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Building envelope values and fenestration areas determined in accordance with Equation 4-2 shall be an alternative to compliance with the U-, F- and C-factors in Tables C402.1.4 and C402.4 and the maximum allowable fenestration areas in Section C402.4.1. Fenestration shall meet the applicable SHGC requirements of Section C402.4.3.
(Equation 4-2)

where:

A = Sum of the (UA Dif) values for each distinct assembly type of the building thermal envelope, other than slabs on grade and below-grade walls.
UA Dif = UA Proposed — UA Table.
UA Proposed = Proposed U-value × Area.
UA Table = (U-factor from Table C402.1.3, C402.1.4 or C402.4) × Area.
B  = Sum of the (FL Dif) values for each distinct slab-on-grade perimeter condition of the building thermal envelope.
FL Dif = FL Proposed — FL Table.
FL Proposed  = Proposed F-value × Perimeter length.
FL Table  = (F-factor specified in Table C402.1.4) × Perimeter length.
C  = Sum of the (CA Dif) values for each distinct below-grade wall assembly type of the building thermal envelope.
CA Dif  = CA Proposed — CA Table.
CA Proposed  = Proposed C-value × Area.
CA Table  = (Maximum allowable C-factor specified in Table C402.1.4) × Area.
Where the proposed vertical glazing area is less than or equal to the maximum vertical glazing area allowed by Section C402.4.1, the value of D (Excess Vertical Glazing Value) shall be zero. Otherwise:
D  = (DA × UV) — (DA × U Wall), but not less than zero.
DA  = (Proposed Vertical Glazing Area) — (Vertical Glazing Area allowed by Section C402.4.1).
UA Wall = Sum of the (UA Proposed) values for each opaque assembly of the exterior wall.
U Wall  = Area-weighted average U-value of all above-grade wall assemblies.
UAV = Sum of the (UA Proposed) values for each vertical glazing assembly.
UV = UAV/total vertical glazing area.
Where the proposed skylight area is less than or equal to the skylight area allowed by Section C402.4.1, the value of E (Excess Skylight Value) shall be zero. Otherwise:
E = (EA × US) — (EA × U Roof), but not less than zero.
EA = (Proposed Skylight Area) — (Allowable Skylight Area as specified in Section C402.4.1).
U Roof = Area-weighted average U-value of all roof assemblies.
UAS = Sum of the (UA Proposed) values for each skylight assembly.
US  = UAS/total skylight area.

Related Code Sections

C502.2 [CE] Existing Buildings, Change in Space Conditioning
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C503.2 [CE] Existing Buildings, Change in Space Conditioning
with this code. Exceptions: Where the component performance alternative in Section C402.1.5 is used to comply with this section, the proposed UA shall ...