Acrylic interior window inserts

How window inserts work

Old single-pane windows and leaky sashes lose heat mostly through air movement and the cold glass itself. An insert adds a second pane on the room side, a couple of inches from the glass. The still air between them works like the gap in a double-pane window. The insert also stops drafts coming around the sash, and less warm, moist room air touches the cold glass, so you get less condensation.

They're popular with renters and in older homes where the original windows have to stay. Nothing is drilled or glued: the insert presses into the window opening and pulls out again in spring.

What thickness to choose

• 1/8 in. for draft and cold control on most windows up to about 36 × 60 in. It's light (about 0.7 lb per square foot) and easy to take in and out.
• 3/16 in. for bigger windows, where 1/8 in. flexes too much to stay seated.
• 1/4 in. when noise is the main problem. Sound blocking depends mostly on mass and a good seal, so the heavier panel helps. It weighs about 1.46 lb per square foot, so a 30 × 54 in. insert is around 16 lb.

Use the acrylic weight calculator to check a specific size. Clear extruded acrylic is the lower-cost choice and is perfectly clear for this; cast is a bit more scratch-tolerant if you'll handle the panels a lot.

How to measure

1. Decide where the insert will sit: usually inside the window casing, against the interior stop, so it covers the whole sash.
2. Measure the opening width at the top, middle, and bottom, and the height at the left, middle, and right. Old frames are rarely square.
3. Take the smallest width and height, then subtract the gasket allowance. With a 1/4 in. compressible foam or silicone tube around the edge, cutting the acrylic about 1/4 in. smaller than the opening in each direction gives a firm press fit.
4. If the frame is badly out of square, measure both diagonals and send us a sketch. We can cut a slightly trapezoid panel to match.

Gaskets, handles, and seasonal storage

The gasket is what makes an insert work. Self-adhesive foam weatherstrip, silicone tubing, or rubber edge trim all work; the goal is an even, airtight seal on all four sides. Add a small pull tab or a couple of clear adhesive knobs near the bottom so you can get the panel out without prying at the edge.

Store inserts flat or standing upright against a wall with the protective film back on or wrapped in a blanket. Leaning them at an angle for months can leave a slight bow.

What inserts won't do

An insert isn't a replacement for a broken or rotten window, and it won't stop water coming through a failed exterior seal. For sheds, barns, and campers where the acrylic is the window, see acrylic replacement windows. And because a well-sealed insert holds the window closed, don't fit one to a window you need as an emergency exit unless it pulls out quickly by hand.

Questions