Water infiltration around windows: understanding the causes, establishing a diagnosis and choosing the right corrective work

Signs to watch for around a window

The first clues are often subtle and appear indoors, below or beside the opening. These are the most common:

• Rings, halos or brownish stains under the sill or in the lower corners
• Blistering or peeling paint; drywall that swells, softens or crumbles
• Wood trim, stools or frames that darken, swell or deteriorate
• Visible mould in the corners, on interior sealant or behind curtains
• A musty or damp smell, especially after rain
• Stained or cupped flooring, or baseboards pulling away below the window
• Outside: cracked or detached sealant, distorted cladding, missing or misdirected flashing above the opening

Why are these signs not conclusive? Because they show where water becomes visible, not where it enters or why. The same stain can come from condensation, leakage through the window unit itself, a faulty joint between the window and the wall, or an entry point higher up — a flashing, a roof junction, a vent penetration — that travels inside the wall before appearing under the sill.

It is therefore useful to separate three levels of information: the symptom (what you observe), the possible mechanism (the hypothesis about the water’s path) and the verified cause (what the investigation has confirmed). Only the verified cause should guide the work.

Condensation or rainwater penetration?

Condensation forms when humid indoor air touches a surface colder than its dew point. In winter, the glazing and frames of windows are often the coldest surfaces in a room, so that is where moisture settles first [3]. Health Canada notes that excess indoor humidity — cooking, showers, drying laundry, insufficient ventilation — encourages condensation and mould growth [1].

Rainwater penetration, on the other hand, is tied to outdoor weather: it appears during or after rain, often when wind drives water against a specific wall, or during snow and ice melt.

Condensation between the panes: fogging or deposits inside a sealed glazing unit (between two or three panes) generally means the unit’s edge seal has failed. It is not leakage into the wall and cannot be wiped away; the usual solution is to replace the sealed unit or the window [3]. Both situations can, however, occur at the same time.

How the building envelope is meant to manage water

• Exterior cladding: the first line, deflecting most rain without being fully watertight
• Drainage plane and weather-resistive barrier (sheathing paper or membrane): they collect water that gets behind the cladding and direct it down and out [4]
• Flashings: metal or membrane pieces that intercept water above and below openings and send it outside; they must be lapped shingle-fashion, with the upper layer overlapping the lower one [4][5]
• Air barrier: limits air leakage, which can carry large amounts of moisture into the wall [2]
• Vapour barrier: on the warm side of the insulation, it limits the diffusion of indoor vapour toward the cold parts of the wall [2][4]

The envelope is everything that separates inside from outside. To work, it must control liquid water, air, water vapour and heat [2]. A window is an opening in this system: every layer of the wall must connect properly to it, or water and air will find a way through.

Canadian wood-frame construction guidance relies on several lines of defence rather than a single perfectly sealed layer [4]:

The key word is continuity. A membrane properly installed across the wall but interrupted around the window does not protect the most vulnerable area.

Possible causes of infiltration around a window

These causes are hypotheses to verify, not conclusions. Several may act at the same time.

How water moves through a wall

For infiltration to occur, there must be water on the surface, an opening, and a force driving it inward [5]. The main forces are:

• Gravity: water runs down any slope toward the inside, hence the importance of flashings sloped outward and laps in the right direction
• Capillarity: water is drawn into narrow gaps and porous materials, even upward; a small gap or capillary break can interrupt it
• Kinetic energy: wind-driven rain can enter directly through open joints
• Air pressure difference: when wind creates higher pressure outside than inside, water on the façade is pushed through openings
• Surface tension: water can cling to the underside of a horizontal element and run back to the wall if there is no drip edge

This is why water can travel a surprising distance inside the wall — along a membrane, a stud or an electrical wire — before becoming visible. The location of the stain does not necessarily indicate the place to repair.

The professional investigation, step by step

A good investigation moves from least to most invasive and aims to confirm a cause before proposing work.

• 1. History and weather: since when, how often, with what kind of rain and wind, what repairs have already been made, what photos exist
• 2. Interior inspection: precise location of the damage, condition of finishes, record of the affected areas and their extent
• 3. Exterior inspection: flashings, sealants, cladding, sills, gutters, roof and anything located above the opening
• 4. Moisture measurement: moisture meters help map wet areas, but readings vary with material, instrument and timing; a dry material on the day of the visit does not rule out intermittent infiltration
• 5. Further testing, only where needed: for example controlled water testing of a specific area, or a limited exploratory opening of the cladding or drywall to see hidden layers
• 6. Conclusion and definition of corrective work: confirmed cause, or most likely hypothesis clearly identified as such, scope of work, and what remains to be validated once opened up

In an older wall, some conditions can only be known once it is opened up. An honest approach says so in advance and plans how to adjust the work if damaged materials are found.

Repair or replace?

When the window is in good condition and the cause lies in its connection to the wall — a poorly installed flashing, an interrupted membrane — the solution is often to rebuild those details, which usually means locally removing the cladding and sometimes the window to reinstall it properly.

Replacement becomes relevant when the unit itself has failed (deteriorated frame, broken sealed glazing, window at end of life) or when envelope work is an opportunity to improve energy efficiency. Natural Resources Canada stresses that a new window must be installed with airtightness and water management around its perimeter; a high-performance window poorly connected to the wall will not solve an infiltration [3].

In both cases, wet or damaged materials (insulation, drywall, wood) must be assessed: some dry out and can stay, others must be replaced.

What drives the cost of the work

Every wall is different; quoting a price without an investigation would be misleading. The main factors are:

• The actual extent of the damage, often only known once opened up
• The type of cladding to remove and reinstall (vinyl, wood, fibre cement, masonry)
• Simple repair of the connections versus window replacement
• The number of openings and walls affected
• Access: upper storey, scaffolding, proximity to a roof
• Interior finishes to redo and materials to replace
• Season and weather, which may require temporary protection

Common mistakes

• Adding sealant over old sealant, or blocking the window’s weep holes
• Repainting or replacing drywall without finding the source
• Replacing the window while keeping a failed flashing or membrane
• Assuming condensation, or infiltration, without checking when the problem occurs
• Waiting several seasons: persistent moisture degrades wood, insulation and finishes and encourages mould [1]

Precautions for homeowners

• Photograph each episode and note the date, duration and wind direction
• Wipe up water, protect floors and furniture, and monitor changes
• Reduce indoor humidity if you suspect condensation: range hood and bathroom fan, ventilation, dehumidifier if needed [1]
• Keep gutters and downspouts clear and directed away from the house
• Do not take apart cladding or flashings yourself: a poorly closed opening can make things worse

Safety: if water reaches an outlet, switch, light fixture or electrical panel, do not touch it; turn off the circuit at the panel if you can do so from a dry, safe spot, and call a licensed electrician. For extensive mould, Health Canada recommends calling on professionals rather than cleaning it yourself [1]. A ceiling that bulges or sags, or a floor that softens, should be kept clear and assessed quickly.

Frequently asked questions

Is caulking enough?

Sometimes, for a visibly failed exterior joint. But sealant is a secondary protection that ages; it does not replace a flashing or a continuous membrane. If the infiltration returns, you need to look further.

My window is recent; can it still cause infiltration?

Yes. A new window can perform well, but infiltration often comes from its connection to the wall rather than from the unit itself [3].

Does fog between my panes mean my wall is leaking?

No. It generally indicates a failed sealed glazing unit. It is a separate problem, although it can coexist with infiltration.

Why does water only appear in certain rains?

Because wind direction, duration and rain intensity change the amount of water on the façade and the pressure pushing it inward [5].

Does the wall always have to be opened?

No. We start with the history and the inspection. A limited opening is proposed only if needed to confirm the cause or assess damage.

My insurer requires an expert report: does your assessment count?

Our assessment is used to plan the work. It does not replace an expert report required by an insurer or in a dispute; check with your insurer.

Entreprise SGB’s approach

Entreprise SGB carries out the investigation and diagnosis of water infiltration itself before proposing corrective work. The same team that analyses the problem plans and carries out the work: cladding, flashings, membranes, windows and interior finishes.

Our goal is simple: separate what is observed from what is confirmed, explain the cause and options clearly, then correct the source rather than the symptom.