Flashing, foam and sealant: the parts that stop leaks
Most window leaks are installation leaks. The detail work around the frame matters more than most homeowners expect.
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Three layers that must work together
- FlashingWaterproof tape or a sill pan sends water that gets past the window back to the outside.
- InsulationLow-expansion foam or insulation fills the gap between the frame and the opening so air cannot flow through it.
- SealantA continuous exterior bead sealed against the frame and the brick or trim keeps rain out of the joint.
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Things that go wrong
- High-expansion foam that bows the frame and makes the sash stick.
- No sill pan or tape, so water that enters has nowhere to go.
- Sealant over weep holes, trapping water in the frame.
- Missing sealant at the top of the window where rain runs down the wall.
- Gaps filled with fiberglass stuffed in so loosely it leaves air paths.
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Brick and stone in Philadelphia
- Brick fronts
- Rowhouse brick is porous and the mortar can be soft. Ask the installer to check mortar joints at each opening and to seal against sound masonry.
- Wood trim and lintels
- Older houses often have wood lintels and sills that rot. Look at them when the old window comes out.
- Interior cold spots
- Condensation and cold drafts after installation often point to missing insulation in the gap rather than to the window itself.
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Keep reading on this site
Install-day timeline
What happens, in what order, and what you should do.
Egress, safety glass and code
Bedroom escape sizes, tempered glass and Philadelphia permits.
Window installation in Queen Village and Bella Vista
Narrow trinities, tight stairs and busy streets.
Window installation in Port Richmond
Brick rowhouses near the Delaware with long-owned homes.
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Questions people ask
What foam is best around windows?
Installers commonly use low-expansion foam made for windows and doors, because high-expansion foam can push the frame out of square.
How long should the sealant last?
It depends on the product and exposure. Ask the installer which sealant is used and what the maker says about lifespan.
Can I check the flashing myself?
Not after the work is finished. Ask for photos before trim goes on.