4.4 Building Envelope Trade-Off Option
The building envelope complies with the code if the proposed building satisfies the Mandatory Requirements and the envelope performance factor of the proposed building is less than or equal to the envelope performance factor of the budget building.
4.4.1 The envelope performance factor shall be calculated using the following equations.
where
where
EPFRoof Envelope performance factor for roofs. Other subscripts include walls and fenestration.
As, Aw The area of a specific envelope component referenced by the subscript "s" or for windows the subscript "w".
SHGCw The solar heat gain coefficient for windows (w). SHGCs refers to skylights.
Mw A multiplier for the window SHGC that depends on the projection factor of an overhang or sidefin. These values are determined by the procedures in § 4.4.2.
Us The U-factor for the envelope component referenced by the subscript "s".
RBFs Radiant barrier factor for roof surface "s". A radiant barrier factor (RBF) of 0.33 shall be assumed for roof constructions with a qualifying radiant barrier, otherwise RBF shall be assumed to be 1.00.
as For mass and metal building roofs, an absorptance (a) of 0.70 shall be assumed for roofs that do not qualify as high albedo. For high albedo roofs, an absorptance (a) of 0.30 shall be assumed. The coefficients for use in the EPF equations are contained in Table 4.4.1.
cRoof,Mass A coefficient for the "Roof, Mass" class of construction. Values of "c" are taken from Table 4.4.1 for each class of construction.
| Table 4.4.1 - Envelope Performance Factor Coefficients | ||||
| Component, Class | Daytime |
24-Hour |
||
| Roofs, Mass | 1.47 |
3.61 |
||
| Roofs, MtlBldg | 15.83 |
25.26 |
||
| Roofs, Other | 2.84 |
3.82 |
||
| Wall, Mass | 2.53 |
6.14 |
||
| Wall, MtlBldg | 6.36 |
9.28 |
||
| Wall, MtlFrm | 6.36 |
9.28 |
||
| Wall, Other | 6.36 |
9.28 |
||
| Fenest, East | 53 |
86 |
||
| Fenest, North | 31 |
51 |
||
| Fenest, South | 58 |
98 |
||
| Fenest, West | 50 |
85 |
||
| Fenest , Skylights | 101 |
163 |
||
4.4.2 Credits for fixed shading devices (M) such as overhangs or side fins shall be calculated using the following equation. A separate calculation shall be made for each orientation and unique shading condition.
| Table 4.4.2 - Overhang and Side Fin Coefficients | |||||||
| Device | Coefficient |
North |
South |
East/West |
|||
| Overhangs | a |
0.16 |
0.21 |
0.10 |
|||
b |
-0.61 |
-0.83 |
-0.58 |
||||
| Side Fins | a |
0.23 |
0.12 |
0.14 |
|||
b |
-0.74 |
-0.59 |
-0.52 |
||||
4.4.3 The following rules shall be used to define the budget building.
a. The budget building shall have the same building floor area, gross wall area and gross roof area as the proposed design. If the building has both 24-hour and daytime occupancies, the distribution between these shall be the same as the proposed design.
b. The U-factor of each envelope component shall be equal to the criteria from § 4.3 for each class of construction.
c. The vertical fenestration area shall be equal to the proposed design or 40% of the gross exterior wall area, which ever is less. The skylight area shall be equal to the proposed design or 5% of the gross exterior roof area, which ever is less.
d. The SHGC of each window or skylight component shall be equal to the criteria from § 4.3.