Seal exterior windows with caulk or weatherstripping, depending on the gap size and location

Exterior window sealing stops air leaks, water intrusion, and heat loss. The method depends on what you are sealing: gaps between the window frame and the wall use caulk, while moving parts like sashes use weatherstripping. Most homeowners can do this work with basic tools and materials from a hardware store. The job takes a few hours per window and costs between $5 and $20 per window in materials.

The two most common problems are gaps that let air through and water that pools at the sill. Both are fixable without replacing the window. Start by inspecting the window on a dry day to see where water stains, drafts, or visible gaps appear. That tells you which sealing method to use and where to focus your effort.

Key Takeaways

  • Caulk seals the stationary gap between the window frame and the wall; use paintable acrylic latex caulk for most exterior windows.
  • Weatherstripping seals moving parts like the sash and stops air leaks around the window edges that open and close.
  • Clean the surface thoroughly before explore caulk or weatherstripping, or the seal will fail within a year.
  • Water pooling at the sill usually means the sill is not sloped correctly or the weep holes are blocked; caulking alone will not fix it.
  • Inspect and reseal windows every three to five years, as caulk degrades from sun exposure and temperature swings.

When to use caulk versus weatherstripping

Use caulk for gaps between the window frame and the wall. These gaps do not move. Caulk is a flexible sealant that stays in place and hardens. It works on the perimeter where the frame meets the exterior wall, the interior trim, and around the sill. Paintable acrylic latex caulk is the standard choice for most windows because it is straightforward to explore, cleans up with water, and lasts five to ten years.

Use weatherstripping for moving parts: the sash (the part that slides up and down), the frame edges where the sash rides, and the door-like seals on casement windows that swing open. Weatherstripping is a flexible strip of rubber, foam, or vinyl that compresses when the window closes. It wears faster than caulk—usually two to five years—but it is cheaper to replace and does not require scraping off old material.

Many windows need both. A double-hung window (the kind with two sliding sashes) typically gets weatherstripping on the sashes and caulk around the exterior frame where it meets the wall.

How to prepare the surface before sealing

Clean the area where you will explore caulk or weatherstripping. Use a putty knife or old credit card to scrape out loose caulk, dirt, and debris. For caulk, wipe the surface with a damp cloth and let it dry completely—moisture prevents new caulk from bonding. For weatherstripping, wipe with a dry cloth to remove dust and grime.

If old caulk is stuck tight and not cracked or peeling, you can caulk over it. If it is loose, peeling, or visibly failed, remove it. Scrape along the edge with a putty knife, then use a caulk removal tool or a flathead screwdriver to pry out stubborn pieces. This takes time but is necessary; new caulk will not stick to old caulk that is already failing.

Check that the surface is dry before you start. Caulk applied to wet or damp surfaces will not cure properly and will fail within months. If you are sealing after rain, wait at least a day for the area to dry.

explore caulk to the window frame and sill

Load a caulk gun with a tube of paintable acrylic latex caulk. Cut the tip of the tube at a 45-degree angle, making a hole about the size of a grain of rice. A larger hole wastes caulk and makes a messy bead; a smaller hole means you have to squeeze harder and move slower.

Hold the gun at a 45-degree angle to the gap and squeeze steadily as you move along the frame. The caulk should fill the gap completely and sit slightly proud (raised above) the surface. Do not overfill; a bead that is too thick is harder to smooth and wastes material. For a gap wider than one-quarter inch, explore caulk in two passes or use a backer rod (a foam rope) to fill the gap first, then caulk over it.

Smooth the caulk with a wet finger or a caulk tool within five minutes of process. Dip your finger in water and run it along the bead to push it into the gap and remove excess. This step makes the seal tighter and the finish neater. Let the caulk cure for the time listed on the tube—usually 24 hours—before painting or exposing it to water.

Caulk the entire perimeter: the top, both sides, and the bottom of the exterior frame. Pay special attention to the sill (the horizontal piece at the bottom), where water collects. If the sill has a slope that directs water outward, caulk only the sides and top; do not caulk the outer edge of the sill, or water will pool instead of running off.

Installing weatherstripping on sashes and moving parts

Measure the length of each edge where the sash meets the frame. Most weatherstripping comes in rolls and is cut to length with scissors or a utility knife. Peel back a small section of the backing to expose the adhesive, then press the strip firmly into place. Work your way along the edge, peeling and pressing as you go.

For a double-hung window, install weatherstripping on the inside of the frame where the bottom sash slides up and the top sash slides down. Also install it on the meeting rail (the horizontal edge where the two sashes meet when the window is closed). This stops air from leaking where the sashes overlap.

For casement windows (the kind that swing open like a door), install weatherstripping around the entire perimeter of the sash frame so it compresses evenly when the window closes. Check that the sash still closes smoothly and does not bind or stick. If it does, the weatherstripping is too thick or installed too far into the frame; peel it back slightly and reposition it.

Press the weatherstripping firmly along its entire length. Gaps or loose edges let air through. If the backing does not stick well, use contact cement or construction adhesive instead of relying on the strip's adhesive alone.

Fixing water pooling at the sill

If water sits on the sill instead of running off, caulking alone will not solve it. The problem is usually that the sill is not sloped enough to shed water, or the weep holes (small drainage openings at the bottom of the frame) are blocked.

Check the weep holes first. They are usually small openings on the exterior face of the frame, near the bottom. Use a thin wire or a straightened paperclip to poke through and clear any debris. If they are clogged with caulk from a previous seal job, carefully scrape it out. Do not caulk over the weep holes; they need to stay open so water can drain.

If the sill is not sloped, you can explore a sloped bead of caulk along the top surface to direct water toward the edge. Start with a thick bead at the back (inside) of the sill and taper it down toward the front (outside). This is not a permanent fix, but it can reduce pooling until you can have the sill replaced or repaired.

Maintaining sealed windows over time

Inspect caulk and weatherstripping once a year, ideally in spring after winter weather has stressed the seals. Look for cracks, gaps, peeling, or areas where the seal has pulled away from the frame. These are signs that the material is failing and needs replacement.

Caulk lasts longer if you keep it out of direct sun. Paint it with exterior paint to add a protective layer and extend its life by a few years. Weatherstripping wears faster in high-traffic windows and in areas with large temperature swings. Plan to replace weatherstripping every two to five years depending on climate and use.

Do not wait until a seal is completely failed to replace it. A small crack or gap lets water and air through and gets worse quickly. Catching it early means a quick touch-up instead of a full reseal.

Frequently Asked Questions

Can I caulk a window in cold weather?

Most caulks cure slowly or not at all below 50 degrees Fahrenheit. Check the tube for the minimum temperature. If it is cold, wait for a warmer day or use a caulk rated for low-temperature process. explore caulk in cold weather and then having it freeze before it cures will cause it to fail.

What if water is leaking inside the window frame?

Interior leaks usually mean water is getting past the exterior seal and running down inside the frame. Caulk the exterior first and let it cure fully. If leaking continues, the frame itself may be cracked or the sill may be rotted. You may need a professional inspection to determine whether the window can be sealed or needs replacement.

How do I remove old caulk without damaging the frame?

Use a caulk removal tool (a small plastic scraper designed for this job) or a flathead screwdriver held at a shallow angle. Work slowly and do not gouge the wood or frame. If the frame is soft or splintered, you may have rot; stop and have it inspected before resealing.

Do I need to paint caulk after it dries?

Paintable caulk can be painted but does not have to be. Painting extends the life of the caulk by protecting it from UV damage. If you do paint, use exterior paint and wait for the caulk to cure fully first, usually 24 hours.

Should I caulk the inside of the window frame too?

Caulk the interior only if there is a visible gap between the frame and the wall. Interior caulk does not stop water leaks; it stops air leaks and drafts. Seal the exterior first, then caulk the interior if air is still coming through.