How To Choose Weather-Resistive Barriers - 11/15/2004 - Home Exterior Roofs Siding

How To Choose Weather-Resistive Barriers

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. All of the information necessary to make practical choices when considering water-resistive barriers is detailed below including the benefits, drawbacks, and suggested applications for each available material.

WHY USE THEM?

Weather-resistive barriers are an effective way to decrease maintenance and utility costs while increasing comfort levels for your customers. 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.

WHEN SHOULD THEY BE USED?

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 may also require the use of weather-resistive barriers.

However, in some cases, weather-resistive barriers may not be needed, such as when foam sheathing is used and the seams are taped to prevent air and water entry.

Housewraps and building felts/papers may differ in some fundamental ways. When selecting one, consider whole-wall design according to climate, including proper selection and placement of vapor retarders, air barriers, insulation, use of drainage planes, and flashing. Below is a guide to the different attributes of each system.

WHAT MATERIALS SHOULD BE USED?

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.

Recent research indicates that water resistance decreases when surfactants (surface-acting substances), such as those found in cedar and redwood siding and sometimes in other exterior finishes, are introduced to water. As surfactants are picked up when moisture moves through these finishes, they reduce the surface tension of water, allowing it to pass through a much smaller opening. Most non-perforated housewraps still have better water resistance than building paper even after surfactants are introduced.

To alleviate this problem, coat all wood siding sides with a clear water-repellent wood preservative, then install, prime, and paint with two finish coats. Furring out siding will also help prevent moisture problems.

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.

For additional information on weather-resistive barriers, call the the ToolBase Hotline at (800) 898-2842 or send an email to: toolbase@nahbrc.org.