Replacement windows that leak and draft: insert or full tear-out, sealing and flashing

Window replacements are often sold on the glass: triple glazing, low-E, argon. The complaints that follow are usually about the installation: a draft around the trim, water stains under the sill, condensation on the new glass. NRCan's guide is blunt that poor installation can cause drafts and water leaks even with an efficient product.

Insert or complete tear-out

NRCan describes two ways to install. In an insert retrofit, a new unit goes into the existing frame. It costs less and disturbs the wall and trim less, but NRCan says to use it only if the old frame has not deteriorated and is properly sealed and insulated. In a complete tear-out, the old unit and frame come out, which allows an airtight, insulated seal to the rough opening. If the old frame is rotten or the gap behind it was never sealed, an insert simply hides the problem behind new trim. Inspect the frame for rot, mould and staining before accepting an insert price.

What a good installation must achieve

NRCan lists the installation criteria: an airtight, insulated seal; the unit kept in its original shape, level and square; no load on the unit beyond its own weight unless it is designed for it; exterior flashing that directs water away from the window and frame; and sealing tapes and caulking used only as the manufacturer recommends. It advises using a professional who follows the manufacturer's instructions, which usually keeps the product warranty valid. Ask the installer to show you the manufacturer's installation instructions and say which method they will follow.

Seal the inside first

For the gap between frame and wall, NRCan says gaps of 6 mm (1/4 in.) or less can be insulated and then caulked with indoor-rated caulk; larger gaps take a backer rod with caulking or low-expansion foam. For very leaky windows with removable trim, remove the trim, insulate, seal and reinstall. Exterior caulking comes only after interior sealing is complete, because caulking the outside first can trap warm, moist house air in the wall and damage it over time.

Flashing does the real work

NRCan calls exterior caulking the last and weakest defence against rain; the main defences are proper flashing and a properly detailed drainage plane. Top flashing sits under the air barrier, and side and bottom flashing sit on top of it, so water running down the wall is always shed outward. A bead of caulk around the outside of the frame with no flashing behind it is a common reason new windows start leaking after a few seasons.

Expect more condensation, and keep one escape window

New windows seal the house more tightly and can raise indoor humidity, so condensation may appear where it did not before. NRCan suggests lowering indoor humidity and improving ventilation. It also reminds readers that building codes require at least one bedroom window that opens from inside for emergency escape; check this before any window is sealed shut or replaced with a fixed unit.

Prevention Checklist

• Have the old frames inspected before accepting an insert.
• Ask which manufacturer installation method will be used, and keep a copy.
• Seal the inside gap first: caulk small gaps, backer rod or low-expansion foam for larger ones.
• Confirm flashing at the head, sides and sill, lapped with the air barrier.
• Keep at least one openable escape window in each bedroom as the code requires.
• Plan for humidity control after the house becomes tighter.

References

1. Natural Resources Canada. Keeping the Heat In - Section 8: Upgrading windows and exterior doors, Web page, 8.1.6-8.1.9, 8.1.10 technical note, 8.3 Contracting the work. Accessed 2026-11-03.

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