How to Measure

Innerglass Interior Storm Window Measuring Guidelines

The Innerglass Interior Storm Window is mounted on the inside of the stops of the primary window, where it seals tightly, eliminating drafts and condensation problems. It conforms to the window opening and pops in and out easily without tools.

Please watch the video on the left to see the simple steps necessary to measure your existing windows so we can build your custom interior storm windows.  If you have any questions, please contact us.

How to Measure

Tools needed

3” Case Dimension Locking tape measure
16”x 24” framing square (to check for out of square)
6” or 12” ruler
Window Worksheet to record measurements


Measure all 4 sides to 1/16” and record the actual measurement. Do not average. Don’t bend the tape into the corner. Instead add the tape case dimension to your measurement. Measure exactly where the window will be mounted.


The Innerglass Compression-Fit window needs 3/4 “depth in the window opening for mounting, 5/8” is possible but call us. The double hung and horizontal sliding interior storms need 1 1/8” depth in the window opening. Watch for obstructions such as window hardware. Screw heads and recessed pockets for the window stops are generally not a problem if they don’t stick out more than 1/16″.


To check for out of square, notice the framing squares at the left and right bottom corners and the 2 lines under each framing square where you record the gap on the worksheet. Start tight to the left side and slide the 24” side down until it touches the sill at either the corner or the end. The framing square is always held tight against the side so that any gap will show up at the bottom. If the window is square at the bottom, you would record a “0” at each of the 4 lines under the framing squares. If the gap is 1/16 or less it is effectively square.

For example: If a window sags down to the right the gap measurements could be 0 ¼, 0 ¼.

Place the framing square against the right side of the window and repeat the procedure. Think of this as a snapshot of each bottom corner. In reality they overlap but for clarity they are separated and are not to scale.

Next lay the 24” side of the framing square on the sill to check if the sill is bowed up or down, if so give us a center vertical measurement and draw an arc showing the bow.


Innerglass Windows will accommodate 3/16” vertical and ½” horizontal play. On deep openings measure where you want the window to be and measure the opening at the wall to check that it does not get smaller than these tolerances. If the opening is smaller or the bottom of the opening is obstructed, call us.


If you are measuring for a double hung storm or are doing a large opening as separate upper & lower windows with a mullion, measure from the sill to the top of the meeting rail. (Where the dust collects.) This is where the top of the mullion will be. A window opening may be done as one window or two windows with a mullion. Generally double hung windows taller than 72″ inches should be done as 2 windows with a horizontal mullion. Measure the length for the mullion where it is to be installed. The dimensions of the vinyl covered wood vertical mullions are 1 5/8” deep x 1 1/8″ wide. The vinyl color will match the window. The horizontal mullion is ¾” thick x 1″ wide. Measure the width and height of the entire opening.

Do not make allowance for the mullion size. We will do that. For pricing purposes, remember that it is one opening but TWO windows, so the width & height of each needs to be added together to come up with the united inch measurement.
Maximum size for double strength glass is 25 sq ft or 125 united inches.

Larger than that 3/16″ or 1/4″ laminated or tempered glass will be used at extra cost.

Within 12″ of a door or 18″ of the floor, Acrylic or tempered glass must be used at extra cost.

Measure Form

Let us help you choose the right solution at the right price

Glass interior storm windows for energy conservation & soundproofing that maintain the integrity and beauty of your historic windows, and automatically conform to the opening. Easy do-it-yourself installation.

How to get a price quote or place an order:

If you have the sizes, we can give you an instant price on your project.
Call to get an instant price from the owner at 860-651-3951

We calculate the price of a 30” by 50” compression window in Low E glass as follows:

30 + 50 = 80 United Inches x $3.25 = $260
11 square feet in clear glass x $3 = $33 + $260 = $293 Total
11 square feed in Low E glass x $6 = $66 + $260 = $326.00 Total

Start by downloading the worksheet below:

Worksheet examples 2 pdf 789x1024 1 - innerglass window systems