For most floating floors, yes — but the honest answer has three parts most people never get told: some planks already have it attached, concrete needs a separate vapour barrier underlayment does not replace, and adding a second layer on top of either one is a common mistake that can void your warranty. This guide covers what underlayment actually does, the types, and how to read a sound rating if your building requires one.
What's in this guide
Do You Actually Need It?
Underlayment does three jobs: it cushions the floor underfoot, it dampens impact sound, and it evens out very minor subfloor texture. It does not fix a subfloor that is out of flatness tolerance — most manufacturers still require flatness within about 3/16 inch over 10 feet regardless of what underlayment sits on top, and levelling compound is the only real fix for that.
Whether you need to buy it separately depends on the product: floating floors (laminate, click-lock LVP, floating engineered hardwood) almost always need a cushioning layer of some kind, either purchased separately or built into the plank. Glue-down and nail-down installs use different products entirely, covered below. Still weighing which floor to buy in the first place? See our laminate vs vinyl plank vs hardwood comparison.
Types of Underlayment
| Type | Typical Thickness | Best For |
|---|---|---|
| Standard foam | 2–3mm | Laminate and LVP floating floors — the default choice |
| Combination foam + vapour barrier | 2–3mm | Floating floors over a concrete slab, in one product |
| Cork | 3–6mm | Sound reduction under laminate or floating hardwood, upper floors |
| Rubber | 2–5mm | Best sound ratings; common in multifamily buildings with IIC requirements |
| Felt / fibre | 3–6mm | Nail-down solid or engineered hardwood — not foam |
| Rosin paper | Thin sheet | Under nail-down hardwood, mainly to reduce squeaks, not cushioning |
Foam is the default for a reason — it is cheap, adequate for a typical single-family home, and does the job floating floors need. Cork and rubber cost more but are worth it specifically for noise, which foam does not solve well.
Underlayment by Flooring Type
- Laminate and click-lock LVP: foam, combo foam+vapour-barrier, or a pre-attached pad — never a felt or fibre product made for nail-down wood.
- Floating engineered hardwood: foam or cork, same logic as laminate.
- Nail-down solid or engineered hardwood: felt or rosin paper, not foam. Foam compresses too much under fasteners and nails driven through a compressible pad are more prone to squeaking over time.
- Glue-down hardwood or LVP: usually no separate underlayment — the adhesive itself is the bonding layer, though some manufacturers specify a membrane for moisture or sound control.
- Carpet: uses carpet pad, a different product entirely — typically 6–8 lb density and 7/16 inch thick for residential use, sized to the carpet manufacturer's warranty requirements rather than a generic flooring underlayment.
Pre-Attached Pad: When You Skip the Roll
Many mid-range and premium laminate and LVP lines have a thin foam or cork pad fused to the back of each plank. If yours does, you generally do not buy a separate roll — check the product's installation instructions to confirm, since this varies by manufacturer and even by product line within the same brand. Our laminate installation guide covers exactly where this fits into the full install sequence.
Check whether your product needs a separate roll
Open the Laminate Flooring Calculator →Vapour Barriers Over Concrete Slabs
Standard foam or cork underlayment is not a moisture barrier by itself. Over a concrete slab — on-grade, above-grade, or below-grade — almost every manufacturer requires a 6-mil polyethylene vapour barrier under the underlayment, unless you are using a combination product with one already bonded in. This applies even to a slab that looks and feels completely dry, because concrete wicks ground moisture upward continuously.
Before you install, run a moisture check. A DIY indicator: tape a 2 ft square of plastic sheeting to the slab, seal the edges, and check after 24 to 48 hours for condensation. For a proper install, especially over a slab with any history of dampness, manufacturers often specify a calcium chloride test (ASTM F1869) or an in-situ relative humidity probe (ASTM F2170) — both are professional tests with published pass/fail thresholds in the manufacturer's installation guide.
Why Doubling Up Is a Mistake
Stacking a separate underlayment roll on top of planks that already have a pre-attached pad — or stacking two layers of vapour barrier and underlayment not designed to be combined — is one of the most common and easily avoidable installation errors.
- It over-cushions the joints. Click-lock connections are engineered for a specific amount of give. Too much softness underneath lets the joints flex more than designed, which accelerates wear and can cause separation at the seams over time.
- It feels spongy underfoot and can create a hollow sound when walked on, which reads as a defect even if nothing has technically failed.
- It can void the manufacturer's warranty outright, since doubling up exceeds the documented installation spec the warranty is conditioned on.
Check the box and the installation sheet before you buy a separate roll — it is a five-minute check that avoids a floor that feels wrong for its entire lifespan.
Sound Ratings: IIC and STC Explained
If you live in or are installing flooring for a condo, apartment, or any multi-unit building with a floor over occupied space below, the HOA or building code very likely sets a minimum sound rating you have to meet.
- IIC (Impact Insulation Class) measures impact noise — footsteps, dropped objects — transmitted through the floor-ceiling assembly. Higher is better.
- STC (Sound Transmission Class) measures airborne noise — voices, music. Also higher is better, and it is a separate number from IIC.
- A common minimum in multifamily buildings is IIC 50 field-tested (or 45 lab-tested) under the International Building Code, though many HOAs set their own, often stricter, requirement in the condo documents.
- Basic 2–3mm foam typically tests around IIC 30–45 as part of a full assembly — usually not enough to clear a 50+ requirement on its own.
- Rubber and denser cork underlayments, properly specified as part of a tested assembly, commonly reach IIC 55–72 depending on the product and the floor construction underneath.
Check your condo or HOA documents first, and buy an underlayment with a published, third-party tested IIC/STC number for a matching assembly — not just a marketing claim. Ratings are assembly-specific, meaning the same underlayment can test differently over a concrete slab versus a wood joist floor. Get written confirmation the specific product meets your required number before installing.
How Much to Buy
Underlayment rolls are commonly sized around 100 square feet per roll, though some products come in different roll sizes — check before ordering. Buy enough full rolls to cover your total floor area, rounded up, plus a small allowance (around 5%) for seam overlap and trim waste. Combination vapour-barrier products are sized the same way.
Get the exact area and roll count for your room
Open the Flooring Calculator →Frequently Asked Questions
Do I need underlayment under vinyl plank flooring?
Usually yes for click-lock LVP, unless the planks have a pre-attached pad on the back — check the product specs first. Glue-down and loose-lay LVP typically skip a separate underlayment.
What is the difference between underlayment and a vapour barrier?
Underlayment cushions and dampens sound; a vapour barrier blocks moisture from a concrete slab reaching the flooring above. They are different jobs done by different (or combined) products — standard foam underlayment alone is not a moisture barrier.
Can I use two layers of underlayment?
It is not recommended. Doubling up — a separate roll on top of a pre-attached pad, or two unrelated layers stacked — over-cushions click-lock joints, can make the floor feel spongy, and often voids the manufacturer's warranty.
Does laminate flooring need underlayment?
Almost always, yes, unless the planks already have a pre-attached pad. Underlayment cushions the floor, dampens sound, and evens out very minor subfloor texture — check the manufacturer instructions to see if yours is built in.
What underlayment is best for reducing noise?
Rubber or dense cork outperform standard 2–3mm foam by a wide margin, commonly testing around IIC 55–72 as part of a tested assembly versus roughly IIC 30–45 for basic foam. If a building sets a minimum sound rating, confirm the specific product meets it — the number is assembly-specific.
Do I need underlayment over a concrete slab?
Yes, and specifically a 6-mil polyethylene vapour barrier under whatever cushioning underlayment you use, unless you buy a combination product with one built in. This applies even if the slab feels completely dry, because concrete wicks moisture upward continuously.
What is IIC and why does my condo require it?
IIC (Impact Insulation Class) measures how much impact noise — footsteps, dropped objects — passes through a floor-ceiling assembly to the unit below. Many multifamily buildings and HOAs set a minimum, often IIC 50, to limit noise complaints between units.
Can I use foam underlayment under nail-down hardwood?
No — nail-down solid or engineered hardwood should use felt or rosin paper, not foam. Foam compresses too much under fasteners, and nails driven through a compressible pad are more prone to squeaking as the floor settles.
How much underlayment do I need to buy?
Enough full rolls to cover your total floor area rounded up, plus roughly 5% extra for seam overlap and trim waste. Rolls commonly cover around 100 square feet each, but check the specific product before ordering.
Does pre-attached pad count as underlayment?
Yes — if your planks have a fused pad on the back, that is the underlayment, and buying a separate roll on top is the doubling-up mistake to avoid. Always confirm in the product's installation instructions rather than guessing by feel.