Understanding heating loads in commercial buildings

From my latest post on the Arborus Efficiency Journal:

We recently encountered some head-scratching results in an energy analysis of an existing office building in downtown Ottawa. This is an old building with out-of-date, metal-framed, leaky windows and we wanted to know what would happen if we replaced the old windows with well-sealed insulating windows. Expectations for energy savings for high, but when we finished the analysis the annual energy savings were only 7%. What happened?