Windows are the most complex sealing challenge in most homes. Unlike doors, they have multiple moving parts and failure modes. And unlike utility penetrations, they're large openings that can't simply be filled with foam. Here's the systematic approach.
Understanding Window Air Leakage
Windows leak at three distinct zones:
1. Operable sash vs. frame: Where the moving window meets the fixed frame. In double-hung windows, this is often the primary leak path. Weatherstripping degrades over 5–10 years.
2. Frame vs. rough opening: Where the window frame meets the wall. This gap is often filled with fiberglass batt insulation (not airtight) or even unfilled. This is actually the largest leak zone in older windows.
3. Interior trim vs. drywall: The gap where window trim meets the wall surface. Caulk here is purely interior—it addresses infiltration that has already entered the wall cavity.
Diagnosing Your Window Leaks
On a cold day, with the windows closed, hold your hand near each leak zone while someone holds a lit incense stick on the other side. Any smoke movement reveals the path. For the frame-to-rough-opening gap, remove the interior window stops (if accessible) and inspect the insulation situation.
Fixing Operable Sash Leaks
Replacement weatherstripping is the solution. Most double-hung windows use a foam or rubber V-strip or pile weatherstrip along the sash edges. Replacement kits are widely available for common window brands. For a perfect seal on the meeting rail (where upper and lower sash meet), use a compressible foam tape.
Keliiyo EPDM Weatherstrip Seal Strip
High-resilience EPDM rubber foam for window sashes and frame edges. 66 feet total—enough for an entire house's worth of windows.
Check Price on AmazonFixing Frame-to-Wall Gaps
This is the most impactful and most overlooked window sealing task. Remove interior casing (trim) carefully with a pry bar. You'll likely find fiberglass batt insulation stuffed in the gap—or nothing. Fill the gap around the perimeter with a low-expansion spray foam (not regular foam, which expands forcefully and can bow the window frame). Replace the trim and caulk the gap between trim and drywall.
Great Stuff Gaps & Cracks Foam Sealant
Use the 'gaps and cracks' formulation (not window and door formulation) for frame-to-rough-opening sealing. Lower expansion than the 'big gap filler' version.
Check Price on AmazonInterior Caulking: The Last Line
Caulk every gap between window trim and drywall with paintable latex caulk. This won't stop air that's infiltrating through the frame cavity, but it stops air that's already in the wall from entering the living space through trim gaps.
DAP Latex Window & Door Caulk
Paintable acrylic latex with 40-year durability claim. Use along all interior window trim where it meets the wall.
Check Price on AmazonStorm Windows: A Major Upgrade
If your windows are single-pane or older double-pane units with compromised seals, interior storm windows are a highly cost-effective upgrade. They add a second air barrier and improve thermal performance. Magnetic interior storm window kits fit most standard window sizes and can be installed without tools.
When to Replace vs. Repair
Repair (weatherstrip + caulk + storm windows): effective for windows in good structural condition with isolated sealing failures. Cost: $10–$50 per window in materials.
Replace: when frames are warped, glass seals have failed (visible condensation between panes), or the window is single-pane with no storm window option. New Energy Star windows with low-e coatings and argon fill represent a significant long-term upgrade but require professional installation and significant upfront investment.