Hi Praveen,

I've copied below a discussion from some time back outlining my general
practice for "where to draw the line" when it comes to envelope/wall
boundaries. This may be generally useful and seems to answer at least
part of your inquiry.

When I'm doing electrical and/or HVAC design alongside the model, my
area takeoffs for each inevitably will match up, because I hate doing
the same work twice.

I use the outermost surfaces when defining my building footprint and
midpoints for all internal partitions for all calcs. Space-by-space LPD
calcs in a strict reading do not require the space areas to be measured
to the outermost surface of an exterior wall (they do say to use the
midpoint of interior partitions, as of 90.1-2007). I'd feel comfortable
saying the extra square-footage "handicap" I'm imposing on myself as a
lighting designer is an insignificant fraction in 99.9% of cases when
determining baseline LPD...

Inevitably, areas summed for all spaces in a building between
architects, HVAC, and lighting designers will not match - that's a fact
of life and in my book that's okay, so long as nobody is way off. Model
reviewers will inevitably/reliably gripe when the numbers don't match
exactly, and it's usually an easy thing to either fix or explain after
the fact. If you to try to make everyone use the same numbers from the
DD/CD design phases, you've chosen a losing battle. For my part in the
role of the project's energy modeler, I'm satisfied to allow my fellow
designers do their own calcs, and just ensure nobody is way off along
the way... quibbles over small differences in the final tallies, if they
come up, are easy to reconcile.

~Nick

NICK CATON, E.I.T.