From Longman Dictionary of Contemporary Englishunderfeltun‧der‧felt /ˈʌndəfelt $ -ər-/ noun [uncountable] British EnglishDHH soft material that you put between a carpet and the floor
Examples from the Corpus
underfelt• Together with thick carpet and underfelt, it should go a long way to pleasing the occupants of the flat below.• It is loose laid on underfelt like an enormous rug with a surround of polished wooden floors. 