Storm Window Replacement: Step-by-Step

Storm window replacement becomes unavoidable once the frame corners loosen, the glazing gaskets shrink, or the sashes no longer glide in their tracks. I have swapped out hundreds of tired units over the years, and the honest news is that storm window replacement is one of the friendliest exterior projects a homeowner can tackle: the whole job is four screws, a bead of caulk, and careful measuring. A new mid-grade unit runs $90 to $250, an hour of labor per opening, and the payoff is a window that seals again instead of rattling in every gust.

Part of our Windows & Doors guide — explore the full guide →

Signs the Old Unit Is Done

Not every scruffy storm window needs to go. Repair makes sense when the problem is a torn screen, a cracked pane, or a lost sash clip — all cheap parts. Replacement is the right call when you see structural failure:

• Corner joints that flex or have visible gaps, letting the frame rack out of square
• Vinyl glazing splines shrunken and pulling away, so glass rattles in the sash
• Tracks worn or bent enough that sashes jump their channels or will not latch
• Pitting and white oxidation so advanced the sashes bind no matter how much you lubricate
• Weep holes and sill expander corroded shut, trapping water against the primary window sill

One more trigger: if the frame was screwed on without caulk and water stains are appearing on the interior stool, the flange seal has failed. Once water gets behind the mounting flange, the unit comes off anyway — so replace it while you are there.

Measure the Opening, Not the Old Frame

Old storm windows were sometimes ordered wrong, wedged in, or shimmed with paint sticks, so never copy the old unit’s dimensions. Measure the blind stop opening — the flat surround the new flange will land on:

1. Measure width between the inside faces of the blind stops at the top, middle, and bottom. Record the narrowest figure.
2. Measure height from the sill to the head stop at the left, center, and right. Again, keep the smallest number.
3. Note the sill angle. Most manufacturers build a sloped expander into the bottom rail that accommodates sills pitched up to about 15 degrees, but a steeply weathered sill is worth mentioning at ordering.
4. Deduct nothing — order to the tight measurement. Factories size the unit about 1/4 inch under your figures so it drops in with wiggle room.

Write measurements width-first; every storm window maker in the country reads 32 x 54 as 32 wide. Transposing those numbers is the most common ordering mistake I see, and custom units are not returnable.

Where to Buy and What You Will Pay

Stock sizes cover a surprising share of openings. Larson — the biggest name in the category — sells through Lowe’s and Menards, and ProVia and Quaker units come through window dealers. Home Depot carries its own stocked sizes as well. Rough 2025 pricing per window:

• Basic double-track aluminum, stock size: $90 to $140
• Triple-track with low-e glass, stock size: $150 to $250
• Custom size or custom color (bronze, white, sandstone beyond mill finish): add $50 to $120
• Oversize units above roughly 40 x 80 inches: $250 to $350
• Professional installation, if you hire it out: $75 to $150 per opening on top of the unit

Lead time on custom sizes runs two to four weeks. If your house has fifteen identical 1950s openings, measure three of them anyway — foundations settle and no two will match exactly.

Tools and Materials for the Swap

Everything fits in one bucket: cordless drill with #2 Phillips and 1/4-inch hex bits, flat pry bar, stiff putty knife, utility knife, caulk gun, exterior-grade polyurethane or silicone-hybrid sealant (OSI Quad and Vulkem 116 are my two standbys), #8 x 1-inch stainless or coated screws, rubbing alcohol and rags for cleanup, and a helper or a couple of suction-cup handles for anything over 36 inches wide. Add a tube of paintable caulk in the trim color if the blind stops are painted wood.

Removing the Old Storm Window

Score the caulk joint around the entire perimeter with a utility knife first — decades of paint and sealant will otherwise tear chunks of wood off the blind stop when the frame lets go. Back out the mounting screws, starting at the bottom and finishing at the top so the unit cannot fall on you. Corroded slotted screws from the 1960s often strip; grab the head with locking pliers and turn, or drill the head off and pull the stub later. With screws out, work the putty knife behind the flange and break the caulk bond, then lift the unit free. Scrape the blind stops back to clean, sound material and let any exposed bare wood get a coat of primer before the new unit covers it for the next thirty years.

Installing the New Unit

1. Dry-fit first. Set the window in the opening and confirm it sits against the blind stops all around with the sill expander touching the sill. Check the sashes slide and latch while it is loose — far easier to exchange a bad unit now.
2. Run a 1/4-inch bead of sealant on the back of the top and side flanges. Leave the bottom flange dry — the sill joint must stay open for drainage.
3. Set the unit on the sill, tip it into the opening, and center it with an even reveal both sides.
4. Drive one screw in an upper corner, square the frame, then fasten the remaining pre-punched holes. Snug, not gorilla-tight — overdriving bows the frame and the sashes will bind immediately.
5. Slide the sill expander down until its vinyl sweep compresses lightly against the sill, then lock it with the two screws at its ends.
6. Tool a neat caulk joint along the top and sides where flange meets stop. Again: never caulk the bottom.

Weep Holes and the One Mistake That Rots Sills

Every aluminum storm unit has two or three weep slots in the bottom rail, and they are not optional. The storm window is intentionally not airtight; a little vapor migrates into the cavity, condenses on cold glass, and must drain out. Sealing the bottom — the classic well-meaning homeowner error — traps that water on the wood sill, and I have replaced more rotted sills caused by over-caulked storms than by any other single cause. After install, poke each weep with a toothpick to confirm it is clear, and add it to the fall maintenance list alongside gutter cleaning.

Post-Install Checks and First-Season Care

Cycle every sash and the screen through their full travel and confirm each latches at the marked positions. Check the interior side on the next windy day with an incense stick; smoke drifting at the meeting rail means the sill expander needs another 1/8 inch of compression. Wash the glass before winter — low-e coatings on new units are on a protected surface, so ordinary glass cleaner is fine outside. A shot of dry silicone spray in the tracks each fall keeps sashes gliding; never use oil-based lubricants, which gum up with grit. Done right, the new unit should give you 25 to 40 quiet, sealed years before you do this again.

Special Cases: Basement, Egress, and Odd Shapes

Basement storm windows are their own animal — usually a fixed single pane in a steel or aluminum surround, sized around 32 x 14 to 32 x 20 inches, and cheap at $40 to $80 stock. Measure the masonry or buck opening the same way and expect to bed the frame in sealant on three sides. Bedroom windows that serve as egress need hardware you can operate without tools from inside; most triple-tracks qualify because the sashes lift out, but confirm your inspector agrees before ordering. Half-round, octagon, and other specialty shapes are custom-only territory — ProVia and regional fabricators will build them from a cardboard template, at $300 to $600 each. Trace the template directly off the blind stop, mark the exterior face, and ship it flat; a warped template becomes a warped window.

Repair or Replace: The Quick Math

Replacement glass for a storm sash costs $25 to $60 cut to size, a re-screen is $15 in materials, and companies like Swisco sell obsolete corner keys and latches for a few dollars each — so a single failed part never justifies a new unit. My threshold: when a repair estimate crosses half the cost of a new stock window, or when the frame itself is racked, buy new. You get fresh gaskets, tighter tracks, and usually a low-e upgrade for barely more than piecemeal parts and your Saturday.