Modeling infiltration in the baseline case for an existing bldg?

I put this comment in a reply below, but I'm afraid it wont be seen, so I'm starting a new thread... We are renovating an existing historic building which included window replacements, furring out the walls and insulating. How do you suggest accounting for poor infiltration in the existing/baseline building, when a building envelope project was a part of the renovation which improved/decreased infiltration in the proposed building? We recieved the review comment, "...the infiltration rates must be modeled identically between the baseline and proposed cases. Revise the model, as required, such that the infiltration rates are modeled identically." The building was leaking like a sieve before the renovation and we modeled it with "poor" tightness in TRACE for the baseline. This resulted in the entered values in the TRACE model being 1 ACH in the baseline and 0 ACH in the proposed. How should we alternatively model this to account for the improvement in infiltration between the old/existing building and the new/proposed building? I appreciate any advice!