Part II: Conservation FirstChapter 3. Seal, Insulate, and Cover

Clear Interior Storm Windows You Can Make

A removable interior glazing panel that raises the R-value of a single-pane window.

Difficulty: Beginner
Cost: $15–40 per window

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A single pane of glass is near U-1, which is another way of saying R-1. A second clear layer on the room side, sealed at the edges, traps a still air gap and can move that window toward the performance of a decent double pane, roughly near U-0.5 if the seal is real. If the seal is sloppy, the panel does little more than a picture frame would, so put most of your care into the seal.

This is a winter panel for a window you can live without opening every day. Bedroom egress windows have to open without tools when you need them, so the panel uses clips or turn buttons, not a bead of permanent sealant. South windows should still be able to see the sun. The panel is clear on purpose. A quilt, in the next guide, is the night strategy when you want a higher R-value and you are willing to give up the view.

Difficulty

Level: Beginner
Time: An hour or two per window after the first
Skills: Measuring, cutting acrylic square, and fitting a gasket

Cost estimate

$15–40 per window

Acrylic sheet is the swing. A small sash can land near the low end. A large window, plus magnetic tape, lands near the high end. Polycarbonate costs more and takes abuse better.

Materials

Item	Qty	Notes
Acrylic or polycarbonate sheet	Cut a little smaller than the opening	About 1/8 inch thick for a small sash. Thicker if the panel is wide and you do not want it to bow.
Closed-cell foam tape or magnetic tape	The perimeter	The seal. Magnetic tape wants a steel strip on the stop if the stop is not already steel.
Clips or turn buttons	4 or more	So the panel can come off. Screws into sound wood, not into a lead-painted stop you have to sand.
Edge protection	Optional	A simple wood or plastic edge if the cut acrylic will be handled often.

Fit and gap

Measure the interior stop at more than one height and more than one width. Old windows are not rectangles. Cut the sheet about an eighth to a quarter inch smaller than the smallest measurement so it goes in without bowing the sash. Leave an air gap between the new sheet and the glass of roughly half an inch to a few inches. A very large gap starts to circulate and gives some of the benefit back. You do not need to hit a magic eighth of an inch.

Sand or scrape the cut edge so it is not a knife. Fit the gasket on the panel or on the stop so it compresses when the clips are turned. You should not see a daylight crack, and a tissue on the room side of the gasket should not flutter on a windy night.

Label the room side. Acrylic scratches. Clean it with a soft cloth and no ammonia if the sheet maker says so. The outer window still does the weather. This panel is not a substitute for broken glazing putty on the sash.

The new sheet sits on the room side of the existing glass, sealed at the edge, and held with clips so it can come out.

Condensation

Moisture in the room will try to reach the cold outer pane. If your interior panel is tighter than the old sash, water can collect on the outer glass and sit on the wood. Check the first cold week. If you see standing water, vent the panel on milder days, improve the outer sash putty and weatherstrip, or leave a small planned weep at the bottom that you can live with. Do not trap a wet sash until spring.

A bathroom window is a harsh test. The panel may be wrong there if you cannot control the humidity.

What it will not do

It will not make the window insulate like a wall. It will not fix a leak at the trim that bypasses the glass entirely. It will not survive as an egress cover if you glued it in. And it will not help a south room in the daytime if you used an opaque sheet.

Choose windows that are not the only exit
Or detail the clips so a bedroom panel comes off in a second, from the inside, without a screwdriver you have to find.
Measure twice and cut small
Old stops are out of square. A panel that has to be forced will crack or will not seal.
Gasket the edge and clip it
Compress the foam. Confirm with a tissue on a windy night.
Watch for water for a week
Wipe what you find and decide whether the outer sash needs putty before you leave the panel up all winter.

Difficulty

Cost estimate

Materials

Fit and gap

Condensation

What it will not do

Choose windows that are not the only exit

Measure twice and cut small

Gasket the edge and clip it

Watch for water for a week

Further reading