Appendix G Interpretation: Existing Building Envelope

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I?ve run into one of those grey areas of the Appendix G language on a
project. We are looking at a major renovation project where the shell of
the building will be upgraded.

Clearly the baseline building will be modeled with insulation U-values
that correspond to the existing conditions of the building rather than the
insulation levels prescribed by tables 5.5-1 to 5.5-8. I think this is
pretty well established.

My question is about the quantity of glazing in each case. For this
project we will be punching new glazing into the existing fa?ade as part
of the renovations.

? Table G3.1, Section 5 states that ?Vertical fenestration areas
for new buildings and alterations shall equal that in the proposed design
or 40% of gross above-grade wall area, whichever is smaller.? So if we add
new windows, it would seem that the baseline building should also have
those windows, but what should their U-value be? Same U-value as the other
existing windows? Lookup the value for new glazing?

? Lower in that same section of Table G3.1 it states: ?Existing
Buildings. 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.? To me, this seems to say that you
would go with the existing quantity of glazing, and the proposed model
would have more glazing than the baseline, even if the proposed does not
exceed the 40% cap.

There seems to be a little contradiction between the two requirements. My
inclination is that the existing building clause overrides the requirement
that the same amount of glazing be used in each model.

Have any of you worked on a project where this came up? Any thoughts? It
seems like this could potentially be one of those hot topics of


Nathan Miller, PE, LEED? AP

Nathan Miller's picture
Joined: 2011-09-30
Reputation: 200


Your conclusion is the same one I have reached in the past in this situation - the existing building clause is the overriding factor. For existing buildings I interpret that the intent is comparing the proposed model to what would occur if you did not change anything about the existing building. It does contradict some of the other provisions of Appendix G, but many of those provisions seem designed to develop that arbitrary baseline of what would be considered "minimally compliant" for comparison when a rating authority uses Appendix G. It gets even fuzzier when you have an addition and renovation together since then you have an existing building baseline for the renovated part of the building and an Appendix G baseline for the addition.

Jeremy R. Poling, LEED AP

Jeremy Poling's picture
Joined: 2011-09-30
Reputation: 0

Dear All,

I am proposing the following methodology:

? Prepare a model based on the current envelope and systems and then
to compare them with utility bills for past 12 / 24 months and establish AS
IS case

? The AS IS case values can be compared with Appendix G values to
establish their status (either %better or %worse than ASHARE base case) and
prepare an baseline.

? The proposed Energy Conservation Measures (ECMs) can be applied to
the AS IS case to calculate % savings

Shirish M. Deshpande

Shirish Deshpande's picture
Joined: 2011-10-02
Reputation: 0