Does base building performance now impact CI performance?

Overall you are correct. This is outlined in the changes from 2009 to v4 in the Reference Guide. Your conclusions sound right to me but I have not researched each one.

I downloaded the summary changes and there was no reference to the exception pertaining to the existing building envelope. The language online states "Exception: the baseline project envelope must be modeled according to Table G3.1(5) (baseline), Sections a–e, and not Section f." Section f being the part that allows the existing envelope to be used as is. I find the choice of the word "exception" to be confusing, if it in fact means the existing conditions can not be used.

We are looking for a more clear explanation of the intent if the "exception".

Any feedback would be appreciated.

I think the "exception" is LEED as the adopting authority is creating an exception to the application of Appendix G. It is excluding the use of the existing envelop in the baseline. I would guess that the rationale for this is that your project should not be rewarded with additional savings for choosing to be located in a building with insulation levels below that required by 90.1.

Hi. Sorry to reply to the old post, but I could not find in the reference guide stating "lighting that is not in the scope of the project, should be modeled identically (energy neutral)."
Could someone kindly tell me where I can reach the information?

Thank you.

I think that comes from the fundamental idea that CI projects are only held to and claim credit for what is in the scope of work of their project.

Thank you Marcus, for your prompt reply.
I understand the idea that we can claim credit for what is in the scope of work, however that is not reflected to envelope and HVAC in energy modeling already. It is like the selection of the high performance basebuilding is awarded as credit. So I thought lighting fixtures in the tenant space but outside of the scope of work could be treated as same as envelope and HVAC.

Reading reference guide carefully again, I found it saying "Existing conditions must be modeled as new construction in the baseline case, following Appendix G, for all components and systems." in the last sentence under BUILDING THE BASELINE ENERGY MODEL, FURTHER EXPLANATION, EA prerequisite Minimum Energy Performance, Page 135 of the Reference Guide I have.
Do "all components and systems" limit to HVAC? Couldn't that include lighting fixtures and controls?

As I said I did not research each one. Based on that sentence you may be correct.

First off let me say that this thread is extremely helpful - possibly more clear than the reference guide on this topic!

We are modeling two floors (typical TI scope, fan boxes, lighting, etc) of office space going into a building (10 floors) that is designed for labs.

Should the model include the whole base building or just the tenant space (with adiabatic walls/floor/ceiling as necessary)? Since all building systems must be modeled as designed, we would use a fractional capacity based on SF for systems that serve more than just the tenant space. It seems that the savings % would be much lower if we modeled the whole building as compared to if we modeled the tenant space alone. 

You only model the tenant space. I think you could proprtionalize the load based on area if the loads are very similar throughout the building. 

Regarding the envelope rule for LEED-CI V4 projects (i.e. Baseline envelope is ASHRAE min, not existing), I just did a little research and noted an addendum (#10454, 17 October 2017) that offers an alternative approach. This addendum allows using the existing envelope features but in doing so increases the minimum performance level to 5% (not 3%), and then requires that the point thresholds for EAc1 are increased across the board by 2%. The entire addendum is located here: https://www.usgbc.org/leedaddenda/10454

Good to know, thanks.

If we are not touching the existing envelope, but we are adding a low-e window film to the existing glazing, do we need to include the window film in our baseline model? Or can we keep the original windows in the baseline model?

I would think you could claim savings for this measure.