Condensation can form within a wall, attic, or floor cavity, where it remains concealed. It can accumulate unnoticed, where it can cause rot of framing members and sheathing, to a point where structural collapse is possible; deterioration of interior room finishes, from gravity flow of water; corrosion and failure of metal truss plates and clips; and decrease of thermal resistance or deterioration of insulation. Thus, it is very important that unintended paths that infiltrating moist air may follow be permanently sealed. It is also important that an impediment exist that limits diffusion of moisture into the building envelope cavity because reducing the amount of water vapor entering a cavity will reduce the quantity of condensation in the cavity. Tbl. 17 characteristic water vapor permeance (perm) ratings for selected building materials. Data is presented to permit comparisons of perm ratings. Exact values for permeance should be obtained from the manufacturer of the materials under consideration or secured as a result of laboratory tests. Data was compiled from an assortment of the manufacturer and governmental sources. Material Vapor Retarders: Aluminum foil STEGO WRAP Under-slab polyolefin geo-membrane Polyethylene film asphalt-impregnated kraft facing Building felt/tar paper (15#) Air Barriers Prime Wrap Tyvek Commercial Wrap Tyvek HomeWrap Insulation Polystyrene (extruded) Polystyrene (expanded) Polyurethane Cellulose Fiberglass Building Materials Built-up roofing Glazed structural (clay) tile masonry Wood (solid board) Oriented strand board (OSB) Plywood, Douglas fir (exterior glue) Plywood, southern pine (exterior glue) Brick masonry Concrete 4 Gypsum wallboard (painted) Concrete masonry Building felt/tar paper (15#) Hardboard (tempered) Hardboard (standard) Finishes Gypsum wallboard-knockdown texture, semigloss paint Gypsum wallboard, 2 coats latex flat paint Gypsum wallboard-knockdown texture, flat paint Gypsum wallboard (unpainted) Plaster on wood lath Plaster on metal lath Plaster on gypsum board lath Paint Asphalt paint on plywood Latex VDR paint or primer Oil-based paint on plaster 2 Moisture Control Moisture control in building envelope cavities is generally accomplished by eliminating (or minimizing) water intrusion, ventilation of indoor spaces where moisture is generated, and construction techniques that properly retard water vapor. Eliminating Water Intrusion Designing and constructing proper details and using appropriate materials will prevent the likelihood of water intrusion. After construction, intrusion can still take place if maintenance of these details and other critical areas (e.g., caulk joints) is neglected. In addition, even slight structural movement as a building settles during its early life can create openings that were not there initially. Damage can be significant if moisture intrusion goes undetected, so early detection of water intrusion is crucial to minimizing or preventing such damage. The location of water entry is often difficult to detect (e.g., damage to the substrate and structural members behind the exterior wall cladding frequently cannot be detected by visual inspection). In cases where the intrusion is suspected, testing must be conducted. A moisture meter is used to detect elevated levels of water in building materials. Two types of moisture meters should be used: a noninvasive moisture meter that scans through the assembly for the presence of moisture without penetrating and damaging the surface; and an invasive, probe-type moisture meter that penetrates the assembly surface and gives moisture readings of materials in contact with the probes. In cases where water intrusion is discovered, repairs should be made promptly. The primary objective of any repair is to first eliminate water intrusion. Areas of elevated moisture with an absence of damage or decay may require no more than eliminating the source of water intrusion. It has been discovered that undamaged but wet substrate can dry out over time once the source of the water intrusion has been eliminated. Where structural damage has occurred, replacement of decayed lumber is required. One of the chief sources of water intrusion into a building interior is a basement or under-floor crawl-space moisture. Water vapor from a wet or damp basement or crawl space travels into the indoor spaces and ultimately migrates into the building interior and envelope. Improper grading around the base of the exterior of the building foundation directs stormwater towards the house. Not extending downspouts so they discharge away from the foundation also contributes to the problem. Solutions to a basement or under-floor crawl-space moisture are improving the slope of grade around the foundation and adding downspout extensions, which are extended as far away from the foundation as practical.