Thermal curtains and a Pakenham winter.

The convection loop, properly explained.

On a cold Pakenham night the glass in your window sits close to the outside temperature. In the hills around Pakenham Upper and Emerald, overnight minimums near freezing are normal, so that pane can be the coldest surface in the house by a long way.

Here is what happens next. Warm air from your heater rises and drifts along the ceiling. It reaches the window and touches the cold glass. It gives up its heat to the pane, becomes cooler and denser, and falls straight down the glass. It lands on the floor and spreads across the room as that familiar cold draught around your ankles. And because air has left the top of the window, more warm air is pulled across the ceiling to replace it, where it will be cooled in turn.

That is a loop, and it runs continuously, all night, for as long as there is a temperature difference. It is not a slow leak through the glass. It is a moving circuit that actively carries the heat you paid for over to the coldest surface in the room and dumps it there. Understand that and everything else about thermal curtains follows.

A curtain is an air barrier, not a blanket.

People assume a thermal curtain works like a jumper, by being thick. It does not. It works by trapping a still layer of air against the window and stopping the circuit. Still air is a decent insulator. Moving air carries heat away. So the entire value of a thermal curtain lies in whether it stops the air moving, which means it has to close the loop in three places.

1. The top.

This is the one almost everyone gets wrong. A curtain on an exposed rod with a gap above it is an open chimney. Warm ceiling air pours straight over the top, down the back of the curtain, across the cold glass, and out at the bottom. The fabric is not in the circuit at all. Sealing the top means either a pelmet across the head of the curtain, or fixing the track hard against the ceiling or a plasterboard bulkhead so there is no gap to pour through. See pelmets explained for the options and what each costs.

2. The sides.

A curtain that simply ends in mid air at each side lets the falling cold air slip around the edge and straight into the room. What stops it is a return: the track is bent back to the wall at each end so the curtain carries around and physically meets the wall. On a pair of curtains you also want an overlap arm in the centre so the two leading edges cross rather than butt together. Returns and overlaps cost very little and they are the difference between a sealed window and a decorated one.

3. The floor.

Cold air that has fallen down the glass has to go somewhere. If the curtain stops at the sill, it pours out underneath and across the floor exactly as if the curtain were not there. Floor length, just brushing the carpet or hard floor, is what closes the bottom of the circuit. It costs more fabric and it is the least negotiable part of the specification for a room you want to be warm.

Lining types compared for warmth.

• Unlined. No real thermal contribution. Fine for a sheer layer, not a warmth solution.
• Standard lining. Adds a second layer and a small trapped air gap. Modest benefit, protects the face fabric from UV.
• Thermal or coated lining. A denser backing that resists heat transfer better than standard lining. Adds $60 to $180 per window and is usually the best value upgrade on a quote.
• Interlining, or bump. A soft flannel layer sewn between face fabric and lining. Adds genuine insulating bulk plus noticeable sound deadening. $600 to $1,400 per window all up. Heavier, so the track and fixings must suit, and it needs more stack space.

The full lining guide compares these on weight, drape, cost and the condensation trade off.

Realistic expectations.

A properly sealed, floor-length, thermally lined curtain can noticeably reduce how fast a heated room cools overnight, and it removes the cold draught coming off the glass, which changes how the room feels more than the thermometer suggests. What it will not do is turn a single-glazed window into a double-glazed one, or fix a house with gaps around the frames, poor ceiling insulation or an undersized heater. Be sceptical of anyone quoting you a specific percentage saving for a curtain, because the honest answer depends on your glass, your heating, your ceiling, how long you run it and how well the curtain is sealed.

The condensation trade off.

Sealing a curtain properly keeps more heat in the room, which means the glass behind it runs colder overnight, which means more moisture can condense on the pane and the reveal. In tighter new estate homes around Officer and in damp hills houses this is a real effect, not a theoretical one. The answer is to open the curtains during the day so the reveal dries, keep some ventilation in the wet rooms, and pick the lining to suit each room rather than putting the heaviest coated lining on every window in the house.

Summer works the other way.

In a Pakenham summer, with stretches above 35 degrees and harsh northerly and westerly sun, the job flips. Now you are trying to stop radiant heat getting in. A light-coloured lining that reflects rather than absorbs, closed early in the morning before the sun swings around, keeps a lot of that load out of the room. Closing it once the room is already hot is far less useful. For the worst-hit west-facing rooms a blind behind the curtain handles the direct sun better than fabric alone.

Frequently asked questions.