Curtains and Heat: Cutting Window Heat Loss

A curtain creates an air layer between window and room, but how much it helps depends on how it hangs. Practical rules for keeping heat in during winter and blocking sun in summer.

Sitting by a window on a winter evening, you do not need it to be open to feel the chill. The glass is colder than the room air; the air next to it cools, sinks and spreads across the floor. Curtains can soften this considerably, but only if they are chosen and hung correctly. A badly hung heavy curtain can even channel a radiator's warmth onto the glass and make the room colder.

How curtains hold on to heat

Heat escapes through a window in several ways: by conduction through the glass and frame, by air circulating against the cold glass, and by air leaking through gaps in the frame and seals. A curtain creates a relatively still layer of air between the glass and the room and reduces direct contact between cold glass and room air.

For that layer to work, air must not circulate freely behind the curtain. If there is an open gap above it and a large gap between hem and floor, a loop forms: warm room air enters at the top, cools against the glass and flows back into the room at the bottom. Such a curtain may look heavy, but thermally it does little.

Rules for hanging curtains effectively

Close off the top: a ceiling-mounted track, a concealed niche or a pelmet over the pole stops air flowing in from above.
Make the curtain wider than the window opening so it overlaps the wall at the sides and cold air cannot escape at the edges.
Let the curtain finish close to the floor or sit snugly at sill level, without a large gap.
Make sure the two panels overlap in the middle.
Add a lining to thicken the fabric layer. Thermal linings and tightly woven fabrics give better results.

When the radiator is under the window

This is the most common mistake. A heavy curtain falling to the floor in front of a radiator directs warm air into the narrow space between curtain and glass. Part of the heat goes straight to the cold glass before it ever reaches the room.

There are several ways round this:

a sill-length curtain, or a roller or Roman blind close to the window;
a two-layer set-up: a layer close to the glass that stops at the sill, plus a long curtain that is drawn only in the evening and leaves the radiator clear during the day;
if the sill is deep enough, letting it deflect the warm air rising from the radiator out into the room.

Cellular blinds: a special case

Cellular or honeycomb blinds are made of two layers of fabric that form air pockets shaped like honeycomb in cross-section. These pockets hold an air layer within the blind itself. Fitted snugly into the window opening with side channels, they perform better thermally than an ordinary roller blind. They are also made for roof windows and large glazing.

Daily habits

Curtains only help if they are used well:

in winter, open curtains on south- and west-facing windows on sunny days so the sun's warmth can come in;
close them as it starts to get dark, since losses rise at night;
in summer, keep curtains drawn during the day on the sunny side;
light-coloured curtains or those with a reflective lining send some of the summer sun's heat back out.

One point about summer heat is important: a curtain on the inside meets the sun after it has already come through the glass, so some heat is trapped in the room anyway. External shades, shutters or awnings block heat before it reaches the glass, which is why they work better in hot climates.

Condensation and what curtains cannot do

In cold weather, glass behind tightly closed curtains gets even colder, because warm room air reaches it less. If indoor humidity is high, condensation may form on the glass and at the bottom of the frame. Opening the curtains in the morning, airing the room briefly and keeping moisture sources under control prevent the problem. If water droplets on the glass are persistent and the edges of the frame are going black, look for the cause in humidity and ventilation rather than in the curtains.

Curtains do not make up for a poor window either. If air leaks through the frame, the seals have worn out or there are gaps where the window meets the wall, those problems must be solved first. A curtain is most useful as an extra layer over a well-installed window.

Because Zaferoğlu İnşaat's renovations run through to the finished interior, windows, radiators and curtain recesses can be considered on one plan. When the radiator position, curtain niche and window sill are planned together, curtains become part of a comfortable thermal environment rather than decoration alone.

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