Testing is also the best way to learn where your project’s strengths and weaknesses lie. Each home reveals its own unique areas of air leakage, which are typically the result of a complex framing detail that makes proper insulation and air sealing difficult. Learning the fundamentals of building science and coming to understand the forces of whole-house performance will protect your reputation, your profitability, and your pride in craftsmanship. I have learned more about building science from watching and participating in diagnostic testing than through any other method.
At a minimum, there are two diagnostic tests that you should perform on every home you build: the blower door test and the duct blaster test.
Blower Door Test
TOP LEFT: Blower Door Test
A large fan connected to a computer simulates wind pressure against the exterior of a home and measures rate of air infiltration into the building shell.
BOTTOM LEFT: Duct Blaster Test
A computer-connected fan pulls air from a sealed duct system, including sealed-off supplies and returns, and measures the pressure difference in the ducts to measure air leakage. You should conduct a separate test on ducts running outside of conditioned space.
TOP RIGHT: Infrared Imaging
Infrared scanners “read” heat loss through walls and roof to identify problem areas where insulation has not been properly installed.
A blower door test measures air infiltration. It estimates the amount of air that is leaking into or out of the house under average weather conditions. It also measures the cumulative size of all the holes, cracks, and crevices in the house. The size of this opening may be comparable to a small window left open year-round, or it may be the size of a patio door.
Here’s how a blower door test works: After tightly closing every window and door in the house, the technician places a device called a “blower door” into the front door of the home. This device contains a large fan that’s attached to a portable computer. During the test, the computer takes the fan through a specific range of speeds designed to simulate a 20-mph wind pressing against all sides of the structure. As the house tries to equalize this pressure from outside, air infiltrates through various leaks in the building envelope. The leakage rate is calculated, and a test report is generated.
A blower door test typically takes about an hour. Have an experienced professional technician, like a certified home energy rater, conduct the test to ensure the data is acquired correctly. This person or a third-party professional from one of the energy certification programs, such as Energy Star, can be a valuable resource.
Ask your rater to generate a test report listing the home’s natural infiltration rate in air changes per hour (ACHnat). Have your mechanical contractor enter this infiltration rate into his Manual J load calculation software. Does it change the size of the HVAC system you are installing? Yes, with better information, you will find right-sizing these systems often results in smaller equipment and lower cost. A correctly sized HVAC system can have a significant effect on energy load, indoor air quality, and home durability.
Make sure your rater also calculates the air leakage rate for the total surface area of the home (CFM50 per square foot of surface). This is the cumulative surface area of all the floors, walls, and ceilings exposed to unconditioned areas. Depending on the program, this target is usually 0.25 CFM50/square foot or less. Builders in Minnesota routinely air-seal to levels between 0.10 and 0.15 CFM50/square foot of surface area.