Energy Efficiency vs. Historic Integrity

Many customers have said to me, "I don't want to lose the character of my old windows but what can I do make them energy efficient?" Less that 10% of the windows I have worked on had any type of weather stripping. A properly weather stripped old window will provide much improved energy efficiency and functional operation.
The most common type in use by restorers is spring bronze. Why spring bronze? It lasts a long time, looks good and provides an excellent sliding surface for the sash. It is typically applied to the window jamb. It comes in 2 basic styles, roll and V style. There are similar styles in plastic but will not last nearly as long as the bronze. Link

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Another type is silicon tube seals that are generally used on the bottom or top of the sash where it meets the sill or header of the window frame. These are available in kerf mounted and stick on versions.