How to Weatherstrip Windows for Summer AC Efficiency

Drafty windows make your AC work overtime in summer. Here is how to weatherstrip windows for AC efficiency — 4 methods matched to common window types, plus what saves the most on bills.

By Chris Mendenhall

6 min read

Most homeowners think about weatherstripping in fall, sealing against cold winter drafts. They miss the bigger payoff: weatherstripping for summer. Hot, humid air leaking in past loose window seals makes your AC run constantly to keep up. A properly sealed window can drop your summer cooling costs by 10–15% — and the fix takes a weekend and about $30 in materials.

This guide walks through how to weatherstrip windows for summer AC efficiency, matched to the type of window you have.

Why This Matters More in Summer

In winter, warm air leaks OUT of your house through gaps — annoying but a known issue. In summer, hot humid air leaks IN, bringing with it moisture that your AC has to work extra-hard to remove (cooling AND dehumidifying). The energy cost of cooling humid air is dramatically higher than cooling dry air. So sealing summer air leaks can save more than sealing winter leaks.

The hottest, most humid air leaks usually come from:

  • Old single-pane windows
  • Double-hung windows where the upper and lower sashes meet
  • Casement (crank-out) windows with worn gaskets
  • Gaps between window frames and the surrounding wall

What You’ll Need

  • A candle, incense stick, or thin paper (for leak detection)
  • Caulk gun and exterior-grade silicone caulk
  • Self-adhesive foam weatherstrip
  • V-strip (tension seal) weatherstrip
  • Felt weatherstrip (for casement windows)
  • A utility knife or scissors
  • Painter’s tape and damp rag
  • A flashlight

Step 1: Find Your Leaks

Before buying weatherstrip, identify where the leaks are.

Method 1: Candle test (best for windy days)

  1. Light a candle.
  2. Hold it 1–2 inches from window edges — top, bottom, sides, where sashes meet.
  3. Watch the flame. If it flickers, air is moving through.

Method 2: Paper test (no flame)

  1. Close the window on a thin piece of paper, leaving an inch sticking out.
  2. Try to pull the paper out.
  3. If it slides freely, the seal at that point is poor.

Method 3: Hand test

  1. On a hot day, run your hand slowly around the window perimeter.
  2. Warm air against your skin = leak.

Mark all leak spots with a piece of painter’s tape.

Step 2: Identify Your Window Type

Different windows need different weatherstrip:

  • Double-hung (slides up and down, two sashes): most common. Use V-strip on sides where sashes ride in tracks, plus foam on the bottom rail.
  • Casement (cranks outward on hinges): uses a compression gasket along the frame. Replace with felt or rubber compression strip.
  • Sliding (horizontal slide): same as double-hung but rotated. V-strip on tracks.
  • Awning (tilts outward at the top): same as casement but smaller. Compression strip.
  • Fixed/picture windows (don’t open): only the perimeter caulk needs attention — no moving seals.

Step 3: Weatherstrip a Double-Hung Window

These have the most leak points and the biggest summer savings potential.

Side tracks where sashes slide

  1. Open both sashes partway.
  2. Clean the side tracks with a damp rag (years of dust and grime here).
  3. Cut V-strip slightly longer than the height of the lower sash.
  4. With the sashes open, peel the backing and stick the V-strip into the side track, with the V opening facing inward toward where the sash will close.
  5. Close the window — the sash compresses the V, creating a seal.

V-strip is much better than foam tape on tracks because foam tape gets dragged out when sashes move.

Where upper and lower sash meet (the meeting rail)

This is the #1 leak point on double-hung windows.

  1. Open the lower sash.
  2. Stick a strip of foam weatherstrip along the top edge of the bottom sash’s meeting rail, OR the bottom edge of the upper sash’s meeting rail.
  3. Close the window. The foam compresses where the two sashes meet, sealing the gap.

If the gap is big enough that foam won’t bridge it, the sashes themselves may need adjusting (a more involved repair).

Bottom rail (where the lower sash meets the sill)

  1. Open the bottom sash.
  2. Apply foam weatherstrip along the bottom edge of the sash (the part that contacts the sill when closed).
  3. Close the window — foam compresses against the sill.

Step 4: Weatherstrip a Casement Window

Casement windows seal by compression — the sash presses against a gasket as you crank it closed.

  1. Open the window fully.
  2. Examine the existing gasket around the frame perimeter. Is it cracked, flattened, or missing?
  3. If yes, remove the old gasket — usually pulls out of a slot in the frame, or peels off if adhesive-backed.
  4. Cut new compression weatherstrip (rubber or vinyl bulb, often called “Q-Lon”) to match the frame perimeter.
  5. Press into the slot or stick to the frame, following the original path.
  6. Close the window — the sash should now compress firmly against the new gasket.

If the window is hard to crank closed, the new gasket is too thick. Look for a thinner option or trim a bit off.

Step 5: Seal Around the Outside of the Window Frame

Air leaks through window-to-wall gaps just as much as through the window itself.

  1. Outside: Inspect the caulk where the window trim meets the siding or brick. Cracked, missing, or gappy caulk = direct path for hot air.
  2. Tools: Scrape out old failed caulk with a putty knife or razor.
  3. Re-caulk: Apply exterior-grade silicone caulk (or polyurethane caulk for very large gaps). Smooth with a wet finger.
  4. Inside: Check around interior window trim for visible gaps. Smaller — usually just paintable acrylic caulk works.

This single step alone can stop major air leaks that no amount of in-window weatherstripping addresses.

Step 6: Address Glass Itself (For Old Single-Pane Windows)

If you have single-pane windows, even perfect weatherstripping won’t make them efficient. Options:

  • Storm windows (interior or exterior) — adds a second pane of glass, dramatically reduces heat transfer. About $50–100 per window.
  • Window film — a clear plastic film that adds a thin air layer. About $20 per window, DIY-able.
  • Replace with double-pane — the right long-term answer but expensive ($500–1,000+ per window).

For renters or short-term fixes, storm windows or film are the most cost-effective.

What to Avoid

Foam tape on sliding tracks. Gets dragged off within a season. Use V-strip on moving surfaces.

Caulking shut windows you might want to open. Don’t seal operable windows closed unless you genuinely don’t plan to use them.

Cheap weatherstrip from dollar stores. Falls off in weeks. Spend the extra $2–3 per package for quality (3M, Frost King, etc.).

Skipping the leak detection step. Random weatherstripping without finding the leaks first wastes material and effort.

Heavy weatherstrip on warped windows. If a window is warped enough that thin weatherstrip doesn’t seal, the window itself needs repair, not more weatherstrip.

Frequently Asked Questions

How much will weatherstripping save on my AC bills? For a home with leaky windows: 10–15% on cooling costs is realistic. If your summer electric bill is $200, that’s $20–30/month saved. Pays back the $30–60 in materials in one summer.

Should I caulk windows shut for summer? If you have specific windows you never open, yes — caulking them shut maximizes seal. For windows you might open for ventilation, weatherstrip instead.

My windows are new. Do I need to weatherstrip? New windows come with built-in seals. They shouldn’t need additional weatherstripping for 5–10 years. After that, seals may degrade.

Are there summer-specific weatherstrip materials? Same materials as winter (V-strip, foam, rubber bulb). The application is the same; the goal differs (sealing OUT hot humid air vs. sealing IN warm air).

Should I use rope caulk? Rope caulk (a removable putty-like strip) is great for sealing windows shut for the season. Peels off cleanly in fall when you want to open them again. Lasts 1 summer.

A Cool, Efficient Home

Weatherstripping windows for summer is one of the highest-leverage, lowest-effort home upgrades — about $30 in materials, an afternoon of work, and 10–15% off your cooling bills for the next several years. Find the leaks first, match the weatherstrip type to the window type, and address the frame perimeter while you’re at it. Your AC will thank you.

#Summer Prep
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