It’s difficult to determine how much air is entering a house through walls that are built, not only for shelter and support but also to prevent airflow as an energy conservation measure. However, several ASTM tests are available to accurately measure airflow through common walls to aid in determining problem areas and finding ways to abate unwanted airflow.
Measuring Air Flow in the Common Wall
Accuframe partnered with the New York State Energy Research and Development Authority (NYSERDA) to gather data using the ASTM E283 test method and ascertain how common wood-frame walls perform under different pressure loads generated by outside airflow (wind). In each test run, the pressure loads were increased stepwise to simulate the normal range of wind conditions a house is likely to experience.
For the test:
- Six stand-alone walls were prepared identically with 2x4 framing (16”OC), 7/16” OSB sheathing with taped seams, batt insulation, ½-inch drywall, one coat of tape, and openings for one outlet and one switch.
- Three specimens were prepared without a framing gasket, which is the case for 99.9 percent of the current housing market.
- Three specimens were prepared with a framing gasket applied to the top and bottom frame-face at the OSB interior, an innovation that hadn’t been previously tested.
ASTM E283 Test Results
Wall cavities showed air intrusion beginning at 4.0 pascal (pressure generated at approximately 2.5 mph wind speed).
Walls without a framing gasket (OSB sheathed directly to the frame face) stopped 19% of air intrusion at 50 pascals (20 mph wind speed).
In contrast, walls with gaskets, at the top and bottom plates between the frame face and OSB interior side, stopped 81 percent of air intrusion at 50 pascals (20 mph wind speed). Click here to see a video of a typical smoke test at 50 pascals of a test wall without gaskets.
Reduction of unwanted airflow in walls
The easiest way to lower household energy bills is to limit unwanted cold air intake when heating or hot air intake when cooling. The NYSERDA testing revealed that airflow could be significantly reduced with a simple gasket application during construction. The reduction of air infiltration and/or exfiltration with gasketing is a practical option to reduce energy costs.
In general, builders tape their OSB sheathing seams and caulk visible gaps to help reduce airflow. Unfortunately, when the home is pressurized by heat or AC airflow, air enters through other openings left unattended in the original sealing process. These remaining openings are often unseen or unreachable without major effort and expense. Ironically, the best weather-resistive barriers often aid the transfer of air to open gaps and points that are left unsealed.
Any air intrusion reduction strategy will leave openings that can be eliminated easily in some cases, or only with great difficulty (expense) in others. Modern builders need increasingly more stringent strategies to meet lower estimated-energy demand in new housing and increased regulatory requirements. Improving an existing strategy can be accomplished by adding simple methods that reduce airflow, such as gasket application during framing, described here.
A comprehensive approach to air sealing
Suppose final ambient air intrusion in the wall cavity is at 80 percent volume. In that case, the rise and fall of air currents in a wall known as ‘convection loop’ will be disrupted, since the high intake of air distributes it and overrides the loop. With a high enough volume of air intake occurring, it may also draw air into the cavity from the conditioned space, which is clearly not desired but can be reduced or eliminated with a more comprehensive air strategy.
Sealing strategies augment each other and a combination of several methods is most often employed, depending on the level of airflow the builder desires. One of the main reasons builders fail their first blower door (50 pascals) test is from failure to prevent air intake directly into the wall cavity.
Glass/Wool batts are the primary insulation solution for housing in the United States. They and are enhanced with additional air sealing methods to stop outside ambient air entering and leaving the wall cavity as well as air leaving the conditioned space. This tried-and-true method does, however, leave many areas for air infiltration that a framing gasket solution can eliminate.
In real-world blower-door testing, two identical batt-insulated houses built by Habitat for Humanity were used to test gasket-sealing against traditional caulking of visible gaps and openings. When tested @50 pascals, the home sealed only with gaskets installed during framing passed the test while the caulked house failed. Further sealing measures were needed for the caulked house to reach the passing level before it could be eligible for a Certificate of Occupancy.
Application of a gasket to framing prior to sheathing is an effective, economical method when used to reduce airflow in walls and can be an important tool for advancing energy efficiency in residential buildings and increasing profits.
Michael Schettine is president of Schettine Associates, Inc. manufacturer of AccuFrame®
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