Moisture Control. It is necessary to design walls that restrict moisture vapor from entering and condensing within the exterior wall cavity. Proper design will ensure the insulation’s thermal efficiency and the overall structure’s performance. Design must provide continuous vapor retarding equivalent to a rating of 1 perm installed on the living space side of the insulation. Vapor retarders are also required on the ground in all crawl spaces and under concrete slabs.
Acceptable products include: 6 mil polyethylene, asphalt impregnated craft or foil-backed insulation (tabs must be stapled to the narrow face of the studs, not to the wide face), vinyl-faced or foil-backed gypsum, sandwich type craft with an asphalt or polyethylene core, or equivalent. Vapor retarding paints are also available for interior walls.
Site Drainage: Slope ground away from structure for a minimum of 3 feet.
Roof Drainage: Incorporate an overhang or drainage system into the structure design to prevent water from running down sidewalls.
Ventilation: Both attics and crawl spaces require adequate ventilation. Clothes dryers must be vented outside; kitchen and bathroom fans are recommended to vent localized moisture outside.
Attics: Attic vents should provide a minimum of 1 square foot of net free vent area for every 150 square feet of attic space. Unrestricted air circulation is best achieved by a combination of soffit vents and ridge or roof vents.
Crawl Spaces: Crawl space vents should provide a minimum of 1 square foot of net free vent area for every 25 lineal feet of exterior wall. They should be placed to allow for cross-ventilation.
Siding Return at Roof (Dormers, etc.): Use flashing, allowing at least a 2-inch clearance between siding and roof-line (see illustration at bottom right). Cut edges of siding must be finished in accordance with finishing instructions described in this guide.