How to Install Acrylic Storm Windows (DIY Guide)

Measure, cut, mount, and seal an interior acrylic storm window, whether you buy a custom compression insert, build a magnetic panel from a kit, or make one from sheet. Includes the mistakes that cause fogging.

An interior acrylic storm window is one of the few energy upgrades a homeowner can do in an afternoon with hand tools and see a difference the same night. The panel traps a layer of still air against the existing glass, stops the drafts that leak around old sashes, and cuts street noise. The Department of Energy's Building America program recommends interior storms as a retrofit specifically because they leave the original window untouched.

The job has three versions depending on what you buy. A custom compression insert arrives cut to size and presses in. A magnetic kit gives you the strips and adhesive to turn a sheet of acrylic into a snap-on panel. A from-scratch build uses sheet, magnetic tape, and foam. The measuring and the sealing rules are the same for all three. This guide walks through them in order, then covers each installation method, then the problems that show up afterward.

Before you start: fix the window first

An insert seals against the inside of the frame. It does not fix anything behind it. The DOE guide is explicit: inspect the existing window for peeling paint, cracks, rotten wood, and failed glazing putty, and repair them before you add a storm. Water that gets into the sash will now be trapped behind an airtight panel, which is worse than the draft you started with. Reglaze loose panes, replace any weatherstripping on the sash, and let paint cure.

Also check egress. A bedroom window that must open in a fire must still open with the insert in place, or the insert must come out in seconds. Compression and magnetic panels pass this test. Screwed-on panels do not.

Step 1: Measure like a fabricator

Every insert failure that is not a moisture problem is a measuring problem. Old windows are never square, and an insert cut to one measurement will bind on the tight side and leak on the loose side.

Measure from inside the window frame, where the panel will sit, not from the trim.
Measure the width in three places: top, middle, and bottom.
Measure the height in three places: left, center, and right.
Record the smallest of each. That is the dimension the panel must fit through.
Measure to the nearest 1/16 inch. Compression inserts depend on it.
Check square with a framing square in each corner and note which corners are out. On a badly racked frame, measure all four sides individually rather than averaging.
Check depth. A compression insert typically needs about 3/4 inch of flat frame depth to seat in. Measure from the face of the stop to the inside edge of the frame.
For a compression insert, send the manufacturer the raw numbers and let them calculate the cut. For a DIY panel, subtract about 1/8 inch from the smallest width and height so the sheet does not bind.

Write the numbers on masking tape stuck to the window. You will need them again.

Step 2: Choose the panel

Thickness. 1/8-inch acrylic for insulation. 3/16 or 1/4 inch if noise is the goal or the panel is larger than about 12 square feet, where 1/8 will bow.
Acrylic, not polycarbonate. Polycarbonate scratches faster and yellows in sun. Acrylic is UV-stable and can be polished. Our acrylic vs polycarbonate guide covers why.
Cast or extruded. Extruded is fine for a flat panel in a frame and costs less. If you plan to polish or cement it, buy cast. Our suppliers roundup says where.
Let the supplier cut it. A plastics shop saw gives a square, chip-free edge. If you must cut it yourself, sheet under 3/16 inch can be scored deeply with a plastic-scoring knife along a straightedge and snapped; thicker sheet needs a fine-tooth blade run slowly with the protective film on.
Leave the film on until the panel is mounted.

Step 3A: Installing a compression insert

This is the easy one, and the reason Indow and Climate Seal charge what they do.

Clean the frame surfaces the tubing will bear on.
Pick a mild day. Acrylic expands and contracts with temperature more than glass; installing at a mid-range temperature keeps the movement in both directions small.
Hold the panel by its edges, tubing out, and seat the bottom edge into the frame first.
Press the sides in, working up. The silicone tubing compresses against the frame and grips.
Press the top corners last. On an out-of-square frame the maker will have cut the panel to match, so it only goes in one way; check the orientation mark.
Run a finger around the perimeter. You should feel even compression with no gaps. A gap means a measuring error; call the manufacturer.

Removal is pulling one top corner toward you. Store panels flat or on edge, never leaning at an angle that will bow them.

Step 3B: Building a magnetic panel from a kit

The Window Savers kit and similar products give you flexible magnet strip for the panel, vinyl-covered steel strip for the frame, trim tape, and pressure-sensitive adhesive. The company says under 40 minutes per window once the sheet is in hand.

Decide where the steel goes. A molding mount sticks the steel strip to the face of the window casing. It is easier and usually hides behind curtains. A jamb mount tacks 3/4-inch square pine cleats inside the jamb, 1 to 5 inches from the glass, and mounts the steel on those; use it when the casing has a profile the strip will not sit flat on. At the sill you can skip the bottom cleat and use foam weatherstrip instead.
Magnets to acrylic. Peel a short section of film back along one edge. Run the magnet strip along the edge of the sheet using the adhesive, cut the ends square, and mask over it from the other side with trim tape so the edge looks finished. Repeat on all four sides.
Steel to window. Cut four steel strips to length. Mount them around the opening with the same tape and adhesive, aligned so the panel's magnets will land on them. Use a level; a crooked steel strip is a leaky corner.
Snap it on. Bring the panel to the window magnets first. It jumps into place. Check that the seal is even all the way around.
Removal is peeling at a corner.

The magnet is doing real work here: it lets the sheet grow and shrink with temperature without breaking the seal. Do not add screws through the acrylic to "help."

Step 3C: From scratch with sheet and tape

Same idea as the kit with hardware-store parts. Buy adhesive-backed steel tape and matching magnetic tape, or use a framed panel.

Follow the kit steps above for the magnetic route. Hardware-store magnetic tape is weaker than a kit's strip, so keep panels small, under about 3 by 4 feet, and use the widest tape you can find.
For a framed version, build a light wood frame to the panel size, fasten the acrylic to it with screws through oversized holes and rubber washers so the sheet can move, and run foam weatherstrip around the frame perimeter. Foam is more resilient than felt and seals tighter. Start the strip at the bottom, extend it about a quarter inch past each side, then run the sides so the corners meet snugly. Press the frame into the opening.
Turn-button cleats or a couple of screws through the frame, never through the acrylic, hold it in.

Step 4: Seal and check

Whatever the method, the perimeter seal is the product. Run a lit incense stick or a strip of tissue around the edge on a windy day. Any movement is a leak. Compression and magnetic seals can be reseated; a framed panel needs another layer of foam at that spot.

After installation: condensation

If fog or water shows up between the insert and the glass, the insert is leaking warm indoor air into the cold gap. The DOE guide's fixes, in order:

Tighten the storm's seal. Reseat the panel, add weatherstrip at the gap.
If the primary window is a double-hung, crack the lower sash open a hair so the gap can vent to the outside instead of the room. Counterintuitive, but it works, because the cold-side leak dries the gap.
Lower indoor humidity: run bath and kitchen exhaust fans, vent the dryer, and put a vapor barrier over a damp crawlspace.

If fog shows up on the room side of the insert, the room is humid enough to condense on any cold surface and the insert is doing its job; treat the humidity.

Seasonal removal and care

Pull compression and magnetic panels in spring if you open windows, or leave them year-round if you run air conditioning; they help in summer too. Label each panel with its window on the trim tape. Store flat, film side protected. Clean with water and a microfiber cloth or a plastic-safe cleaner. Never ammonia, never a dry paper towel. Scratches come out with the kits in our scratch repair roundup.

Which method should you use?

One to four windows, or any window that is badly out of square: a compression insert. The measuring is done for you and the seal is the best available. Our storm window roundup compares the brands.
Five or more windows and a steady hand with a tape measure: a magnetic kit. Our Window Savers review covers the leading one.
A cold frame, a shed, a basement window, or a rental where nothing can be attached: sheet and tape, kept small.