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Drainage Key to Minimize Moisture in Exterior Walls
This is the first of a series of occasional articles on water intrusion and entrapment and ways to prevent moisture from penetrating beyond the exterior envelope.
Water intrusion and entrapment is recognized as one of the most important concerns to the building and construction products industries because it can pose a potential threat to the durability and usability of residential construction. The challenge facing the industry is assuring that moisture in exterior walls does not exceed an acceptable maximum level. This issue is not limited to the United States. (See M. Bassett, “Examining Drying Rates in Walls,” BUILD magazine, June/July 2007, summarizing a three-year study in New Zealand of drying rate measurements gathered from four water management categories).
The International Residential Code (IRC) requires exterior walls to “provide the building with a weather-resistant exterior wall envelope” and that Water Resistive Barriers (WRBs) be installed behind every type of exterior siding, or cladding, material.
The code recognizes that eventually all types of cladding will allow some moisture to penetrate. Section R703.1 was modified in 2006 to require “a means of draining water that enters the assembly to the exterior.” The code does not define what constitutes “drainage.”
Section R703.1 of the 2006 edition of the IRC includes several new exceptions that are listed in the Code Comparison Table, Appendix 1. One of these drops the requirement for a “means of drainage” if the exterior wall enclosure has been tested (per ASTM E-331) and demonstrated to resist wind-driven rain. This new testing requirement does not appear to be commonly conducted or commonly published by manufacturers of many types of claddings.
Moisture management choices — such as "rain screen"/cavity wall or face-sealed design concepts; absorptive or non-absorptive claddings; and absorptive or non-absorptive substrates — affect the overall performance of the building enclosure. Additionally, claddings, windows, doors, water- and air-resistive barriers, and flashings must all be properly integrated and joined together to help achieve a successful enclosure.
Moreover, "acceptable" building enclosure performance cannot depend on the expectation of "perfect" workmanship on the part of contractors because such a standard cannot be realistically achieved.
One potential method for addressing moisture-related issues is a systems-based approach to protecting the underlying wall construction. This is accomplished by selecting and integrating appropriate membrane and flashing materials to provide a secondary Water Resistive Barrier that promotes Drainage and Drying (WRB/D2). The WRB/D2 provides the necessary foundation for installation of any number of water-shedding components — such as claddings, windows, doors and joints. The WRB/D2 extends behind the water shedding components to provide continuous protection.
Manufacturers of building products have the responsibility to ensure that product design and instructions comply with all relevant code requirements. To provide a water-resistive barrier and a means of draining water, as required by code, exterior cladding installation instructions provided by manufacturers should meet the following criteria:
- Will prevent moisture-related problems from occurring inside the building enclosure for the range of climates where it may be installed
- Can be practically and consistently accomplished by contractors in the field
- Adequately define the proper interface of the cladding with other elements of the wall system such as the WRB, windows, flashing and adjacent claddings
- Are supported by adequate testing and evaluation that help ensure that the above criteria are satisfied
If these criteria have not been met, consideration should be given to using products that include an integral drainage/drying space or to creating such a drainage/drying space within the exterior wall construction using spacers such as furring strips, drainage mats or other specialty drainage products.
These solutions can often minimize the potential for water-related problems caused by discontinuities within and between individual water-shedding components in a given building enclosure.
For more information, those who are logged into the NAHB Web site can click here (English version) or click here (in Spanish) for “Moisture Protection of Exterior Walls — An Installer’s Guide;” or e-mail Craig Drumheller at the NAHB Research Center, or call him at 301-430-6307.
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