How to choose a vapor barrier
Water-resistive barriers, including housewraps and building (tar or felt) paper that are applied after sheathing and prior to siding, are quickly becoming common practice in the building industry.Weather-resistive barriers are an effective way to decrease maintenance and utility costs while increasing comfort levels.They are becoming common practice in the industry because they reduce air infiltration and improve building durability by protecting wall sheathing and framing from moisture.
Because almost all exterior finishes allow at least some water penetration, weather-resistive barriers are a practical idea in virtually all cases. They should be considered part of the flashing system and sometimes part of the air sealing package. Building codes require the use of weather-resistive barriers.weather-resistive barriers are not needed, when foam sheathing is used and the seams are taped to prevent air and water entry.
Water Resistance:
Perforated housewraps do not resist water as well as the non-perforated types. Building felts/papers are resistant to water penetration but the extent of resistance varies greatly depending on their construction. Typical 15# building felt is less water resistant than most non-perforated housewrap types but more resistant than the perforated types.
Vapor Permeability: All weather-resistive barriers are vapor permeable, which is generally desirable because it promotes drying. However, in hot, humid climates, a vapor retarder may be preferable. Building papers and felts become more permeable as they get wet, which may be an advantage if moisture gets behind the barrier.
Air Resistance: All these materials resist air movement, but it is critical that all seams and penetrations are sealed. Building papers are not usually sealed in this manner, so they are generally not considered air barriers.
Moisture Absorption: Housewraps are plastic-based materials and do not absorb moisture. Building papers and felts vary widely in their construction but all may absorb some moisture under certain conditions. In some cases this may be an advantage-if moisture gets behind the barrier, the barrier will absorb it then slowly dry out, protecting wood materials.
Durability: Housewraps are very tear resistant. Building papers/felts vary widely in this regard but are generally more prone to tearing than housewraps. Tear resistance may be an advantage during installation and perhaps afterward if left uncovered and subject to wind damage. Housewraps vary widely in UV resistance, as well; building papers/felts are generally not very UV resistant. This may be an important consideration if siding will not be applied for some time. Housewraps are very moisture tolerant, while building papers/felts may deteriorate over time if repeatedly subjected to water.
MOLD & MOISTURE
Moisture is the leading nemesis of wood homes. Water in the form of rain, humidity, and plumbing leaks can cause mold, undermine the structural integrity of the home and ruin finished surfaces. In some energy-efficient buildings, moisture that makes its way into exterior walls must fight harder to find its way out.
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