One thing to consider by requiring project teams to include windows in the calculation is areas where the exisitng windows cannot physically be retrofitted to meet the new performance criteria. Replacement of windows and other building openings such as doors and louvers are not just done for energy purposes, but to also meet Building Code requirements for windborne debris in certian regions such as hurricane zones. Additionally, with the times we live in, many buildings (federal projects especially) have force protection requirements that must be met. Replacement windows and doors can match the historic appearance and not detract from the historic exterior. There should be an option for replacement windows that meet the historic preservation requirements where these requirements exist.