The window shutter specialists at California Shutters claim it can take up to half the time to heat a room if you have a radiator placed next to a window - and it only takes two seconds ...
And it won't cost a lot. £1, in fact, could be all you need. The team at The Underfloor Heating Store is bigging up the £1 tin foil technique - although an inverted, cleaned crisp packet might work ...