What kind of costs would I be looking at for cavity wall insulation in a 1940's terraced house? Its a 3 bedroom and an inner one of a block of four, (no gable end). How long to recoup the costs, (approximately). We have GCH and double glazing, (UPVC to rear, wooden to front). Loft is insulated and centre section T&G Boards for walking/storage. Is there a risk that if we get it done it will also spread sideways and partially insulate the houses on each side or are there barriers to prevent this? (Don't see why I should pay to help warm their houses!!)