Will double glazing actually fix condensation on your bedroom windows?

Condensation on bedroom windows is the most common reason NZ homeowners start looking at double glazing. Two adults breathing in a closed room overnight produce around 1.5L of water vapour, and on a single-glazed window that vapour finds the coldest surface in the house and condenses there. The morning ritual of wiping the sill is a uniquely Kiwi annoyance. Double glazing is most of the answer — but not all of it, and a poorly-spec'd retrofit will frustrate you.

Why single glazing condenses

Inside-pane temperature on single glazing in a Wellington July night sits around 6–9°C when the room is at 18°C. Air at 18°C and 60% relative humidity has a dew point of around 10°C — so the moment that air touches the glass, it dumps water. The colder the glass, the more water. The only ways to stop it are: warm the glass, drop the humidity, or stop the air reaching the glass.

What double glazing does to the inside-pane temperature

A retrofit IGU with argon fill and a warm-edge spacer typically holds the inside-pane temperature around 14–16°C in the same conditions — high enough to be above the dew point of normal indoor air. This is why double glazing 'fixes' condensation in most cases: not because the glass is dry, but because the inside pane is warm enough that the air-against-glass no longer chills past dew point.

Where double-glazed condensation still appears

• On the spacer edge — if the IGU has an aluminium spacer (older or cheaper specs), the perimeter cold-bridges and condenses there even when the centre-of-glass is dry. Always spec a warm-edge spacer.
• On the frame, especially aluminium frames without a thermal break — the frame becomes the new coldest surface.
• When humidity is genuinely too high — ensuite-adjacent bedrooms with poor ventilation, drying clothes indoors, unflued gas heaters. Double glazing will still drip if the room is at 75% RH.
• On the OUTSIDE pane on cold clear nights — this is normal and good. It means the IGU is working and the outer pane is being cooled by night-sky radiation faster than the inside is heating it.

The boring stuff that still matters

• Run the bathroom extractor for 15 minutes after every shower, with the door closed
• Vent the kitchen during cooking — most of the indoor moisture in a NZ home comes from cooking and showering
• Don't dry clothes inside without a dehumidifier or vent fan running
• If you have a gas heater that isn't flued externally, replace it — unflued gas dumps roughly a litre of water vapour per litre of LPG burned
• Crack a window in each bedroom by 5-10mm overnight — even with double glazing this dramatically improves the morning humidity

When double glazing won't fix it

If your house has a wet subfloor, leaks in the building envelope, or genuinely under-ventilated wet rooms, no glazing upgrade will fix the condensation problem — you're treating the symptom not the cause. A good installer will tell you this when they come for the site visit; a bad one will sell you the windows anyway. Spend $300 on a building-performance survey first if you're not sure.