How to Measure Windows
FRAMES WITH BLIND-STOPS:
The width of the sash opening is measured between the side casings. It is best to take measurements at the top, middle, and bottom of the sash opening. Record the largest of the measurements as the sash opening width.
The height of the sash opening is taken from the underside of the head casing to a point on the sill flush with the outside of the casing. If the height measurement is taken against the blind stop, the measurement will be too short in height because allowance was not made for the sill drop from inside to outside of the storm sash. Remember that we are wanting the overall width and height dimensions.
The thickness of the storm sash should be the same as the thickness of the casing, so that the inside of the sash will fit against the blind stop and the outside of the sash will be flush with the casing. The height measurement should be taken at both the left and right side of the frame, and the larger of the two measurements recorded as the sash opening height.
The usual thickness for storm sash is 1-1/8”. Depending on geography, 1-1/8” was a standard blind and storm sash (casing) thickness from the 1870s until the 1980s. If storm sash other than 1-1/8” are wanted, inquiry must state.
FRAMES WITHOUT BLIND-STOPS:
For window frames commonly found on very old buildings in New England that do not have blind-stops, the storm sash must be hung on the exterior of the window casing. The size of the storm sash is typically 1" wider than the opening between the side casings and 1/2" higher than the opening between the window head and sill. This allows the storm sash to be located directly on the window sill (typically a very small gap is allowed to provide for condensation to drain from between the window and storm sash) and will provide for storm sash stiles, rails, and overall glass sizes to match the primary window, while the storm sash overlaps the side and head casings by 1/2". Special offset sash hangars are used to attach the storms.
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Measuring Window Frames for Storm Sash
Determining Sash Opening Width and Height
Obtaining Correct Measurements for Width and Height of Storm Sash Opening