See the Texas Design Guide for more recent information here.
Windows are character-defining features, defined by the National Park Service as “a prominent or distinctive aspect, quality, or characteristic of a historic property that contributes significantly to its physical character.” Replacing windows can have a devastating impact on the integrity of the structure or “the authenticity of a property's historic identity, evidenced by the survival of physical characteristics that existed during the property's historic or prehistoric period.”
The repair of historic windows is likely to be a better long-term investment than their replacement. Historic windows were designed to come apart so that individual elements could be repaired or replaced without replacing the entire window unit. They were also constructed from old growth wood, which has denser growth rings than new growth which results in a higher strength. New windows will have a much shorter lifespan and when their components degrade, they cannot be repaired.
TMSP Design Staff has created two tech sheets on window repair and replacement and how to properly shade windows. Download them here:
Design Topic: Window Shading Strategies for Downtown Buildings
Design Topic: Window Repair and Replacement
Additional guidance is provided through the National Park Service:
Preservation Brief #9 - The Repair of Historic Wood Windows - http://www.nps.gov/tps/how-to-preserve/briefs/9-wooden-windows.htm.
Preservation Brief #13 - The Repair and Thermal Upgrading of Historic Steel Windows - https://www.nps.gov/tps/how-to-preserve/briefs/13-steel-windows.htm
The General Services Administration (GSA) also provides technical sheets on wood windows that list straightforward steps to follow and list companies/suppliers.
Restoring Wood Window Sash and Frames - https://www.gsa.gov/real-estate/historic-preservation/historic-preservation-policy-tools/preservation-tools-resources/technical-documents?Form_Load=88188
Rehabilitating Wood Windows - https://www.gsa.gov/real-estate/historic-preservation/historic-preservation-policy-tools/preservation-tools-resources/technical-documents?Form_Load=88071
Epoxy Repair for Deterioration and Decay In Wooden Members – https://www.gsa.gov/real-estate/historic-preservation/historic-preservation-policy-tools/preservation-tools-resources/technical-documents?Form_Load=88139
Surface Preparation for Painting Wood – https://www.gsa.gov/real-estate/historic-preservation/historic-preservation-policy-tools/preservation-tools-resources/technical-documents?Form_Load=8844
Why should I repair my historic windows?
It is a common misconception that it is cheaper and easier to rip out wood windows and replace them with metal or vinyl alternatives rather than repair the existing material. This is NOT TRUE. The following resources help make the case for keeping historic windows.
Association of Preservation Technology - "What Replacement Windows Can't Replace"
Center for ReSource Conservation - "The Effects of Energy Efficiency Treatments on Historic Windows"
Forbes - "Don't Buy Replacement Windows For Your Old House"
Keith Haberern, Engineer,Collingswood, NJ Historic District Commission - "Old Wood Window / Replacement Window Energy Analysis"
National Alliance of Preservation Commissions
National Trust for Historic Preservation
Preservation Green Lab - Saving Windows, Saving Money: Evaluating the Energy Performance of Window Retrofit and Replacement
Preservation Resources, Inc. - "Of Paint and Windows: Replace or Repair - That is the Question"