How to Replace a Bottom Window Seal
Raise the sash, peel or pry out the old seal from the bottom rail, clean the channel, measure and cut a new compression or bulb seal to length, press it into the groove, and close the window to test. Takes about 15 minutes per window.
- Time
- 15 min
- Frequency
- every 3-5 years (or when drafty at the sill)
- Difficulty
- easy
- Cost
- $10
What you'll need
- Utility knife or scissors
- Tape measure
- Damp cloth or rubbing alcohol
- Putty knife (optional, for stubborn old seals)
The steps
- 1
Identify the seal type on your window
Open the window and look at the bottom edge of the lower sash. Most double-hung and sliding windows have a rubber, vinyl, or foam seal pressed into a groove or adhered to the sash bottom. Common types are compression seals (a solid rubber strip), bulb seals (a tube-shaped strip), and adhesive-backed foam tape. Take a photo or remove a small piece to match at the hardware store.
- 2
Remove the old seal
Pull the old seal out of the groove by hand, starting at one end. If it is press-fit, grip the end and pull steadily along the length. If it is adhesive-backed, peel it slowly. Use a utility knife to cut through stubborn sections. Remove all remnants. Leaving old material behind prevents the new seal from seating properly.
- 3
Clean the channel or sash bottom
Wipe the groove or surface with a damp cloth to remove dirt, old adhesive residue, and paint flakes. For heavy adhesive buildup, use rubbing alcohol on a cloth. Let the surface dry completely before installing the new seal. A clean, dry surface is critical for adhesive seals and ensures press-fit seals seat flush.
- 4
Measure and cut the new seal to length
Measure the width of the sash bottom. Cut the new seal strip to match, leaving about a quarter inch of extra length. A slightly long piece can be trimmed in place, but a short piece leaves a gap at one end that lets air through. Use scissors for foam and vinyl seals, or a utility knife for rubber.
- 5
Install the new seal
For press-fit seals, align the fin or T-shaped base with the groove and push it in firmly along the full length, working from one end to the other. For adhesive-backed seals, peel the backing in six-inch sections and press firmly into place. Do not stretch the seal during installation. It should sit flat without bunching or gaps.
- 6
Close the window and test the seal
Close and lock the window. The sash should compress the new seal against the sill or frame. Try the paper test: close the window on a piece of paper at the bottom edge. If the paper grips firmly and resists pulling, the seal is working. If the paper slides out easily, the seal may be too thin or not properly seated. Check for daylight gaps along the bottom.
Types of bottom window seals
The seal along the bottom of a window sash is not the same as the weatherstripping in the side channels. It sits on the bottom rail of the sash and compresses against the sill when the window closes. Three common types exist, and knowing which one your window has determines what replacement to buy.
Compression seals are solid rubber or vinyl strips with a flat base and a rounded or angled top. They press-fit into a groove in the sash bottom. When the window closes, the rounded portion compresses against the sill to create a tight seal. These are the most durable option and are standard on most modern vinyl and aluminum windows. They last three to five years and maintain their shape through thousands of open-close cycles.
Bulb seals have a hollow tube shape attached to a flat fin or T-shaped base. The tube compresses more easily than a solid strip, making them a good choice for windows with uneven sills or slight warping. The hollow center rebounds well but can crack and split after a few years of UV exposure, especially on south-facing windows.
Adhesive foam tape is the simplest option. Peel the backing and stick it to the sash bottom. Foam tape is cheap and easy but compresses permanently under the weight of the sash within one to two years. It works as a temporary fix for a single winter but is not a long-term solution. Replace it with a compression or bulb seal when possible.
Why the bottom seal wears out first
The bottom of the window takes more punishment than any other edge. The sash weight rests on the bottom seal every time the window is closed, compressing it thousands of times over its lifespan. Gravity pulls moisture, dirt, and debris to the sill, where it sits against the seal and accelerates deterioration.
South and west-facing windows get the most UV exposure on the bottom rail, which dries out rubber and vinyl. The combination of mechanical compression, moisture, UV, and dirt makes the bottom seal the first to fail on nearly every window in a home.
You will often notice drafts at the bottom of windows while the side and top seals are still in good condition. This is normal and does not mean all the weatherstripping needs replacing. Just replace the bottom seal and check the others while you are at it.
Matching the right replacement seal
The easiest way to get the right replacement is to bring a piece of the old seal to the hardware store. Pull out a two-inch section, put it in a bag, and match the profile shape and width at the store.
If no old seal remains, measure the groove width with a ruler. Common groove widths are 3/16 inch, 1/4 inch, and 5/16 inch. The seal packaging lists the groove size it fits. For adhesive-backed seals, measure the flat surface width where the seal will stick.
Do not force a seal that is too wide into a narrow groove. It will bunch up and prevent the window from closing fully. A seal that is too narrow will not fill the groove and will fall out. Exact width matters more than length, since you can always trim length.
The paper test
After installing a new seal, the paper test tells you immediately whether it is working. Close the window on a piece of paper or a dollar bill at the bottom edge. Try to pull the paper out. If it grips firmly and tears or resists, the seal is making good contact. If the paper slides out freely, there is a gap.
Test at the center and both ends of the bottom rail. The ends are the most likely spots for gaps, especially if the seal was cut slightly short. If the center seals well but the ends do not, trim and reinstall the last inch of seal to close the gap.
On a windy day, you can also hold a lit incense stick along the bottom of a closed window. If the smoke moves horizontally toward the interior, air is getting past the seal.
Frequently asked questions
- What type of seal is on the bottom of my window?
- Most modern vinyl windows use a press-fit compression seal or bulb seal in a groove along the bottom of the sash. Older wood windows may have adhesive-backed foam or felt. Open the window and look at the bottom edge of the sash to identify which type you have. If the seal pops out of a groove, it is press-fit. If it peels off, it is adhesive-backed.
- When should I replace the bottom window seal?
- Replace it when you feel a draft at the bottom of a closed and locked window, when the seal is visibly cracked, compressed flat, or missing pieces, or when you can see daylight between the sash bottom and the sill. Most bottom seals last three to five years depending on how often the window is opened.
- Can I seal the bottom of a window from the outside?
- The bottom seal is part of the sash, not the exterior frame, so it must be replaced from the inside by opening the window. If the gap is between the frame and the exterior wall rather than between the sash and sill, that is a caulking issue and not a seal issue.
- Does replacing the bottom seal help with noise?
- Yes. A tight seal at the bottom reduces both air infiltration and sound transmission. It is not as effective as a full window replacement, but it noticeably reduces street noise coming in through the sill gap.
Products you'll need
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Frost King V-Seal Weather Strip (17ft)
Self-adhesive weatherstripping for windows
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