This article explains the use of blower door tests to evaluate building air infiltration and tight compared with leaky houses. This website discusses how to inspect, diagnose problems in, and install or repair building insulation & ventilation systems including air leaks, air infiltration, heating cost, heat loss, moisture, & interior stains.
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For decades, energy specialists have known that air infiltration accounts for one-third to one-half of a typical home's heat loss. Yet efforts to curtail convective heat loss in both new and retrofit building projects have been for the most part haphazard and not always successful.
The blower door test provides a scientific approach to identifying and controlling air infiltration (air leaks) in buildings. Developed in the mid-1970's at Princeton University's Environmental Studies Branch, the portable blower door successfully moved from the university to the construction industry and energy conservation consultant use.
The blower door also provided leads for active solar space heating, teaching contractors where they need to tighten the homes they build, and blower doors have added vigor to the home insulation and home weatherization business. In Sweden pressurization testing is required by the building code; in Canada similar standards have been developed.
What is a Blower Door Test?
The heart of the blower door is a common fan, usually about 500 cfm (cubic feet per minute of air movement capacity), the same size as a typical 30-inch whole house fan. The blower door fan mounts into a door-sized housing that cleverly adjusts and seals to fit snugly in the main doorway of the building to be blower-door tested.
One type of blower door seals with an inflatable band around its perimeter. The unit then pressurizes (blows into) or de-pressurizes (blows out of) the house, typically to 50 pascals of pressure. This forces air to flow through cracks or leaks in the building envelope.
In the heat loss and air infiltration article AIR LEAK DETECTION TOOLS the energy team begin the building analysis with a blower door test to determine the starting point for a building energy savings tune-up. In that example case, the house leaked at 32 air changes perhour (ach) at 50 pascals.
Equivalent Leakage Area - ELA - is a Window Open?
Built-in instrumentation compares the air blown through the fan to the pressure drop across the blower door assembly (thus across the building doorway), giving a measure of the relative tightness of the house. Charts or computers convert the pressure drop to an equivalent leakage area (ELA) - the size of a single gaping hole in the building shell that would result in the same amount of air leakage.
The equivalent leakage area (ELA) helps consumes understand the cumulative effect of the many small leaks that are present at a typical building as they may indeed add up to the equivalent of an open door or window on the building.
Air Changes per Hour - ACH - At What Rate is Air Leaking Into or Out Of the Building?
The computer can also generate an estimate of air changes per hour (ACH) of the house under normal winter conditions - that is, with windows and doors closed. The presence or absence of wind will, of course, change the ACH of a leaky home, as we discuss just below.
Air changes per hour (ACH) measures are readily recognized by building researchers and code officials and join R-Value as popular measures of the energy efficiency of a building. (See INSULATION R-VALUES & PROPERTIES).
Using a Smoke Test to Pinpoint Air Leaks & Direction of Air Movement: Infiltration & Exfiltration in buildings
We (DJF) use a smoke pencil or smoke gun (shown below) in buildings to demonstrate that even without a blower door test one can observe air movement in buildings. Without a blower door to pressureize or de-pressurize the entire building at once, we can observe air convection currents caused by temperature differences at each building level, and we see air movement caused by various building blowers and fans such as furnace or air conditioing air handlers, bath vents, kitchen vents, whole house fans and similar devices.
Often there are surprises: significant leak points that were not recognized and air movement in opposite direction to that anticipated such as warm air moving down from a roof vent rather than up and out in an air-conditioned two story home.
What is a Blower Door Test Really Measuring?
As our photographs above demonstrate (D. Friedman using a smoke tester to screen for leaks into an air handler cabinet), air movement through an opening in a building or through openings in the building's HVAC equipment and ducts changes dramatically depending on whether wind is blowing or not, or whether a fan is on or off in the building's HVAC system or ventilation system.
At above left the blower fan is off, and at above right the HVAC system blower fan was on. There was no air movement into the HVAC system until the fan turned on, but at with fan-on the leakage rate was significant, drawing moldy air from a wet basement in this particular case, blowing these particles into the living area upstairs.
In fact, the blower door really measures only one thing. This is the amount of air leakage that would occur if an enormous wind blew or drew with equal force at all sides of the house -- which of course never actually occurs. Typically, the leakage rate at 50 pascals is extrapolated down to 5-10 pascals to find the ELA (equivalent leakage area).
Deriving the natural ACH (air changes per hour in the building) is the more artful step. The actual rate of air exchange depends on the interaction of the two driving forces -- the stack effect and wind -- with the shape of the house and the location of the cracks having an important effect on the actual leakage and thus the heating or cooling energy costs of the building
How Building Shape Affects Air Leakage
A tall skinny home in cold windy weather will leak more than a squat home in a milder climate with low or no wind present. Gaps open to north winds will leak more than gaps of similar size and shape that are buffered by porches or plantings around a home. Algorithms for predicting ACH are available and are continuously refined.
What is the Typical Rate of Building Air Leakage?
The best ACH algorithms still claim accuracy within 25 percent. For a rough estimate of normal air leakage at a building, divide the ACH at 50 pascals by 7.
How Should Blower Door, Air Leak, & Air Movement Tests be Used?
Sealing Leaks in A Building & Equivalent Savings in Heating Cost
As reported in Solar Age magazine in the 1980's, James McGarvey, a licensed dealer with Canada-based Ener-Corp Management, Ltd., seals any gap that the smoke test reveals at 10 pascals, equivalent to a 9-13 mph head wind. Sealing anything beyond that, he said, is not cost effective. The ELA of a typical Victorian home, according to McGarvey, might be cut in half to 300 to 400 square inches, reducing the air infiltration rate to .8 or .9 ACH.
The annual fuel bill in a leaky Victorian home can be cut by 30-365 percent using this approach. A large old house will take 3000 - 5000 linear feet of silicone caulk applied indoors, and a variety of weather strips and seals applied carefully.
Also as reported in Solar Age, Princeton Energy Partners (PEP) took a slightly different approach. An outgrowth of Princeton University's Center for Energy and Environmental Studies - birthplace in 1977 of the first portable fan door - PEP offers franchised crews marketing and technical support but sells no products. Franchised crews in the Eastern U.S. improve home energy efficiency by using a combination of infrared thermography and building pressurization testing methods to identify areas of heat loss and air leaks. PEP remarked that plenty of time is spent in the attics of homes where convective loops from wall partitions into building attics pump more heat out of a house than most people realize. Air infiltration is thus only part of the heat loss story.
The contractors then perform the highest priority procedures and leave the client with recommendations for additional savings. Also see ENERGY SAVINGS PRIORITIES for our discussion of setting priorities when saving on heating or cooling costs at a building.
See AIR SEALING STRATEGIES and also AIR LEAK DETECTION TOOLS for details about sealing air leaks in buildings. See BRICK VENEER WALL INSULATION for a discussion of leaks at brick veneer walls insulated with foam board.
Here we include solar energy, solar heating, solar hot water, and related building energy efficiency improvement articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.
Readers should see Blower Door Test Data Results and also see PASCAL CALCULATIONS where we describe air infiltration rate rules of thumb, and also see AIR LEAK DETECTION TOOLS (including an example of use of the blower door test to measure building air changes per hour) and see INDOOR AIR QUALITY IMPROVEMENT GUIDE which includes details about whole house ventilation systems. Our page top photo, courtesy of Steven Bliss, shows an Infiltec blower door test being performed at a home.Accompanying text is reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.
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