ASHRAE 90.1 Definition of Window-to-Wall-Ratio (WWR) - Only Walls to Conditioned Spaces?

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Dear All:

I know a related topic has been discussed here on the list serve before.
However, after reviewing all the correspondence on this topic, and the 90.1
standard itself, I still have a few uncertainties I hope at least one of you
are able to clarify. If not I sincerely value as many of your thoughts as
you can offer so I can understand what the common interpretation is. This
question pertains to the 2004 edition. I don't even understand if using
addendum A can solve this problem for me, so please also inform me if it

In table G3.1.5 it is stated:

"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 uniformly in horizontal bands across
the four orientations."

Addendum A states:

"Vertical fenestration areas for new buildings and additions shall equal
that in the proposed design or 40% of gross above-grade wall area, whichever
is smaller, and shall be distributed uniformly in horizontal bands across
the four orientations and shall be distributed on each face of the building
in the same proportion as on the Proposed Design."

Let us assume there is a building with mostly above grade parking garage and
a few offices that are conditioned. The parking garage has a wall to the
outside, but no windows and is not conditioned. If I am following the
prescriptions above, what do I do of the following two options, or is there
a third option, when following Appendix G?

1 - I take the area of windows in the proposed building and apply them
equally (or not for Addendum A) on all facades in the baseline building, for
all floors in horizontal bands. This way even the parking garage gets
windows in the baseline building, and the offices in the baseline building
has very little window area compared to the proposed (because so much of the
are is on the parking garage in the baseline), so little that this model
significantly penalizes the project for not putting windows in the parking
garage! I here apply the windows to whatever surface I would bump into if I
approached the building by foot (and I can be as tall as the building
itself, so it's not just the lower floors' walls).

2 - I take the area of the windows and only apply them to walls that
separate conditioned space and the outside. This way the parking garage in
the baseline building does not receive any windows. In this instance I can
get savings from placing the glazing strategically within the offices. The
sum of the area in all offices are the same for the baseline and proposed
(since there are only two types of spaces: parking and office).

I have seen in previous correspondence that only below-grade walls to
conditioned spaces are included (I did not understand why only conditioned
but at least I know it's common practice), but is it the case that only
walls between the conditioned spaces and the outside conditions get windows
for above-grade walls too? In the above quotes from the standard and the
addendum, the problem arises with the use of "orientations" and "face". The
statement that fenestration is supposed to be distributed on the
"orientation" or "face" of the building does not explain much, since neither
of these terms seems to be defined in 3.2 or elsewhere in the standard. If
the texts had used the term "exterior building envelope", I would have been
quite confident in using option 2 above. Since the quotes does not mention
that only parts of the baseline building will receive glazing, I am inclined
to follow option 1 above, although it seems to go against the intention of
standard 90.1and Appendix G.

The terms "orientations" and "face" is substituted by the term "exterior
wall" in the User's Manual on page G-15. This is yet another undefined term.
Initially I was inclined to think that exterior wall is the same as
"Nonresidential Exterior" and/or "Residential Exterior" wall on page G-35 of
the User's Manual which I think might be the same as exterior building
envelope, but this link is not established anywhere. Moreover, on this very
same page, the term "exterior wall" is used to describe an "Unconditioned
Exterior" wall, so clearly the term "exterior wall" is not unique to only
walls separating conditioned spaces from outdoor conditions. In fact, this
use of exterior wall leads me to again consider option 1, above, and apply
windows in the baseline building to the parking garage in the hypothetical
building above.

Closely linked to the discourse above, I do not see why windows in
unconditioned spaces in the proposed building must be included in either
option 1 or 2 above. Let's examine another hypothetical building. This
building is half storage, half office. The offices are conditioned and have
windows. The storage space is unconditioned, and well ventilated so that the
walls between the offices and the storage rooms are treated as exterior
building envelope. However, there are many windows in the storage spaces
too. Disregarding the discourse above of allocating the fenestration area in
the baseline building, I am seeing difficulties in what the fenestration
area is in the first place. If I consult section 3.2 of 90.1 I find:

"fenestration area: total area of the fenestration measured using the rough
opening and including the glazing, sash, and frame. For doors where the
glazed vision area is less than 50% of the door area, the fenestration area
is the glazed vision area. For all other doors, the fenestration area is the
door area. (See door area.)"

Where fenestration is defined as:

"fenestration: all areas (including the frames) in the building envelope
that let in light, including windows, plastic panels, clerestories,
skylights, glass doors that are more than one-half glass, and glass block
walls. (See building envelope and door.) (a) skylight: a fenestration
surface having a slope of less than 60 degrees from the horizontal plane.
Other fenestration, even if mounted on the roof of a building, is considered
vertical fenestration. 8 ANSI/ASHRAE/IESNA STANDARD 90.1-2004 (b) vertical
fenestration: all fenestration other than skylights. Trombe wall assemblies,
where glazing is installed within 12 in. of a mass wall, are considered
walls, not fenestration."

Is the fenestration area all the fenestration in the building, both for the
conditioned and for the unconditioned spaces? Or is it only the fenestration
for the conditioned spaces? When I look at option 2 above, if I take all the
fenestration in the proposed building and apply it to only to the
conditioned spaces for the baseline, we can get huge energy savings just
from the type of building, not from the way this type of building was
designed. We can even get more windows than there are walls in the
conditioned offices which in my instance (Trace 700) can cause problems
running the simulation. Even if the software ran with more windows than
walls, the output could potentially be of little value for analysis. It
seems like I am supposed to take the entire fenestration area, both in
conditioned and unconditioned spaces, of a building and this is another
reason I am inclined towards option 1 above. However, option 1 still remains
very unfair when I consider a building with more above grade car park than
offices, as explained above.

Thank you for reading through this. Please post your answers or opinions, if
any. Also remember to reference any section of the 90.1 Standard, its
addenda or its User's Manual if you have any clarifications regarding my
uncertainties pertaining to the various definitions discussed in this email.

Faithfully yours,

*Ulrik Welle-Strand Horn *

Ulrik Welle-Strand Horn's picture
Joined: 2011-10-01
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