Underlayment
A thin layer installed between the subfloor and the finish flooring for cushioning, sound dampening, or moisture protection.
Underlayment is anything that sits between the structural subfloor and the visible finish flooring. Common types include foam (LVP click-lock), cork (engineered hardwood, sound dampening), and felt (carpet). Some products ship with attached underlayment built in.
Choosing the right underlayment matters for moisture management, foot comfort, and warranty coverage. Concrete slab installs almost always need a vapor barrier underlayment to keep slab moisture out of the finish flooring. Multi-family installs often need an IIC/STC-rated underlayment for sound transmission compliance.
Always include underlayment as a separate line item on your estimate — it's a real cost and customers should see it itemized so you don't get pressured into "absorbing" it.