Blower Door Testing a Small Home

Code compliant blower door testing of a small home can be very frustrating for both the person performing the test and the homeowner or contractor who are required to have the test. A few months ago, I tested a 952-square-foot, newly constructed home with a volume of 7616 cubic feet, one of the smallest I’ve tested. The house had a leakage rate of 416 CFM, which resulted in a 3.28 ACH50 number. A failed blower door test (my climate requires 3 ACH50 or less). If we take that same CFM rate but increase the ceiling height to 9 feet, which changes the volume to 8568 cubic feet, 2.91 ACH50, a passing test.  Either way, 416 CFM of air moving through the blower door isn’t much, some kitchen exhaust hoods can move more air than that. Seems unfair to punish smaller homes when the blower door test is testing the surface of a structure, not its volume.

The energy code in the 2021 IRC makes an exception for testing small homes. The code states:

N1102.4.1.2 (R402.4.1.2) Testing

Exception: When testing individual dwelling units, an air leakage rate not exceeding 0.30 cubic feet per minute per square foot [0.008 m³/(s x m²)] of the dwelling unit enclosure area, tested in accordance with ANSI/RESNET/ICC 380, ASTM E779, or ASTM E1827 and reported at a pressure of 0.2 inches water gauge (50 Pa), shall be permitted in all climate zones for:

1. Attached single and multiple family building dwelling units.
2. Building or dwellings that are 1500 square feet (139.4 m²) or smaller.

Mechanical ventilation shall be provided in accordance with section M1505 of this code or section 403.3.2 of the International Mechanical Code, as applicable, or with other approved means of ventilation.

Basically, what this means is if a home is 1,500 square feet or less, you are able to use the CFM50/square foot of enclosure area for the metric instead of the ACH50 metric. What would that mean for our slab on grade example home from above? To calculate, you count the surface area of all sides plus the floor and ceiling. The example slab on grade home was 28 ft. by 34 ft., 952 square feet. (28 x 2 + 34 x 2 = 124 lineal feet x 8-foot side wall height = 992 ft² of wall surface area.) Add that to the floor and ceiling surface areas (the ceiling in this home is flat, cathedral ceilings complicate the calculation) (952 x 2 = 1904 ft²) result in a total surface area is 2896 ft².  Plug that number and our 416 CFM50 into our formula, CFM50/ft² of enclosure area and we get 0.14 CFM/ft² of dwelling enclosure area. This house easily passes the code exception in the 2021 IRC.

One of the issues with this small home exception is the 2021 IRC has not been adopted in many areas yet. This is the case in my home state of Minnesota where we are still on a version of the 2012 energy code. I had a conversation with our local building inspectors about using the 2021 IRC exception on another small home I recently tested that failed the blower door test. They asked for the code reference allowing the exception, along with the test results showing the home would pass using the CFM50/square foot of enclosure area metric. The building department accepted the test results, and the home was given its occupancy certificate.

If you’ve never played around with the CFM/ft² metric, it can be hard to get your head wrapped around what is a good score and what is not. After all, there is no direct relationship between ACH50 and CFM/ft²; it’s all dependent and the shape of the home. I couple years ago I had a great conversation with Gary Nelson, one of the founders of Minneapolis Blower Door and the Energy Conservatory (you can read about that conversation here: Q&A With Gary Nelson.) He felt we should be shooting for around .075 CFM/ft². As it turns out, .075 is in the neighborhood of roughly 1 ACH50, again, it’s not a direct relationship.

What’s the future of the two metrics, the cubic feet per minute of air flow at 50 Pascals per square foot of enclosure area compared to the air changes per hour at 50 Pascals? With the addition of the testing exception in the 2021 IRC, I believe that is the first step to making the change. Will it end up in the same place as the US’s adoption of the metric system? Time will tell. But as I stated earlier, the purpose of the blower door test is to test the continuity and integrity of a structure’s surfaces, not its volume.

This article originally appeared on Green Building Advisor and is reprinted with permission.