Window and Door Resealing in Thornhill Before Winter

Published 2026 — Rhodex

Reseal by late October: caulk the fixed joints, weatherstrip the parts that open, and work in dry weather above about 5°C. In Thornhill, check the age of the frames and whether you are in Markham's heritage district before you choose repair or replacement.

Thornhill has a lot of houses from the 1960s to the 1980s, and a lot of them still have their original frames, patio sliders and storm doors. Those are exactly the parts where caulk shrinks, felt strips flatten and a draft starts. This is the practical order of work for a Thornhill home, and the three questions that are specific here: how old the paint is, which side of Yonge Street you are on, and whether replacing is worth more to you than repairing.

Find the leaks before you buy any caulk

Manitoba Hydro and Hydro-Québec both give the same low-tech test: on a cool, windy day, hold a lit candle or a thin strip of paper near the frame, and if the flame or paper moves, air is getting through. Check the joint between the frame and the wall, the gap where the sash meets the frame, and the bottom edge of every exterior door. Mark each spot with painter's tape so you fix what you found, not what you guessed.

Caulk and weatherstripping do different jobs

Caulk seals joints that never move: where the frame meets the siding, brick or trim. Weatherstripping seals parts that open and close: sashes, door slabs and the track of a slider. Caulking a gap that is supposed to move cracks within a season, and stuffing weatherstripping into a fixed joint does nothing. A real job uses both, and a quote that says only "caulk everything" is a quote to question.

Prepare the surface first. Hydro-Québec's guidance is to clean it and cut out the old split, dried compound with a utility knife, then caulk in dry weather and not below 5°C. In practice that means the exterior work should be finished by late October; the interior joints can wait for a cold day, since they are heated.

Check the paint age before you scrape anything

Health Canada says a home built before 1960 probably has lead-based paint, and that the exterior of homes built from 1960 to 1990 may. Much of Thornhill sits in that second range. Its advice is not to sand, scrape or burn leaded paint, because that makes dust and fumes, and to have paint tested if you suspect lead. Cutting a bead of old caulk out of a painted frame is a very small disturbance, but aggressive paint scraping on an original 1970s window frame is not. If the frame needs stripping rather than a fresh bead of caulk, ask the contractor how they will test and contain the paint.

Your side of Yonge Street matters for replacement, not for resealing

Resealing needs no building permit in either city. Replacing is different. Markham's permit page exempts replacing windows or doors with no change to the size of the rough opening, and Markham's heritage page lists Thornhill as one of its four heritage conservation districts. There, "removal and replacement of windows or doors" is an example of work that needs a minor heritage permit, while the same page lists "general repairs to weather-stripping" and "repair of broken window panes to original specifications" as exempt. So inside the district, repairing and resealing is the easy path and replacing is the one that needs paperwork.

Vaughan has its own Thornhill heritage conservation district plan, and its published pages do not make clear which window work is exempt. If your house is on the Vaughan side near Yonge Street, ask Vaughan's heritage staff before you plan a replacement. Making an opening bigger, or adding one, is a different job in either city, so ask the building department first. The broader split is in how your side of Yonge Street changes the permit rules for small jobs.

When resealing is not the fix

Fogged glass between the panes means the sealed unit has failed, and no caulk fixes that. Soft, spongy wood at the bottom of a sash, or a frame that has rotted at the sill, is also past resealing. A patio slider that no longer closes square may need new rollers or a new door, not new weatherstripping. Replacing makes sense when the repair cost starts to approach the cost of a new unit, and that is the point to get a second look.

The rebate rules if you do replace

Ontario's Home Renovation Savings Program, delivered by Save on Energy and Enbridge Gas, lists $100 back per rough opening for windows and doors, which must be ENERGY STAR certified models. Air sealing is listed at up to $250. These are bundled upgrades: the program's page says to start with a home energy assessment, then complete two or more upgrades to receive the rebates. That means the assessment comes before the work, not after. Amounts change, and Enbridge warns of fake rebate websites, so confirm everything on the official Save on Energy or Enbridge Gas page before you book anything.

A short sequence that works

Do the candle or paper test on a windy day and mark the leaks.
Decide repair or replace, and check the heritage rules first if you are on the Markham side near Yonge Street.
Reseal exterior joints in the last dry, mild week of October.
Replace worn weatherstripping and door sweeps from the inside.
Leave a bathroom fan and kitchen hood working. Sealing the house tighter makes ventilation matter more, not less.

Quick answers

When should I reseal windows before winter in Thornhill?

Do it by late October, while days stay dry and above about 5°C. Exterior caulk needs those conditions to cure, so a cold, damp November week is too late.

Do I need a permit to reseal a window in Thornhill?

No building permit is needed to caulk or weatherstrip. Markham also lists general weather-stripping repair as exempt from heritage permits, while replacing windows or doors in its Thornhill heritage district needs one.

Is there a rebate for new windows and doors in Ontario?

Yes, the Home Renovation Savings Program lists $100 back per rough opening for ENERGY STAR certified windows and doors. You start with a home energy assessment and complete two or more upgrades.

How can I find a window draft myself?

Hold a lit candle or a thin strip of paper near the frame on a windy day. If the flame or paper moves, air is getting through that spot.

Should I sand old paint off a Thornhill window frame before caulking?

Not without testing first. Health Canada says homes built from 1960 to 1990 may have lead paint on exterior surfaces, and it advises against sanding or scraping leaded paint.

Do the exterior first, while the weather allows

The exterior joints are the only part of this job with a deadline, so that is where to spend the dry days. If you would rather hand it to someone, describe the windows and doors on Rhodex and ask for the scope in writing: which joints get caulk, which parts get weatherstripping, and what happens to the old paint.

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Window and Door Resealing Before Winter in Thornhill: Repair, Heritage Permit or Replace?