Putting Tightness to the Test

August 13, 2015

Constructing tight buildings is a requirement in an ever-increasing number of communities, including those that have adopted the 2012 or 2015 IECC as local code. Even where not required, it's a good idea and it's not as expensive as you might think. First, you must test your buildings and understand the metrics to know where you stand. Once you're armed with this basic knowledge, you'll be able to improve the training of your crews and subcontractors to optimize air leakage control in the homes you build.

Understanding the 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" or the CAN/ CGSB 149.1 "Determination of the Airtightness of Building Envelopes by the Fan Depressurization Method." These standards follow similar test protocols and give very similar results.

A large fan is installed in one exterior door of the house, creating a pressure difference of 50 Pascal across all sides of the house at once-roughly equal to the pressure of a 25 mph wind. While a house would never experience this naturally, it enables simple and accurate leak detection. The fan is calibrated and the operator records the cubic feet per minute (CFM) that the fan is delivering at 50 Pascals of pressure. Often a range of pressures and airflows is recorded so that results can be reported in a variety of ways, including the three preferred metrics discussed below.

Metric 1: CFM@50Pa
(CFM required to create a 50-Pascal pressure difference across the house.)

The early weatherization programs used the CFM@50Pa as a way to track air sealing efforts in an individual house and it's still the most common metric used in the United States. In a typical 2,000-square-foot single-story home without a basement, anything under 450 CFM@50Pa reflects a very tight house.

Metric 2: ACH@50Pa
(Air changes per hour at a pressure difference of 50 Pascal.)

This metric takes the CFM@50Pa number, multiplies it by 60 (minutes in an hour), and divides that result by the volume of the heated area of the house. This makes comparing houses of different sizes possible and has become the most common metric referenced in codes and energy efficiency programs. A good goal for most builders is anything under 2 ACH@50Pa. meaning that the entire volume of air in the house is replaced twice per hour during this exaggerated test condition.

Metric 3: ELA@10Pa
(Equivalent leakage area-technically this is a calculated, extrapolated number used for energy modeling, but for your purposes you can think of it as the size of a theoretical hole in a wall if you combined all the air leakage points in a house into one spot at a pressure of 10 Pascal.)

This metric allows for simple communication as crews, sub-contractors, and homeowners can often relate to the size of a hole in square inches better than trying to imagine CFMs of air flowing through multiple small leaks. Using the 2,000-square-foot house again, a really tight house would be less than 80 square inches of leakage. (A sheet of 8.5×11 paper.)

Tight is Good, but Ventilation is Critical

While we used to rely on natural infiltration of air into a home to maintain air quality, it wasn't particularly effective and certainly wasn't energy efficient. However, tighter homes impact the availability of adequate air for the safe operation of combustion appliances and may impact air quality. The best choice is a tight enclosure with a controlled, continuous mechanical ventilation system to meet expectations for fresh, safe, healthy air in all homes.

Comparing Blower Door Test Results

Common Air Tightness Metrics for a Typical 2000 ft2, Single Story Home Without Basement
ACH@50 Pa CFM@50 In2ELA@10Pa
Older Home 7 - 20 2100 - 6000 250 - 800
IECC 2009 Reference 7 2100 250
IECC 2012:
C.Zone 1-2 5 1500 200
C.Zone 3-8 3 900 150
Canadian R-2000 1.5 450 70
Passive House 0.6 180 30
Tightest House Measured by Ci 0.16 50 10

Any of these metrics can be used to help focus on the real task: looking for ways to improve the air tightness of houses. Be sure to test your homes and get to know the results so that you can measure your progress on each project. But at the same time, don't neglect proper ventilation. Combining ever-improved levels of air tightness with the capacity for modest amounts of continuous ventilation ensures you are building comfortable, durable, healthy, and efficient homes.

By Gord Cooke, President