underfelt vs underlayment

underfelt and underlayment both are nouns.

Word	Noun	Adjective	Verb	Adverb
underfelt	Yes	No	No	No
underlayment	Yes	No	No	No

As nouns, underlayment is a hypernym of underfelt; that is, underlayment is a word with a broader meaning than underfelt:

Other hypernyms of underfelt include carpet pad, rug pad, underlay.

underfelt (noun)	underlayment (noun)
a carpet pad of thick felt	a pad placed under a carpet