PPS 6 Planning Archaeology and the Built Heritage·Page 61·E17
Energy efficiency alternatives for historic windows
Where double glazing is difficult, alternatives such as secondary glazing, thicker single-sheet glass, draft strips, weather seals, shutters, and good quality curtains can provide equivalent thermal performance without requiring listed building consent. Plastic strips simulating glazing bars are not acceptable.
Where there are difficulties, the first step is to discover, by calculation, whether or not double glazing would provide a material benefit in controlling heat loss. Traditional internal timber shutters and good quality curtains will give an equal performance during the hours of darkness. However, heat may be lost through other routes. Commonly this is via the air gaps around badly fitting frames. The installation of draft strips and weather seals is a simple and very effective procedure. Reglazing with thicker single sheet glass or the installation of secondary glazing are other options that will not normally require listed building consent. Plastic strips simulating glazing bars and sandwiched into the cavity of the double glazed unit are not an acceptable conservation option.
Source — /Users/richardhill/Documents/planning-arch-project/data/documents/regional/PPS 6 - Planning Archaeology and the Built Heritage.pdf