How to Insulate Windows

Drafty windows make a Connecticut winter feel longer and cost you money every month. The U.S. Department of Energy reports that heat gain and loss through windows drives 25% to 30% of home heating and cooling energy use. Here is how I tell homeowners to insulate windows, from the cheapest fix to the most permanent one.

1. Find the Air Leaks First

Pick a cold, windy day. Hold a lit incense stick or a strip of tissue near the window edges. Watch for movement. Check these spots:

• Where the interior trim meets the wall.
• The meeting rail where the two sashes overlap.
• The bottom of the lower sash and the sill.
• Around the outside frame where it meets the siding.

2. Caulk the Gaps Around the Trim

Use a paintable acrylic latex caulk on the inside trim. Use an exterior-grade silicone or polyurethane caulk outside. Remove old cracked caulk first. Run a steady bead and smooth it with a damp finger. Caulk works best on gaps under 1/4 inch.

3. Add Weatherstripping to the Sashes

Weatherstripping seals the moving parts. Choose the type that fits the gap:

• Adhesive foam tape for the bottom of the sash and the sill.
• V-strip (tension seal) for the side tracks of double-hung windows.
• Rubber or vinyl tubular gaskets for larger, uneven gaps.

4. Fill Larger Gaps With Low-Expansion Foam

Gaps between the window frame and the house framing need low-expansion spray foam made for windows and doors. Standard foam expands too hard and bows the frame. Remove the interior trim to reach this gap, then reinstall it.

5. Use Window Insulation Film

Shrink film kits stretch clear plastic across the inside of the frame. A hair dryer tightens it. The trapped air acts as an extra pane. Film costs little and comes down in spring.

6. Hang Insulated Curtains or Cellular Shades

Thermal curtains and honeycomb cellular shades trap a layer of air against the glass. Close them at night and open south-facing ones on sunny days to collect free solar heat.

7. Replace Windows That Are Past Saving

Insulation fixes drafts around a window. It does not fix a failed window. Plan on replacement when you see:

• Fog between the glass panes, which means the seal failed.
• Rot in the frame or sill.
• Single-pane glass with no storm windows.
• Sashes that will not lock tight.

ENERGY STAR certified windows for the Northern zone, installed with proper air sealing, deliver the biggest long-term savings. Learn more about our replacement windows in Connecticut.

Sources

• U.S. Department of Energy: Energy-Efficient Windows

Last reviewed October 2026 by Adam H. Cooke, Premier Improvements.