by Jon Haehnel
BVH was recently asked to undertake envelope commissioning as part of a university science building renovation that included a new, subterranean clean room to accommodate a highly sensitive electron microscope. This instrument clean room included isolation from acoustic noise, airborne vibration, electromagnetic fields and envelope tightness capable of maintaining moisture migration into the sub-30% relative humidity space conditions.
Controlling space pressure and humidity in the built environment comes down to two issues: HVAC systems and building envelope. The tighter you can make the envelope, and in particular the better the air barrier performance, the better control over space pressure and humidity.
Our building envelope team was brought in to determine an acceptable level of leakage, test the space and undertake any steps necessary to address the findings. Based on our experience with similar spaces, we recommended .25 cfm/sf as an appropriate target. Of note, the calculated surface of the room included floor, wall, and ceiling area in square feet (sf).
Since this room was built inside another building six floors below grade – essentially a room within a room – the design and construction team had endeavored to seal the new space by building exceptionally thick walls (approximately 20 inches thick). The outer wall was a single-sided partition with metal studs. Acoustical batt insulation was installed to absorb acoustic noise and airborne vibrations. The inner wall, which forms the shell for the instrument, was a double-sided metal studded partition. Three layers of gypsum board, batt insulation, electromagnetic shielding and acoustical absorbing layer comprise this inner wall which is why the team expected the room to be pretty tight once we began testing. Unfortunately, at nearly six times the target (1.34 cfm/sf), it was anything but. Why was the room so leaky? This was because they failed to think about air sealing at critical points during construction. Clean rooms require special attention to wall, ceiling and floor assemblies and need to account for the pipes, ductwork, wires and other lines that penetrate these surfaces. Every transition needs to be carefully sealed.
BVH crews went to work diagnosing the space and recommending repairs. In the first round we substantially improved the air barrier performance, reducing the cfm by two thirds to .43 cfm/sf. After another, we tested again, this time achieving .28 cfm/sf.
From the beginning, the owner’s requirements were to maintain tight tolerances for temperature and humidity in this space. A tight envelope is necessary to meet these requirements. While establishing a basis of design wouldn’t typically be employed for a single room, it should be if it’s a critical room; we would definitely want to start with that discussion if we were undertaking a high-performance building project. Beyond that, three key steps would ensure success in meeting even the most stringent sealing requirements:
- Hold a plan review – What little changes can be made that would make envelope more efficient, tighter, easier to construct?
- Undertake regular visual inspections early in construction.
- Do a preliminary blower door test; it’s the best way to check if you’re on the right track.
A high-performance building envelope is well worth the investment, whether you’re building clean rooms or bedrooms. A tighter envelope will allow the equipment to operate at a much higher efficiency and maintain space pressure than one that has a high leakage rate. This in turn minimizes the risk of post-occupancy issues, all while saving energy and operating costs over the life of the building.
Jon Haehnel is the director of building envelope services at BVH Integrated Services, P.C. His expertise in building envelope testing and design focuses on institutional and commercial projects throughout the Northeast.
CFM (cubic feet per minute) is a measurement of the velocity at which air flows into or out of a space.
On a per square foot basis*, current rates range:
- Average construction – .6-.8 cfm at 50 pascals (0.2 inches w.c.)
- High-performance threshold – .25 at 50 pascals (0.2 inches w.c.)
- US Army standard – .25 at 75 pascals (0.3 inches w.c.)
- US Marines standard- .15 at 75 pascals (0.3 inches w.c.)
- Passive House standard for commercial buildings: .05 at 50 pascals (0.2 inches w.c.), which can significantly reduce the size of the HVAC system.
* The surface area calculation includes all six sides of the conditioned space.