Blower Door and Air Leakage Numbers

As in most professions there is jargon.  Here is guide to the most common jargon related to air tightness and water testing.

Air Flow Rates

CFM- cubic foot of air per minute.  A cubic foot of air is about the size of a basketball.

ACH – the total building volume of air per hour.  This unit is often used in sizing HVAC systems and has carried over to blower door testing.

Pressures

Pascals – the international unit of pressure and the unit most used in air tightness testing. It takes 6894.75 Pascals to equal 1 psi.

Common pressure benchmarks - 50 and 75 pascals are specific pressure benchmarks used in blower door testing of buildings.  Most buildings in the United States are tested at 50 Pascals as a means of comparison. 50 Pascals is about 5 times the pressure a low level building might experience on a cold winter day.  Tall buildings use 75 Pascals as the pressure benchmark because the test pressure has to be greater than the stack effect pressure but 75 pascals can also be used to overcome wind pressures.  25 Pascals is used to test ductwork for air tightness. Finally. 75 and 300 Pascals are common pressure benchmarks used to test windows and curtain walls for air tightness.  75 is more often used for field tests and 300 is more often used for laboratory tests although there are no hard rules on this.

“ w.c. (Inches water column) – a common engineering unit of pressure used in the U.S., used often in HVAC applications.  248 Pascals equal 1” w.c.

psf (pounds/sq foot) – another common engineering unit of pressure used in the U.S. 48 Pascals equals 1 psf

psi (pounds/sq inch)- the most familiar unit of pressure in the U.S. What we use to gauge the pressure in our tires.  This unit is rarely used for air tightness testing.

25 Pascals = 0.004 psi = 0.1 “w.c. = 0.52 psf = ~14 mph wind

50 Pascals = 0.007 psi = 0.2 “w.c. = 1.04 psf = ~20 mph wind

75 Pascals = 0.011 psi = 0.3 “w.c. = 1.56 psf = ~25 mph wind

300 Pascals = 0.04 psi = 1.2 “w.c. = 6.26 psf = ~50 mph wind

Note: Pascals can be equated “loosely” to wind speeds but speed and pressure are not equivalent units.

Flow Rates at Certain Pressures  – these are “combination” units used often to describe blower door results.  They are air flow rates when the system has reached a specific pressure benchmark.

CFM50- the most common blower door measure.  Cubic feet of air per minute when the building is at 50 pascals of pressure difference from inside to outside.

CFM75 - Cubic feet of air per minute when the building is at 75 pascals of pressure difference from inside to outside. This is used more often on tall buildings although the United States military test all its buildings at 75 Pascals.

CFM25 - Cubic feet of air per minute when ductwork is at 25 Pascals of pressure difference from inside the ductwork to the building. This unit is used mostly for air tightness testing of ductwork.

ACH50 – air changes per hour at a pressure of 50 Pascals.  Because this unit incorporates the building volume as a measurement it can be used as a unit for comparison between one building and another. In fact, it is the most common method used for comparing homes.  However, it is only a fair unit of comparison between buildings of similar size.   1 air change in a mobile home is very different from 1 air change in a gym.

ACH natural- air changes per hour at natural pressures.  “Natural pressure” Fluctuates from moment to moment on a building and is subject to wind, inside and outside temperatures and HVAC induced pressures.  Still we try to guess at what it is on average so we can predict air infiltrations rates in a building day to day so we can predict how much energy is lost due to infiltration. This value is not directly measured by a blower door but is mathematically estimated based on blower door readings at 50 or 75 Pascals (decidedly “unnatural” pressures). The math that is used to make these predictions is still being studied, the translation is not an easy one. For this reason ACH natural is not a good way to specify and air tightness standard for a building, it is open to interpretation.

Despite its weaknesses, ACH natural has been used for years by the HVAC industry in heat load calculations for sizing heating and AC systems.

CFM natural- directly analogous to ACH natural just in cubic feet per minute rather than air changes per hour.

Building Measurements  – these are used to normalize the blower door test results for comparison between buildings.

Floor square footage- used often to indicate the size of a building for real estate and construction.  NOT used often in describing the size of a building for blower door testing.  When square feet is mentioned in the context of blower door testing it is probably referring to above grade surface area or total surface area (see below).

Above Grade Surface Area -  the surface area of the external shell of the building that encloses conditioned space but is not in contact with dirt.  The surafec area buried behind dirt i.e.  Below grade foundation walls or on dirt i.e. slab on grade is NOT counted in this measurement.  The idea is that anything behind dirt or on dirt is not really subject to air infiltration.  Also note, this is a measure of the surfaces that enclose conditioned space so the surface area of a roof over a vented attic would NOT be counted in this measure.

Total surface area – the ENTIRE surface area of the external shell of the building that encloses conditioned space.  Put another way, this is all 6 sides of the building- top, bottom, and 4 sides. Slab on grade and below grade foundation walls are counted in this measurement.  The idea is this is a fairer way to compare buildings than just counting above grade surface area.  similar to Above Grade Surface Area, only surfaces that enclose conditioned space are counted.

An Air change- the volume of the conditioned space of the building, i.e. the total volume of the building that is actively heated or cooled. Put another way this is total volume of air inside a building. This is often used to express a flow rate known as an air change per hour or ACH.

Flow Rates at Certain Pressures and Normalized by Building Size – There are also “combination” units used to compare one building to another.

CFM50/sf - Cubic feet of air per minute at 50 Pascals of pressure difference per square foot of surface area. Just be sure you know which surface area is being applied, floor square footage (rare), above grade square footage (also called the Minneapolis Leakage Ratio), or total surface area.  This unit is the most accurate way to compare buildings of all types and sizes to each other as long as the method of calculating the surface area is the same for all buildings.

CFM75/sf - Cubic feet of air per minute at 75 Pascals of pressure difference per square foot of surface area.  Same as CFM50/sf but a different test pressure.

liters/s/meter2 (liters per second per square meter) – The metric equivalent to CFM50/sf.  Used in Canada and Europe but often seen in U.S. specifications because standards are based on more established standards in other countries.

ELA - effective leakage area. The equivalent area of a hole in square inches if all the holes in the building were summed to one.  This is a mathematical extrapolation form the CFM50 number and is based on all the holes being nozzle shaped.

EqLA - equivalent leakage area.  The equivalent area of a hole in square inches if all the holes in the building were summed to one.  This is a mathematical extrapolation from the CFM50 number and is based on all the holes being a sharp edged orifice.

ELR – effective leakage ratio.  The equivalent area of a hole if all the holes in the building were summed to one in square inches per 100 sf of building shell.