Explanation of Blower Door Terms and Results

Explanation of Blower Door Terms and Results
Information taken from “TECTITE BUILDING AIRTIGHTNESS TEST” by The
Energy Conservatory
CFM50: This is the airflow (in Cubic Feet per Minute) needed to create a change
in building pressure of 50 Pascals. CFM50 is the most commonly used measure
of building airtightness.
ACH at 50 Pa: The Air Changes per Hour (ACH at 50 Pa) is another commonly
used measure of building airtightness. ACH at 50 Pa is the number of complete
air changes that will occur in one hour with a 50 Pascal pressure being applied
uniformly across the building envelope. ACH at 50 Pa is a useful method of
adjusting (or normalizing) the leakage rate by the size (volume) of the building. If
you did not enter the building volume on the Building Information screen,
ACH50 will not be calculated.
ACH at 50 Pa =
(CFM50 x 60) / building volume in cubic feet
CFM50/square foot of floor area: This is the CFM50 reading for the building
divided by the floor area of the building. CFM50/square foot adjusts (or
normalizes) the leakage rate by the size (floor area) of the building. If you did
not enter the floor area on the Building Information screen, this variable
will not be calculated.
CFM50/square foot = CFM50 / floor area in square feet
CFM50/square foot of surface area (MLR): Also known as the Minneapolis
Leakage Ratio (MLR), this is the measured CFM50 divided by the above grade
surface area of the building. MLR is a useful method of adjusting (or normalizing)
the leakage rate by the amount of envelope surface through which air leakage
can occur. The MLR has been particularly useful for weatherization crews
working on wood frame buildings. Experience to date has shown that for
buildings with a MLR above 1.0, very large cost-effective reductions in infiltration
can often be achieved using blower door guided infiltration and insulation
techniques. In buildings with a calculated MLR in the 0.5 to 1.0 range, it is often
more difficult to achieve economical improvements in airtightness. If you did not
enter an Above Grade Surface Area value into the Building Information screen,
MLR will not be calculated.
MLR = CFM50 / above grade surface area in square feet
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Once the leakage rate for the building has been measured, it is useful to
estimate the cumulative size (in square inches) of all leaks or holes in the
building's air barrier. The estimated leakage areas not only provide us with a
way to visualize the physical size of the measured holes in the building, but they
are also used in infiltration models to estimate the building's natural air change
rate (i.e. the air change rate under natural weather conditions).
The results screen includes two leakage area calculations, based on differing
assumptions about the physical shape of the hole, which are compatible with the
two most commonly used infiltration models.
Equivalent Leakage Area (EqLA):EqLA is defined by Canadian researchers at
the Canadian National Research Council as the area of a sharp edged orifice (a
sharp round hole cut in a thin plate) that would leak the same amount of air as
the building does at a pressure of 10 Pascals. The EqLA is used in the AIM
infiltration model (which is used in the HOT2000 simulation program).
Effective Leakage Area (ELA): ELA was developed by Lawrence Berkeley
Laboratory (LBL) and is used in their infiltration model. The Effective Leakage
Area is defined as the area of a special nozzle-shaped hole (similar to the inlet of
your Blower Door fan) that would leak the same amount of air as the building
does at a pressure of 4 Pascals.
Notes on Leakage Areas: When using leakage area calculations to demonstrate
physical changes in building airtightness, we recommend using the EqLA
measurement. Typically, EqLA more closely approximates physical changes in
building airtightness.
For example, if you performed a Blower Door test, and then opened a window to
create a 50 square inch hole and repeated the test, the estimated EqLA for the
building will have increased by approximately 50 square inches from the initial
test results.
Coefficient (C) and Exponent (n):Once an automated airtightness test sequence
(or manual entry of data into the table) has been completed, a best-fit line (called
the Building Leakage Curve) is drawn through the collected Blower Door data.
The Building Leakage Curve can be used to estimate the leakage rate of the
building at any pressure. If you conduct a single point test (i.e. input a single
target pressure into the custom pressure list), the program assumes an exponent
(n) of 0.65 in its calculation procedures.
The Building Leakage Curve is defined by the variables Coefficient (C) and
Exponent (n) in the following equation:
Q = C x P^n
where: Q is airflow into the building (in CFM).
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C is the Coefficient.
P is the pressure difference between inside and outside of the
n is the Exponent.
Example: Use the Building Leakage Curve to estimate the exhaust fan
airflow in a building needed to create a 5 Pa negative pressure. From our
Blower Door test we determined the following Building Leakage Curve variables.
C = 110.2 n = 0.702
From the equation above:
Airflow (at 5 Pa) = 110.2 x 5^.702 = 341 CFM. In other words, we
estimate from the Building Leakage Curve that it would take exhaust fans with a
combined capacity of 340 CFM to cause a 5 Pa pressure change in this building.
Correlation Coefficient: The correlation coefficient is a measure of how well the
collected Blower Door data fit onto the best-fit Building Leakage Curve. The
closer all data points are to being exactly on the Building Leakage Curve, the
larger the calculated correlation coefficient (note: the largest possible value for
the correlation coefficient is 1.0). Under most operating conditions, the
correlation coefficient will be at least 0.99 or higher.
Testing in very windy weather can sometimes cause the correlation coefficient to
be less than 0.99. In this case, you may want to repeat the test, or increase the
number of Samples Per Station. Achieving a correlation coefficient of 0.99 or
higher is particularly important in the estimation of Leakage Areas, or when using
the Building Leakage Curve to estimate leakage rates at low building pressures.
Estimating the natural infiltration rate of a building is an important step in
evaluating indoor air quality and the possible need for mechanical ventilation.
Blower Doors do not directly measure the natural infiltration rates of buildings.
Rather, they measure the building leakage rate at pressures significantly greater
than those normally generated by natural forces (i.e. wind and stack effect).
Blower Door measurements are taken at higher pressures because these
measurements are highly repeatable and are less subject to large variations due
to changes in wind speed and direction.
In essence, a Blower Door test measures the cumulative hole size, or leakage
area, in the building's air barrier (see Leakage Areas above). From this
measurement of leakage area, estimates of natural infiltration rates can be made
using mathematical infiltration models. TECTITE uses the calculation procedure
contained in ASHRAE Standard 136-1993 to estimate the average annual
natural infiltration rate of the building.
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CFM, ACH and CFM/person: The estimated annual natural infiltration rate
(based on ASHRAE Standard 136-1993) is expressed in Cubic Feet per Minute
(CFM), Air Changes Per Hour (ACH), and CFM per person. When determining
occupancy for the CFM/person calculation, the program uses the number of
bedrooms plus one, or the number occupants, whichever is greater.
Notes on Estimated Infiltration Rates: Daily and seasonal naturally occurring air
change rates will vary dramatically from the estimated average annual rate
calculated here due to daily changes in weather conditions(i.e. wind and outside
The physical location of holes in the building air barrier compared to the
assumptions used in the infiltration model will cause actual annual infiltration
rates to vary from the estimated values. Research done in the Pacific Northwest
on a large sample of houses suggests that estimated infiltration rates for an
individual house (based on a Blower Door test) may vary by as much as a factor
of two when compared to measured infiltration rates using PFT tracer gas. (PFT
tracer gas tests are one of the most accurate methods of measuring actual
natural infiltration rates). The annual average infiltration estimates from ASHRAE
Standard 136-1993 should be used only for evaluating detached single-family
dwellings, and are not appropriate for use in estimating peak pollutant levels or
energy loss due to infiltration. If any of the building leakage is located in the
forced air distribution system, actual air leakage rates may be much greater than
the estimates provided here. Duct leaks result in much greater air leakage
because they are subjected to much higher pressures than typical building leaks.
In addition to estimating an annual infiltration rate above, the program estimates
the design winter and summer infiltration rates for the building. The design
infiltration rates are the infiltration rates used to calculate winter and summer
peak loads for purposes of sizing heating and cooling equipment. The
calculated design infiltration rates can be used in ACCA Manual J load
calculations in lieu of the estimation procedures listed in Manual J. The
estimation procedure uses the design wind speed and temperature difference
values input into the Climate Information Screen, and are based on the
calculation procedures listed in the ASHRAE Fundamentals Handbook, Chapter
on Infiltration and Ventilation.
Winter and Summer: CFM, ACH: The estimated design infiltration rates are
expressed in Cubic Feet per Minute (CFM), and Air Changes per Hour (ACH).
It is possible (even easy) to increase the airtightness of a building to the point
where natural air change rates (from air leakage) may not provide adequate
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ventilation to maintain acceptable indoor air quality. To help evaluate the need
for mechanical ventilation in buildings, national ventilation guidelines have been
established by ASHRAE. The recommended whole building mechanical
ventilation rate presented in this version of TECTITE is based on ASHRAE
Standard 62-2003, and is only appropriate for low-rise residential structures.
Recommended Whole Building Mechanical Ventilation Rate: This value is the
recommended whole building ventilation rate to be supplied on a continuous
basis using a mechanical ventilation system. The recommended mechanical
ventilation rate is based on 7.5 CFM per person (or number of bedrooms plus
one, whichever is greater), plus 1 CFM per 100 square feet of floor area. This
guideline assumes that in addition to the mechanical ventilation, natural
infiltration is providing 2 CFM per 100 square feet of floor area.
For buildings where the estimated annual natural infiltration rate (based on the
Blower Door test) is greater than 2 CFM per 100 square feet of floor area, the
recommended mechanical ventilation rate is reduced to provide ventilation credit
for excess infiltration. In these cases, the recommended mechanical ventilation
rate is reduced by the following amount:
0.5 x (est. annual natural infiltration rate (CFM) minus 0.02 CFM x sq. ft. of floor
Notes on the Ventilation Guideline: ASHRAE Standard 62.2-2003 also contains
requirements for local kitchen and bathroom mechanical exhaust systems.
These local exhaust systems may be incorporated into a whole building
ventilation strategy. Consult Standard 62.2-2003 for more information on
ventilation strategies and specific requirements and exceptions contained in the
Compliance with the ventilation guideline does not guarantee that a moisture or
indoor air quality (IAQ) problem will not develop. Many factors contribute to
indoor air quality including ventilation rates, sources and locations of pollutants,
and occupant behavior. Additional testing (including combustion safety testing)
is needed to fully evaluate air quality in buildings. In many cases, a combination
of pollutant source control and mechanical ventilation will be required in order to
ensure adequate indoor air quality.
Previous versions of TECTITE used ASHRAE Standard 62-1989 to determine an
annual ventilation guideline. The Standard 62-1989 guideline (which was
superceded by Standard 62.2-2003) was based on 15 CFM per person or 0.35
Air Changes per Hour (whichever was greater).
The program estimates the annual cost associated with air leakage, both for
heating and cooling. Because these cost estimates are based on estimated
infiltration rates and many other assumptions, actual cooling costs may differ
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significantly from the estimates. The equations used to calculate the annual cost
for air leakage are:
Heating =
26 x HDD x Fuel Price x CFM50
-------------------------------------- x 0.6
N x Seasonal Efficiency
- HDD is the annual base 65 F heating degree-days for the building location.
- The Fuel Price is the cost of fuel in dollars per Btu.
- N is the Energy Climate Factor from the Climate Information Screen (adjusted
for wind shielding and building height). See Appendix E of the Model 3 Blower
Door Operation Manual for more information on this calculation procedure.
- Seasonal Efficiency is the AFUE rating of the heating system.
Cooling =
.026 x CDD x Fuel Price x CFM50
---------------------------------------N x SEER
- CDD is the base 70 F cooling degree days for the building location.
- The Fuel Price is the cost of electricity in dollars per kwh.
- N is the Energy Climate Factor from the Climate Screen (adjusted for wind
shielding and building height). See Appendix E of the Model 3 Blower Door
Operation Manual for more information on this calculation procedure.
- SEER is the SEER rating for the air conditioner.
Note: The Cooling Cost procedure does not include latent loads. In humid
climates, latent loads due to air leakage can be greater than the sensible loads,
which are estimated by this procedure.
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