What Causes Condensation on Windows and How Do I Stop It?

Waking up to streaming windows or wiping mould off sills is a frustrating reality for many New Zealand homeowners — especially through winter. Condensation forms when warm, moist indoor air meets a cold surface like a window pane, and the moisture in the air turns to liquid water. Left unchecked, it can rot timber joinery, trigger mould growth, and contribute to the damp, unhealthy homes that Tenancy Services' Healthy Homes standards are specifically designed to address.

Why Condensation Happens: The Root Causes

Understanding the cause is the first step to fixing it. Condensation is essentially a humidity problem — too much moisture in the air relative to the temperature of your surfaces.

Common causes include:

• Single-glazed windows — old aluminium-framed single glazing is extremely cold on the inside surface, making condensation almost inevitable in NZ winters
• Poor ventilation — cooking, showering, breathing, and drying laundry indoors all add moisture; without airflow, it builds up
• Inadequate heating — cold rooms mean cold surfaces; condensation forms more readily
• No or poor insulation — cold walls and ceilings compound the problem
• Unflued gas heaters — these release water vapour directly into the room as a byproduct of combustion, dramatically raising indoor humidity
• Subfloor moisture — in older NZ homes, moisture rising from the ground can enter the living space

How to Diagnose Your Condensation Problem

Not all condensation is the same. Use this step-by-step approach to identify what you're dealing with.

Step 1: Work out where the condensation is forming

• On the inside surface of the glass — this is the most common type and is caused by high indoor humidity
• Between the panes of double glazing — this means the sealed unit has failed and the inert gas or desiccant inside has been compromised; the unit needs replacing
• On the outside of the glass — this is actually a sign of a well-insulated window and is harmless; it means the outer pane is cooler than the outside air

Step 2: Check your indoor humidity

Pick up a hygrometer (a small humidity meter) from Bunnings or Mitre 10 — they're inexpensive and very useful. Healthy indoor humidity in NZ should generally sit between 40–60%. If you're regularly seeing readings above 65–70%, you have a moisture problem that needs addressing.

Step 3: Identify your moisture sources

Walk through your home and note:

• Is there a rangehood venting outside when you cook, or does it recirculate?
• Does your bathroom extractor fan actually vent outside, or into the ceiling cavity?
• Are you drying laundry indoors?
• Do you use an unflued gas heater?
• Is there visible mould on walls or ceilings beyond just the windows?

Step 4: Check your subfloor (older homes)

In homes built before the 1990s, a damp subfloor is a major hidden contributor. If you can safely access your subfloor, look for pooling water, wet soil, or inadequate ground vapour barrier.

What You Can Fix Yourself

Many condensation issues can be significantly improved with straightforward DIY steps.

Improve ventilation

• Open windows daily, even briefly — 10–15 minutes of cross-ventilation makes a real difference
• Install trickle vents in window frames (some joinery allows this without specialist work)
• Make sure bathroom and kitchen extractor fans vent outside the building envelope, not into the ceiling
• Consider a heat recovery ventilation (HRV) or positive pressure ventilation system — these are installed by specialists but are worth investigating for persistently damp homes

Reduce moisture at the source

• Always use lids on pots when cooking
• Run the rangehood when cooking
• Dry laundry outside or use a vented dryer
• Replace unflued gas heaters with a flued gas heater, heat pump, or electric panel heater — BRANZ has good guidance on heating choices and indoor air quality

Add secondary glazing or window insulation film

For single-glazed windows, DIY secondary glazing film (available at Bunnings or Mitre 10) creates an insulating air gap and raises the inner surface temperature, dramatically reducing condensation. It's not as effective as proper double glazing but is a low-cost interim fix.

Seal draughts — carefully

Draughts feel unpleasant, but completely sealing a home without adding controlled ventilation can make moisture problems worse. Seal obvious gaps around windows and doors with draught-excluding tape, but ensure you still have adequate fresh air coming in through other means.

Wipe down and treat affected surfaces

For existing mould on sills and frames, clean with a diluted bleach solution or a purpose-made mould treatment. Resene produces mould-resistant paints that can help on timber sills once they're clean and dry.

Comparing Window Solutions

How to Prevent It Recurring

• Maintain indoor humidity between 40–60% year-round
• Heat your home consistently — cold rooms condensate more
• Ensure all wet areas (bathrooms, laundry) have fans venting outside, as required under NZ Building Code clause G4
• If renovating, upgrade to thermally broken double-glazed joinery — this is now standard under the Building Code for new work and makes a transformative difference
• Check and maintain subfloor ventilation and ground moisture barriers
• Review your insulation — Energywise has information on government support for insulation upgrades

When to Call a Professional

Some condensation fixes go beyond DIY and require a qualified tradesperson.

Call a builder or glazier if:

• Your double-glazed units have failed (fogged between panes) — the sealed unit needs professional replacement
• You want to upgrade to full double glazing or thermally broken joinery — this is building work and should be done by an LBP-registered builder
• You need a subfloor vapour barrier installed properly
• Timber window frames or sills are rotting and need repair or replacement

Call a licensed plumber if:

• Your bathroom extractor fan needs to be re-routed to vent outside — ducting that penetrates the building envelope may require restricted building work sign-off

⚠️ Electrical safety note: Installing or relocating extractor fans involves electrical wiring, which is restricted electrical work in New Zealand. This must be carried out by a registered electrician — see the Electrical Workers Registration Board (EWRB) for more information. Do not attempt to wire fans yourself.

If you're not sure where to start, a qualified local builder can assess your home, identify the main moisture pathways, and recommend the most cost-effective solutions. You can find experienced tradespeople in your area at tradedirectory.co.nz.

Sources & Further Reading

• BRANZ — Condensation and moisture in homes
• MBIE Building Performance — Building Code moisture requirements
• Energywise — Home insulation and heating guidance
• Tenancy Services — Healthy Homes Standards
• WorkSafe NZ — Indoor air quality and unflued heaters
• Resene — Mould-resistant paints and coatings