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Continuous Air Barriers in Multi-family Buildings

AJ web photoAlistair Jackson, CSBA and LEED for Homes QAD discusses a recent development in multi-family building performance.

With the implementation of the Washington State Energy Code of 2009 came the requirements (Section 1314.6) that multi-family buildings in Washington State include a continuous air barrier in the building envelope and, more challengingly, test that envelope for infiltration performance.  Projects that chose to pursue certification under the LEED for Homes Midrise Pilot ran into a similar requirement in that program a year or more before the WSEC implementation and so got a head start on the process.

The value of a tight building envelope has been broadly accepted in the world of single family construction for several years, supported by sound building science, as an effective way to reduce space conditioning energy, improve thermal comfort and improve moisture management.

In multi-family buildings, this broad acceptance has not yet occurred.  Part of the reason for this is the common assumption that multi-family buildings are dominated more by internal loads than by envelope loads (because of the skin to volume ratio), so the thermal and air-exchange loads of the envelope have tended to take a back seat to mechanical conditioning, ventilation and lighting issues.  In reality, however, the taller form of these buildings leads to significant pressure differences from bottom to top that can drive much greater air exchange across the building envelope for a given surface due to stack effect.   This must be countered by larger mechanical systems to keep the building balanced and comfortable. 

By improving air barrier continuity and hence controlling the leakage through the envelope, we tip the first in a line of dominoes that topple, one by one, towards better overall building performance.

Generally speaking, there are two approaches to creating a continuous air barrier in a building envelope.  The more “conventional” approach, originating in single family construction and long supported by Energy Star, RESNET and others, is to use rigid, self-supporting components of the building assembly – such as sheathing, framing, gypsum board, – to form the air barrier, ensuring continuity by sealing them together with resilient and often flexible materials (glues, caulks, foams) to form an air tight box.  This is a reliable approach as long as the craftspeople assembling the building understand the intent of the details and install them carefully.

On the scale of a six story midrise building, however, with 300 residential units in it and multiple transitions between wall assembly types, across floor plates, and so forth, this approach can prove challenging to manage and implement.

More recently, the availability of high-performance building wraps or membranes, designed originally as weather resistive barriers, has led to the exploration of using these products to also serve as a continuous air barrier – we’ll call this an “exterior membrane air barrier” system.  Manufacturers are providing installation guidelines for their products as an “Air Barrier System” with additional details that go beyond weather barrier requirements – notably addressing sealing to the structure, sealing of lap seams and fastening systems. 

This “exterior membrane air barrier” approach is attracting a lot of attention among the current raft of multi-family projects pursuing LEED for Homes Midrise certification. We’ve seen systems using Vaproshield and Tyvek CommercialWrap D on recent project specifications. With many of these projects using a qualified envelope durability consultant, there’s good confidence of a successful implementation. 

To date, we have been cautious about this approach because we’ve seen, first hand, that efforts to properly seal lap seams between sheets and at openings can be challenging and that the typical, liberal application of regular staples to fasten the wrap to the sheathing results in an equally liberal distribution of torn perforations in the wrap rendering it useless as an air barrier.  Furthermore, some wrap products can degrade with prolonged exposure to sun and wind, resulting in reduced performance over the long term.

So adapting this material and system requires some changes in approach and in understanding of the intent.  As we have seen in the past with changes of construction practice, adoption follows the typical bell curve – early adopters jump in and figure it out, then practice percolates into the mainstream.  Both the rigid air barrier approach and the exterior membrane approach demand attention to detailing to be effective. What remains to be seen is how quickly the construction sector will build the capacity to properly implement the details to a standard that will deliver performance. 

The good news is that the addition of a performance testing requirement means we'll see real measurements of the effectiveness of this new approach.  Assuming that the market place can actually deliver the capacity for large building infiltration testing (another story) by the time these buildings are completed, we should get good empirical data on the various approaches to creating a continuous air barrier fairly quickly as buildings are completed.  I’m on the edge of my seat . . .

Alistair Jackson, CSBA is a Principal at O’Brien & Company. He was involved in the development of the original LEED for Homes Pilot Rating System and is, among other things, the Quality Assurance Designee (QAD) for O’Brien & Company’s role as LEED for Homes Provider. He is a strong advocate for market-led innovation and market transformation towards sustainability. He is also a father of two – and impatient for change towards a better future.

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