Condensation:

Its Cause and Control

Today's construction techniques usually produce a far better sealed and insulated building than those built in the past. At the same time, the modern house is likely to be equipped with appliance that give off moisture. It is more important than ever before, therefore, to provide adequate measures for controlling moisture and condensation. Lack of attention to this area may cause difficulties and costly callbacks.

Studies show that moisture originates booth inside and outside the structure. Trouble may start with condensation on the underside of roof decking in the attic. Moisture transmission through walls can cause paint failures on exterior walls. The solution is adequate ventialati9ohn and properly placed moisture vapor retards. The latter should always be located on or near the warm side of walls and floors. Ceiling vapor retarders are typically omitted where attics are well ventilated.

Minimum ventilation requirements for both attic and crawl space appear in the CABO One and two Family Dwelling Code, and in other model building codes. The requirements are based on the ratio of the free ventilation area to the area to be ventilated. The required ratio is 1 to 150, applicable to both crawl spaces and attic areas. When a ground cover is placed in the crawl space, the crawl space ventilation ratio may be reduced to 1 to 1500. The ratio in the attic area may be reduced to 1 to 300 provided: 1) a vapor retarder having a transmission rate not exceeding 1 perm is installed on the warm side of the ceiling, or 2) at least 50 percent of the required ventilating area is provided by ventilators located in the upper portion of the space to be ventilate,d with the balance of the required ventilation provided by eave or cornice vents.

In general, condensation can be controlled with a vapor retarder placed on the warm side of the walls and below concrete slabs or as a ground cover in crawl spaces, along with adequate ventilation in attics and crawl spaces.

Ventilation should not be cut off at any time during the year when it is the only means of moisture control in crawl spaces. In cold climates, low temperatures beneath the first floor may be expected in a ventilat4ed crawl space, and insulation will be required in the floor and around exposed mechanical lines. When a vapor retarder is installed for ground cover, vents may be closable and the perimeter foundation wall may be insulated in lieu of the floor.

Moisture control recommendations for low slope APA panel roof decks are described in Engineered Wood systems (EWS) Technical Note, Moisture Control in Low Slope Roofs, Form EWS R525, available from APA.

Additional information is available in Engineered Wood Systems (EWS) Technical Note, Controlling Decay in Wood Construction, Form EWS R495.