Existing Building Envelope Calculations - Help!

Jonathan Weiss

Jacobs Buildings & Infrastructure

LEEDuser Premium Member

140 thumbs up

February 21, 2011

According to the Appendix G definition, "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 (Table G3.1)," but a colleague showed me a CIR from 2009 (2/20/2009) that ruled that this only applies if the envelope is not being touched. This doesn't make sense to me. If that were true, why would the standard include the phrase "prior to any revisions..."? AND in the "Advanced Energy Modeling for LEED" document, it lists (p. 26) that NOT modeling the baseline as the existing building shell condition is a "common error" for ASHRAE 90.1. So which is right? I am confused! On a project where we are proposing a new envelope, should our baseline be the existing envelope, or the ASHRAE prescriptive requirements in section 5? And why? Thanks in advance!

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David Hubka

Director of Operations

Transwestern Sustainability Services

LEEDuser Basic Member

355 thumbs up

Mon, 02/21/2011 - 20:01

I am currently modeling an existing building that is undergoing a major renovation. There will be a new roof and all new glass. The existing exterior walls will remain "as is". ASHRAE 90.1-2007 baseline values are required for my new roof and glass. The baseline for the existing walls are the U-values of the actual existing walls, even if this value is worse than ASHRAE 90.1 values.

Jonathan Weiss

Jacobs Buildings & Infrastructure

LEEDuser Premium Member

140 thumbs up

Tue, 02/22/2011 - 16:22

Thanks, David. In my project, the exterior walls will be replaced in portions, and all the windows are going to be new. I am still confused about the conflict between the ASHRAE wording and the LEED documents (or between the CIR and the Energy Modeling Guide.). Anyone have ideas for what that distinction would be?

Marcus Sheffer

LEED Fellow

7group / Energy Opportunities

LEEDuser Expert

4454 thumbs up

Sat, 02/26/2011 - 00:42

This issue is pretty straight forward. For the Baseline Building - in an existing building follow the guidance in Table G3.1.5(f) for a new one follow Table G3.1.5(b). Folks often confuse what you have to do for code compliance with what you have to do for Appendix G. For code compliance the "if you touch it" thing takes effect. It does not matter for Appendix G. As usual you always model what has been designed and built for the Proposed Building.