Curtains and Radiators

by Jamie on March 8, 2010 · 3 comments

This topic came up tonight: what do you do with a curtain hanging over a radiator? And why are radiators sited under windows anyway?

Back in the day, radiators were located under windows for a counterintuitive, but valid reason, although it has more to do with comfort than energy efficiency.

Imagine a room with single glazing and a radiator on the opposite wall. The air beside the window will be cooled and will sink as it becomes denser. At the opposite end of the room the air is warmed by the radiator and will therefore rise.

These two effects combine to circulate the air around the room, but this circulation will draw cold air from the window down into the centre of the room where you are sitting.

Stick the radiator under the window and it will warm the air beside the window, causing it to rise into the room. At the other end of the room it will descend, but it should be at a warmer, more comfortable temperature. It’s worth noting that with double glazing, this isn’t really a problem and you can locate your radiator wherever is most convenient.

So radiators are usually found under windows which have curtains which, more often than not, hang down below the top of the radiator.

Having them hang in front of the radiator is a big no no. All that lovely heat will rise up behind the curtain, right against the single glazing and will be trapped there before leaking out to warm the outside air.

Tucking them behind the radiator can also be counterproductive. Radiators work best when they have a free and unobstructed flow of air around them, helping to set up the convection* currents that circulate warm air around the room. For this reason you definitely want to avoid radiator shelves and radiator cabinets.

If you take a look at a typical radiator, you’ll see there are the thick bits through which the hot water flows, and thinner fins that transfer the heat to the air. It’s these fins that you need to keep clear and unobstructed, and tucking a curtain around them will reduce their effectiveness.

So what’s the solution? Cut the curtains so that they hang a bit above the radiator and then find some way of holding them flush against the wall. Hooks? Velcro? Elastic? Whatever works best. Just make sure that as little hot air as possible escapes behind the curtain and as much air as possible can flow around the radiators.

By holding the curtain flush against the wall, you will not only prevent warm air from rising behind the curtain, but you’ll also trap a layer of air there. Air is a fantastic insulator, as long as it is kept still, so if you can keep the air between the curtain and window  as motionless as possible, you’re on to a winner.

* radiators are really badly named – they’re convectors, not radiators; but radiators they shall be known as for ever and ever…

{ 3 comments }

1 petek April 7, 2011 at 10:33 pm

I understand that having a short curtain will be the best thing in terms of airflow. However, short curtain will look like a man in business suit but with short trousers.

2 Jamie May 9, 2011 at 9:18 pm

All the rage in Bermuda!

3 peterliv72 October 26, 2013 at 5:05 pm

What about victorian cast-iron style radiators? Do they convect air in the same way as modern radiators and would you be able to tuck curtains behind them without any negative effects?

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