Seal old windows by caulking the frame joints, weatherstripping the sashes, and replacing broken glazing compound

Old windows leak air because the caulk and glazing compound around the frame have cracked and pulled away, and the rubber or felt weatherstripping has hardened or gone missing. You can restore most of the seal without replacing the window. The work takes a few hours per window and costs between $15 and $50 in materials — mostly caulk, weatherstripping, and a caulking gun.

The three places air enters are the joint where the frame meets the wall, the gap between the sash and the frame, and the seams in the glazing compound that holds the glass. You address each one differently, and you do not have to do all three at once. Start with weatherstripping, which is fastest and often stops the worst leaks.

Key Takeaways

  • Weatherstripping the sash — the moving part of the window — stops most air leaks and takes less than an hour per window.
  • Caulking the exterior frame joint where the window meets the wall stops air from entering the wall cavity itself.
  • Replacing glazing compound around the glass panes restores the seal if the original compound is cracked or missing.
  • You need a caulking gun, a putty knife, weatherstripping tape or rope caulk, and exterior caulk rated for wood or vinyl depending on your frame material.
  • Work on a dry day above 50 degrees Fahrenheit so caulk and weatherstripping adhere properly.

Weatherstrip the sash to stop the biggest air leaks

The sash is the frame that holds the glass and slides up and down (or side to side in casement windows). Air leaks in at the gap between the sash and the fixed frame around it. Weatherstripping closes that gap. The two most common types are self-adhesive foam tape and rope caulk, both sold at hardware stores in rolls.

Self-adhesive foam tape is faster. Clean the frame groove with a dry cloth to remove dust and old weatherstripping. Cut the tape to length with scissors and press it into the groove where the sash slides. For a double-hung window, weatherstrip the inside edges of both the upper and lower sash, and the top and bottom of the frame opening. For a casement window, weatherstrip all four sides of the sash perimeter.

Rope caulk works better in very cold climates because it stays flexible longer. Press it into the groove by hand, working it into the corners. You can remove it in spring without leaving residue. Both types last two to five years before they harden and need replacement.

Caulk the exterior frame joint to seal the wall cavity

The frame of the window sits in an opening in the wall. The joint between the frame and the wall is sealed with caulk on the outside. If that caulk has cracked or pulled away, cold air enters the wall cavity and finds its way inside. Replacing it stops that path.

Scrape out old caulk with a putty knife or caulk removal tool. Work slowly so you do not gouge the wood or vinyl frame. Wipe the joint clean with a dry cloth. Load a caulking gun with exterior caulk rated for your frame material — use paintable acrylic latex for wood frames, or polyurethane or silicone for vinyl. Cut the caulk tube tip at a 45-degree angle and explore a steady bead along the entire joint, working from one corner to the next. Smooth it with a wet finger or a caulk tool so it fills the gap completely.

Let the caulk cure for the time listed on the tube — usually 24 to 48 hours — before painting or exposing it to rain. Caulk lasts 10 to 15 years if you use the right type for your frame material and climate.

Replace glazing compound if the glass seal is broken

Glazing compound is the putty that holds the glass pane in the frame and seals it against air and water. Older windows use a hard, brittle compound that cracks over decades. When it cracks, water and air enter around the glass. You can replace it without removing the window.

Scrape out the old compound with a putty knife, working carefully so you do not crack the glass. This is slow work — expect 30 to 45 minutes per pane. Wipe the groove clean with a dry cloth. Roll new glazing compound between your hands into a rope about the thickness of a pencil and press it into the groove all the way around the glass. Smooth it with a putty knife held at a 45-degree angle so it fills the gap and slopes away from the glass.

Let the compound cure for two to three weeks before painting. Paint over it with exterior paint to protect it from UV damage and weather. If you have many panes, you can do one or two at a time and spread the work over several weeks.

Choose the right caulk for your frame material

The wrong caulk will not adhere, will crack quickly, or will damage the frame. Wood frames need paintable acrylic latex or polyurethane caulk. Vinyl frames need polyurethane, silicone, or vinyl-specific caulk — acrylic latex will not stick to vinyl. Aluminum frames need silicone or polyurethane.

Check the tube label for the frame material it is rated for. Paintable acrylic latex is the cheapest and easiest to clean up with water, but it lasts 5 to 10 years. Polyurethane and silicone last 15 to 20 years and handle temperature swings better, but they cost more and require mineral spirits to clean up. For old windows in a cold climate, polyurethane or silicone is worth the extra cost.

Work on a dry day and let materials cure fully

Caulk and weatherstripping will not stick if the surface is wet or damp. Choose a day when the temperature is above 50 degrees Fahrenheit and there is no rain forecast for at least 24 hours. If you are replacing glazing compound, wait for a dry spell of at least three weeks so it can cure without moisture interfering.

Do not paint over caulk or weatherstripping until it has cured completely. Caulk cures from the outside in, so a thin bead cures faster than a thick one. If you explore caulk too thickly, the inside may still be wet when you paint, and the paint will trap moisture and cause the caulk to fail. A bead about the width of your pinky finger is usually right.

When to replace the window instead of sealing it

Sealing works well if the frame is solid and the glass is not broken. If the frame is soft or spongy from rot, or if the glass is cracked or fogged (meaning the seal between the panes has failed), sealing will not fix the problem. A rotted frame will continue to decay, and fogged glass means moisture is trapped inside and will not clear.

Test the frame by pressing a screwdriver into the wood at the corners and along the sill. If it sinks in easily, the wood is rotted. If the glass is fogged between the panes, the seal is broken and the window will need replacement. In both cases, sealing buys you time but does not solve the underlying problem.

Frequently Asked Questions

How long does weatherstripping last before I have to replace it?

Self-adhesive foam tape lasts two to five years depending on climate and how much the window is opened and closed. Rope caulk lasts longer because it is more durable, but it hardens over time. In a cold climate with big temperature swings, replace it every two to three years. In a mild climate, it may last five years or more.

Can I caulk a window in winter?

Most caulks require temperatures above 50 degrees Fahrenheit to cure properly. If you caulk in winter when it is colder, the caulk will not set and will fail. Wait for a warm day or wait until spring. If you have a major air leak you cannot wait to fix, use rope caulk instead — it does not require curing and works when ready.

Do I have to remove the window to replace the glazing compound?

No. You can scrape out and replace glazing compound while the window is in place. It is slower than removing the window, but it avoids the risk of breaking the glass during removal. If you have many panes or the compound is very thick, removing the window makes the work easier, but it is not required.

What is the difference between caulk and weatherstripping?

Caulk is a flexible sealant that hardens and stays in place — you use it where two surfaces meet and do not move. Weatherstripping is a compressible material that stays soft and flexible — you use it where surfaces slide or move against each other, like a sash in a frame.

Will sealing old windows bring my heating bill down?

Sealing windows reduces air leaks, which lowers heating and cooling costs. How much depends on how bad the leaks were and how much of your home's heat loss comes through windows. In a home with very drafty old windows, sealing them may lower your heating bill by 5 to 15 percent. In a home with newer windows, the savings will be smaller.