Condensation pooling on windowsills is one of the most common complaints from New Zealand homeowners, particularly through our cool, damp winters. The good news is that with a combination of better ventilation, humidity control, and — where needed — glazing upgrades, you can significantly reduce or eliminate the problem.
Why Does Condensation Form on Windowsills?
Condensation forms when warm, moist indoor air meets a cold surface. Single-glazed aluminium-framed windows are especially prone because the glass and frame get very cold, dropping below the dew point of the indoor air. Water then beads on the glass and runs down onto the sill, where it can pool, cause rot in timber sills, promote mould growth, and damage paint or sealant.
NZ homes — particularly older weatherboard or brick houses — often have poor insulation and ventilation, which makes the problem worse. High indoor humidity from cooking, showering, drying clothes indoors, and even breathing all contribute.
Step 1 — Reduce Indoor Humidity
The single most effective thing you can do is lower the amount of moisture in your indoor air:
- Ventilate when cooking and showering — always run the rangehood or bathroom extractor fan, and leave it running for 10–15 minutes after you finish.
- Dry clothes outside or use a vented dryer — drying a load of washing indoors can release up to 2 litres of moisture into the air.
- Open windows briefly each morning — even 5–10 minutes of cross-ventilation flushes out overnight moisture. This is a core principle behind the Healthy Homes ventilation standards.
- Use a dehumidifier in rooms that consistently suffer — portable units are widely available from hardware stores.
- Avoid unflued gas heaters — these release significant moisture as a combustion by-product.
Step 2 — Improve Heating
Cold rooms condense more moisture. Keeping your home consistently warmer (the NZ Healthy Homes standard requires a minimum heating capacity in living areas) reduces the temperature gap between the air and your window surfaces. Heat pumps are efficient and don't add moisture to the air, making them a good choice.
Visit Energywise for guidance on efficient home heating options.
Step 3 — Manage the Windowsill Itself
While you work on the root causes, protect your sills from water damage:
- Wipe sills down daily with a dry cloth or squeegee during cold months.
- Check and reseal any gaps between the window frame and sill with a flexible, paintable sealant (such as a silicone or polyurethane product available from Bunnings or Mitre 10) — gaps allow water to track into the wall cavity.
- Repaint timber sills with a moisture-resistant enamel or trim paint. Resene and Dulux both offer suitable exterior-grade trim paints that can be used on interior sills for extra durability.
- Install a condensation strip or drip channel — some glaziers can fit a small plastic channel at the base of the glass unit to direct water away from the sill.
Step 4 — Add Secondary Glazing or Window Insulation Film
If you can't yet afford full window replacement, secondary glazing is a cost-effective middle step. A secondary panel (acrylic or glass) is fitted inside the existing window reveal, creating a still-air gap that raises the inner surface temperature significantly.
DIY window insulation film kits are also available from hardware stores — these are stretched across the frame and shrunk with a hairdryer to create a thin air gap. They won't last as long as proper secondary glazing but can make a noticeable difference over winter.
Step 5 — Upgrade to Double Glazing
The most permanent solution is replacing single-glazed windows with double-glazed units (IGUs — insulated glass units). Double glazing keeps the inner pane much warmer, dramatically reducing condensation. Thermally broken aluminium frames or uPVC frames go even further by eliminating the cold metal bridge that conducts heat out through the frame.
Window replacement is generally exempt from building consent under Schedule 1 of the Building Act when it is a like-for-like replacement in the same opening — but you should always confirm this with your local council before starting work, as conditions apply. Refer to MBIE Building Performance for guidance on consent exemptions.
Note that any structural changes to the opening (enlarging it, for example) will almost certainly require consent.
When to Call a Professional
Call a registered glazier if you need:
- Double-glazed unit replacement or full window upgrades
- Secondary glazing professionally fitted
- Assessment of failed IGU seals (when you see fogging between the panes — this means the unit needs replacing, not just cleaning)
If your windowsills show signs of rot, structural damage, or mould penetrating the wall, you may also need a Licensed Building Practitioner (LBP) for repairs — find one at LBP.
For persistent whole-home condensation and moisture issues, a building surveyor or BRANZ-accredited assessor can diagnose underlying causes. See BRANZ for technical resources.
You can find qualified local glaziers and building tradespeople on tradedirectory.co.nz.
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