# G3.1.1.4 Modeling Building Envelope Infiltration

The air leakage rate of the *building envelope* *I _{75Pa}* at a pressure differential of 0.3 in. of water shall be converted to appropriate units for the

*simulation program*using one of the following formulas:

For methods describing air leakage as a function of *floor* area,

*I _{FLR}* = 0.112 ×

*I*×

_{75Pa}*S/A*

_{FLR}For methods describing air leakage as a function of the area of *above-grade walls* that separate *conditioned spaces* and *semiheated spaces* from the exterior,

*I _{AGW}* = 0.112 ×

*I*×

_{75Pa}*S/A*

_{AGW}When using the measured air leakage rate of the *building envelope* at a pressure differential of 0.3 in. of water for the *proposed design*, the air leakage rate shall be calculated as follows:

*I _{75Pa}* =

*Q/S*

where

I_{75pa} | = | air leakage rate of the building envelope (cfm/ft^{2}) at a fixed building pressure differential of 0.3 in. of water, or 1.57 psf |

Q | = | volume of air in cfm flowing through the building envelope when subjected to a pressure differential of 0.3 in. of water, or 1.57 psf, in accordance with ASTM E 779 |

S | = | total area of the building envelope (ft^{2}), including the lowest floor, any below-grade walls or above-grade walls, and roof (including vertical fenestration and skylights) |

I_{FLR} | = | adjusted air leakage rate of the building envelope (cfm/ft^{2}) at a reference wind speed of 10 mph and relative to the gross floor area |

A_{FLR} | = | gross floor area, ft^{2} |

I_{AGW} | = | adjusted air leakage rate of the building envelope (cfm/ft^{2}) at a reference wind speed of 10 mph and relative to the area of the above-grade walls of the building envelope |

A_{AGW} | = | total area of above-grade walls of the building envelope, ft^{2} |

**Exception to G3.1.1.4**

A multizone airflow model alternative method to modeling *building envelope* air leakage may be used, provided the following criteria are met:

- Where the calculations are made independently of the
*energy simulation program*, the proposed method must comply with Section G2.5. - The method for converting the air leakage rate of the
*building envelope*at 0.3 in. of water, or 1.57 psf, to the appropriate units for the*simulation program*is fully documented and submitted to the*rating authority*for approval.

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