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) |
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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.
Quieter underfoot with a built in moisture barrier, 200 sq ft per roll. Best for condos, upper floors and basements under laminate or vinyl. IIC 72 and STC 73.
1 roll covers ~200 sqft → 500 sqft needs 3 rolls
The dimpled air-gap underlay for basements and concrete slabs — lifts laminate, click vinyl or engineered wood off the slab so moisture can evaporate instead of soaking into the floor. Built-in vapour barrier, thermal break and sound control in one layer.
1 roll covers ~100 sqft → 500 sqft needs 5 rolls
Budget standard underlay for laminate floors, 200 sq ft per roll. IIC 52 and STC 66 for basic sound control.
1 roll covers ~200 sqft → 500 sqft needs 3 rolls
Dimpled air-gap underlay for max moisture protection over concrete or basement subfloors, 100 sq ft per roll. Works under laminate or vinyl. IIC 72 and STC 73.
1 roll covers ~100 sqft → 500 sqft needs 5 rolls
Sound mat for click-LVP floors, condo-board grade acoustic rating. Sold in 150 sq ft and 300 sq ft rolls.
1 roll covers ~300 sqft → 500 sqft needs 2 rolls
Natural cork roll underlayment for laminate, engineered hardwood, and vinyl floors, cutting impact sound and adding cushioning. Choose 1/8 inch or 1/4 inch thickness.
Combined foam and moisture barrier underlayment roll for floating vinyl and laminate floors.
1 roll covers ~100 sqft → 500 sqft needs 5 rolls
Sold-by-the-sheet cork underlayment for laminate, engineered hardwood, and vinyl floors, good for patching rolls or small rooms. Available in 6 mm and 12 mm thickness.
2 mm foam underlayment with a built-in vapour barrier for laminate and engineered hardwood floors. Sold in 100 sq ft and 630 sq ft rolls.
1 roll covers ~100 sqft → 500 sqft needs 5 rolls
Basic foam underlayment for laminate floors. Pair it with a poly moisture barrier when installing over concrete.
1 roll covers ~100 sqft → 500 sqft needs 5 rolls
Sound-absorbing acoustic mat for engineered hardwood and laminate floors, sold in a 269 sq ft roll.
1 roll covers ~269 sqft → 500 sqft needs 2 rolls
6-mil poly sheeting laid under floating engineered and laminate floors on concrete subfloors to block moisture from below.
1 roll covers ~120 sqft → 500 sqft needs 5 rolls
Rosin/wax paper under nail-down solid hardwood. 750 sq ft 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.