Replacement of Severely Deteriorated Windows

Any replacement other than a custom-crafted replica involves some measure of change in appearance. Even a wood window, if it is a modern manufactured product, will have profiles and details that vary from historic windows. The acceptability of any replacement is based on the overall visual effect of the new window as compared to the historic window.

Documentation
Deterioration justifying replacement needs to be documented with good close-up photographs. When possible, a trial repair, photographed and evaluated, is an excellent way to determine whether or not repair is reasonable.

An alternative to scale drawings is a mock-up or sample installation photographed together with an existing window. A mock-up may be particularly helpful in determining the importance of variations between the deteriorated historic window (left) and the proposed replacement window (right).
Photos: NPS files

Substitute materials
Much of the character of a window derives from its materials. Using a substitute material for a replacement window adds another element of change. On small-scale residential buildings or any building where the interior appearance of the window is important to the character of the building, wood windows may be the only appropriate replacement for historic wood windows. A substitute material is not appropriate if it does not permit the profile and dimension of the historic window to be reproduced accurately.

Clad wood windows generally have the same profiles as other manufactured wood windows, but a clad window will be noticeably different if the cladding is applied with joints that do not match the lines of typical wood window joinery. Also, clad windows do not offer the range of profiles necessary to match some windows, particularly very large windows.

Muntins
True divided lights are desirable, but accurately matching the size and profile of the historic muntins is just as important. Beaded and ogee-shaped glazing beads are a poor match for the simple putty bevel of a historic window. Exterior applied muntins--when backed up by between-the-glass spacers and/or interior muntins--may adequately convey the effect of divided lights on high upper floors. On lower floors, the historic windows should be retained and preserved.

A new sample muntin (above), with its shallow, irregular profile, looks very different from the simple, deeper profile of the glazing putty on the historic window. The new muntin would not be an acceptable replacement. Photo: NPS files

Installation
Successful replacement windows depend not only on the match of the windows themselves, but also on the way they are installed in the wall. The relationship of window to wall plane should not change when a window is replaced. Installing a full replacement unit without removing the existing jambs will usually reduce the glass size and unacceptably compromise the match.

Operation
The way a historic window operates is a major component of its appearance. While a replacement window should capture the visual effect of that operation, it does not have to operate in the same way or at all.

Right: The heavier frame of the pivot sash in this historic industrial building creates a distinctive pattern in the overall grid of muntins. This visual distinction can clearly be reproduced in any replacement window, regardless of how or whether the window operates.
Photo: NPS files