Energy performance is a significant aspect of what drives this project. Having a performance standard for rehabilitated historic windows will not only provide a standard for judging the success of a project, but also can be a selling point when an owner makes a decision whether to save their historic windows or replace them.
We can take a page from the window manufacturing industry. AAMA/WDMA/CSA 101/IS2/A440 is a nationally-recognized performance standard which evaluates new windows based on several factors, including structural performance, air infiltration, water infiltration, break-in resistance, and thermal performance. Most categories are measured against a design performance factor, which is based on the anticipated wind pressure in pounds per square foot on the window's surface. The minimum performance standards listed are pretty soft in some areas. We usually ask for additional performance in certain areas over and above the standard design performance level. Most manufacturers can produce windows which beat these standards.
The AAMA/WDMA/CSA standard is not applicable to evaluation of existing windows, but the basic performance criteria and categories can help to define the key areas where these Standards can offer assurance that the finished product will perform in an acceptable fashion. It seems that the Standards should set forth minimum performance requirements based on air infiltration (with the Pine Mountain tests we are on our way to having data to establish this), water infiltration, and thermal transmittance (u-value). Window manufacturers establish their performance grade by testing sample products in the lab every few years. For historic windows, it seems that the performance data can be established by testing on installed conditions, similar to how it was done at Pine Mountain. The test data can serve not only to help sell window restoration, but also to determine appropriate treatments, and if necessary, to provide the performance criteria for a testing and inspection regimen on an individual project.
Having a demonstrated "service record" for these treatments, and established grounds for performance expectations, will go a long way in helping the case for window restoration.