Hi all,
I am working on a project that has a large amount of glass. ASHRAE 90.1-2007 states this about baseline modeling for glass:
The fenestration area in the baseline building shall be equal to the fenestration area in the proposed building or 40% of the exterior wall, whichever is less. If it is necessary to reduce window area in the baseline building, then corresponding increases are made in the opaque walls such that the gross exterior wall area is unchanged between the proposed building and the baseline building.
Fenestration area in the baseline building shall be distributed among the thermal blocks and around the building in a uniform manner. In each thermal block, the fenestration area shall be equal to the window wall ratio for the baseline building (the maximum of 40% or the proposed building WWR) multiplied times the exterior wall area for the thermal block. If the thermal block has more than one exterior wall, window area is placed proportionally on each.
The window area is positioned in continuous horizontal bands on each exterior wall, although the configuration of window area on each wall is not significant since neither daylighting or self shading from the building is modeled. See Figure G-D.
While the prescriptive envelope requirements provide an exception for street-level, street-side vertical fenestration (e.g., store display windows), the building performance rating method does not. Window area in the baseline building is always determined as described above.
Based on the wordage above, I must develop the baseline model with either the same glass areas as the proposed building (essentially copying the proposed model and just updating U-values, etc.) or 40% of the exterior wall, whichever is smaller. Since I have roughly 60% of glazing, I will have to redistribute the glass into continuous horizontal bands on each exterior wall, similar to how ASHRAE 90.1-2004. However, as I began to look at the glass % of each exterior wall, I ran into a problem:
Wall
Window SQ FT
Wall SQ FT
Total SQ FT
Window %
North
10727.76
8485.95
19213.71
55.83%
North-East
1812.26
2272.83
4085.09
44.36%
East
2645.52
1846.83
4492.35
58.89%
South-East
1683.46
1222.87
2906.33
57.92%
South
12595.36
3958.92
16554.28
76.09%
South-West
623.46
829.56
1453.02
42.91%
West
3282.3
2924.38
6206.68
52.88%
North
0
104.26
104.26
0.00%
Total
33370.12
21645.6
55015.72
60.66%
There is not enough exterior wall space to redistribute glazing to drop the percentages for each wall to 40%. Do I just reduce each wall to 40% glazing with continuous horizontal bands?
Thanks!
William Mak
