Vermont Commercial Energy Code

Have You Seen the New Air Barrier Requirements in the 2020 VT CBES?
Part 2: Taking the Blower Door Compliance Path

Vermont’s 2020 Commercial Building Energy Standards (CBES) has been in effect since last September. In addition to many minor modifications to the code, there are also several notable changes related to the air tightness of the building.

These code changes are relatively easy to incorporate into the project — unless they are ignored during design and/or construction. BVH is here to help architects and builders save time, money and major aggravation.

In this four-part series we will highlight the new code requirements and strategies to meet them.

In Part I of our four part series related to the new air barrier requirements for the 2020 VT CBES, we looked at the general requirements set forth, and two possible paths to achieving whole-building air barrier compliance.

Part 2 of this series takes a deeper dive into the blower door pathway for compliance. Is it the right option for your project? Please read on to find out.

How feasible is it to reach 0.30 CFM/sf at 75 pascals (0.3 inch water gauge)? How tight is this? 

It is very feasible if the air barrier is addressed in both design and construction. In other words, the design must show a continuous air barrier on all sides of the building (walls, fenestration, roof/ceiling, and foundation) and a continuous air barrier must actually be installed during construction.  If this is done, the building will meet target.  If the building is designed and/or built with little consideration to a continuous air barrier, it probably won’t meet the target.  By way of comparison, average construction built today ranges between 0.50 CFM/sf and 0.80 CFM/sf at 75 pa. On the other hand, we have tested buildings much tighter than this in Vermont with numbers at 0.04 CFM/sf at 75 Pa. and lower. 

As mentioned before, there are two possible compliance paths to meet the air barrier provisions in the code – testing the whole building with blower doors at the end or commissioning the air barrier during construction. However, there is benefit to doing both. The commissioning process provides opportunities for course corrections during construction and helps the construction team to see if the air barrier is on track to meet target long before the building is complete. With a solid commissioning process, you will have a good idea about how the building will perform at the blower door test. It won’t be a day of hand-wringing.

One other point, some of you may be familiar with blower door testing that only counted the above grade surface area of the building for the test. In CBES 2020, the building surface area calculated for the test is all 6 sides of the building: roof/ceiling, walls, and below grade slabs and foundations. The additional below grade surface area, buried in soil and typically very tight, makes it easier to meet the standard than the other method. See C402.4.1.1 (link here and scroll down:

What if the building is difficult to blower door test because it is too large? There are no size limits to buildings that can be tested.  Larger buildings just need more blower door fans.  However, the code allows buildings over 50,000 sf of floor area to be tested in representative sections if needed (C402.4.1.1) but often this is not needed.  We have routinely tested new buildings as large as 200,000 sf. Sometimes it is actually easier to test a whole building, even if it is large, than to try to test it in sections.  

What if the building is difficult to blower door test because it is too interconnected with another building? 

The code language is clear that new additions must be tested. It says that additions must meet the provisions of the code but the original unaltered buildings they are attached to do not have to meet the code (C502.1).  In other words, the addition has to be blower door tested even if it is interconnected to another building. However, the original building does not have to meet the air tightness standard if it has not been improved or renovated.    

One of the problems with testing an addition is, how do we know how much air leakage is in the addition, versus air leakage in the original building?  The code does not get into the specifics of how to separate the air leakage of the addition from that of the original building. Likely, because the answer is highly dependent on the situation. There is a method, called guarded, or pressure-balanced, blower door testing that we can use in these situations.  Effectively, it allows us to separate the addition from the existing building in terms of air movement.  From our experience, blower door testing an addition is relatively easy if the addition is connected to the original building at just a few points, like a wing connected to the main building by a single hallway.  However, if the addition is wrapped around, or surrounded by, the original building and/or it shares ductwork with the original building, blower door testing with accuracy becomes increasingly difficult.  In a few cases, even pressure-balanced blower door testing cannot be used effectively so air barrier commissioning may be the best way to show compliance with the code. 

At what point in construction should we do the test?

The code is not explicit about precisely when the blower door test should occur but it is implied that it should occur once the air barrier is complete. Typically, this would mean near the end of envelope construction, after exterior doors have been installed along with supplemental weather-stripping and hardware. In our experience, this usually falls somewhere around a month or two before building occupancy. Waiting until this point better ensures that all air barrier components have been installed. If waiting until that point just makes you too nervous, then take the commissioning compliance path, or if it is feasible for your project, do a preliminary blower door test. For commercial projects that are relatively small, the air barrier can often be largely complete around the entire building before it is completely covered by insulation and finishes. Doors might not be installed, and there may be a missing window here and there, but by masking these areas off, you can test the building unofficially to gauge whether or not you are on track to meet target. The benefit here is that your air barrier is still largely accessible for improvement if necessary, and there is less of that hand-wringing we mentioned when it comes time for the final compliance test. Preliminary tests don’t really work for large buildings though. One end might have finish brick going up while the other end is only erecting light gauge framing.

How do we prepare for the test?

Blower door testing is disruptive to the flow of the workday that the test is scheduled for and has potential to be quite involved. For these reasons, it is best to hold a pretest coordination meeting to work out the bugs so that all proceeds smoothly on test day. This coordination meeting should include reviewing the target and the surface area of the building (which determines the final CFM pass/fail threshold), the building configuration for testing, the building access limitations, schedule of the test, the coordination of any personnel for specific needs like building/room access, and  HVAC control. For most blower door tests, building configuration can be broken down into three general parts:

  1. Allowing air flow throughout: Blower door fans will be distributed at select entrances of the building. In order to ensure those fans have access to adequately depressurize all parts of the envelope, we need to eliminate interior restrictions. This means opening all interior doors that otherwise separate conditioned spaces from each other, including stairwells and corridors.
  2. Masking: Since the air barrier is what we are officially quantifying, we need to isolate it by eliminating other non-envelope components that might influence the blower door results, namely intentional openings. HVAC intakes, exhausts, dryer exhausts, etc. need dampers closed and then they should also be masked with tape and poly sheeting.  This also means that all HVAC systems need to be shut off for the test. 
  3. Diagnosing the Barrier: Should the air barrier not meet its target, we need a way to identify outstanding sources of air leakage in the air barrier. This can be accomplished by pressurized fog or infrared imaging. Pressurized fog requires fire alarms to be deactivated and the local fire department notified. Infrared imaging requires a temperature differential of at least 15 degrees between indoors and out. In the winter, this is easy to obtain, but it may require temporary set point adjustment in the swing seasons and in summer. Infrared imaging is generally the preferred diagnostic method when possible.

What if the building does not meet the air tightness requirement? 

This is the key question, isn’t it?  One way to avoid this question entirely is to take the commissioning path to meet the air barrier requirements in the code. Then, a blower door test is not required. We will discuss that path more in the next post: “Part 3: Taking the Commissioning Compliance Path.” 

If you blower door test the building and it is greater than 0.30 CFM/sf at 75 Pa., but still falls between 0.30 CFM/sf and 0.40 CFM/sf, then air leaks must be identified in the building and corrected.  A written report showing the corrective actions must be submitted to the code official and the owner.  The code does not say what is required if the building fails to meet the target and the leakage rate is higher than 0.40 CFM/sf but it implies that more invasive measures will be required to bring the building envelope into compliance.      

Should you have further questions or need any assistance about how to meet these code provisions in your project, please email or call Jon Haehnel, BVH’s Building Envelope Specialist at or 802-522-9713.