Straight Green: Continuous Air Barriers—Upping the Ante
The requirement for buildings to be designed with a continuous air barrier has been making its way into U.S. building codes ever since the state of Massachusetts first did in 2001 with the introduction of Section 1304.3.1 Air Barriers.
The requirement for buildings to be designed with a continuous air barrier has been making its way into U.S. building codes ever since the state of Massachusetts first did in 2001 with the introduction of Section 1304.3.1 Air Barriers:
“The building envelope shall be designed and constructed with a continuous air barrier to control air leakage into, or out of the conditioned space.”
The requirement includes descriptive language about sealing joints, penetrations and connecting adjacent air barrier systems as well as prescriptive language requiring air barrier permeability not to exceed 0.004 cfm/ft² under a pressure differential of 0.3 inches water.
Massachusetts borrowed its building code air barrier language almost word-for-word from Canada’s National Building Code, which has required air barriers for more than 15 years.
The Massachusetts code does not address verification—submit a material test report for the air barrier material (typically the weather barrier), slap together a few details showing sealed penetrations and adjacent assemblies tied together, and you are on your way out of the building department, permit in hand.
In 2009, the U.S. Army Corps of Engineers issued an Engineering and Construction Bulletin directive requiring all new Army construction projects for building air tightness and building air leakage testing. As with the NBC and Massachusetts building code, air barriers are required and a combination of descriptive and prescriptive language is provided. However the USACE goes further, requiring that building construction documents clearly define the air barrier and show details of “the joints, interconnections and penetrations of the air barrier components.” Additionally, the directive addresses verification that the air barrier was designed and constructed correctly by requiring whole building air tightness pressure testing and Infrared Thermography.
U.S. state and city jurisdictions are taking notice of the USACE verification requirements. Many are considering the incorporation of these requirements into building codes. In 2009, Washington state and the city of Seattle did incorporate such requirements (and both have since been adopted). The WA State and Seattle energy codes require a continuous air barrier (all buildings under Seattle jurisdiction, buildings over five stories in WA State) and verification of performance as follows:
“Compliance of the continuous air barrier for the opaque building envelope shall be demonstrated by testing the completed building and demonstrating that the air leakage rate of the building envelope does not exceed 0.40 cfm/ft² at a pressure differential of 0.3 inch w.g. (1.57 psf).
Both codes also require: “All air barrier components of each envelope assembly shall be clearly identified on construction documents and the joints, interconnections and penetrations of the air barrier components shall be detailed.”
Air tightness testing requirements have also recently been incorporated into California Energy Code (scheduled to take effect in 2014) and are being considered by the Army, Navy, Air Force, U.S. General Services Administration, the State of Massachusetts, and Salt Lake City. Assuming this trend continues, it won’t be long before all U.S. jurisdictions adopt similar requirements.
TESTING LEADS TO
During the first year the USACE began testing buildings for air tightness, every building failed. However, over time, the testing has resulted in improvements of better air tightness. Once designers and contractors were put to task through the testing requirement, problem areas were identified and addressed during design and construction.
The USACE now routinely gets results below the 0.25 cfm/ft² at 1.57 psf requirement for air tightness. So many results come in with low numbers below the requirements, that USACE is now considering adopting a more stringent requirement as low as 0.05 cfm/ft² at 1.57 psf.
CHASING THE AIR BARRIER—
THE PEN TEST
Architects and engineers are not regularly thinking about the continuity of the four control layers of the building enclosure: water, vapor, air, and thermal. In a typical two-dimensional detail in a set of construction documents, one will commonly find materials called out as moisture barriers, vapor barriers, insulation, and flashing, but I have never seen any drawings that follow each of these control layers through the building enclosure details. For architects and engineers in WA, this is now a requirement for the air barrier assembly. But what does this look like?
For years now, building enclosure professionals have been trying to get designers to pay attention to this, and for years they have failed. Consider the detail in Figure 1. With a colored pen, try to draw a line representing the air barrier layer from bottom to top without lifting it from the paper. Were you successful? Now think about every part of the enclosure where the air barrier changes—from material to material, and assembly to assembly. Each one of these must be indicated on the construction documents in accordance with energy codes in Washington state and Seattle. As the above exercise demonstrates, this is no easy feat, and a skill that architects and engineers will need to get very good at, very quickly.
AIR TIGHTNESS TESTING
When Washington state and the City of Seattle incorporated, and eventually adopted, building air tightness testing requirements into the energy code, Phil Emory of Neudorfer Engineers Inc. could barely contain his excitement. Emory is a Mechanical Engineer specializing in building air tightness testing in Seattle. He expects to be very busy as the code is enforced. He has conducted several air tightness tests since the code requirement came into effect and has learned some interesting lessons:
Testing a small mockup of an air barrier assembly is not an effective method in getting to an air tight building. Although he will conduct such tests if asked, he also makes it a point to let the client know that this is typically a waste of time and money.
Major air tightness testing failures are almost always due to holes at roof/wall transitions and similar air barrier assembly interfaces. Paying attention to getting these holes plugged during design and construction is the key to success. Fretting over how permeable the weather barrier and peel and stick transition tape might be is a waste of effort without first adequately addressing on the HOLES.
Jurisdictions’ rapid and unquestioning adoption of air tightness performance requirements and testing standards are a potential ticking time bomb. The USACE picked its 0.25 cfm/ft² at 1.57 psf performance benchmark completely arbitrarily. And although now this is a performance criterion that most of its buildings are able to meet, most of its buildings are essentially one- to two-story wood-framed houses. The USACE air tightness requirements have not been studied for relevance to large, commercial-multistory buildings. It is unknown how these requirements will be fulfilled for such buildings, what the cost impacts will be, and whether or not any significant results will be achieved.
There are no scientific studies that exist to examine the correlation between air tightness and energy savings. The theory that a building—any building—is a better energy performer the tighter its enclosure becomes has not been proven. How much more energy efficient is a 20 story high rise office building located in downtown Seattle going from an air tightness of 0.5 cfm/ft² at 1.57 psf to 0.25 cfm/ft² at 1.57 psf?
Air barrier continuity is likely to be something that more and more design professionals and contractors are required to address in ways they have never had to. The adoption of air tightness performance and testing requirements is on the uptake across the United States. Will these new requirements result in the energy savings promised? Or will they be just another unenforced and unsubstantiated “project killing” regulation?
Air tightness is good for buildings, and paying close attention to air barrier details is something that designers should be doing in every jurisdiction. But, instead of blindly adopting arbitrary performance and testing standards, it seems a more prudent path might be to first examine the relationship between the air tightness of different building types—size and use, against the potential energy savings, and then make decisions about air tightness performance and testing requirements. W&C