances that produce moisture, such as clothes dryers, stoves, and kerosene heaters to the outside where possible.  (Combustion appliances such as stoves and kerosene heaters produce water vapor and will increase the humidity unless vented to the outside.)
• Use air conditioners and/or de-humidifiers when needed.
• Run the bathroom fan or open the window when showering. 
• Use exhaust fans or open windows whenever cooking, running the dishwasher or dishwashing, etc.
• Reduce the humidity (see above).
• Increase ventilation or air movement by opening doors and/or windows, when practical. 
• Use fans as needed.
• Cover cold surfaces, such as cold water pipes, with insulation.
• Increase air temperature.

Moisture control is the key to mold control, so when water leaks or spills occur indoors - ACT QUICKLY.  If wet or damp materials or areas are dried 24-48 hours after a leak or spill happens, in most cases mold will not grow. 
Clean and repair roof gutters regularly. 
Make sure the ground slopes away from the building foundation, so that water does not enter or collect around the foundation. 
Keep air conditioning drip pans clean and the drain lines unobstructed and flowing properly.