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GLASS & SEALED UNITS

Water Inside Window Frame

Water inside a uPVC or aluminium frame may be normal within drainage channels, but it should discharge outside rather than spill indoors.

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Treat cracked or loose glass as hazardous

Do not press, lever or dismantle glazing unless the pane is safely supported. Large, high-level and safety-critical glass needs professional handling.

MOST LIKELY CAUSES

Start with the glass, seals and frame support

Blocked drainage slots Water cannot escape through the designed route.
Failed gaskets Excess water enters around the sash or glass.
Blocked or missing drainage caps External outlets are obstructed.
Failed frame or sill joints Water bypasses the intended drainage path.
◫ Symptom variations Match the misting, cracking, rattling or water pattern. ⌄
  • Water visible in lower frame channel
  • Leak appears after driving rain
  • Water spills indoors when window opens
  • Drainage slots hold standing water
  • Damp appears at sill corners
✓ Quick checks Safe checks before disturbing the glazing. ⌄
  1. Locate internal and external drainage slots.
  2. Clear loose debris carefully.
  3. Pour a small controlled amount of water into the channel.
  4. Inspect gasket and sill joints.
  5. Check external sealant and sill end caps.
Important: Avoid pressure near cracked glass and never block window drainage slots.
1 Step 1 — Restore the drainage path Make the smallest safe correction and retest. ⌄

Clear slots and confirm water exits outside.

2 Step 2 — Repair failed seals Make the smallest safe correction and retest. ⌄

Replace damaged gaskets or reseal suitable joints.

3 Step 3 — Investigate persistent ingress Make the smallest safe correction and retest. ⌄

Check sill, frame joints and installation details.

⌁ Why the problem happens How sealed units, glazing support and drainage affect performance. ⌄

Many modern frames intentionally collect small amounts of rainwater and drain it outside. Problems occur when the route is blocked or joints fail.

⌘ Advanced checks Further diagnosis for persistent glazing faults. ⌄
  1. Check pressure-equalisation chambers.
  2. Inspect sill end dams.
  3. Assess glazing gasket leakage.
  4. Check frame fixing penetrations.
  5. Hose-test sections gradually.
◇ Tools and information needed Useful items for accurate checks. ⌄
  • Torch
  • Soft cleaning brush
  • Plastic probe
  • Absorbent paper
  • Sealant tools
▣ Parts that may need replacing Common glazing components linked to this fault. ⌄
  • Drainage caps
  • Gaskets
  • Sill end caps
  • Exterior sealant
  • Frame joint seals
! When to call a professional Conditions that should end basic DIY diagnosis. ⌄
  • Water reaches internal finishes.
  • The frame is loose or distorted.
  • Drainage repeatedly blocks.
  • The sill or wall below is damp.
  • Water may be entering through the building fabric.
? Frequently asked questions Common glass and sealed-unit questions. ⌄

Is some water in the frame normal?

Yes, provided it drains outside quickly.

Should I seal the drainage slots?

No. They are essential to the frame design.

Why does water appear only in strong wind?

Wind pressure can drive rain past weak seals or overwhelm drainage.