How Do I Reduce Heat Loss Through Windows in Winter?

New Zealand winters can be surprisingly brutal, especially in the South Island and central North Island — and your windows are often the weakest link in your home's thermal envelope. Single-glazed aluminium-framed windows, which are common in older NZ homes, can lose several times more heat than a well-insulated wall. The good news is there are practical steps you can take, from simple weekend DIY fixes to more involved upgrades, to keep the warmth in and the cold out.

Why Windows Lose So Much Heat

Heat escapes through windows in three main ways:

• Conduction — heat travels directly through the glass and frame
• Radiation — warmth radiates outward through the glass on cold nights
• Air infiltration (draughts) — cold air leaks in around gaps in frames, seals, and sills

Single-glazed windows with aluminium frames are particularly poor performers because aluminium is highly conductive. BRANZ research consistently identifies windows and glazing as a major contributor to heat loss in typical NZ housing stock.

Step 1 — Draught-Stop Your Windows First

Before spending money on anything else, eliminate draughts. This is the most cost-effective fix and can make a noticeable difference immediately.

• Self-adhesive foam tape — press into the gap between the sash and frame on sliding or double-hung windows. Available at Bunnings or Mitre 10 for a modest cost.
• V-strip (tension seal) weatherstripping — more durable than foam tape and suits sliding sash windows well.
• Silicone sealant — use around fixed frames where there are gaps between the frame and the wall. Don't seal openable sashes shut.
• Door snakes / window draught excluders — a quick fix for the gap at the bottom of French doors or large sliding doors.

Check for draughts by holding a lit incense stick or a damp hand near frames on a windy day — any flicker or chill reveals a gap.

Step 2 — Use Heavy, Well-Fitted Curtains or Blinds

Quality curtains are one of the most effective and affordable thermal upgrades available to NZ homeowners. To work properly, curtains must:

• Reach floor to ceiling (or as close as possible)
• Have a pelmet or close-fitting track at the top to stop warm air circulating behind them
• Be made from heavy, lined fabric — thermal lining adds significant performance
• Be closed at dusk before the room cools down

Roman blinds and cellular (honeycomb) blinds that fit snugly inside the window recess also trap a layer of still air against the glass, improving insulation.

The Energywise programme (run by EECA) has good guidance on curtaining as part of a warm, healthy home.

Step 3 — Apply Window Insulation Film

Secondary glazing film is a clear plastic film stretched over the window frame on the inside and shrunk tight with a hairdryer. It creates a still-air gap that meaningfully reduces heat loss and condensation. It's temporary (usually one season), low-cost, and a good option for renters or those not ready to commit to permanent upgrades.

For a more permanent DIY option, rigid acrylic or polycarbonate secondary glazing panels can be cut to size and clipped or magnetically attached inside the existing frame — a popular solution in older villas and bungalows.

Step 4 — Consider Secondary Glazing or Double Glazing

If draughts and curtains aren't enough, upgrading the glazing itself is the most effective long-term solution.

Full double glazing replacement involves removing existing sashes or entire window units and installing thermally broken, double-glazed units. This is a significant investment but aligns with the NZ Building Code requirements for new builds and major alterations (clause H1 — Energy Efficiency).

uPVC or thermally broken aluminium frames make a big difference alongside double glazing, as standard aluminium frames conduct cold even when the glass is upgraded.

Condensation — A Sign of a Bigger Problem

If you're seeing heavy condensation on your windows every morning, that's a sign of excess moisture in the air combined with cold glass surfaces. Reducing heat loss through glazing upgrades helps, but you should also:

• Ventilate bathrooms and kitchens with extractor fans
• Use a heat pump or efficient heater to maintain a steady indoor temperature
• Check your home meets the Healthy Homes Standards if you're a landlord — these set minimum requirements for heating, insulation, and ventilation in rental properties

What a DIYer Can Safely Do

Most of the measures above — draught sealing, curtains, film, and DIY secondary glazing panels — are well within a capable homeowner's ability. No building consent is typically required for these minor works.

However, replacing window units (full double glazing installation) may require a building consent depending on the scope, and work that affects the weathertightness of the building envelope should be done carefully. Check with your local council if you're unsure.

When to Call a Professional

Call a qualified tradesperson when:

• You're replacing entire window units — a joiner or window installer should handle this to ensure weathertightness and compliance
• You suspect your window frames have rot or structural damage
• You want professionally fitted secondary glazing systems for the best thermal performance
• The work involves changes to the building structure or exterior cladding

For any electrical work associated with heated curtain tracks or motorised blinds, this is restricted electrical work and must be carried out by a registered electrician — see the Electrical Workers Registration Board (EWRB) for more information.

You can find qualified local insulation installers, joiners, and window specialists on tradedirectory.co.nz.

Sources & Further Reading

• BRANZ — Thermal Performance of Windows
• Energywise — Heating and Insulation Advice
• MBIE Building Performance — NZ Building Code
• Tenancy Services — Healthy Homes Standards
• Consumer NZ — Double Glazing Guide