Whether you are using equipment from Minneapolis Blower Door, Retrotec, or any other blower door manufacturer, one of the first steps in conducting a blower door test is to measure the home’s floor area, volume, and surface area. Coming up with the floor area is the simplest, length times width. Calculating volume also isn’t hard, width times length time height, simple right? Not always, getting the volume of a geodome or a complex cathedral ceiling often takes some time. And then there’s the surface area calculation, measuring the area of the floor, walls, and ceiling. There is some discussion and a little controversy on where to measure. Do you take the outside dimensions from a set of plans or measure to the inside finish? I did manage to find some guidance in the ANSI/RESNET/ICC 380-2019 standard. One of the three air leakage rate testing standards required by the 2018 IRC. (The other two are ASTM E779 and ASTM E1827.)
Conditioned Space Volume² – The volume within a building or Dwelling Unit serviced by a space heating or cooling system designed to maintain space conditions at 78 °F (26 °C) for cooling and 68 °F (20 °C) for heating. The following specific spaces are addressed to ensure consistent application of this definition:
- If the volume both above and below a floor assembly meets this definition and is part of the subject Dwelling Unit, then the volume of the floor assembly shall also be included. Otherwise, the volume of the floor assembly shall be excluded.
o Exception: The wall height shall extend from the finished floor to the bottom side of the floor decking above the subject Dwelling Unit for non-top floor level Dwelling Units and to the exterior enclosure air barrier for top floor level Dwelling Units.
- If the volume of at least one of the spaces horizontally adjacent to a wall assembly meets this definition, and that volume is part of the subject Dwelling Unit, then the volume of the wall assembly shall also be included. Otherwise, the volume of the wall assembly shall be excluded.
o Exception: If the volume of one of the spaces horizontally adjacent to a wall assembly is a Dwelling Unit other than the subject Dwelling Unit, then the volume of that wall assembly shall be evenly divided between both adjacent Dwelling Units.
- The volume of an attic that is not both air sealed and insulated at the roof deck shall be excluded.
- The volume of a vented crawlspace shall be excluded.
- The volume of a garage shall be excluded, even when it is conditioned.
- The volume of a thermally isolated sunroom shall be excluded.
- The volume of an attic that is both air sealed and insulated at the roof deck, the volume of an unvented crawlspace, and the volume of a basement shall only be included if the volume is contiguous with the subject Dwelling Unit and the party conducting evaluations has either:
o 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,
o 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 evaluations, are capable of maintaining the heating and cooling temperatures specified by the Thermostat section in Table 4.2.2(1) of ANSI/RESNET/ICC 301.
- The volume of a mechanical closet, regardless of access location, that is contiguous with the subject Dwelling Unit shall be included if:
o it is serviced by a space heating or cooling system designed to maintain space conditions at 78 °F (26 °C) for cooling and 68 °F (20 °C) for heating, and
o it only includes equipment serving the subject Dwelling Unit, and
o the mechanical room is not intentionally air sealed from the subject Dwelling Unit.
I don’t know what you think, but to me, that is a tough read. What I got out of it is that the conditioned space volume (heating or cooling) is what we are calculating, this includes all the conditioned areas including between floors where the above and below floors are conditioned, conditioned attic spaces and any interior partition walls. What’s not included are attached garages, even if they are conditioned, unconditioned attic and vented crawl spaces along with any other attached structure that are outside the thermal boundary.
If you are measuring to achieve a HERS rating, RESNET requires the building to be measured from the exterior. This better aligns with how the real estate industry calculates square footage of a home. In reality, measuring from the interior or exterior of the structure will have little effect on the test outcome for all but the smallest structures.
How I measure. I start by walking around the outside of home noting the shape. Often there are bump outs, recesses or cantilevers that are hard to tell when inside. I might do a quick sketch if the design is complex. This is also a good time to complete any measurements that might be needed for the CFM per square foot of surface area.
I next go inside and start measuring the floor area. Floor area is part of the information requested by the software I use while blower door testing. I use both Tectite from The Energy Conservatory and FanTestic by Retrotec. The floor area is also the first part of the equation needed to calculate volume.
Floor area times the wall height gets me the volume of the home. It’s usually not that simple, varying ceiling heights, multiple levels, and cathedral ceilings all complicate the calculation.
After measuring for the volume, you’ll have most of the information needed for square feet of surface area. This measurement includes the entire floor, whether it’s the concrete slab of a slab on grade, crawl space or basement. It also includes the entire perimeter of wall, including areas below grade, and the ceiling. The ceiling may be a vault, which then will require the length and width of the slope. Vaulted or cathedral ceilings will also include the gable end area.
A quick sketch I used recently to calculate floor area, volume, and surface area for a blower door test on a duplex. Both sides were identical, speeding up the procedure for completing two tests. This calculation was simple, slab on grade construction with flat ceilings. Had there been a basement or cathedral ceilings, that volume and surface area would also need to be included.
I always measure inside dimensions. As I see it, the majority of air control layers in my climate are located on the inside, not the exterior. I use a laser measuring device to speed up the process. They work well if you have a line of sight from wall to wall, otherwise you are adding smaller interior spaces together to calculate the floor area. I do round up or down slightly, 1 inch gets rounded down to the full foot, two-inch I use one-quarter foot in the calculation. I feel this averaging has little effect on the final blower door results.
One of the communities where I conduct code compliant blower door testing requires volume to be measured in the ANSI/RESNET/ICC 380-2019 standard, they even go as far as requiring an inspector to be present during the measuring and test. Other areas where I have tested trust the testing people to be honest with the test and results. I have heard that some jurisdictions allow the building contractor to perform their own testing. It’s important to understand the code requirements in your area and have a conversation with the local authority having jurisdiction as to what they want to see.
The average volume for houses I test is under 15,000 cubic feet. The sketched example calculated at 13,640 cubic feet. The largest home I’ve tested wasn’t that big as far as square footage of floor area, but the volume was substantial compared to most homes I test. It was a geodome with, if I remember correctly, 23-foot-high ceilings to the top of the dome. The volume was over 46,000 cubic feet, with that much volume, you almost need to try to fail a blower door test.
Often, calculating areas and volume takes more time on the test day than running the test. A way to help speed the process is to calculate area and volume is by using the house plans. Depending on your involvement in the project, you may have been given or have access to a set of plans. Small adjustments may be required if you are using the interior finish as the air control layer to calculate the volume. Most house plans will be dimensioned to the exterior, you may need to subtract the wall thickness from the measurement. If you have a digital copy of the plans, there are other options. Both Paul Morin from The Energy Conservatory and Sam Myers from Retrotec suggest using software for calculations. Several programs and apps will allow PDF’s to be imported to their software, which can then be scaled to allow accurate calculations. The calculations can be completed off site, and then verified at the time of the test.
Is having the calculation completed for us an option? It is possible for the architect or designer to calculate volume using software, though it’s not as easy as simply clicking a button. Alexandra Baczek (on Instagram as @alexandrabaczek) from Steven Baczek Architect had this to say about using software to calculate volumes.
“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.”
Having someone else calculate area and volume helps speed the process, but I would also verify. Changes may have been made during the build which could affect the results.
I was recently made aware of a local code that modifies the interior volume calculation in the state of Washington. Bryan Uhler recently had a post on his Instagram feed (@pioneerbuildersinc) showing a video of him blower door testing and calculating the test results on one of his new homes. He was showing the new Washington State Energy Code requirement volume calculation which limits the ceiling height to 8.5 feet, this cap includes any vault or cathedral ceilings. The Washington State Energy Code went into effect February 1st of this year. Rayel Hulsing, (on Instagram as @cascade_aerobarrier) an Aerobarrier contractor who also works in Washington State commented on Bryan’s post:
“This is going to be a challenge on a lot of bigger houses. We performed an air-seal project using the 2018 WSEC (Washington State Energy Code) 8.5’ ceiling height. The actual volume was over 10,000 cubic feet more than the code allows. This resulted in around 1.2 ACH50 difference. Meaning if we came in at 2 ACH50 using the actual volume we tested at 3.2 ACH50 under the new guidelines.”
Basically, the Washington State Energy Code is attempting to equal the playing field between small and large homes. This tweak in the code is moving the air changes per hour at 50 Pascals (ACH50) requirement closer to the cubic feet per minute per square foot of surface area (CFM/Sq Ft) calculation.
I’d like to hear from other readers on their experiences in measuring homes for blower door testing. Do you have any unique codes in your area like the recently adopted Washington State Energy code? Are code officials present when you conduct code compliant blower door tests? Should that be a requirement? I look forward to the discussion.