Used carpet underfelt is discarded and replaced by foam underlay when old carpets are replaced.    From Wordnik.com.  [NEW IDEAS FOR GREEN INSULATION | Inhabitat] Reference

There was a roll of carpet underfelt in the room with the stored furniture and he spent the rest of the afternoon cutting it into strips and tacking it round the windows and doors to keep the draught out.    From Wordnik.com.  [Cal]

December 31, 2007 at 11:17 pm evy nite sins teh ICHCB begun, iz be reedin an lovin dis place, but dis B teh fursty time iz achually has post cuz iz wants to saiz how wunnerf…underfelt…wuh..grooovy an chahmin B teh tale ov klules kitteh. joo rule demon KTHXBAI.    From Wordnik.com.  [a fud is eatn mah fud - Lolcats 'n' Funny Pictures of Cats - I Can Has Cheezburger?] Reference

They roost visibly in open spaces, hanging from roof beams or underfelt.    From Wordnik.com.  [WalesOnline - Home] Reference

Giuseppe Tornatore trudges out his usual devices in the overlong and underfelt Baaria, in which three generations of Sicilians brave the historical dust of the 20th century.    From Wordnik.com.  [Bangkokpost.com : Breaking News] Reference

Elizabeth Cloete, a descendant of the Khoi herders who once roamed the parched plains and granite outcrops that give the region its distinctive character, lives under an assemblage of plastic tarpaulins and old carpet underfelt draped over precariously balanced zinc sheets.    From Wordnik.com. Reference