The best replacement for underlay is underlie.

The best alternatives to underlay are rated as following:
★★★★★  underlie
★★★  rubber better
★★★  rubber
★★★  foam
★★  mattress protector
★  tredaire
★  goblin glider
  overlay
  halftone

Your alternative for underlay is not listed? Suggest!
Speaking about underlay, one probably shouldn't forget about felt backed carpet.

Other search terms related to underlay and their ratings are:
★★★★★ felt backed carpet
★★★★★ carpet tiles
★★★★ lino
★★★ laminate flooring
★ carpets
You have in mind a companion for underlay and it is not yet here? Propose!



Google Trends

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underlay
underlie
rubber better