How to Install Impact-Sound Underlayment
Impact-sound underlayment is the quiet layer under every floating floor – it damps footstep noise and evens out tiny irregularities. This guide walks through it step by step – and shows you how to get the exact floor area before you buy.
Work out your material first →
Underlayment is sold by the roll or pack, and unlike planks it produces almost no waste – you simply need the floor area. Draw your room in the free planner and it gives you the exact square feet for your actual floor – then follow the five steps below to install it.
Installing underlayment in 5 steps
Choose the right underlayment
Match the thickness and rating to your floor and subfloor, and pick a heating-compatible grade if you have underfloor heating. Unlike planks, underlayment barely wastes anything – the amount you need is simply the floor area, which the free planner reads straight off your room.
Prepare the subfloor
The subfloor must be clean, flat and dry. Sweep and vacuum it thoroughly – grit, crumbs or a raised nail telegraph straight through a thin underlayment and into the finished floor, so a spotless surface is well worth the few minutes it takes.
Lay the vapor barrier on a mineral subfloor
On screed or concrete, moisture can rise into the floor, so lay a vapor barrier first – or use a combined underlayment that already has the barrier built in. On a dry wood subfloor you can skip it and lay the underlayment on its own.
Roll it out butt-jointed, not overlapped
Roll the strips out edge to edge and tape the seams – never overlap them, because a double layer makes a ridge that telegraphs into the floor above. Run the strips perpendicular to the plank direction so the joints never line up.
Trim at the walls and install the floor on top
Cut the underlayment to the walls, leaving room for the expansion gap the floating floor needs. Then install the floor straight on top – the underlayment stays loose underneath, held in place by the weight of the planks.
Know your floor area before you shop
Underlayment barely wastes anything, so all you need is an accurate area. The free planner reads the exact square feet off your room, so you buy the right number of rolls or packs – no leftover, no second trip.
Open the free planner →Frequently asked questions
- What's the difference between impact-sound underlayment and a vapor barrier?
- The underlayment damps footstep noise and cushions small unevenness; the vapor barrier is a thin film that stops moisture rising out of a mineral subfloor into the floor. On screed or concrete you need both – often as one combined underlayment with the barrier already built in.
- Should I overlap or butt-joint the underlayment seams?
- Always butt-joint them edge to edge and tape the seam. An overlap makes a double-thick ridge that telegraphs up through the plank above and can even work the floor joints apart over time. Keep the whole layer flat and single-thickness.
- How much underlayment do I need to buy?
- Simply your floor area – underlayment produces almost no waste, unlike planks, so there's no need to add a waste margin. The free planner gives you the exact square feet for your room, which you round up to whole rolls or packs.