Radiators are best placed under windows because the rising warm air from the radiator counteracts the cool air that tumbles down from the cold window. Radiators are designed with a large surface area so they dissipate heat from the water inside. This warm surface then radiates heat to the rest of the room.

Anything that impedes that transfer of heat, such as a radiator cover, will diminish the performance of the radiator. Also, long curtains that cover the radiator are bad news as they tend to trap the heat in the space between the curtain and window. So the boiler will need to work harder to warm the room.

Maybe you should replace the radiators with better looking versions that you can happily leave uncovered. This is an area of design which has blossomed in the last few years and there is now a huge range of radiator styles available, from ornate column to slab fronted, from the sculptural to the geometric. All in all, replacing the radiators may be no more expensive than installing radiator covers, and will certainly save on your energy costs in the future.