Underlay & Moisture Barriers
Underlay and moisture barriers solve two different problems that often get confused. Underlay is about comfort and sound — it cushions a floating floor underfoot and quiets footsteps, which matters most in a condo or a second-storey room. A moisture barrier is about protection — it blocks water vapor from wicking up through a concrete slab and warping or clouding the new floor from underneath. Some subfloors need one, some need both, and getting it backwards (or skipping the one you need) is one of the most common flooring-failure causes we see.
The rule of thumb: floating laminate over a wood subfloor above grade usually just needs a standard underlay. Floating laminate, vinyl, or engineered wood over a concrete slab — especially below grade, in a basement, or over radiant heat — needs a real moisture barrier as well, either as a separate poly film or an underlay roll with one built in. Vinyl with a pad already attached to the back skips underlay entirely. The table below is the short version; every roll on this page lists its coverage so the calculator can size exactly how many you need.
Which underlay or barrier for which floor?
| Floor & subfloor | Use |
|---|---|
| Laminate over wood subfloor, standard room | 2.5mm white foam underlay |
| Laminate in a condo (sound requirements) | 3mm black acoustic underlay (IIC 72/STC 73) |
| Laminate or engineered wood over concrete / below grade | 5mm airflow bubble underlay, or standard underlay + 6-mil poly barrier |
| Vinyl with no attached pad | Thin vinyl-rated underlay (AcoustiTECH LV) — skip if plank already has a pad |
| Nail-down solid hardwood | Rosin/wax paper (not a foam underlay) |
Underlay & Moisture Barriers
Sold per full roll. Underlay cushions and quiets a floating floor; a moisture barrier blocks vapor from a concrete or below-grade subfloor.
Our pick — quieter underfoot with a built-in moisture barrier. Best for condos, upper floors & basements. IIC 72 / STC 73.
1 roll covers ~200 sqft → 500 sqft needs 3 rolls
Budget standard laminate underlay. IIC 52 / STC 66.
1 roll covers ~200 sqft → 500 sqft needs 3 rolls
Max moisture protection — dimpled air-gap for concrete/basement subfloors. IIC 72 / STC 73.
1 roll covers ~100 sqft → 500 sqft needs 5 rolls
Sound mat for click-LVP — condo-board grade acoustic rating.
1 roll covers ~300 sqft → 500 sqft needs 2 rolls
Foam + moisture layer for floating vinyl and laminate.
1 roll covers ~100 sqft → 500 sqft needs 5 rolls
Premium foam with built-in vapour barrier — the laminate upgrade pad.
1 roll covers ~100 sqft → 500 sqft needs 5 rolls
Basic foam pad, no vapour layer — pair with poly on concrete.
1 roll covers ~100 sqft → 500 sqft needs 5 rolls
Sound-absorbing acoustic mat for engineered hardwood and laminate.
1 roll covers ~269 sqft → 500 sqft needs 2 rolls
Subfloor vapour barrier under floating engineered floors on concrete.
1 roll covers ~120 sqft → 500 sqft needs 5 rolls
Rosin/wax paper under nail-down solid hardwood. 750 sqft roll.
1 roll covers ~750 sqft → 500 sqft needs 1 roll
How-to guides
FAQ
What underlay do I need for laminate flooring?
Floating laminate almost always needs a separate underlay unless the plank has a pad already attached to the back or is a thick 14mm board. In a condo, use the 3mm black acoustic underlay (rated IIC 72/STC 73) to clear sound-transmission requirements; over concrete or below grade, step up to the 5mm airflow bubble pad or add a poly moisture barrier underneath.
Do I need underlay under vinyl plank flooring?
Usually not — most vinyl (SPC/LVP) sold today has a pad already attached to the back, and adding a second pad (pad-on-pad) can feel spongy and void the flooring warranty. Only bare vinyl with no attached pad needs a thin underlay; check your plank spec or ask us.
What moisture barrier thickness do I need over concrete?
6-mil poly (or thicker) is the standard minimum most independent flooring guides and manufacturers point to for laminate, SPC, and engineered wood over a concrete slab. Thinner film isn’t considered a real vapor barrier, and skipping it on a slab is a common cause of a floating floor cupping or failing early.
Do I need both an underlay and a separate moisture barrier?
It depends on the product. Our 6-mil poly moisture barrier (Roberts Moisture Barricade) is a standalone film that goes under laminate or engineered wood over concrete. Some of our underlay rolls (like Roberts First Step / Unison "2-in-1" or "3-in-1") already combine a cushioning pad with a built-in moisture layer — those replace both products in one roll. Check the product spec or ask us which your subfloor needs.
What goes under nail-down solid hardwood?
Rosin/wax paper (not a foam underlay) goes down first, stapled to the plywood subfloor, before every nail-down solid hardwood install — it reduces squeaks and blocks moisture wicking from the subfloor. This is a required line item on every solid hardwood Install Kit on our product pages.