If the heater has been running all afternoon and the lounge room still never quite warms up, the problem is usually not the heater. It is the building around it. Government researchers have measured where the warmth in Australian homes actually goes, and the numbers surprise most people: in a typical home, the walls and windows together can account for around 25–45% of winter heat loss.
The figures behind that range come from the Residential Efficiency Scorecard, a national home energy rating program delivered by the Victorian Government on behalf of Australia’s federal, state and territory governments. Its guidance breaks down where an average house leaks warmth in winter and soaks up heat in summer.
What the government figures say
According to the Scorecard’s guidance on insulation and the building shell, winter heat loss splits across an average home roughly like this:
- Ceilings and roofs: 25–35% of heat loss
- Walls: 15–25% of heat loss
- Windows: 10–20% of heat loss in winter, and a much larger 25–35% share of unwanted heat gain in summer
- Floors: 10–20% of heat loss
- Gaps and cracks: 5–25% of heat loss or gain
Add walls (15–25%) and windows (10–20%) together and you get the 25–45% figure: between a quarter and almost half of the warmth you are paying for, escaping through the two surfaces you look at every day.
Sustainability Victoria puts a number on the draught side too: up to 25% of winter heat loss from existing houses is caused by air leakage. That is the cold air slipping in around windows, doors, floorboards and skirtings while your warm air slips out.
Why old windows punch above their weight
Glass is the weakest thermal link in almost every wall. Sustainability Victoria notes that a single pane of glass can lose almost ten times as much heat as the same area of insulated wall. Stand next to an old single-glazed window on a July evening and you can feel it: the glass draws the warmth out of the room and radiates cold back at you.
Older timber windows also contribute twice. Heat conducts out through the glass, and warm air leaks out through the gaps that open up as sashes shrink and frames settle over the decades. That is why a period home can be freshly heated and still feel draughty at the same time, and why condensation streams down the glass on cold mornings.
The good news: you don’t have to replace your windows
Ripping out original timber windows is the expensive answer, and it costs your home its character. It is also rarely necessary, because the two fixes that target the government’s figures directly both work with the windows you already have.
Draught and acoustic sealing closes the gaps between sash and frame, tackling the air-leakage share head on. Your windows still open and close, more smoothly than before, and without the rattles.
Upgrading the glass deals with the conduction share. Sustainability Victoria advises that energy-efficient windows can reduce heat loss through the window by up to 50% compared with single glazing, and the Victorian Energy Upgrades program notes that double-glazed windows retain up to 30% more heat than single-glazed ones. With vacuum insulated double glazing, the new glass is slim enough to fit into your existing timber sashes. The heritage character stays; the cold does not.
What this means in Melbourne and Hobart
Victoria and Tasmania have some of the oldest housing stock in the country, and some of the coldest winters. If you live in a Victorian-era terrace, a Federation weatherboard or an interwar brick home, your walls and windows are very likely sitting at the leaky end of those government ranges.
Here is the practical part: of all the places a home loses heat, windows are the most fixable. Insulating existing solid walls means opening them up or cladding over them, which is disruptive and costly. Sealing and reglazing the windows you already own is typically done in days, with no structural work and no change to how your home looks from the street. It is the biggest slice of the heat-loss pie you can claim back without a renovation.
Thinking about upgrading your windows?
If your home is cold, draughty or expensive to heat, start with the windows. The government’s own numbers say that is where a big share of your warmth is going. To understand your options, these guides are a good place to begin:
- An Easy Way to Understand Glass Performance
- The Quiet Truth About Sash Windows: Seal the Gaps and Your Glass Can Finally Work
- Draught & Acoustic Sealing
- Vacuum Insulated Glass Australia
Or simply get in touch and we will take a look at your windows before winter gets another word in.
Bring your windows back to life.
