Underlay: when you need it, when it’s forbidden, and what voids the warranty

Underlay is the cheapest layer in a floor and the fastest way to void the warranty on the expensive one. It is also the layer with the most contradictory advice attached, because the word names four products doing four jobs: a cushion, a vapour retarder, an acoustic layer and a levelling aid. The one that is right for one job can be actively wrong for another.

So the first question is not “which underlay”. It is: does this floor take one at all, what is the subfloor, and what does the installation guide permit.

The installation method decides before you do

Our catalogue records installation methods on 13,393 products, carrying 20,854 method tags across 8 distinct methods. Each of them answers the underlay question differently:

Two caveats. Those counts describe products, not projects, and one product can carry several tags — a click-lock plank is also a floating floor — so the rows overlap rather than partition. And they count only listings that publish a method, a fraction of a catalogue of 118,651 items.

Attached pad: the most common expensive mistake

Many rigid-core planks ship with a pad already bonded to the back, usually IXPE or EVA foam one to two millimetres thick. Laying a second pad underneath is probably the most common warranty exclusion in the trade, and the reason is mechanical, not bureaucratic. A locking joint is engineered against a maximum compressible thickness beneath it; exceed it and the joint flexes with every footfall, the tongue works and then shears, and the floor gaps and peaks in a way that looks like a product defect and is not covered as one.

The nuance people get wrong in the other direction: a pad is not a vapour retarder. Over concrete you may still need one unless the attached pad is explicitly film-backed and rated for it — a line in the installation guide, not a guess.

Concrete, and the moisture nobody tests

Concrete never entirely stops giving up moisture, and a slab that feels bone dry can read high on a probe. This is the failure that writes off whole floors a year later, and the cheapest to rule out first.

• Test rather than assume. In-situ relative humidity probes to ASTM F2170: three tests for the first 1,000 sq ft, one more per additional 1,000, with the slab at service temperature and humidity. The result is a number you can hand an installer and attach to a claim.
• Six-mil polyethylene is the baseline. Lapped 6–8 in at the seams, taped, turned up the wall behind the baseboard. If the guide gives you a maximum permeance instead, match the perm rating, not the mil count.
• Never sandwich two barriers. A retarder above and below an absorbent layer traps moisture where it cannot dry in either direction. One barrier, on the concrete side.
• A new slab is not a dry slab, and nail-down is off the table. Fresh concrete needs a long dry-down before it passes a probe, and the calendar is not the test. A slab means glue-down, floating, or a sleeper or panel subfloor built first.

Basements and below-grade slabs

A Canadian basement slab is cold, sits below grade in soil whose moisture changes with the season, and in a bad week gets wet. The flooring question there is really a build-up question.

• Dimpled air-gap panels give a thermal break and a drying path, at the cost of roughly 3/4 in to 1 in of height before the flooring starts — height that comes out of your door undercuts and your bottom stair riser, which is what people discover last.
• The site-built version — poly, rigid foam, plywood — costs less in material and far more in labour, and must be detailed properly at the perimeter to be worth doing.
• A floating floor on a film-backed pad, straight on the slab, is the low-build-up option. Legitimate, and cold: comfort underfoot in a basement comes from the assembly, not the plank.
• Below-grade approval is written per product. It is a manufacturer statement about a specific floor, not a property of a category. If the technical sheet does not say so, ask before ordering.

Acoustic underlay: what you are actually buying

Two different noise problems want different products. One is the sound you hear — the hollow click of a floating floor, mostly a question of how well the plank is supported. The other is what your neighbour hears through the structure: mass, resilience and edge isolation. A thick soft foam can improve the first and make the joint worse; a dense rubber mat does far more for the second and feels like nothing underfoot.

In a condo none of this is a matter of taste — the board sets the requirement, and that requirement belongs to the whole floor-and-ceiling assembly rather than to the underlay. Its own subject, and we have written it up separately.

Thickness is a tolerance, not a preference

The instinct is that a thicker underlay must be a better one. For a click floor it is closer to the opposite: the joint is designed against a maximum compressible thickness and a minimum compressive strength, and thicker-and-softer is exactly what it cannot tolerate.

• Rigid core (SPC / WPC) click floors commonly specify 1–1.5 mm maximum, high density — where a separate underlay is permitted at all.
• Laminate and floating engineered hardwood commonly specify 2–3 mm.
• Carpet underpad: never. It is soft by design, and softness under a locking joint is the failure mode, not the feature. This is the most reliable way to have a claim denied.

Two specs matter more than the millimetres: compressive strength and density. A 3 mm high-density fibre board and a 3 mm open-cell foam are not the same product. A datasheet with no compressive figure is a consumer convenience item, not a system component.

Over radiant heat there is a third number: thermal resistance. Floor plus underlay together usually has to stay under about R-1.0 — roughly 0.18 m²·K/W — or the layer you added fights the heating system you already paid for. Look for a stated R-value or TOG and add the floor’s own.

The mistakes that actually void warranties

1. Adding a pad under a plank that already has one bonded to it.
2. Using an underlay the installation guide excludes by name or by type — carpet underpad under laminate, rubber against a vinyl that prohibits it.
3. No vapour retarder over concrete, or the wrong one — the most commonly denied claim in the trade, because the damage appears a year later and the slab test that would have proven the cause was never done.
4. Two vapour retarders with something absorbent between them.
5. Closing the expansion gap. Underlay does not rescue a floor pinned at the perimeter — and in an acoustic install, a baseboard nailed through the floor bridges the isolation you paid for.
6. Lapping underlay seams meant to butt, or leaving grit beneath — a permanent crunch only lifting the floor will fix.

A ten-minute checklist before you order

1. Read the installation guide for the exact product — not the collection — first.
2. Turn a sample plank over. Attached pad, or not?
3. Identify the substrate: plywood, OSB, slab on grade, below-grade slab, or a floor you plan to cover.
4. If it is concrete, test it to ASTM F2170 and write the number down. If it is a condo, get the board’s acoustic requirement in writing first.
5. Add up the whole build-up and check your doors, stair risers and appliance clearances.
6. Put the underlay on the same order as the floor — it is bulky, light, and a whole second freight rating if it travels alone.

Shop what this guide covers

• underlayment — foam, fibre, cork and rubber, with the density on the datasheet.
• vinyl planks (LVP and SPC) — check the back of the plank for an attached pad before you buy anything for under it.
• laminate planks — the 2–3 mm case, and the one carpet underpad ruins.
• electric floor heating — where the third number, thermal resistance, starts to matter.
• STC and IIC for condo boards — the requirement that is set by the assembly, not by this layer.