Cooking, showering, breathing, and other day-to-day activities generate a moisture load that needs to be removed. If the house is sealed tight with spray foam, it is much harder for this moisture to leave the house naturally. It builds up inside the home and can raise relative humidity to uncomfortable and even dangerous levels.

While most spray-foam homes are required to have some form of mechanical ventilation (ventilation to ASHRAE 62.2 Standard is recommended), there are many questions on how best to provide it.

One solution for mechanical ventilation in humid climates is a whole house dehumidifier, such as the Ultra-Aire whole house ventilating dehumidifier. There are several benefits to using a whole house ventilating dehumidifier in tight spray foam insulation homes including occupant health and comfort, energy efficiency, and property protection.

Comfort is improved because of ideal indoor humidity. Health is improved because of purging of the indoor pollutants found in homes with fresh air. Also typical allergens producing biological organisms are minimized. Energy is conserved by minimizing the need for air conditioning when the home is unoccupied and improves comfort at higher indoor temperatures.

Property protection is achieved through the control of high indoor moisture levels thus minimizing the potential for mold.

Airtight homes without fresh air ventilation become humid with moisture from the occupants in a couple days. Inadequate ventilation can increase indoor pollutant levels by not bringing in enough outdoor air to dilute emissions from indoor sources and by not carrying indoor air pollutants out of the home. Unless they are built with special mechanical means of ventilation, homes that are designed and constructed to minimize the amount of outdoor air that can “leak” into and out of the home may have higher pollutant levels than other homes. All airtight homes require a minimum amount of fresh air to purge indoor pollutants and renew oxygen per ASHRAE 62.2 2007. Consider this equation:

0.01 cfm per square-foot, plus

7.5 cfm x (# of bedrooms + 1)

1500 sq. ft. / 3 bedroom = 45 cfm

As the fresh air passes through the home, the moisture from the occupants is also removed. When the outside air is less than 55˚F dew point (dry), the moisture from the occupants is also purged without exceeding 60 percent relative humidity (RH). When the outside air is more than 60˚ (moist), the moisture from the occupants raises the indoor relative humidity and will exceed 65 percent RH. A fresh air change in an occupied home is recommended every four to five hours.

An additional benefit to controlled fresh air ventilation is the ability to create a positive pressure on the home. Positive pressure reduces unfiltered and untreated air from entering the home. It also provides make-up air when exhausting vented appliances, like the bath fans, kitchen hood exhaust, and clothes dryers are used in the home. Fireplaces and gravity chimney appliances also need make-up air to function.

When outdoor dew points are more than 60˚F, the moisture in the fresh outside air that is needed to purge indoor pollutants and renew oxygen contains excess moisture that contributes to the excess moisture in the home. Supplemental dehumidification is needed to maintain the ideal less-than 50 percent RH. During hot weather, a properly setup air conditioner, operating for extended hours throughout the day, usually removes enough moisture to maintain less than 50 percent RH.

Ideally, supplemental dehumidification is provided when the inside space of a home is more than 55 percent RH. This includes basements and crawlspaces. A few hours of greater than 60 percent RH can be tolerated but should not exceed more than 24 hours.

Information provided by Ultra-Aire.

Publication date: 8/6/2012