On this page
It is one of the most common small jobs in Canadian homes, and one of the easiest to get wrong. Sealing the outside before the inside, filling a window's drainage holes or laying fresh sealant over crumbling old caulk can do more harm than leaving the joint alone.
This page explains what interior and exterior caulking each do and which comes first, how to find where air actually leaks, which window joints to seal and which to leave open, how to choose a sealant, how old caulk comes out, when the weather allows outside work, and why paint on older frames should be tested before anyone scrapes it.
Interior vs exterior window caulking: what each side does and which comes first
Each side of a window has a different job. Inside, caulking is air sealing: Natural Resources Canada's window guide recommends a continuous bead where the casing meets the wall, at the mitred corners of the casing and between the casing and the frame, using a good-quality caulk made for indoor use that can be painted. It specifically warns against using exterior caulking indoors. Outside, the bead between the frame or its trim and the cladding is mainly there to keep rain out of the wall.
Order matters as much as the product. NRCan advises caulking the outside of a window only once interior sealing is complete, because a window sealed on the outside first can trap warm, moist indoor air in the wall, which can damage it over time. A careful contractor seals the inside, checks the outside joints, and only then closes them. A draft between the sash and the frame is a different problem, handled by weatherstripping on the window repair side.
Finding air leaks around windows: incense, windy days and exhaust fans
NRCan's advice for windows is to check for air leakage around the frame and at every movable joint, pairing a visual inspection with a simple leak test, before any old caulk comes out.
- Choose a windy day, since NRCan notes that air leaks are easier to find when the wind is blowing.
- Close all windows and doors and turn on the exhaust fans, a rudimentary fan test NRCan describes that pulls outdoor air in through the gaps.
- Skip the fan test if a fireplace is burning or the house has a fuel-burning appliance vented by a chimney, since lowering the indoor pressure can draw its combustion gases back inside.
- Move a lit incense stick slowly along the casing, its corners and the frame; NRCan says strong leaks scatter the smoke and make the tips glow, and two or three sticks held together are easier to follow.
- Look for the clues winter leaves behind: dust lines on the trim, frost at a corner of the casing, or peeling paint and water stains under an outside joint.
Which window joints to caulk, and which to leave open
Windows are built to drain away water that gets past their outer seals, so a good installer closes some joints and deliberately leaves others open.
- Caulk indoors: the casing to the wall, the mitred corners of the casing and the casing to the frame, the three spots NRCan lists.
- Caulk outdoors: the sides and the head of the joint between the frame or its brickmould and the siding or masonry.
- Leave open: the weep holes and drainage slots along the bottom of many vinyl and aluminum frames, which let water that gets inside the frame drain back out.
- Leave open: the gap between an opening sash and its frame, which needs weatherstripping, since caulk there simply seals the window shut.
- Storm windows: NRCan says the main window must be sealed more tightly than the storm window so moist indoor air stays out of the space between them, so an exterior storm is never sealed tighter than the window behind it.
Choosing caulk for window trim, frames and siding joints
NRCan's materials guide compares common caulks by durability, joint movement and the surfaces they bond to, and the types differ widely, so the label and the window maker's instructions settle the choice.
- Interior casing and trim: a paintable indoor caulk, often acrylic latex, which cleans up with water and suits joints that barely move.
- Glass and smooth frames: silicone bonds well to glass and many plastics, but NRCan advises avoiding it on bare concrete, masonry and steel, and most silicone won't take paint.
- Brick or stone around a frame: polyurethane grips most surfaces and can be painted, and NRCan lists it for exterior use only.
- Mixed materials, such as a vinyl frame against brick: hybrid sealants are a common outdoor choice because they bond to many surfaces and accept paint.
- Vinyl and prefinished aluminum frames: ask for a sealant the window maker approves, since some products bond poorly to these finishes or discolour them.
Removing old window caulk and preparing the joint
Most of a window caulking job is removal and cleaning. A new bead only lasts if it bonds to clean, sound and dry surfaces on both sides of the joint, and very little will stick to old silicone or to caulk that is already failing.
- The old bead is cut free along both edges and pulled out, without scoring a vinyl frame or tearing the membrane or flashing tape installed behind the trim.
- Leftover residue is scraped and wiped away, with extra care for silicone traces, since fresh caulk slides off them.
- The joint is brushed or vacuumed clear of dust and loose paint, then left to dry fully after any washing.
- Soft or rotted wood found at the sill or the casing is repaired first, because no sealant holds on crumbling wood.
- Indoors, the room is aired out while the caulk cures, as NRCan's materials guide calls for with most types, and painting waits for the time on the label.
Weather and temperature for window caulking, and winter stopgaps
Interior window caulking can be done in any season, and the heating season, when drafts are easy to feel, is a good time to find and seal them. Outside work is another matter: each sealant lists an application temperature range and a requirement for dry, frost-free surfaces, and those limits vary by product, so exterior window joints are best planned for mild, dry days, following the label rather than a rule of thumb.
When a window leaks badly in midwinter and the outside can't be sealed yet, NRCan describes inexpensive, temporary heat-shrink plastic film kits: double-sided tape holds the film on the window trim and a hair dryer shrinks it tight across the opening. It cuts the draft until spring, but it doesn't replace sealing the joints.
Lead paint on old window frames: testing before scraping out caulk
Casings and sills in older homes often carry many coats of paint, and cutting out old caulk means scraping right against them. Health Canada says a home built before 1960 probably contains lead-based paint and that the exterior of one built between 1960 and 1990 may. Paint chips can be sent to a lab for analysis, or a contractor with x-ray equipment can check the painted surfaces on site.
Health Canada warns against sanders, heat guns and blowlamps on lead-based paint, since they create toxic dust and fumes, and advises keeping children and pregnant women away from the work area. Scrapings and chips go in a sealed container marked as hazardous waste and are disposed of as the municipality directs. Health Canada also lists caulking among the materials in older buildings that may contain asbestos, so hard original caulking should be tested before it is cut out.
Window Caulking: frequently asked questions
Should windows be caulked on the inside or the outside?
Both, but in order. Natural Resources Canada recommends sealing the interior first and caulking the exterior only after that, since an outside seal on its own can trap moist indoor air in the wall. Inside, caulk stops drafts; outside, it keeps rain out.
Why do I still feel a draft after my windows were caulked?
The air may be coming through a place caulk doesn't reach: worn weatherstripping between the sash and the frame, a latch that no longer pulls the sash tight, or a gap between the frame and the wall hidden behind the casing. An incense test on a windy day usually shows which.
Should I caulk the small holes at the bottom of my vinyl window?
No. Those are drainage holes that let water inside the frame escape to the outside. Sealing them can send that water into the wall or onto the sill indoors.
Can windows be caulked in winter?
Inside, yes. Outside, most sealants need temperatures and dry surfaces that winter rarely offers, so check the label. Until the weather allows, a heat-shrink plastic film kit on the interior trim can hold back the draft.
Is old window caulking or paint dangerous to remove?
In older homes it can be. Health Canada says homes built before 1960 probably have lead-based paint and lists caulking among materials that may contain asbestos, so have the paint and hard original caulk tested before anyone scrapes, sands or cuts them.
Sources
- Keeping the Heat In, Section 8: Upgrading windows and exterior doors — Natural Resources Canada
- Keeping the Heat In, Section 4: Comprehensive air leakage control — Natural Resources Canada
- Keeping the Heat In, Section 3: Materials — Natural Resources Canada
- Lead-based paint — Health Canada
- Health risks of asbestos — Health Canada
Written by the LokalMatch editorial team. Reviewed by Zubair Afzal on September 10, 2026. How we write and check our guides