About 88,000 homes are built each year in the Atlanta, Georgia, Washington, D.C., and Raleigh-Greensboro, North Carolina markets. The climate in which these homes are built is hot and humid for half the year and experiences year-round thunderstorms, as well as winter storms. The weather in this climate zone can have harmful effects on building envelopes, so it's important to plan for temperature, humidity, and rainfall in this mixed-humid climate zone.
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Moisture intrusion. |
Based on inspections and tests of homes of seven homebuilders IBACOS worked with in 2002, we found that six building construction practices presented the greatest challenges to improved building performance. The venting of crawl spaces promoted moisture intrusion. The typical heating, ventilating, and air-conditioning (HVAC) system, which consisted of two air-handling units often both outside of conditioned space, provided an inefficient means for heating and cooling. The use of polyethylene as the vapor retarder in exterior walls could seriously hamper the drying potential of the wall. The effectiveness of the drainage layer behind the exterior cladding was questionable because it often included gaps and typically consisted of an exterior sheathing with poor water shedding characteristics. Flashing practices were found to be inconsistent, sometimes channeling water into a wall instead of away from it, and, in some cases, were absent altogether. Lastly, building airtightness was overlooked as an issue. Based on these challenges, we determined that the following practices need to be emphasized further:
• Crawl spaces should be built unvented, insulated, and conditioned (with a return to the first floor) instead of being vented
• Engineered designs are needed for HVAC systems in order to make them more energy effective while minimizing additional cost
• Kraft-faced batts should be installed instead of unfaced batts with polyethylene so that the drying potential of exterior walls is increased
• Lapped thin structural sheathing is the preferred drainage layer for exterior walls
• Effective flashing at roofs, windows, and doors is essential for controlling water intrusion
• Draftstopping is needed to maintain air barrier continuity and optimal insulation performance.
These practices will reduce the risk of moisture problems and make
homes more comfortable and energy efficient.
Improving Flashing Practices
With the amount of rain the mixed-humid climate region receives, we expected that good roof, window, and door flashing practices would have been integrated into building practices, but this was not the case. We found the following common situations in one builder’s practices:
Step flashing at roof and wall intersections and at roof saddles was not integrated with the wall drainage plane and did not extend up the wall high enough to completely prevent being circumvented by rainfall.
Flashing membranes were not used on some windows, and, if used, they were often installed in the incorrect sequence. Often materials not approved as a flashing membrane, such as housewrap tape, were used to tape window flanges to the exterior sheathing. Non-adhesive flashing was stapled to the exterior sheathing and would not prevent water moving down the sheathing from entering under the flashing and into the home. Plastic housewrap or building paper was rarely used on walls, resulting in the exterior sheathing acting as the main source of water diversion around windows (underneath the cladding).
Sill flashing at exterior doors was found to be incomplete, with areas of the band joist left uncovered. Sill flashing at doors was sometimes installed underneath a lower piece of flashing, thereby diverting water into the band joist area.
IBACOS developed an extensive education and training effort on flashing with each builder and produced architectural drawing and photographic details that highlighted effective flashing practices at roofs, windows, and doors. Through these details, the use of kick-out flashing to divert water away from obstructions and acceptable flashing materials were introduced. In on-site training sessions and classroom presentations we conducted and in the details we created, we stressed the correct sequencing of flashing so that positive drainage away from openings occurs. We also stressed that flashing must be integrated with the wall drainage plane system.
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