Windows are an integral part of a building system. They transmit light, control heat flow, are a means of egress, frame exterior views, and are significant elements that contribute to the design of the building. Windows are complex units and are made up of many different components that can be decorative, functional or both.
Why are historic windows important?
Historic windows are often character-defining elements. Character-defining elements are the materials, form, location, spatial configurations, uses and cultural associations or meanings that contribute to the heritage value of a historic place and must be retained in order to preserve its heritage value. Furthermore, historic windows are elements that directly reflect a site’s craftsmanship and design and are usually constructed out of particular materials. They are usually quite detailed and in some instances retain original glazing units.
Common misconceptions about historic wood windows
On a daily basis I field questions about the replacement of historic wood windows. A typical case is a historic site with historic windows that have not been looked at in some time and have deteriorated to some extent due to weathering. A common misconception is that replacement of these historic windows with a modern unit is cheaper and will increase the thermal efficiency of the building through higher R values.
Research in performance standards for timber sash and case windows in Scotland has taught us that estimated costs including painting of window components, repairing damaged putty and re-caulking where necessary within a regular maintenance program eliminated the cost of a major restoration project every five years. Another thing to consider is that modern sealed units, when they fail are not maintainable and must be replaced outright.
It is also interesting to learn that a single glazed window in conjunction with an exterior or interior storm window is comparable to a modern sealed unit. A single glazed window has an R value of 0.6 while a single glazed window with wood storm has an R value of 2.0. The top of the line triple glazed window with low E coating and argon has an R value of 3.5. Overall windows in general are thermally inefficient in comparison to a typical wall with 4” batt insulation that has an R value of 12.
Planning for historic wood window conservation work
When planning for any conservation work we always take the approach of minimal intervention. Preserving historic material and maintaining historic material is the first step and outright replacement, if necessary, is the last option.
In most cases simple epoxy repairs to wood, adequate prepping of the wood surface (manual scraping), the application of an appropriate primer and brushed on layers of exterior paint is all that is needed to repair historic windows and to prevent deterioration.
For more severe cases, putty replacement, replacement of broken or damaged glazing, and dutchman (splicing in of new wood) may be required.
- Historic windows have heritage value.
- Historic windows have demonstrated good durability and maintainability.
- Always assess and document each window before proceeding with conservation work.
- Compile historic photographs and refer to the Statement of Significance in your planning process.
- Minimal intervention should be the first approach.
Remember, you can save on costs and achieve the same thermal efficiency through conservation.