Published 18 September 2026.
Water on or around a window can have several different causes. For Melbourne homeowners, it is important to distinguish between indoor condensation, moisture between double-glazed panes and rainwater leakage before deciding whether a window needs repair or replacement.
These problems can look similar at first, but they have different causes and should not be diagnosed from one symptom alone.
Condensation on the Inside Surface of the Glass
Condensation can form when warm, moisture-laden indoor air meets a sufficiently cold surface. This is common on colder glass or frames during cool weather. Indoor humidity, ventilation, room temperature, glazing and frame performance can all influence the amount of surface condensation.
Surface condensation does not by itself prove that rainwater is leaking through the window.
Moisture Between Double-Glazed Panes
If moisture or fogging appears inside the sealed cavity between the panes of an insulated glass unit, the sealed unit may have failed. This is different from moisture that can be wiped from the room-side glass surface. The condition of the frame and the glass unit should be assessed before deciding whether glass-only work or complete replacement is appropriate.
Water Appearing During or After Rain
Water associated with rain may involve the window, but it can also originate from surrounding construction. Flashings, sealants, cladding interfaces, masonry cavities, sills, drainage paths and other building-envelope details may need to be investigated. The visible location of water inside a room is not always the exact point where water entered.
Can Condensation Cause Mould Around Windows?
Persistent surface moisture can contribute to conditions that support mould growth on nearby finishes. However, mould can have multiple moisture sources. Cleaning visible mould without addressing persistent moisture or ventilation conditions may not resolve the underlying issue.
Will Double Glazing Stop Condensation?
Double glazing can raise the room-side glass surface temperature compared with basic single glazing under comparable conditions, which may reduce the likelihood of surface condensation. It cannot guarantee that condensation will never occur because indoor humidity, ventilation and external conditions also matter.
Homeowners considering a complete upgrade can read our double-glazed window replacement guide.
Does a Leaking Window Always Need Replacement?
No. If the problem is a serviceable sealant joint, drainage issue or another repairable detail, replacement may not be necessary. Conversely, replacement may become relevant when the frame is deteriorated, poorly operating, unsuitable for the intended upgrade or one part of a broader renovation. Correct diagnosis should come before specifying the remedy.
See our repair-versus-replacement homeowner guide.
What Should I Photograph?
- The full window from inside and outside
- The location where moisture first appears
- The sill and bottom corners
- Any visible gaps, failed seals or damaged frame areas
- Surrounding brickwork, render or cladding
- Fogging between panes if present
What Should I Send JL Windows?
If you are considering replacement, send your suburb, photos, approximate window dimensions and a description of when the moisture occurs — for example, on cold mornings, between panes, or specifically during rain. This helps distinguish the initial replacement enquiry, although some moisture problems may require investigation by an appropriate building or waterproofing professional rather than a window replacement quote.
For draught-related problems, see our draughty windows guide. For replacement pricing, see our window replacement cost guide.
