Condensation on Windows: Causes and Ventilation Fixes

Summary

Condensation on windows is the most common damp complaint UK tradespeople and landlords face, and it is widely misunderstood. The water is not a leak and, in most cases, not a window defect — it is moisture that was already in the indoor air, condensing out onto the coldest surface in the room. Glass is usually that coldest surface, which is why windows show the problem first even when the real issue is whole-house humidity and inadequate ventilation. Modern airtight homes make it worse: draught-proofing and double glazing trap the moisture that older, leakier houses used to lose naturally.

There are three distinct situations that all get called "window condensation", and they need different responses. Internal condensation (water on the room side of the glass) is the everyday case — too much moisture, too little ventilation, cold glass. External condensation (dew on the outside of the glass on a clear morning) is actually a sign of a well-insulated, efficient window — the inner pane is so warm that the outer pane stays cold enough to collect dew; it needs no fix. Interpane condensation (mist trapped between the panes) means the sealed double-glazed unit has lost its seal and its desiccant is saturated — the unit must be replaced.

For the everyday internal case, the durable fix is almost always about ventilation and moisture management, governed in new and refurbished homes by Building Regulations Approved Document F (Ventilation). A tradesperson who reaches for "just replace the windows" without addressing extract fans, trickle vents, heating and lifestyle moisture is treating the symptom and will be back. This article sets out the causes, the diagnostic split between the three condensation types, the Part F ventilation rates that fix the common case, and the practical hierarchy of fixes from free behavioural changes to installing continuous mechanical extract.

Key Facts

• Dew point — condensation forms when air cools to the temperature at which it is saturated; cold glass is usually the coldest surface, so it condenses first.
• Target humidity — keep relative humidity roughly below 60% RH to avoid surface condensation and mould; above ~70% RH sustained surface mould risk rises sharply.
• Moisture sources — a typical household produces several litres of water vapour a day: cooking, washing, drying clothes indoors, showering, breathing (a sleeping adult releases moisture overnight), and unflued sources.
• Internal condensation = warm moist room air on cold glass → ventilation/moisture/heating problem.
• External condensation (outside the glass, clear nights) = a sign of an efficient, well-insulated unit; no remedy needed.
• Interpane condensation (mist between the panes) = failed sealed unit, blown desiccant → replace the double-glazed unit.
• Approved Document F (Ventilation) intermittent extract rates (dwellings): kitchen 30 l/s (or 60 l/s if not over the hob) / 13 l/s continuous; bathroom 15 l/s intermittent; utility 30 l/s intermittent; WC 6 l/s.
• Trickle vents — Part F (2021 edition) requires background ventilators (trickle vents) on replacement windows in most cases; closing them off to "stop draughts" causes condensation.
• Black mould (Aspergillus/Cladosporium) on window reveals, sills and frames is the visible consequence of sustained surface condensation — a health and (for landlords) legal concern.
• Awaab's Law / HHSRS — damp and mould in rented homes is a recognised category 1 hazard under the Housing Health and Safety Rating System; landlords have duties to address it (reinforced by Awaab's Law provisions for social housing).
• PIV (Positive Input Ventilation) units gently pressurise the home with filtered loft/outside air, diluting and displacing moisture — a common whole-house retrofit cure.
• Cold bridging at reveals and lintels creates cold spots that condense even when glass is upgraded — insulate reveals where mould tracks the frame edge.

Quick Reference Table

Detailed Guidance

Identify which condensation you have first

The single most important diagnostic step is establishing where the water sits, because it determines whether this is a behaviour/ventilation job, no job at all, or a glazing replacement. Wipe the inner glass: if it wipes dry, it is internal condensation; if the mist is between the panes and cannot be wiped, the sealed unit has failed.

Decision tree: diagnosing window condensation

CONDENSATION ON WINDOWS
│
├─ Can you WIPE the moisture off the glass?
│     │
│     ├─ NO — mist is BETWEEN the panes
│     │     └─ FAILED SEALED UNIT (blown desiccant)
│     │         → Replace the double-glazed unit (not the whole window usually)
│     │
│     └─ YES → which FACE is wet?
│           │
│           ├─ OUTSIDE face, clear/cold mornings, clears as day warms
│           │     └─ EXTERNAL condensation = efficient window
│           │         → No action required (normal, harmless)
│           │
│           └─ INSIDE face (room side) → INTERNAL condensation
│                 │
│                 ├─ Worst in KITCHEN / BATHROOM?
│                 │     └─ Source moisture
│                 │         → Fit/upgrade EXTRACT FAN to Part F rate
│                 │         → Run during + 15 min after; lid pans; shut door
│                 │
│                 ├─ Worst in BEDROOMS overnight?
│                 │     └─ Occupant moisture + closed room
│                 │         → Open TRICKLE VENTS, crack window
│                 │         → Maintain low background HEAT
│                 │
│                 ├─ Drying WASHING indoors / unvented tumble dryer?
│                 │     └─ Major moisture source
│                 │         → Vent dryer outside; dry in ventilated room/outside
│                 │
│                 └─ Whole house humid, multiple rooms, persistent MOULD?
│                       └─ Inadequate whole-house ventilation
│                           → Consider PIV or continuous MEV
│                           → Check/insulate cold bridges at reveals

The ventilation fixes (in order of cost)

Free / behavioural (always do these first):

• Keep trickle vents open — closing them to stop a draught is the commonest self-inflicted cause.
• Shut kitchen and bathroom doors while cooking/showering to contain the moisture, and run the extract fan.
• Dry washing outside or in a ventilated room; never on radiators in a closed room. Vent tumble dryers outside (or use a condenser/heat-pump dryer).
• Lid pans, crack a window when cooking, wipe condensation off sills each morning.
• Provide consistent background heat rather than short hot bursts — warm surfaces don't condense, and steady low heat keeps glass above dew point.

Low–medium cost interventions:

• Fit or upgrade extract fans to the Approved Document F rates: a kitchen fan rated to 30 l/s (intermittent) and a bathroom fan to 15 l/s, ideally with humidity sensing and overrun timers so they run after use.
• Add or open trickle vents on windows — required on most replacement windows under Part F (2021).
• Insulate cold reveals and lintels where mould tracks the window edge (cold bridging).
• Use a portable dehumidifier in problem rooms as an interim measure.

Whole-house systems (for persistent, multi-room damp):

• PIV (Positive Input Ventilation) — a loft- or wall-mounted unit gently supplies filtered air to dilute and displace moist indoor air; effective and low-running-cost for whole-house condensation.
• Continuous MEV (Mechanical Extract Ventilation) — a central fan continuously extracting from wet rooms.
• MVHR (Mechanical Ventilation with Heat Recovery) — for airtight new builds/deep retrofits, supplying fresh and extracting stale air while recovering heat. This is the Part F System 4 approach for very airtight dwellings.

When it is the window, not the ventilation

Two cases genuinely point at the glazing. Interpane misting means the sealed unit's edge seal has failed and the desiccant is saturated — replace the unit (often you can replace just the glazed unit, not the whole frame). And if single glazing or very poor old units leave the inner glass extremely cold, upgrading to modern low-E double or triple glazing raises the inner surface temperature above dew point — but only address ventilation at the same time, or the condensation simply migrates to the next-coldest surface (often a wall corner or reveal) and grows mould there.

The mould consequence

Sustained surface condensation feeds black mould on reveals, sills and frame edges. Beyond the unsightly staining, mould spores are a respiratory health concern, and for rented property it is a recognised hazard under the Housing Health and Safety Rating System (HHSRS), with landlord duties reinforced by Awaab's Law provisions. Treating the mould (wash down with an appropriate fungicidal wash) without fixing the ventilation only delays its return — the cause is moisture, and the cure is ventilation plus heat.

Frequently Asked Questions

Why do I get condensation only since I had new windows fitted?

Because old windows leaked air constantly, ventilating the house by accident and carrying moisture away. New, well-sealed double glazing stops that uncontrolled air loss, so the moisture your household produces now stays inside and condenses on the (still coldest) glass. The fix is to restore controlled ventilation: keep the trickle vents open, run extract fans, and ventilate when cooking and showering. It is a ventilation change, not a window fault.

Condensation is on the OUTSIDE of my new windows — is that a defect?

No — it is the opposite. External condensation forms on clear, cold mornings when the inner pane is so well insulated that the outer pane stays cold enough to collect dew from the outside air, exactly like dew on a car. It shows the glazing is performing efficiently and losing very little heat. It clears as the day warms and needs no remedy.

What's the difference between condensation between the panes and on the surface?

Surface condensation (you can wipe it off) is an indoor-air moisture/ventilation issue. Condensation between the panes that you cannot wipe means the sealed double-glazed unit has failed — its perimeter seal has broken and the moisture-absorbing desiccant is saturated, so moist air now sits permanently inside the unit. That unit must be replaced; no amount of ventilation will clear it.

Will a dehumidifier fix the problem permanently?

A dehumidifier reduces humidity and is a useful interim or supplementary measure, especially in a single problem room. But it treats the symptom while running and costs electricity. The durable fix is source extraction and background ventilation (trickle vents, extract fans, or a whole-house PIV/MEV system) combined with steady heating, so the house manages its own moisture without a machine running constantly.

How much ventilation do I legally need?

For new dwellings and many refurbishments, Approved Document F (Ventilation) sets minimum extract rates — broadly 30 l/s for a kitchen and 15 l/s for a bathroom (intermittent), plus background ventilators (trickle vents) on replacement windows in most cases. Existing homes are not retrospectively forced to meet these, but they are the sensible benchmark a tradesperson should aim for when curing condensation..

Regulations & Standards

• Building Regulations Approved Document F (Ventilation), 2021 edition — extract rates for wet rooms, background ventilators (trickle vents) on replacement windows, whole-dwelling ventilation strategies (Systems 1–4).

• Building Regulations Approved Document L (Conservation of fuel and power) — works alongside Part F; airtightness improvements must be paired with adequate ventilation.

• Housing Health and Safety Rating System (HHSRS) — under the Housing Act 2004; damp and mould growth is an assessed hazard for rented housing.

• Awaab's Law — provisions (Social Housing (Regulation) Act 2023) requiring social landlords to address damp and mould within set timescales..

• BS 5250 — Code of practice for the control of condensation in buildings — the definitive UK condensation-control guidance.

• Building Regulations Approved Document C — resistance to moisture (interstitial/surface condensation context).

• GOV.UK — Approved Document F (Ventilation) — statutory ventilation requirements and extract rates

• BSI — BS 5250 control of condensation in buildings — definitive UK condensation guidance

• GOV.UK — Understanding and addressing the health risks of damp and mould in the home — health guidance and landlord duties

• Energy Saving Trust — condensation and ventilation — practical homeowner guidance

• bathroom refit — Part F extract requirements and ventilation in bathroom installations

• heating controls — maintaining consistent background heat to keep surfaces above dew point

• boiler selection — adequate, well-controlled heating as part of a condensation strategy

• hot water systems — heating system context for whole-home warmth and humidity control