Basecase Building Envelope

Dear All:- we have two major renovated Building in India Building 01. No change in building envelope (used the existing wall, floor, window) but changes in whole MEP Design Building 02. Changes in building envelope & MEP Design we have some doubts in selecting the basecase building envelope for both buildings 1.Can we consider ASHRAE 90.1 basecase envelope for both buildings instead of existing building envelope (is it wrong) 2.If we consider existing building envelope for building 01 basecase means we need to consider the same wall, floor, window for both cases because there is no changes in building envelope(is it correct) 3.Can we consider the ASHRAE 90.1 basecase envelope for both buildings or Building 01 4.Is it mandatory to consider the existing building envelope for basecase building envelope 5.Is it anything wrong to consider ASHRAE building envelope for major renovated building basecase instead of existing building envelope Please note that we have advantage of considering existing building envelope for basecase building envelope instead of ASHRAE building envelope In another major renovation building we don't have existing building details such as (wall, window, floor U value, SHGC, VLT) 1.Can we consider ASHRAE 90.1 base case envelope instead of existing building envelope Thanks in Advance