We have a loft extension that was built before we bought the place. It
is freezing. There is no insulation on the interior of the external
walls (just plaster on brick). The loft has standard loft insulation -
the stuff you can buy in a roll - to a depth of approx 6 inches. We
need to add extra insulation and were planning to put thermal
insulation boards up on the walls. Is it worth adding extra insulation
to the roof space?