Building envelope: Testing and commissioning best practices

Due to the increased interest in high-performance buildings, more attention is being paid to how envelope assemblies affect interior comfort and energy performance.

03/26/2013


Figure 1: A typical wall assembly includes exterior cladding, a water channel layer, air barrier, vapor barrier, and an insulation barrier. Courtesy: NEBB and CCJM EngineersTraditionally, building envelopes have been commissioned by architectural inspection using punch lists for tracking needed corrections. Due to the increased interest in high-performance buildings, more attention is being paid to how envelope assemblies affect interior comfort and energy performance. The traditional contractor’s quality process has not been effectively used on building envelopes due to the fact that the assemblies are field built by multiple contractors at various times during the construction process. The building envelope consists of an exterior cladding, water channel layer, air barrier, vapor barrier, and an insulation barrier, each of which must be installed correctly without holes or voids in any of the layers to provide the intended performance (see Figure 1).

To overcome the lack of effective commissioning of these envelope assemblies, organizations like the National Environmental Balancing Bureau (NEBB) have begun to use the technical commissioning process on building envelopes with improved success. The technical commissioning process uses the expert commissioning provider process, instead of the contractor quality process, to achieve these results, and uses both testing of the envelope components and installation inspections.

To assist in the commissioning process of building envelopes, the industry has begun to adopt envelope testing standards. Standards have been developed for air intrusion, water intrusion, and thermal intrusion, but have not been widely used.

Air intrusion testing is the done by pressurizing and de-pressurizing the space and measuring the leakage at an elevated building pressure approximately 12 to 15 times normal building operating pressure. This is most effective when applied to whole buildings rather than individual assemblies. These tests find and identify leaks that when repaired provide a higher level of energy efficiency and comfort.

Water intrusion testing is done by spraying water against windows, doors, and wall assemblies while the building is under negative pressure to find any water leaks in those assemblies. This is most effective when applied to single transitional elements such as window/wall or door/wall transitions or wall-to-roof or wall-to-footing transitions. Roofs also can be tested by spraying or flooding roofs and then investigating the roof system for leaks by thermal or conductance measurements or observations. Flooding roofs is not best practice due to the possibility of damage to the structure or roof elements.

Thermal intrusion testing is done by performing thermal imaging of the envelope components when under a temperature difference of at least 15 F or greater. These images will indicate areas of excess thermal intrusion in wall or roof areas and can indicate the presence of thermal bridging of structural elements and also the presence of air leaks when the building is under test pressure.

Testing standards

Most testing standards were developed for residential construction and are being updated to be used on commercial and industrial buildings. The most successful standard to date is the U.S. Army Corps of Engineers Air Leakage Test Protocol for Building Envelopes, which is beginning to be used for new military structures. This standard was based upon ASTM E 779, which is also being updated to meet present technology and standards. There are many ASTM standards that apply to air leakage, water leakage, and thermal intrusion testing. Most of them are geared for a specific assembly or construction type, making application of these standards to commercial buildings very confusing. This has led to just a few highly experienced firms performing these services at very high costs. The lack of expert firms and the lack of understanding of these standards have limited the availability and economy of envelope testing in the construction industry.

Mandating that all military buildings be tested will require many more qualified testing firms and will require that the testing standards be easier to understand and apply to commercial building envelopes. To this end, the Air Barrier Association of America created a working group to assist the U.S. Army Corps of Engineers in updating its standard, which was completed in 2011. To help train for and commercialize these testing services, several organizations—including NEBB—have developed training and certification programs for building envelope testing, allowing more firms to enter the market and lower the total cost of envelope testing.          

So how does envelope performance testing get applied to a commercial building? First, the designer of the building envelope must specify the acceptable leakage rates for air and water and thermal intrusion for the building under design. This sounds simple enough but turns out to be the most difficult part. For water it is pretty simple—none is the right answer—but for air and thermal, it is not so simple.

Thermal intrusion performance presents a trade-off between envelope mass and material thermal resistance versus daylight and views through glazed openings. Higher thermal resistance of the entire envelope can be achieved with smaller areas of glazing, but without adequate glazing, natural light and views are affected. So for thermal performance, there is no right factor to be applied. It is the task of the designer to provide a balanced compromise of design once optimization of building orientation and dynamic light/shade systems have been introduced.


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SIVA , AL, India, 04/21/13 11:41 AM:

excellent
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