Underlayment 101: When the Attached Pad Is Enough (and When It Isn't)

Underlayment is where flooring projects quietly leak money in both directions. Some people buy a pad they didn't need because the floor already had one bonded to it. Others skip a vapor barrier because their floor is waterproof, which is the more expensive mistake of the two.

The confusion comes from treating underlayment as one product. It isn't. It's up to four different jobs, and your attached pad handles some of them and none of the others. Here's how to tell which is which.

Quick Answer

If your floor has an attached pad, you usually don't need to add anything - with one firm exception. Over a concrete subfloor, a 6-mil vapor barrier is required regardless of what's attached to the plank, because a cushioning pad is not a moisture barrier and a waterproof floor doesn't stop moisture from reaching the subfloor beneath it. Add cork underlayment only when you want acoustic performance beyond what the attached pad delivers, typically for condos with HOA sound requirements or upstairs rooms. And never stack pads beyond the manufacturer's stated thickness - on McMillan floors, separate underlayment tops out at 3mm.

Underlayment Does Four Different Jobs

Naming them separately is what makes the decision obvious, because different products do different ones:

• 1. Cushioning and sound. A resilient layer between a hard plank and a hard subfloor absorbs impact, softens footfall, and stops the hollow, clacky sound cheap floating floors are known for. This is the job attached pads are designed for.

• 2. Minor smoothing. A pad conforms to very small subfloor irregularities. It does not level anything - dips and humps beyond tolerance still need fixing at the subfloor.

• 3. Moisture protection. A polyethylene vapor barrier blocks moisture migrating up out of a concrete slab. This is a completely separate function from cushioning, and foam or cork pads do not perform it.

• 4. Thermal comfort. A pad adds a small amount of insulation, making a floor feel less cold underfoot - though over radiant heat you want this minimal, since insulation works against the system.

An attached pad handles jobs one, two, and four. It does not handle job three. Almost every underlayment mistake traces back to that single sentence.

What You Have Already

The practical takeaway: check your specific product page before buying anything. Attached-pad status varies by collection, and it's stated in the specs on every product.

The Concrete Rule (The One That Actually Matters)

This is the section worth reading twice, because it's the mistake with real consequences.

Concrete releases moisture continuously as vapor, for years after it cures. Over a slab - basement, ground floor, garage conversion, anywhere on or below grade - that vapor rises into whatever sits on top of it. A 6-mil polyethylene sheet blocks it. An attached foam pad does not, and neither does cork.

The counterintuitive part is that a 100% waterproof floor doesn't remove the need. SPC's core won't absorb moisture, true - but the vapor doesn't stop at the plank. It collects underneath, where it can encourage mold and mildew in the assembly, affect adhesives, and degrade the subfloor environment. Waterproof describes the plank; the vapor barrier protects everything else.

Non-negotiable over concrete: a 5-6 mil poly vapor barrier, installed directly on the slab, under everything else - including under cork if you're using it. McMillan's heavy-duty 6-mil poly sheeting covers 1,000 sq. ft. per roll at $119.98. On a wood subfloor above ground level, the risk is far lower and a vapor barrier generally isn't required, though it's worth confirming for humid climates.

When Cork Is Worth Adding

Cork is the upgrade that has a real reason to exist, and it's an acoustic one. It's the material sound engineers reach for first, and it does meaningfully more for impact noise than a thin attached pad alone. Add it when:

• Your condo or HOA requires a specific acoustic rating above what your floor's published numbers deliver, or mandates a separate underlayment layer as a condition of approval. Documentation is often part of the approval process.

• There's a bedroom or living space directly below the room you're flooring, and footstep noise matters to whoever is down there.

• You want a softer, warmer feel underfoot, particularly in bedrooms, where the comfort difference is noticeable.

McMillan's SoundGuard™ cork underlayment comes in 1/4” and 1/2” thicknesses and works under SPC, laminate, and engineered hardwood alike. One caution that trips people up: cork handles sound, not moisture. Over concrete it goes on top of the poly vapor barrier, never instead of it - which McMillan covers in detail in Do I Need a Moisture Barrier if I'm Also Purchasing the Cork Underlayment?.

Why Stacking Pads Backfires

The instinct is understandable: if a pad is good, more pad should be better. It isn't, and this is one of the few DIY choices that reliably causes problems:

• It makes the floor spongy. Excess cushion lets planks flex underfoot, and a floating floor is engineered to sit on a firm base.

• It stresses the locking joints. Flex concentrates at the click joints, which fatigues them and eventually separates seams - the same failure mode as installing over an unflat subfloor.

• It can trap air and amplify noise, producing exactly the hollow sound the pad was meant to prevent.

• It typically falls outside warranty terms, because it exceeds the manufacturer's stated installation specification.

The rule on McMillan floors: separate underlayment up to 3mm, and don't add a second pad beneath a floor that already has one attached. If your board requires more acoustic performance than that allows, cork under a no-pad product is the correct configuration rather than layering.

The five-second decision: Does your floor have an attached pad? If yes and you're over a wood subfloor, you're done - install as-is. If yes and you're over concrete, add the 6-mil vapor barrier and nothing else. If your floor has no attached pad, use an underlayment up to 3mm, plus the vapor barrier over concrete. Want extra quiet for a condo or an upstairs room? Cork, on top of the vapor barrier. That's the entire decision tree.

Frequently Asked Questions

Do I need underlayment if my flooring has an attached pad?

For cushioning and sound, no - the attached pad handles it, and it's typically included in the floor's tested acoustic rating. Adding a second pad beneath it makes the floor spongy, stresses the locking joints, and usually falls outside warranty terms. The exception is moisture: over a concrete subfloor you still need a 6-mil vapor barrier, because a foam pad is not a moisture barrier.

Do you need a vapor barrier under waterproof vinyl flooring?

Over concrete, yes. The plank being waterproof means the core won't absorb moisture, but concrete releases vapor continuously and that vapor doesn't stop at the flooring - it collects underneath, where it can encourage mold and mildew and affect the subfloor assembly. A 5-6 mil polyethylene sheet installed directly on the slab prevents it. Over a wood subfloor above ground level, a vapor barrier generally isn't required.

Can you put cork underlayment under vinyl plank with an attached pad?

It's generally unnecessary and can exceed the manufacturer's thickness specification. If you need acoustic performance beyond what your floor's attached pad provides - for a condo board requirement, for example - the better configuration is cork beneath a product without an attached pad, rather than layering cork under a floor that already has one. Check your product's installation guidance for its stated maximum.

How thick should flooring underlayment be?

For McMillan floors installed without an attached pad, use a quality underlayment up to 3mm thick. Thicker isn't better: excess cushioning lets planks flex, which fatigues the locking joints over time. Over radiant heat, thinner is better still, since underlayment insulates and works against the heating system. Cork is the exception where extra thickness serves a purpose, and it comes in 1/4” and 1/2” for acoustic applications.

The Bottom Line

Underlayment stops being confusing the moment you separate the four jobs. Cushioning and sound: your attached pad has it covered. Moisture over concrete: that's a vapor barrier, always, no matter how waterproof the plank is. Extra acoustic performance for a condo or an upstairs bedroom: that's cork, on top of the barrier. And more pad is not better pad - stay within the 3mm spec and let the floor sit on a firm base, which is what it was engineered to do.

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Read: Do I Need a Moisture Barrier With Cork Underlayment? →

Read: Best Flooring for Apartments and Condos (HOA Noise Rules) →

Read: Flooring Over an Uneven Subfloor →