By Phil Emory, on Feb 3, 2018, The NEBB Professional – Q1 2018 Edition
Recently I had a few discussions with mechanical design firms regarding the effects of tighter building enclosures and how they affect the HVAC systems within the enclosure.
As buildings become tighter, the back pressure the enclosure creates is influencing the HVAC systems, most notably the elevator/stairwell fans. Several mechanical design firms are realizing the pressure gradients are changing due to the tightness of the building. Attention needs to be taken to ensure the proper relief so the pressure gradients can be achieved to meet the local fire code requirements.
I carried the conversation further with respect to the effects on the rest of the air systems. If local code requires an air barrier to be included in the design, and the barrier itself is to be tested to meet a code standard, most typically 0.40 cfm/ft2 of building enclosure at a reference pressure of 0.30 in H20, then are the firms using new models to size the air systems?
At this point the room usually goes quiet.
The point I try to drive home is this: If the code requires an air barrier in the design, showing the barrier in plan view, elevation view, and the appropriate details for its continuity, along with the calculation of the square footage of the enclosure (which would greatly affect the mechanical design), then why are drawings being permitted without the air barrier design? If the mechanical design firm is part of the drawing development process, then should they not press the Architect of Record to include the air barrier drawings as required by law?
Another issue that has risen with the mechanical design firm is the current standard of the building enclosure test, which reports the amount of leakage at the reference pressure of 0.30 in H20. The question is, “How relevant is the current reference pressure to the mechanical design? Why not focus on reporting the leakage based on the anticipated operating pressure of the building?”
The reason the reported leakage of a building enclosure test was raised to of 0.30 in H20 was to make the tests more repeatable from one test to another. The original attempt was to have this testing requirement as part of the building code, not the energy code. But how do you prove that a proper air barrier would increase the life of a building? It was possible to show energy savings and that’s why we now have this as part of the energy code in many local jurisdictions.
If a building enclosure test is performed by sealing and unsealing different items to approximate their contribution to the percentage of leakage, you may find that the leakage, as a percentage of the total leakage, varies dramatically throughout the enclosure when reporting at 0.04 in H20 versus the current standard of 0.3 in H20.
Pressure gradients are critical in pharmaceutical facilities, hospitals, cleanrooms and many other facilities. A tight air barrier will affect the design and final operation of these facilities. All too often we only think of air barrier design at the external skin of the building – when the same concepts can be utilized at the internal walls to help achieve proper pressure gradients.