Continuous Air Barriers in Multi-family Buildings

Alistair 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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Comments

[Tyler Douglas]

I am actually talking to an archetect today about a continuous liquid applied air barrier for a multifamily complex. The benefits are very great to shift from a wrap to a liquid applied simply because of the superior water protection you get with such a product.
Commercial has picked up on the need for better weather / air barriers, and the residential side is just catching up, mainly because building codes are driving the change.
I am currently installing an air barrier with a price point that the residential market in Utah can support. The main holdup for Multifamily is the cost. A commercial grade air barrier might be too expensive to be applied in that application.

[B Mac]

Alistair, nice piece. Just wanted to send word to those interested that Tyvek CommercialWrap and CommercialWrap D recently completed ABAA evaluation and are now listed at the ABAA website as meeting all requirements for commercial Air Barriers (materials and assemblies) and weather resistive barriers. So you're right, we are seeing the addition of performance testing requirements that do measure the effectiveness of the approach.

[Alistair Jackson]

Thanks B Mac. It's great to hear that there are more approved products available. I believe the ABAA listing only addresses material performance against ASTM E2178; assessing the material's resistance to air infiltration. That's an important first step, but on its own, is insufficient. Equally important are the material's ability to remain effectively sealed at seams, to resist perforating damage where it is mechanically fastened to the building, and to "tolerate" other challenges to its air-tightness during construction - such as abrasion, wind tearing and so forth.
Envelope performance testing is a major step towards better building performance, but failing the test can be an expensive headache. Quality assurance steps taken by the design and construction teams are critical to ensure successful testing at completion.

ABAA's website - www.airbarrier.org - is worth a visit. Can anyone recommend any other great resources for air barrier installation details?

[Alistair Jackson]

Tyler Douglas - thanks for your comment. We've had some limited experience with liquid applied products. Initial impressions suggest they are effective, but costly, as you suggest. They maybe easier to install than membrane systems, which may result in lower overall costs. Does anyone have first hand experience to share?

One other aspect of liquid applied air/weather barriers is that while they may enhance a building's durability, I suspect they also render the sheathing useless for anything except the landfill once that building finally reaches its service life.

[Tyler Douglas]

As for cost, the starting point for cost could be as low as $.75 per sqft installed, depending on the height and total project size. Other commercial products range from $2-$3 at least installed.

Here is the link to the application instructions for the product Envirodri I was talking about. I think the paperwork to have it on Multifamily projects will finish this year. I will keep you up to date. Liquid Applied Air barriers have to pass as an assembly, making it a superior product because of the stricter tests that are enforced. It would be a large step forward since it is less than half the installed cost of other air barriers on the market currently. It is a Vapor Permeable Product allowing water to slowly defuse through the sheathing to the outside, but restricting bulk water from entering in. The system includes detailing over every sheathing joint and flashing over windows to completely replace papers or wraps.

http://www.guaranteeddrybasements.com/fileshare/installation/TBS0564EnviroDriInstallInst.pdf

They manufacture other air barriers under Tremco commercial such as Exoair, but like I said, the price point is higher than EnviroDri.

http://www.tremcosealants.com/commercial/products/product_types.asp?type=26

As for reusability of the sheathing, That might be a minor cost compared to the energy savings one might get with a tighter building and upfront savings in HV/AC. Not to mention the savings in liability with a product that is much more durable and lasting than papers or wraps. It also helps in the energy performance since under 0mph winds to 75mph winds, there is no change on how effective it is as an air barrier.

As for experience, I've installed over 10,000 sqft of EnviroDri on smaller residential projects. The most sucessful one was in Park City Utah where I not only sealed the seal plate but also the windows to the sheathing with the reinforcing mesh, providing a monolithic seal to the window and sheathing.