LEED modeling requirements for existing buildings

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When modeling an existing building, is required to calculate the
proposed building window to wall ratio and equally redistribute the
glass area to the baseline building and prepare models for the four
rotation exposures? My take is that for a proposed existing building,
there is not an opportunity to position the building to the most
advantageous exposure and you could end up being penalized if the
baseline energy use is averaged for all rotations but I have not been
able to find any reference in LEED or ASHRAE to support this. I'm not
sure what to think about equal redistribution of the glass areas.

Mark Sorensen

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Joined: 2011-10-02
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Under ASHRAE 90.1 Appendix G, Table G3.1, Section 5, "Baseline Building
Performance" column, requirement (f) (take a breath...) it states that
"For existing building envelopes, the baseline building design shall
reflect existing conditions prior to any revisions that are part of the
scope of work being evaluated". So the rotation doesn't apply to you.
Also, I interpret that to mean the windows don't need to be
redistributed either. Hope that helps!

Eric O'Neill

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Joined: 2011-09-30
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What about the baseline envelope assemblies? As an example I have an
existing building where the windows are being replaced and insulation is
being sprayed on the outside of a built up roof.

In the same column of Table G3.1, requirement (b) states that "Opaque
assemblies used for alterations shall conform with 5.1.3" and requirement (c)
states that "The fenestration areas for envelope alterations shall reflect
the limitations on area, U-factor, and SHGC as described in 5.1.3."

5.1.3 states that "Alterations to the building envelope shall comply with the
requirements of Section 5 for insulation, air leakage, and fenestration
applicable to those specific portions of the building that are being

My interpretation is: because the windows and roof are being altered, I have
to use the appropriate U-factors and SHGC from the tables for the baseline
windows and roof. However, the walls are not being altered, so I can use the
existing construction for the baseline walls.

Does anybody have a different interpretation?


Jay Keazer, EI

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Joined: 2011-09-30
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It is my understanding that that is the correct interpretation.

However, I don't think Eric's interpretation that the baseline rotation
requirement does not apply to existing buildings is correct. Let me explain
why I disagree.

Requirement f in Part 5 of Table 3.1 refers specifically to the baseline
building design (note the italics which signify that a phrase or word is
defined in Section 3 of the Standard) while Requirement a in Part 5 of Table
3.1 refers specifically to the baseline building performance (again in
italics). The baseline building design is not the same as the baseline
building performance and therefore Requirement f. cannot supercede
Requirement a in any case.

One last comment about the equal window distribution. ASHRAE 90.1-2004
Addendum a removes the equal distribution requirement for windows in the
baseline. If you're working on a LEED project I suggest you consider
adopting Addendum a to avoid this requirement. USGBC has allowed the use of
ASHRAE 90.1-2004 addenda. If you do decide to adopt Addendum a remember
that USGBC requires that you incorporate all the changes included in the
Addenddum. Addendum a has a total of eight changes to Appendix G, most
important of which are the changes to the calculation of the baseline HVAC
system types.


Michael Tillou, PE, LEED

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Joined: 2011-10-02
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Good morning all,

Mike is correct here. My apologies.


Eric O'Neill's picture
Joined: 2011-09-30
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I asked the USGBC for an interpretation (not an official CIR) on the
need for rotating an existing building for modeling purposes and the
response was that the existing building does in fact need to be rotated.

This is their response (the "specified analysis" refers to building
rotation of an existing building):

If the existing building being renovated is pursuing LEED-NC rather than
LEED-EB, then it would indeed be required to undergo the specified
analysis. This analysis is used to establish the baseline for energy
performance using the ASHRAE standard. LEED doesn't have any specific
exemptions for existing buildings in this requirement, but if ASHRAE has
some kind of exemption, we will honor that.


Jeffrey G. Ross-Bain, PE, LEED

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Joined: 2011-09-30
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I thought that the rotation requirement was gone in any case.

Carol Gardner's picture
Joined: 2011-09-30
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Table G3.1 item 5(f) indicates that you would use the U-factors and SHGC
from the existing building envelope prior to any revisions that are part
of the scope of work being evaluated. Someone please straighten me out
on this because I'm having a hard time understanding how it could be
interpreted that you need to use the minimum U-factors and SHGC from the
tables in Section 5 for the baseline building regardless of whether
those envelope components are being altered or not.


Mark Sorensen CEM

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The way I understand it is as follows:

1. All parameters in basecase and as designed case = existing conditions if
not being altered.
2. If improvements will be made to component X in the as designed case, the
basecase component X = 90.1 requirements.

So, for example, if the envelope is not being altered, set it to existing
conditions in both the basecase and as designed models if the information is
available. Otherwise, you can use 90.1 values in both models. If the
glazing is going to be updated as part of the renovation, the basecase =
90.1 requirements, the as designed case = new glazing.

A similar situation applies to system improvements. For example, if a VFD
is going to be added to the cooling tower fan, the basecase will need to be
modeled as a two-speed fan per the requirements of 90.1, regardless of what
is existing.

Essentially, the existing building basecase becomes a moving target as
energy improvements are considered.

If feel this is incorrect, please let me know.

Amanda Bogner

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Joined: 2011-09-30
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My interpretation is different. Table G3.1 5(f) says for the building
envelope only:

"...the baseline building design shall reflect conditions PRIOR to any
revisions that are part of the scope of work being evaluated."

So, if I read this right, the baseline reflects the as-is conditions and
the design case reflects the new conditions. Now when there are
additions, such as adding an atrium or more space, then the baseline
becomes 90.1 because there was nothing there before.


Jeffrey G. Ross-Bain, PE, LEED

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Demba Ndiaye's picture
Joined: 2011-09-30
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Dear All,

I was looking for something else and found this in a LEED-NC EAc1 update
document that I downloaded, but can no longer find on the USGBC site:

"For existing buildings that are being renovated, the building envelope
design parameters for the
Baseline Design should be modeled using the existing (pre-retrofit) building
envelope thermal
parameters rather than the ASHRAE Std. 90.1 prescriptive building envelope
requirements for the
specified climate. Any proposed changes to the building envelope (such as
replacing windows or
increasing roof insulation) should be modeled in the Proposed Design."

James V. Dirkes II, P.E., LEED AP

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Joined: 2011-10-02
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I am inclined to agree with Amanda on this issue.

Appendix G is a methodology for demonstrating energy performance beyond the
minimum requirements of ASHRAE 90.1. If you alter a building envelope
element and want to claim energy savings beyond the minimum performance
requirements of ASHRAE 90.1-2004 then the baseline HAS to be ASHRAE
90.1-2004 minimum performance. Otherwise you are no longer making a
comparison to ASHRAE 90.1-2004.

LEED v2.2 EA-1 credits are based on a comparison to an ASHRAE 90.1-2004
baseline. This fact should supersede the apparent contradiction in Appendix
G that is being discussed.


Michael Tillou, PE, LEED

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Joined: 2011-10-02
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Appendix G is very explicit regarding these issues. Amanda's interpretation
is not consistent with Appendix G (though any mandatory envelope measures
applicable to "alterations/additions") would still apply for the LEED
Prerequisite (EAp2)).

The relevant sections of Appendix G include:

- Table G3.1.5(Baseline)(f): All elements of the existing Baseline
building envelope shall reflect the existing conditions prior to any
revisions that are part of the scope of work being evaluated. [Notes: [1]
this only includes existing portions of the building, and does not apply to
building additions; [2] there is no exception mandating that elements with
changes that are part of the scope of work must be modeled as meeting
minimum ASHRAE prescriptive envelope requirements].
- Table G3.1.5(Proposed): The building envelope shall be modeled as shown
on architectural drawings or as built for existing building envelopes (any
changes to the existing building envelope reflected on the architectural
drawings would be reflected in the Proposed Case).

gail's picture
Joined: 2011-09-30
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Hi Carol,

Do you have a refrenece for the relaxation of the rotation requirement?
The arbitrary rotation requirement has been one of my pet peeves for a
long time, and any information you have confirming its demise would be
welcome news...;-)


Brandon Nichols's picture
Joined: 2011-10-01
Reputation: 0

Mike and others;
The concept in the exception is that for a renovation project, the Standard does NOT REQUIRE replacement of existing crappy windows, for example. So the exception (f) gives credit for an improvement to something that could have been left in the same crappy condition and met the Standard. Even though, once you commit to changing them, the Standard indeed requires those new windows to have the minimum performance. Contradiction resolved in my mind.


Fred Porter's picture
Joined: 2011-09-30
Reputation: 0

Also to backup/justify Amanda's and Fred's posts -- if the project is a renovation where mechanical and lighting are replaced, but there are no envelope modifications, then the building permit probably excludes the envelope and 90.1 would not apply to those components.

It would make sense then to fill the parameters into both models as equal in both cases where it was excluded from the permit.


David S Eldridge's picture
Joined: 2011-09-30
Reputation: 2000

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Mahabir Bhandari, Ph.D., LEED AP

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