Looking up U-Factor for older windows

How can I guesstimate U-Factors for older unmarked windows? I do work legalizing unpermitted dwellings, of which my area has tens of thousands.   These were all built at a time when… How can I guesstimate U-Factors for older unmarked windows? I do work legalizing unpermitted dwellings, of which my area has tens of thousands.   These were all built at a time when…Read MoreGreenBuildingAdvisor