| A wood frame-panel roof deck system can be
effectively insulated through a variety of techniques. Figure 37 shows four APA panel roof
decks- three with insulation and one without. Average U values for below-deck insulation
include adjustment for 8 percent farming area. 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.
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