How to Weatherstrip and Caulk Windows and Doors for Winter

St. Louis winters swing from mild afternoons to nights in the single digits, sometimes in the same week. That freeze-thaw cycle is hard on caulk and weatherstripping. Joints open up, foam flattens, and by January you can feel cold air sliding under the front door. Sealing drafts around windows and doors is one of the cheapest ways to make a house more comfortable and cut heating bills, and most of it can be done in a single weekend with tools from any hardware store.

Difficulty: Beginner · Time: 2–4 hours for a typical house

Tools and materials

• Stick of incense or a candle (for finding drafts), plus a flashlight
• Caulk gun
• Exterior caulk: a high-quality polyurethane, hybrid polymer or siliconized acrylic rated for exterior use and paintable if needed
• Interior caulk: paintable acrylic latex for trim
• Utility knife, 5-in-1 tool or caulk removal tool
• Foam backer rod for gaps wider than about 1/4 inch
• Weatherstripping: V-strip (tension seal), adhesive foam tape, and replacement compression or kerf-in bulb weatherstripping for exterior doors
• Door sweep or replacement door bottom
• Window insulation film kit (optional)
• Rubbing alcohol, rags, painter’s tape
• Sturdy ladder for exterior work above reach

Step 1: Find the drafts

Pick a cold, windy day. Close all windows and exterior doors, turn off the furnace fan, and turn on bathroom and kitchen exhaust fans to pull the house slightly into negative pressure. Then slowly move a lit stick of incense around window sashes, door edges, trim, outlets on exterior walls and attic hatches. Where the smoke wavers or gets pulled sideways, you have a leak. At night, a second person can shine a flashlight from outside while you look for light around door and window frames from inside.

Make a list as you go. Most houses have the same usual suspects: the bottom of exterior doors, the latch side of doors, the meeting rail where two window sashes overlap, and the gap where window and door trim meets siding or brick.

Step 2: Caulk the outside first

Exterior caulk keeps both air and water out, so it does double duty. Work on a dry day above about 40°F, or check the tube for its minimum application temperature; some modern sealants go on colder.

1. Remove failed caulk. Cut and scrape out caulk that is cracked, shrunken or pulling away. New caulk won’t bond well over old failed beads.
2. Clean the joint. Brush out dirt and wipe the surfaces with rubbing alcohol. Let it dry.
3. Fill deep gaps with backer rod. Push foam backer rod into any gap wider than 1/4 inch so the caulk bead is the right depth. Caulk should bridge the joint, not fill a cavity.
4. Apply a steady bead. Cut the nozzle small, hold the gun at about 45 degrees and push the bead ahead of the tip in one continuous pass.
5. Tool it. Smooth the bead with a wet finger or a caulk tool so it presses into both sides of the joint.

One important exception: do not caulk the weep holes at the bottom of window frames or the bottom edge of window trim on siding where water is designed to drain out. Sealing those traps water and rots the frame.

Step 3: Seal interior trim gaps

Gaps where window and door casing meets the wall leak air too. Run a thin bead of paintable acrylic latex caulk along those joints and along the top of baseboards on exterior walls. For gaps between the window frame and the rough opening (visible only if you remove the trim), low-expansion window-and-door spray foam is the right product. Regular expanding foam can bow a frame and make the window stick.

Step 4: Weatherstrip the doors

1. Check the door bottom. If you can see daylight under the door, replace the sweep or the door bottom. Many steel and fiberglass doors use a kerf-in bottom that slides into slots on the door; take the old one to the store to match it.
2. Replace the jamb weatherstripping. Most modern exterior doors use a foam-filled or bulb strip pressed into a kerf in the jamb. Pull the old strip out with pliers and press the new one in. It should compress slightly when the door closes.
3. Adjust the strike plate. If the door closes loosely even with new weatherstripping, move the strike plate slightly toward the stop so the door pulls tighter against the seal.
4. Check the threshold. Many thresholds have adjustment screws that raise the sill to meet the sweep.

Step 5: Weatherstrip the windows

For older double-hung windows, V-strip (a folded vinyl or metal strip) installed in the side channels and along the meeting rail seals well and still lets the window slide. Adhesive foam tape works on the bottom of the lower sash and on casement windows where it compresses rather than slides. Clean surfaces with alcohol first or the adhesive will let go in the first cold snap.

For drafty windows you don’t open in winter, a shrink-film insulation kit adds a sealed air layer. Apply the double-sided tape to the interior trim, stretch the film, then shrink it tight with a hair dryer. It comes off in spring.

Mistakes to avoid

• Caulking over dirt or old caulk. The new bead fails in one season.
• Using interior caulk outside. Cheap acrylic shrinks and cracks with UV and temperature swings.
• Sealing weep holes and drainage gaps. Air sealing should never trap water.
• Over-sealing gas appliances. If you have a gas water heater or furnace that draws combustion air from the room, a very tight house can cause backdrafting. Make sure you have working carbon monoxide detectors on every level, and have your HVAC contractor check combustion air if you do major air sealing.
• Ladder shortcuts. For second-story trim, set the ladder on firm, level ground at a 4-to-1 angle and don’t overreach.

When to call a pro

Caulk and weatherstripping fix air leaks, not failing windows and doors. If you find soft or rotted wood at sills or brick molding, fogged glass between panes, frames that are out of square, or water stains under windows, the problem goes deeper than a bead of caulk. Those are signs that it may be time for repair or replacement. RossBuilt installs replacement windows, patio doors and entry doors through our windows and doors services, and we repair rotted trim and siding through our exterior services.

Rather have it done for you? RossBuilt Construction serves the St. Louis metro. Request a free estimate or call 636-380-2574.