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Infiltration Method |
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Applicability |
All projects |
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Definition |
Energy simulation programs have a variety of methods for modeling uncontrolled air leakage or infiltration.
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Units |
List: effective leakage area, component leakage, air changes per hour, or other method supported by the energy analysis software. |
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Input Restrictions |
The default method is the component leakage method; however, there are no restrictions on other reasonable methods. |
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Baseline Rules |
The infiltration method used for the baseline building shall be the same as that used for the proposed design. |
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Infiltration Data |
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Applicability |
All projects |
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Definition |
Information needed to characterize the infiltration rate in buildings. The required information will depend on the infiltration method selected above. For the effective leakage area method, typical inputs are leakage per exterior surface area in ft² or other suitable units and information to indicate the height of the building and how shielded the site is from wind pressures. The air-changes per hour (ACH) method requires an estimate of the ACH and a schedule which modifies the ACH for various periods during the year. Similar data would be specified for the leakage per component area method. Only zones with exterior wall area are assumed to be subject to infiltration. |
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Units |
A data structure is required to define the effective leakage area model, while a single numeric value can define the ACH or the leakage per area of exterior envelope method. |
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Input Restrictions |
Any reasonable inputs may be specified, consistent with the chosen infiltration modeling method. Acceptable ranges for inputs should be defined for each method supported by rating software. Default is 0.38 cfm/ft² of exterior wall. |
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Baseline Rules |
The infiltration data for the baseline building shall be the same as the proposed design unless the proposed design employs specific measures that go beyond the mandatory measures specified by the baseline standard. When credit is taken for reductions in infiltration (see above), test results or research reports shall be provided that document and support the inputs used for the baseline building and the proposed design. |
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Infiltration Schedule |
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Applicability |
When an infiltration method is used that requires the specification of a schedule |
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Definition |
With the ACH method and other methods (see above), it may be necessary to specify a schedule that modifies the infiltration rate for each hour or time step of the simulation. Typically the schedule is either on or off, but can also be fractional. |
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Units |
Data structure: schedule, fractional |
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Input Restrictions |
The default infiltration schedule is 1.0 (100% of specified infiltration rate for periods of time when the fans serving the thermal block are not operating and a 0.25 for periods of time when the fans are operating. This is based on the assumption that when the fan system is on, the pressure of the interior space above the pressure of the exterior infiltration of outside air is reduced. When the fan system is off, interior pressure drops below exterior pressure and infiltration increases. |
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Baseline Rules |
The infiltration schedule for the baseline building shall be the same as the proposed design. |
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Infiltration Method |
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Applicability |
All projects |
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Definition |
Energy simulation programs have a variety of methods for modeling uncontrolled air leakage or infiltration. • Some procedures use the effective leakage area (ELA), which is generally applicable for small residential-scale buildings. • The component leakage method requires the user to specify the average leakage through the building envelope per unit area (ft²). This could be a function of floor area or a function of area of the above-grade walls. • Other methods require the specification of a maximum rate (like air change hours), which is modified by a schedule. |
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Units |
List: ELA, Component Leakage Method (function of floor area or function of above-grade wall area or function of all exterior surfaces), ACH |
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Input Restrictions |
For Standard 90.1-2019, a fixed infiltration rate can be specified and calculated. • As a leakage per area of exterior envelope, including the gross area of exterior walls, roofs, and exposed floors, but excluding slabs on grade and interior partitions. • A zone ACH input • The default method is component leakage method; however, there are no restrictions on the use of other listed methods. |
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Baseline Building |
The infiltration method used for the baseline building shall be the same as that used for the proposed design. |
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Infiltration Data |
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Applicability |
All projects |
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Definition |
Information needed to characterize the infiltration rate in buildings. The required information will depend on the infiltration method selected above. Infiltration shall be modeled using the same methodology, and adjustments for weather and building operation in both the proposed design and the baseline building design. These adjustments shall be made for each simulation time step and must account for but not be limited to weather conditions and HVAC system operation, including strategies that are intended to positively pressurize the building. For the effective leakage area method, typical inputs are leakage per exterior wall area in square feet or other suitable units and information to indicate the height of the building and how shielded the site is from wind pressures. Only zones with exterior surfaces are assumed to be subject to infiltration. |
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Units |
A data structure is required to define the effective leakage area model |
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Input Restrictions |
When the infiltration rate is determined by whole building air leakage testing in accordance with ASHRAE 90.1 - 2019 Section 5.4.3.1.1, the air leakage rate of the building envelope (I75Pa) at a fixed building pressure differential of 0.3 in. H2O shall be as measured. Note: This requires that the building be tested and the model updated post construction. For buildings providing verification in accordance with ASHRAE 90.1 - 2019 section 5.9.2.1(a) , the air leakage rate of the building envelope (I75Pa) at a fixed building pressure differential of 0.3 in. H2O shall be defaulted to be 0.6 cfm/ ft2 of exterior building enclosure area When infiltration inputs are based on air leakage testing, test results shall be submitted with the other required documentation identified in Section Error! Reference source not found. of this report. The air leakage rate of the building envelope shall be converted to appropriate units for the simulation program using one of the two methods described below. Any reasonable inputs may be specified, consistent with the chosen infiltration modeling method. Acceptable ranges for inputs should be defined for each method supported by rating software. The peak infiltration rate of the building envelope (I75Pa) at a fixed building pressure differential of 0.3 in. H2O will be defaulted to 0.6 cfm/ft2 exterior building enclosure area, unless a different value is provided by the user and approved by the rating authority. ???????????? = ??????? ⋅ ???ℎ????? ⋅ (? + ? ⋅ |????? − ????| + ? ⋅ ?? + ? ⋅ ?? 2 ) Where: Infiltration = Zone infiltration airflow (m³/s-m²) Idesign = Design zone infiltration airflow (m³/s-m²) Fschedule = Fractional adjustment from a prescribed schedule, based on HVAC availability schedules in COMNET Appendix C (COMNET 2017) (unit less) tzone = Zone air temperature (°C) todb = Outdoor dry bulb temperature (°C) ws = Wind speed (m/s) A = Overall coefficient (unitless) B = Temperature coefficient (1/°C) C = Wind speed coefficient (s/m) D = Wind speed squared coefficient (s²/m² The DOE-2 Infiltration methodology coefficients would be used, where: A = 0 B= 0 C= 0.224 D= 0 When Infiltration normalization per above grade wall area is used, the air leakage rate of the building envelope (I75Pa), at a pressure differential of 0.3 in. H2O shall be converted to appropriate units for the simulation program using the following formula describing infiltration as a function of exterior wall area: IEW = 0.112 × I75Pa × S/AEW Source: (ANSI/ASHRAE/IES 2019) When Infiltration normalization per floor area is used, the air leakage rate of the building envelope (I75Pa), at a pressure differential of 0.3 in. H2O shall be converted to appropriate units for the simulation program using the following formula describing infiltration as a function of floor area: IFLR = 0.112 × I75Pa × S/AFLR When using the measured air leakage rate of the building envelope at a pressure differential of 0.3 in. H2O for the proposed design, the air leakage rate shall be calculated as follows: I75pa = Q/S Where: I75Pa = Air leakage rate of the building envelope expressed in cfm/ft2 at a fixed building pressure differential of 0.3 in. H2O, or 75 Pa Q = Volume of air in cfm flowing through the whole building envelope when subjected to an indoor/outdoor pressure differential of 0.3 in H2O or 1.57 PSF in accordance with ASHRAE 90.1 - 2019 Section 5.4.3.1.1 S = Total area of the envelope air pressure boundary (expressed in ft2 ), including the lowest floor, any below or above-grade walls, and roof (or ceiling) (including windows and skylights), separating the interior conditioned space from the unconditioned environment IEW = Adjusted air leakage rate (expressed in cfm/ft2 ) of the building envelope at a reference wind speed of 10 mph and the above ground exterior wall area AEW = Total above-grade exterior wall area, ft2 IFLR = Adjusted air leakage rate (expressed in cfm/ft2 ) of the building envelope at a reference wind speed of 10 mph and relative to gross floor area AFLR = Gross floor area. ft2 |
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Baseline Building |
The air leakage rate of the building envelope (I75Pa) at a fixed building pressure differential of 0.3 in. H2O shall be defaulted to be 1.0 cfm/ ft2 of exterior building enclosure area. Infiltration modeling approach for the baseline building shall be identical to the proposed. The following aspects are required to be the same for baseline and proposed building models: The same methodology as defined in the section above • Adjustments for weather. The coefficients A, B, C, D will be same as designed. If not provided by user, they would be the same as the default values outlined above. • Building operation in both the proposed design and the baseline design • HVAC system operation, including strategies that are intended to positively pressurize the building NOTE: If a value for I75Pa or Q is provided by the user for the proposed design, the infiltration value would be flagged and reported by the software tool. |
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Infiltration Schedule |
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Applicability |
When an infiltration method is used that requires the specification of a schedule to adjust the peak infiltration rate. |
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Definition |
With the ACH method and other methods (see above), it may be necessary to specify a schedule that modifies the infiltration rate for each hour or time step of the simulation. Such schedules are typically used to account for building pressurization due to the HVAC system. |
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Units |
Data structure: schedule, fractional |
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Input Restrictions |
The schedules for infiltration can be specified by the user or the default schedule can be used. The default infiltration schedule shall be set equal to 1 when the fan system is off, and 0.25 when the fan system is on. This is based on the assumption that when the fan system is on it brings the pressure of the interior space above the pressure of the exterior, and decreases the infiltration of outside air. When the fan system is off, interior pressure drops below exterior pressure and infiltration increases. Schedules other than the default shall be permitted to account for building ingress and egress. Infiltration schedule shall be 1 for spaces that include fan systems that do not supply outside air and do not run continuously when the space is occupied in accordance with Standard 90.1-2019 Table G3.1(4) ‘Schedule’. |
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Baseline Building |
The infiltration schedule for the baseline building shall be the same as the proposed design. |