Infiltration - Legacy system

Air infiltration can be a significant component of thermal losses in residential buildings. In the Home Energy Saver, we calculate the energy impact of air infiltration in the DOE-2 simulation model, based on the leakage area of the thermal shell and location-specific weather data. Although leakage area can be measured using diagnostic testing such as with a blower door, few homeowners know the leakage area of their home, so we provide two options: 1) A simplified estimate based on building characteristics and 2) Direct entry of the measured leakage at 50 pascals (CFM50).

Estimate of Building Characteristics

For this method we estimate leakage area using a database of measured leakage values compiled by LBNL. This database has been analyzed to provide average leakage values for single-family homes based on a few key parameters that strongly influence air leakage (Sherman and Matson 1997, Matson 1998). The LBNL leakage database reports leakage values as Normalized Leakage (NL), or square feet of leakage area per 1000 square feet of conditioned floor area. For input to DOE-2, we converted these normalized leakage values to fractional leakage areas (FLA) using the following equation:

where:

FLA = Fractional leakage area, the ratio of envelope leakage area to floor area

                        (square feet/square feet)

                       NL = Normalized leakage (sq. ft. leakage/sq. ft. conditioned floor area)

                       stories = 1 if single-story house, otherwise stories = 2

                       8 = Assumed ceiling height (feet) for the house

                       0.3048 = conversion factor feet to meters

Source: Sherman, et. al. 1997

The key parameters used to select a house's leakage value from the database are: house vintage (pre-1980, 1980 and later), stories (1, more than 1), shell condition (whether or not air leaks have been sealed in a comprehensive way), presence of a ducted heating or cooling system, and air leakage through the floor (slab or conditioned basement, vs. other foundation types). In addition, for houses built in 1990 or later, we assume a leakage value that is consistent with the "tight" thermal shells typically seen in new construction (NL = 0.5). Default leakage estimates are climate zone specific and are mapped to the TMY weather locations; locations that were not part of the original analysis are handled by using data from the closest location.

Direct Entry of Measured Leakage

Building shell leakage is typically measured using a blower door that pressurizes the living space to 50 pascals. This measurement is converted to fractional leakage area using the following equations:

    Q50si = Q50ip x 0.0004719474 ft3/min / m3/sec

    Q4 = Q50si x (4 / 50) 0.65

    ELA = Q4 / (((2 / 1.2) x 4)0.5)

    FLA = (ELA / (0.09290304 m2 / ft2)) / A

Where:

    Q50 - Air flow measured at 50 pascals (ft3/min for ip and m3/sec for si)

    Q4   - Air flow at 4 pascals

    ELA  - Effective  leakage area (in m2)

    FLA  - Fractional leakage area

    A     - Floor Area (ft2)

Further Development

An improved infiltration database is being developed based on the work of the Energy Performance of Buildings group at LBNL (see:http://resdb.lbl.gov/). Release is estimated for spring 2012.