ASHRAE 90.1-2007 Fenestration Areas

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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

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Hi Will,*

*I think your answer is here:*

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.*

Based on your table you need to reduce the window area to be 40% of the
overall wall area, making it a band as described, except for the North wall
with 0% glass as it is already less than 40%.

Happy Holidays!

Carol

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Hi Will,

If I remember clearly, the continuous horizontal bands should be placed
across all orientations and even for exterior walls without window in the
proposed design. In this case, 40% window should be placed at North.

Regards,

Cheney

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The section of the standard states "Vertical fenestration
areas for new buildings and additions shall equal that in
the proposed design or 40% of gross abovegrade wall area,
whichever is smaller, and shall be distributed on each face
of the building in the same proportions in the proposed
design." I've always assumed that you keep all the windows
from the proposed design and just make them smaller. In your
example since each window would be about 2/3 the proposed size.

Jason

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I think you are using the User's Manual for ASHRAE Standard 90.1-2004.

In ASHRAE 90.1-2007, there is no requirement of distributing glazing in horizontal bands.

Please refer Example G-D in 2007 User's manual. (yes, Example G-D and not Figure G-D)

As Jason mentioned, you need to reduce each glazing by 2/3 or 67% (40%/60% = 67%)

Gaurav Mehta, LEED? AP BD+C

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Carol and Jason tend to buy the idea of proportionally reduce the windows.
Well, Yes and No. It very much depends on which standard, 2004 or 2007, you
are going to use.

What Will quoted is from 90.1-2004 (User Manual), so I answered accordingly
in my last email. The proof can be found at Page G-16, *Example G-D?Baseline
Building Model, Building Envelope. *

distributed equally on each facade in horizontal bands. This means that some
rooms that do not have windows in the proposed building will have windows in
the baseline building. Since daylighting is not modeled in the baseline
building, the position of the band of glass above the floor is not
relevant...."

In 2007, however, it says: " ...The baseline building has each window
(including glass doors) area reduced proportionally by a factor of 67%
(40%/60%=67%) from the proposed building. ..."

Does it answer your question, Will?

Regards,

Cheney

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It appears to me nobody has noticed William's actual question... :O

I think the total under the "Wall SQ FT" column in the provided table is causing you some unnecessary confusion, William. While one can (apparently) debate a variety of approaches, you should be able to define 0.40*55,105.72 = 22,042 SF of fenestration on the ~55k SF of envelope area =).

Uniform, horizontal bands as described in 90.1 is one approach, but are not necessary the easiest - it depends on your modeling program and preferences. If you are under 90.1-2007 as you suggest, be cautious that a building in strict compliance with 90.1-2004 may not necessarily maintain the proportions between faces as required under 2007. In effect, 2007 is no longer "rewarding" designers choosing to not place their windows on Northern facades.

The alternative to defining horizontal bands (and also my personal preference) would be to start with your proposed, actual layout, and simply multiply the width (or height) of each vertical fenestration by 40%/[actual WWR] = (40/60.66) = 0.6594... in your case. I'd advise utilizing excel to carry out the numbers and minimize rounding errors as they will add up. After this procedure, your WWR should calculate to 40%.

Neither approach described (uniform reductions of an actual layout & continuous horizontal bands) is "wrong," though 2004's wording appears rigid. The "proportionate reduction" approach described by myself, Carol and Jason has been in use successfully for LEED projects under 90.1-2004.

In any case, 90.1-2007 has removed the phrase "continuous horizontal bands" as Cheney illustrates. Either approach is acceptable, and which one should pursue is more practically a matter of what software package you're using and which method is the easiest for the modeler.

Happy holidays everyone!

NICK CATON, E.I.T.

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Oh! There was the problem... I was looking at the 90.1 2004 User's Manual!

So as long as we proportionally reduce windows to bring the overall building glazing percentage to 40%, I should be good?

William Mak

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Yes, Sir! Merry Christams to all of you!

Cheney

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