A blower door is designed to measure the volume of air moving across the blower door fan at a specific pressure difference between inside and outside the home. The volume of air measurement in the US is cubic feet per minute (CFM) and the pressure differential is measured in Pascals (50 Pascals is the standard for residential construction). One of the more common metrics used to express air leakage in a home is air changes per hour (ACH). The only way we can calculate that metric is by measuring a home’s volume.
Measuring the volume of something is straightforward, length times width times height. The problem is a modern home is rarely built as a simple box. There are crawlspaces and basements, inside and outside corners and convoluted roof designs that all may or may not be included in the calculation. The first step is to understand what spaces are included, then we can measure and calculate the volume.
The most recent energy code from the 2021 IECC requires all blower door testing to be performed in accordance with either the ANSI/RESNET/ICC 380, ASTM E779 or ASTM E1827 standard. This article will be based on ANSI/RESNET/ICC 380.
Conditioned Floor Area
As I stated earlier, calculating the volume of a space is simply multiplying the length times the width times the height. It’s easiest to first measure the conditioned floor area, or length times the width. The 380 standard uses the older ANSI Z765 standard for calculating square footage of a home, which is measured from the exterior finish of the building. All exterior and partition walls are included in the floor area calculation. Basements might or might not be included in the conditioned floor area. If they are serviced by the heating and cooling equipment, the space is included in the conditioned floor area.
Areas that are excluded from the conditioned floor area are attached garages, even when they are conditioned, thermally isolated sunrooms, the floor area of an attic, even if conditioned, the floor area of a crawl space, even if conditioned, and the floor area of an unconditioned basement.
In the simple floorplan shown, the attached garage and sunroom would be excluded from the conditioned floor area. This leaves the main area of the home, 24’ x 30’-6” and the addition, 14’ x 16’ for a total conditioned floor area of 956 square feet as measured from the exterior of the building.
Conditioned space volume
Now that we have the conditioned floor area of the structure, we can begin measuring the height to calculate the volume. This volume calculation will usually include any space between two floors, for example, the space between a first and second level would be included as long as both floors are conditioned. According to the 380 standard, the definition of “conditioned” is:
“Obtained an ACCA Manual J, S, and Either B or D report and verified that both the heating and cooling equipment and distribution system are designed to offset the entire design load of the volume; or”
“Verified through visual inspection that both the heating and cooling equipment and distribution system serve the volume and, in the judgement of the party conducting the evaluations, are capable of maintaining the heating and cooling temperatures specified by the Thermostat section in the Building Component Column of Table 4.2.2(1) of ANSI/RESNET/ICC 301.”
The 380 standard does have a complication, a conditioned attic or crawlspace, which is not included in the conditioned floor area, is included in the conditioned space volume as long as a door or hatch is opened to the space during the blower door test.
Using the simple floor plan as an example, if that home had a conditioned crawlspace or basement, the volume of that space would be included in the overall volume of the home. So, if the home had a three-foot-deep crawlspace, a space between the crawlspace and main level of one foot, and the main level ceilings were eight-foot flat ceilings, the calculation would be:
(3’ crawlspace + 1 foot between the floors + 8’ main level) = 12’ x conditioned floor area of 956 Ft² = 11472 cubic feet of conditioned space volume.
What happens if there is a vaulted ceiling or some other complicated roof detail somewhere in the home? That space would be included in the conditioned space volume. Measuring the volume of a simple vault or cathedral ceiling isn’t difficult, until there are dormers, or some other convoluted roof line also included in the vault. Often calculating the volume of a home will take much longer than actually performing the blower door test, when a home is complicated, I usually perform the math to calculate volume in my office after performing the test. On very complicated homes, I may use architectural software to aid in the calculation. The house in the above photo is one of those homes that will take several hours to calculate the volume.
Alternatives to the standards
Standards are the recommendation; I personally do not follow all of the 380 standard. For example, I disagree with measuring the structure from the exterior. In my heating dominated climate, it’s common for the main air control layer or air barrier to be located on the warm in winter side of the exterior wall and roof assemblies. To me, it makes sense to measure area from the interior. (It is also difficult to measure the exterior of the home during the winter months when snow makes it challenging to move about the outside of a home.) The code officials I have discussed this with agree with the thinking and allow the variation of the standard. On the downside, measuring from the interior does result in a smaller volume which makes it more difficult to pass the blower door test.
It is possible to deviate from the standard but have a discussion with your local authority having jurisdiction and be consistent with your practices.
How I measure
I start my measuring process by first making a simple sketch of the home’s floorplan. This can be done either from the interior or exterior. Moving around the home also gives me the opportunity to check for any concerns before testing. After producing a sketch, I begin measuring. I prefer to use a laser measuring device, but tape measures and measuring wheels are sometimes also used. Lasers work well if you have a line of site between the two areas being measured, occasionally it will be required to add several different rooms together to achieve a total length. I round measurements to the nearest quarter foot.
Once I have the floorplan mapped, I then measure the heights of the various spaces. Flat ceilings are easy, homes with cathedral ceilings and dormers add a complication to the calculation. You may be required to calculate the volume of a triangle, trapezoid, circle or sphere to accurately calculate the volume of a complicated home.
It is possible to use a set of architectural plans to calculate the volume, but I still back that up by confirming measurements. It’s not uncommon for a home to vary from the plans. It may also be possible to import a digital copy of the house plans into a software program, which then can be scaled to allow for accurate volume calculations. This can speed testing by calculating the volume off site, then verifying the measurements at the time of the test.
One last option is to ask the architect or designer to calculate the volume. I had a discussion a while back with Alexandra Baczek (Steven Baczek Architect) on using software to calculate volume, here is what she had to say:
“We have used SketchUp in the past for the purpose of translating volume metrics to energy consultants. SketchUp has the ability to quickly generate the volume of a model, but like anything, that metric is dependent on the accuracy of the model. For a simple house, it’s pretty easily achieved, for a more complicated house, the complexity is relative.”
Again, I would confirm measurements on site to verify the accuracy of the calculation.
Local code variations
Be sure to check any local code requirements when it comes to calculating a home’s volume for blower door testing. I am aware of a specific requirement for the state of Washington. Washington’s State Energy Code caps the ceiling height used for calculating volume at 8.5 feet, this includes any cathedral ceilings. It is easier to pass a blower door test in a home with a large volume, smaller homes are more difficult. I believe this is Washington’s attempt to level the playing field.
The ACH50 calculation
Once you’ve measured the volume of the home and run the blower door test (which will normally be shown as a cubic feet per minute (CFM) of airflow moving across the blower door fan), you can now calculate the air changes per hour at 50 Pascals of pressure, which is what the code is looking for as the pass/fail metric of a blower door test. The formula is:
CFM x 60 minutes in an hour/volume of the home=ACH50
As an example, a home had a blower door test of 750 CFM and a calculated volume of 12,000 cubic feet. Using the formula, 750 CFM x 60 minutes / 12,000 cubic feet = 3.75 ACH50. This home would pass the blower door test in some areas of the country where the test result must be 5 ACH50 or less, but not in others where the requirement is 3 ACH50 or less. It’s important to know the requirements of your area.
The air changes per hour at 50 Pascals is required by code to be a specific value depending on the location of the home, as I mentioned earlier, either 3 or 5 ACH 50. Is this the best metric for determining the tightness of a structure? A home does not leak in its volume, air leaks happen on surfaces, specifically the exterior shell. As an example, here are the calculations for the smallest and largest homes I’ve tested to date.
The smallest is a 696 square foot home built in 2018, this was a code required blower door test. The volume of the home was calculated at 6,333 ft³ with a cubic feet per minute of air flow at 316 CFM50. This home barely passed the blower door test at 2.99 ACH50. (Code requires 3 ACH50 or less in my area.) To put that level of tightness in perspective, a clothes dryer and bath fan operating at the same time in this home might be capable of producing a pressure of -50 pascals, the same as a blower door test. The home is tight.
The largest home I’ve tested was a seasonal log cabin just over 5000 square feet with a cavernous volume of 99,076 ft³. The interior peak of the main cathedral ceiling was 25’ high and the structure had a 4’ conditioned crawlspace. The crawlspace is not included in the conditioned floor area of the home but is included in the conditioned space volume. This blower door test was part of an energy audit. The home’s leakage rate was 5306 CFM at 50 Pascals, 3.21 ACH50.
The air change rates of the two structures are similar, but with very different air flow rates. Back to the question, is the ACH50 number the best metric for determining if a home is “tight”? There are other metrics for showing different leakage rates of a home, the best, in my opinion is the cubic feet per minute at 50 Pascals per square foot of surface area. A discussion for another time.
For the vast majority of blower door tests I conduct, I spend much more time calculating volume than actually performing the test. The final air changes per hour calculation will only be as accurate as the volume calculations. Understanding what areas are and are not allowed to be included in the volume of a home also plays into the test’s accuracy. The ANSI/RESNET/ICC 380-2019 standard is currently the best source of information on performing blower door testing for code compliance.