We have just started on a residential development in a historic district with the conversion of an existing historic building with landmark status into condominiums and the construction of 15 new houses. Because of the requirements placed on us by the Historic Resources Commission, we cannot remove the interior trim of the building or replace the windows. The building currently has no insulation, and only has a .75" cavity to put in new insulation, so we are facing significant challenges to improve the energy efficiency over ASHRAE 90.1 for this one building. The development as a whole will be as green as we can make it, with LEED H certified houses, optimum site location in an urban infield setting, and sensitive, low impact site design. How do we handle the prereq for minimum energy efficiency in this one particular building?