I was intrigued that the recently announced Home Star energy efficiency program has been nicknamed “Cash for Caulkers.” Certainly all of us in the energy efficiency business appreciate the value of air sealing as one of the most cost-effective solutions to reduce energy consumption in houses. Yet there are still those in the industry who are asking whether it’s possible to make houses too tight.
The short answer is “No. Homes can’t be too tight.” But that comes with a caveat: A home can’t be too tight, provided it also addresses the air quality problems that result from reducing air leakage.
As houses get tighter they face more potential for indoor air quality problems due to buildup of moisture, stale air, dust, pollen, and chemical pollutants. Tighter homes also pose combustion safety concerns because chimney-vented furnaces, water heaters, and fireplaces need adequate fresh air for proper operation, which is compromised by negative air pressure. We need to recognize that natural infiltration of air into a home, especially as we reduce air leakage, is not a reliable or adequate source of fresh air to maintain air quality. As we’ve emphasized in previous columns, to meet healthy air quality standards today’s homes need continuous mechanical ventilation and provisions for safe operation of combustion appliances.
Assuming you are taking these steps to address air quality, the goal then is to seal the home as much as possible. But even that is not enough. Unless you test and measure how tight you’re building—and understand the resulting metrics—you won’t know where you stand. And beyond understanding how tight your homes are, you’ll also be able to apply the information to training of your labor crews and subcontractors who are critical to reducing defects and improving the quality of workmanship on your projects.
Testing Procedures and Metrics
The method of choice for measuring a home’s tightness is a blower-door test. The most common techniques follow the ANSI/ASTM-779-99, “Standard Test Method for Determining Air Leakage Rate by Fan Pressurization” and the CAN/CGSB 149.1 “Determination of the Airtightness of Building Envelopes by the Fan Depressurization Method.”
In fact, these standards are very similar and give almost identical results. They also follow the same procedures: Close all windows, doors, and other intentional openings in the enclosure. Open all interior doors and turn off air handlers and exhaust fans. Install the large blower door fan in an exterior door with a pressure tap or housing to the outside. Either use the fan to exhaust air out of the house, thus causing a negative pressure in the house, or blow air in and create a positive pressure.
Usually a pressure difference of 50 Pascals (Pa) is the goal; this is the pressure roughly equal to a 25-mph wind acting on all sides of the house at once. While this is clearly an exaggerated pressure that a house would rarely experience, it does enable simple detection of leaks and helps negate wind effects that could skew results. The fan is calibrated and the operator records the air flow that the fan is delivering at 50 Pascals of pressure. Testing typically records a range of pressures and air flows so that the results can be extrapolated and reported in a variety of ways depending on what form of evaluation is required.