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Windows are the weak point in most houses. The federal government's YourHome guide puts it plainly: up to 40% of a home's heating energy can be lost through its windows. For a Pakenham or Cardinia home, start with the actual windows: note the glazing, frame, room use and curtain gaps. An older aluminium window and a large glazed door may need different improvements even within the same room.
Curtains are one of the few fixes that can be added to an existing window without replacing it, and they can make a real difference. But the difference depends far more on how the curtain is fitted than on how thick the fabric is. A lot of people buy thermal curtains, hang them from a rod above the frame, and notice little change. The reason is air movement, and once you see how it works, the fixes are straightforward.
How heat escapes through a window
Heat leaves through a window in three main ways. Conduction is heat passing through the glass and frame themselves, and single glazing and aluminium frames conduct heat readily. Air leakage is draughts through gaps around the sashes and frame, which energy.gov.au suggests you can spot by rattling or whistling in the wind, or curtains that move when the window is shut. The third is convection inside the room: air touching the cold glass cools, becomes heavier, and falls. This last one is the one curtains deal with, and it is the one most fittings get wrong.
Before spending anything on curtains, check for draughts. Curtain movement can come from draughts, room air circulation or cooled air near the glass, so it does not identify a frame leak on its own. Have suspected draught paths checked and use suitable seals without blocking required ventilation or drainage. Treat window leakage and curtain fit as separate checks.
The cold air loop behind a curtain
When a curtain hangs in front of cold glass with a gap at the top, the space behind it becomes a small chimney running the wrong way. Air behind the curtain cools against the glass and sinks. It spills out under the hem and across the floor. To replace it, warm air from the ceiling area of the room is drawn in through the gap at the top, down behind the curtain, and cooled in turn. The curtain keeps the glass out of sight but keeps the heat loss going, which is why the floor near a curtained window can still feel cold.
YourHome's guidance is that close-fitting curtains with pelmets reduce this by trapping a layer of still air next to the window, and that air gaps around all edges of the curtain and pelmet should be eliminated. It also notes that heavier fabrics and multiple layers increase insulation, but that this benefit is reduced if air moves around the curtain. In other words, sealing comes first and fabric second.
Sealing the top: pelmets, ceiling tracks and recesses
The top gap is the one that matters most, because it is where warm room air enters the loop. There are three ways to close it.
A pelmet is a box or board fixed above the curtain and projecting past it, closing the space between the top of the curtain and the wall. Traditional pelmets are timber or upholstered, and they suit older homes where a box above the window looks at home. The pelmet needs to reach the wall and project beyond the front of the curtain, with its ends closed. A pelmet with open ends only moves the gap.
A ceiling-fixed track can reduce the top gap, but the finished heading and carrier clearance still matter. Ask to see how much space will remain above and behind the fabric; ceiling fixing alone does not create an airtight seal. It also makes the room look taller and the window wider. The tracks versus rods guide covers ceiling and wall fixing in more detail.
A recess or bulkhead built into the ceiling can hide the track and reduce exposed top gaps. Its depth and the finished heading position determine how closely it fits. It is a building decision, easiest at framing stage in a new build.
Rods are the hardest to seal. The curtain sits below the rod and the rod sits on brackets off the wall, leaving a gap both above and behind. A rod is fine on a window where warmth is not the priority, but on the coldest windows it works against a thermal curtain.
Sealing the sides: overlap and returns
Air also leaks around the sides. A curtain that stops flush with the window frame leaves a gap down each edge. Two changes help. First, extend the track past the frame on each side, so the curtain covers the wall as well as the glass when closed. That also lets the curtain stack off the glass when open, which lets in more winter sun. Second, add returns. A return is the end of the track or rod turning back to the wall, with the curtain following it around, so the side edge of the curtain touches the wall instead of standing off it. Where a return is not possible, a curtain made wide enough to wrap to the wall still closes most of the gap.
Where two curtains meet in the middle, an overlap arm or overlapping master carriers stop a vertical slot of cold air between them. It is a small part that is easy to leave off and it makes a noticeable difference in a draughty room.
The bottom edge
The bottom edge is where cold air leaves the loop. A curtain that stops at the sill, above a window that runs to the floor below, or that hangs well short of the floor, lets the cooled air pour out freely. For thermal performance, a curtain should run close to the floor. A small clearance is better than a hem pooling on the floor, because a dragging hem wears, collects dust and, against cold glass, can wick condensation. Our measuring guide covers floor clearance in detail.
There is one exception. If a heater or a heating vent sits under the window, a floor-length curtain can cover it and send the heat behind the curtain straight to the glass. In that case a sill-length curtain with a well-sealed top, or relocating the vent, is the better answer. Never let fabric rest on or close to a heater.
Linings and layers
Once the edges are sealed, fabric makes a real difference. An unlined curtain does little on its own. A standard lining adds a second layer and protects the face fabric from sun. A thermal lining has a coated or brushed back designed to slow heat flow. A blockout lining does a similar job and blocks light. Interlining adds a soft padded layer between face and lining and can add insulation, though the extra weight needs a strong track and a sewn heading such as pinch pleat.
Two layers on a double track, such as a sheer at the front and a lined curtain behind, trap more air than one. It is also a practical way to keep daytime privacy while leaving the lined curtain open for sun. The fabric guide compares face fabrics, and the blockout curtains page covers blockout and thermal linings specifically.
Existing curtains do not always need replacing. If the face fabric is sound, adding a thermal lining to a curtain you already own is one of the alterations that pays back in a cold room.
Window heat leak checker
Describe one window at a time and add it to your list. The checker gives each window a planning priority score out of 10 and lists fixes in order. This is a simple checklist ranking, not a measured heat-loss result or energy rating. Add each window to the saved list, then download the list in priority order. Changes to the current window do not edit an already saved entry; clear the list to rebuild it.
A daily routine for sun and cooler evenings
How you use the curtains matters too. In winter, open curtains on north-facing windows on sunny days. Sun through glass is free heat, and a closed curtain blocks it. Close every curtain before sunset, not after the room has already cooled. Once the glass is cold and the room has lost its warmth, a curtain slows further loss but cannot bring the heat back.
In the morning, open the curtains on windows that have fogged overnight and wipe the glass and sill. Condensation trapped behind a closed curtain all day is what leads to mould on hems and linings; our curtain cleaning guide covers what to do if it has already started.
In summer the routine reverses for west and north windows. Close them before the afternoon sun reaches the glass. A lined curtain with a light-coloured back reflects some of that heat, and a curtain that is closed early keeps the room cooler than one closed after the room has warmed.
Where to start if you cannot do every window
Few people replace every curtain at once. The best order is usually by how much time you spend in the room and how much glass faces the cold. Start with the rooms you heat, typically the living area and main bedroom. Compare the rooms at the time you use them. A shaded south-facing window may receive little useful winter sun, while a north-facing window can still lose heat after sunset. Large doors deserve attention because of their glass area, but do not assume every north window can wait. Balance evening comfort against daylight, ventilation and access.
On any given window, fix the cheapest leak first. Draught seals on the frame, then the top gap, then the sides, then the lining. It is common for an existing lined curtain to perform much better once it is moved to a ceiling track and extended past the frame, without new fabric at all.
Hypothetical example
A Koo Wee Rup household has a living room with a single-glazed south window and a floor-length lined curtain on a rod 150mm above the frame. The curtain stops at the frame edges. The checker flags the open top and short sides. Moving the same curtain to a ceiling-fixed track extended past each side, with returns, addresses both. The unlined curtains in a spare room with a north window stay as they are for now.
Mistakes and how to fix them
| Mistake | Why it fails | Remedy |
|---|---|---|
| Heavy thermal curtain on a rod | Open top feeds the cold air loop | Ceiling track or a closed-end pelmet |
| Curtain stops at the frame | Air flows round the sides | Extend the track and add returns |
| Pelmet with open ends | Air enters at the ends instead | Close the ends to the wall |
| Curtain over a heater | Heat goes to the glass, fire risk | Sill length, or move the heat source |
| Curtains closed after dark | Room has already lost its warmth | Close before sunset |
| Buying curtains before fixing draughts | Wind through the frame bypasses the curtain | Seal the window first |
Want the cold windows checked properly? At the free measure we look at the top, sides and hem of each window and tell you which ones need new curtains and which just need a better fit.
Thermal curtain questions
Do thermal curtains actually keep heat in?
They help, but the fabric is only part of it. Federal YourHome guidance says up to 40% of a home's heating energy can be lost through windows, and that curtains with pelmets work by trapping a layer of still air next to the glass. A thick curtain with an open top and short sides lets that air circulate, so much of the benefit is lost. Sealing the top, sides and bottom matters as much as the lining.
What is a pelmet and do I need one?
A pelmet is a box or board that closes the gap above the curtain. It stops warm room air being drawn down behind the curtain and cooled against the glass. A ceiling-fixed track or recess may reduce the gap, but its finished heading clearance still needs checking. It does not automatically give the same closure as a fitted pelmet.
Should thermal curtains touch the floor?
They should finish close to the floor so cold air cannot spill out underneath, but not drag, which wears the hem and can soak up moisture. A small clearance works well. On a window with a deep sill and a heater underneath, a sill-length curtain may be the safer choice so it does not cover the heater.
Is interlining worth it over a thermal lining?
In the coldest rooms, often yes. Interlining adds a soft padded layer between face and lining and makes the curtain noticeably heavier and warmer. It costs more and needs a sturdier track and a sewn heading. For most rooms a good thermal or blockout lining with a sealed top and sides gets most of the gain.
Do thermal curtains help in summer?
Yes, on west and north windows. Closing lined curtains before the afternoon sun reaches the glass keeps some of that heat out of the room. External shading stops more heat because it catches the sun before the glass, but a closed, well-fitted curtain still helps.
Sources: YourHome (Australian Government), Glazing; energy.gov.au, Windows.