Building envelope testing in existing buildings
- Understand the codes and standards that relate to air pressure testing.
- Know how to conduct a pressure test on an existing building.
- Understand alternative methods of testing for leakage in existing buildings
Air pressure testing of the building envelope or building enclosure is gaining traction in the industry due to the fact that air tightness of a building improves the building’s performance. For increased building performance, it is important to minimize the building’s air leakage to limit the negative impact it has on energy and indoor environmental quality (IEQ) issues within the building. The air pressure testing procedure for new buildings is fairly straightforward and has several testing standards to follow (see below), but testing of existing buildings is another matter. Existing buildings cannot be tested under the same protocols as new buildings, so you have to approach an existing building from many different angles to achieve the end goal.
Prior to testing, all intentional openings (such as exhaust fans, wall louvers, flues, and vents) are sealed. Sealing is normally achieved by using Visqueen and packing tape or carpet tack film to cover the opening in such a manner that the induced test pressure does not blow off the seal. All doors and windows need to have permanent weather seals and sweeps installed. A minimum of 10% of the ceiling tiles must be removed and all interior doors opened to assure a clear air path between the air barrier and the test fans.
Once the building is sealed, test fans and pressure manometers are installed to measure the leakage rate in accordance with the test standard being used. Before the leakage rate is tested, a building bias or baseline pressure is established by measuring the pressure differential between the interior and exterior of the space with the fans off and sealed. This pressure—which is created by either stack effect or wind pressures—will be used in the final test calculations. After the baseline test, the flow/pressure test is completed. The actual leakage test is run by measuring the leakage flow 10 times at each test pressure and using the average pressure and flow rate.
The test results will indicate if the air barrier leakage rate is below the selected leakage rate standard. To pass, the test must show a leakage rate below the standard leakage rate but must also indicate that the test results achieved at least a 95% statistical confidence level. (The accuracy of the test is dependent on the amount of variance between each pressure reading for both the baseline test and the test pressures. If they vary by more than 10%, the test will not be within the 95% confidence level.)
The test is performed once in a positive pressure mode and once in a negative pressure mode, and the results are then averaged; this increases the accuracy of the test if the bias pressure is above zero. Once you know the final leakage rate, you can also calculate the equivalent single opening size for that rate; this is helpful in visualizing the extent of the leaks for showing contractors and owners how critical it is to get these leaks sealed.
To clarify, new building testing procedures normally cannot be used on an existing building unless you can take the building out of service for an extended time to do the test to setup and seal all normally open openings for a pressure test.
Existing building pressure testing
So how do you test the air tightness of existing buildings? The first thing is to determine what goals you are trying to achieve. Normally, existing buildings are tested to:
- Reduce the impact of air leakage on energy usage
- Reduce the effect of air leakage on IEQ such as comfort, temperature humidity, and indoor air quality (IAQ)
- Reduce noise intrusion into the space.
If the goal is to achieve a predetermined leakage rate similar to that of a new building, then the options are limited to taking the building out of service, allowing no occupants in the building, and then performing a new building pressure test. If the goals are to improve the building as stated above, then you can achieve these benefits by applying several methods of testing other than whole building pressure testing under a new building protocol.
The simplest method to identify air barrier leakage is visual inspection of the building enclosure. If you can see through the envelope, then debris, water, air, and vapor can enter the building. Look for damaged or missing weather stripping and door sweeps. Look for gaps or failed caulking at expansion joints and transitions between walls and roof, between walls and floors, and between walls and windows and doors.
When all visual inspection is complete and all deficiencies are resolved, determine how to find the leaks you cannot see. Building leaks allow debris, water, air, and water vapor to enter the building depending on the size of the opening, with debris requiring the largest opening and water, air, and vapor requiring smaller openings.
After you have inspected the building enclosure and have corrected any obvious deficiencies, then the next step is to check the building operating pressure. The typical goal is to maintain a normal building positive pressure of about +0.015 to +0.02 in. of water pressure. If the building is in a very cold climate and has experienced humidity issues where the water vapor was driven into the structure, then a neutral pressure should be maintained. It is undesirable to maintain a negative building pressure because that will negatively impact project goals. In other words, sometimes a positive pressure is not a good thing (especially in a cold climate), but a negative pressure is always bad. Humidity control and its effect on building envelopes is another subject not covered in this article.
Building operating pressure is achieved by bringing in more outside air through the air conditioning system than leaves the building through exhaust fans, reliefs, and building leakage. The correct procedure for an existing building is to measure the building pressure across an outside door, and adjust the outside airflow at the air handling units until the desired building pressure is achieved. Once the desired building pressure is correct, measure the total outside airflow and subtract all exhaust air and relief airflows to discover the total building leakage flow.
Measuring outside airflow by a pitot or velgrid measurement at low pressure is not as accurate as using a calibrated fan blower door at high pressure. Because you are not testing the air barrier to a standard leakage rate, but rather trying to improve the enclosure performance, a high level of accuracy is not required. (Many times the building is being overventilated due to relief fans running when they are not needed. Total outside airflow can be reduced by turning off or adjusting these relief fans.) If the people ventilation cfm, as required by ASHRAE Standard 62.1, is greater than the existing outside air (OSA) cfm, then the OSA must be increased to meet the ventilation rate and the excess air cfm will need to be relieved through the relief air system. Normally, this is not the case; most buildings have more leakage and exhaust than they require for ventilation air.
To determine if your existing building has excessive leaks, use the following rules of thumb:
If you are trying to see how close an existing buildings leakage rate is to a new building equivalent tested leakage rate, then divide the actual measured OSA flow minus the measured exhaust flow by the air barrier area and see how close that factor comes to the rule of thumb shown above. For this calculation, figure the air barrier is equal to the area of the roof and the floor area plus the area of all the exterior walls.
As an example, a single story 100,000-sq-ft office building with dimensions of 250 x 400 x 16 ft, a slab-on-grade floor, and a flat roof would have an air barrier of 220,800 sq ft. If the measured OSA flow was 20,000 cfm with 6,000 cfm of exhaust airflow and no relief airflow, then the air leakage amount would be 14,000 cfm. Dividing the air leakage amount (14,000 cfm) by the air barrier area (220,800 sq ft) results in 0.063 cfm/sq ft leakage rate at 0.02 in. pressure. Interpolating from the rule of thumb table, this would be equivalent to a building that achieved approximately a 0.24 leakage rate. This is, of course, a rough rule of thumb and cannot be compared to the accuracy of a whole building pressure test, but it can guide you to a more tightly sealed building.
Because it is not usually possible to conduct whole building pressure testing on occupied existing buildings, you cannot test to a leakage standard and come away with an overall building leakage rate as you can on a new building. But you can test the existing building for leaks; you just cannot quantify total building leakage accurately.
One method of pressure testing is to test components or assemblies in isolation by building an enclosure around the component and testing only that section of wall. Testing single components will identify the leakage rate of the component assembly, but it will not provide a whole building leakage rate. It is also possible to test a single room that has an exterior wall, but normally the interior walls will not have air barriers, so again the test cannot determine if the air leaks are on the outside wall or the interior walls—unless you equalize the pressure on all other rooms around the test room so the leakage rate of all interior walls equals zero (called a “guarded test”). To do this, place blower doors in all the surrounding rooms and keep the pressure difference between the test room and the surrounding room at zero. If you are doing a lot of exterior wall rooms, this process can become quite expensive due to the number of tests.
Low-pressure testing and thermography
Another method is to use thermography to try to find areas of hidden leaks. This requires a minimum of 15 to 20 F of temperature difference between the inside and outside of the building. Normally, if the ambient is colder than the interior, shoot from the outside of the building with the space pressurized. Or if the ambient is warmer than the interior, shoot from the inside of the building with the space under negative pressure. In reality, it does not matter which direction you shoot from, but we find this to be the preferred method. To do thermography, you need to temporarily place the building under negative or positive pressure. This can be accomplished by using the HVAC system to provide low pressure testing of the building. Building pressure can be temporarily set by doing the following:
- During colder months: To temporarily place a building under positive pressure, open the OSA dampers full and close off any relief dampers and lower exhaust flows to a minimum. If additional flow is needed, place a blower door in an exterior door to assist by bringing in more outside air to add pressure to the building. If possible, try to get the building to at least 20 Pa (0.08 in.) positive pressure. Once complete, return the OSA flow to a normal pressure and verify that all gas pilot flames have not been blown out.
- During warmer months: To temporarily place a building under negative pressure, close the OSA dampers and open the relief dampers and raise exhaust flows to their maximum flow. If additional flow is needed, place a blower door in an exterior door to assist by removing more air from the building. If possible, try to get the building to at least to a negative pressure of 20 Pa (0.08 in.) negative pressure. It is best practice to turn off all gas-fired appliances in the building prior to placing the building in a negative pressure condition. It is important to not induce high negative pressure on a building if any gas appliances are still active in the building because it could draw flue gas back into the space or blow out pilot flames.
With this pressure being maintained, using a thermal camera, review all exterior walls; underside of the roofs; transitions between the roof and walls, and floors and walls; and all window- and door-to-wall transitions. Leaks will show up as areas of temperature intrusion into the building materials or surrounding areas. When doing this type of test on an existing building, it is important to remember that by keeping the building under negative or positive pressure, you will begin to affect the interior temperature and quality of air of the building over time, so this activity must be kept to short enough time periods to not affect the temperature and environmental quality of the space. It is advisable to verify any issue found through thermography by other means to make sure it is a leak and not a material conductance or bridging issue. It is also helpful that the thermographer be qualified in interpreting thermal images. Other means of verifying identified air leaks are by smoke, feel, temperature, or sound.
Testing the existing building enclosure is not a simple or straightforward process, but must entail inspection, low pressure testing using the building HVAC system, and thermal inspection to achieve the goal of reducing existing building leakage effects.
Calculating air barrier area
The ASTM International standards were originally created to pressure test houses, not commercial buildings, so some of the procedures are confusing when applied to commercial buildings. One such issue is how they calculate air barrier. The official ASTM air barrier calculation is the area of the bottom floor, the area of the top floor ceiling, and the area of the air barrier in all the exterior walls. The problem is that when you have a slab-on-grade for the bottom floor, it is still applied in the air barrier calculation, making the leakage rate lower than it really is because the air barrier does not actually have that much area.
Jim Bochat is president at Commissioning Concepts. He has been involved in the Arizona engineering and construction industry for more than 40 years. His experience includes mechanical design, mechanical construction, controls, test and balance, commissioning, retro-commissioning, and ongoing commissioning.