When you replace one of your windows with double glazing, why doesn’t the house fall down? This is because there is a lintel above the window that holds up the bricks. This lintel has to go right the way through the wall. In an insulated cavity wall it holds up the insulation above the window, to stop it crashing down, and the blocks on the inside.
It is possible for a window lintel to simply be made from a sheet of steel, pressed into the necessary shape to fit into the wall. You can still buy steel lintels like this but they’re not very efficient. A lintel like this is a tremendous source of heat loss, as it consists of a large sheet of metal positioned over the top of the window going from inside to outside. In the days of energy conservation in new buildings, it is more common for a lintel to be made from steel, pressed into the necessary shape, with a block of insulation in the centre. This block of insulation usually has metal over the top, as you need something strong to keep the block of insulation from crunching due to the huge forces that are acting on it. An insulated lintel has rigid polymer (extremely strong plastic) over the top of the insulation block instead of metal. This means that there is no metal in the centre of the lintel at all. This is called a thermal break and prevents heat being conducted outside the room along the metal of the lintel.
The use of insulated lintels improves the performance of a house in a SAP test considerably. One of the problems with standard insulated lintels is that you can’t put them in a timber frame wall. SAPS4U can advise you how to position the insulation above the window in a timber frame wall to achieve the same effect as an insulated lintel.