Building envelope: Testing and commissioning best practices

03/26/2013


Applying the tests

Determining the best leakage rate is much more difficult because every building leaks and the engineering goal is to minimize the rate as much as possible for the lowest construction cost. Leakage is due to the permeability of the air barrier itself and how the air barrier is applied, especially due to transitions and penetrations such as pipes and conduits and due to how many doors and windows a building envelope has.  

Buildings normally should operate at a positive pressure of 4 to 5 PA (0.016 to 0.02 in.). At a test pressure of 75 PA (0.3 in.), a leakage rate of 0.4 cfm/sq ft of air barrier would be a very high leakage rate and 0.1 cfm/sq ft would be a very low leakage rate. By specification, these leakage rates are measured while the building is at a pressure difference of 50 to 75 PA, between the interior and exterior. The air barrier area is defined as the area of the first floor, the top floor ceiling, and the total area of the exterior wall air barrier. Table 1 indicates standard leakage rates for various types of structures at either 50 PA or 75 PA building pressures.

Courtesy: NEBB Procedural Standards for Testing of Building Envelopes 2012

Courtesy: NEBB Procedural Standards for Testing of Building Envelopes 2012     

Required building test parameters (specified by designer):

  • Leakage rate in cfm/sq ft of air barrier; standard leakage rates are 0.40 to 0.10 cfm/sq ft
  • Test pressure in PA; standard test pressures are 50, 60, and 75 PA
  • Test standard used ASTM E779, ASTM 1827, U.S. Army Corps of Engineers, or NEBB
  • Number and general locations of water intrusion tests and standard used
  • Specified requirement for thermal testing.

The type of structure and number of openings will have a significant impact on the ability of the contractor to achieve the desired leakage rate. A concrete structure with few openings can achieve a leakage rate of less than 0.1 cfm/sq ft, but a steel stud wall building with many openings may not be able to achieve less than 0.25 cfm/sq ft. The U.S. Army Corps of Engineers standard requires less than 0.25 cfm/sq ft at a test pressure of 75 PA. The last thing the designer must specify is what assemblies the pressure, water, and thermal tests are applied to. It is best practice to apply pressure testing and thermal testing to the whole building envelope, and water intrusion testing to a sampling of types of transitions at windows and doors.  Exact locations of water intrusion tests should be determined by the testing agency, since the locations will provide a sampling of the overall envelope performance. The contractor should not be told where the test locations will be, so that the random sampling will indicate the overall quality of the installation.

Engineers pressure-test a building with a three-blower door system. Courtesy: Commissioning ConceptsAir intrusion testing is normally done with multiple blower doors and fans to achieve the high airflow rate required to test commercial building envelopes. There is a limit to how large a building you can test with blower doors. That limit is approximately 150,000 sq ft, depending upon the leakage rate of the building; the lower the rate, the larger the building can be. If a building is too large for blower doors to be easily used, the HVAC system can be used as a pressure test apparatus. To use the HVAC system, it must be specifically designed to be used for both the standard HVAC requirements and the testing requirements. These design attributes are the capability to place the building under both positive and negative pressure to 75 PA, to produce the high flow rates required, and to accurately measure the flow at those test pressures. The NEBB Procedural Standard for the Testing of Building Envelopes contains standards and requirements for testing using the HVAC system.

Requirements for using the HVAC system as a testing apparatus are:

  • Capability of providing both positive pressure and negative pressure
  • Capability of providing test flow in both positive and negative flow directions
  • Capability of providing very accurate airflow reading for both flow arrangements. 

One interesting impact of building pressure testing is that you can easily calculate the actual building air leakage quantity at normal operating building pressures. Currently, HVAC designers have a difficult time accurately calculating how much a structure will actually leak, and most designers leave this amount out of the makeup air calculation or use a swag of 10% of the supply airflow, neither of which is correct and leads to the building being over pressurized or under pressurized. Our testing experience allows us to accurately calculate the actual leakage rate once we know the testing pressure and the acceptable leakage rate. We can calculate what the actual leakage rate will be at normal operating pressure by using the basic flow calculations for the pressure test.

Example:

For a 100,000-sq-ft two-story building that has 50,000 sq ft per floor, a total air barrier surface area of 126,500 sq ft, and a specified maximum leakage rate of 0.25 cfm/sf, the allowable leakage amount at test pressure of 75 PA will be 31,625 cfm. Using the formula to find the maximum allowable leakage rate at a normal operating pressure of 0.02 in.: 

We see that the maximum leakage at normal building pressure of 0.02 in. will be 8,945 cfm. If the actual test flow is less, then the actual building pressure leakage will be less, but this allows the HVAC designer to plan for the maximum allowable leakage flow rate.

Two of the more important questions from owners and architects: How much does this testing add to the cost of the building, and how much testing should we do? Best practice would indicate that at a minimum whole building pressure testing and an infrared thermal survey should be done on every building because the value of these tests is high. Water intrusion testing can be very expensive if excessively applied. Best practice would indicate that water intrusion testing should only be done on any building that has complex window walls or doors with complex transitions, and it should be applied with a sampling strategy so only a few tests are conducted. The costs of pressure testing varies depending upon the building size, layout, and testing constraints and the quality of the construction, but a rule of thumb for the example building would be $0.20 to $0.35/sq ft of floor area. A rule of thumb for a thermal survey for the example building might be in the range of $0.03 to $0.05/sq ft, and a single water intrusion test might be in the range of $2,000 per test.

Equipped with these modern standards and testing methods, building envelopes should now be tested on high-performance buildings to ensure they are actually providing the performance as designed.


James Bochat is president of Commissioning Concepts. He is a past president of NEBB and past chairman of the NEBB commissioning committee. He was instrumental in developing the NEBB commissioning, retro-commissioning, and building envelope commissioning disciplines and has taught commissioning seminars across the United States and in Australia for NEBB. He is chair of the ASHRAE Performance Measurement Protocols best practices committee and a past member of the ASHRAE building performance metrics multidisciplinary task group. 


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

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