Most Portofino flooring products — SPC vinyl, WPC vinyl, and laminate — come with underlayment pre-attached to the back of each plank. In most above-grade installations over a wood subfloor, you don't need to purchase separate underlayment. However, pre-attached underlayment does not include a moisture barrier. If you're installing over a concrete subfloor — in a basement, on a ground-level slab, or anywhere concrete is below the floor — you need to add a 6 mil plastic vapor barrier regardless of whether your floor has pre-attached underlayment. Engineered hardwood in a floating installation needs a separate underlayment since it doesn't come pre-attached.
This guide covers what underlayment actually does, which floors need it, and when a vapor barrier is required.
What underlayment actually does
Underlayment sits between your flooring and your subfloor. It performs four jobs:
Sound absorption — underlayment reduces the hollow sound that floating floors can produce underfoot, particularly in multi-story homes where sound transmission to the floor below is a concern. This is the most noticeable benefit of quality underlayment — the difference between a floor that sounds solid and one that sounds like you're walking on a drum.
Minor subfloor compensation — underlayment provides a small degree of forgiveness over minor subfloor imperfections. The operative word is minor. Underlayment compensates for surface variations up to about 3/16 inch over 10 feet. Beyond that threshold, subfloor prep is required before installation — underlayment is not a leveling product and cannot fix a significantly uneven floor.
Slight cushioning — underlayment adds a small amount of softness underfoot, making the floor feel slightly more comfortable to stand on for extended periods. The difference is subtle with most underlayment thicknesses but noticeable in comparison.
Moisture barrier — this is where most buyers get tripped up. Underlayment provides a degree of moisture management, but standard pre-attached underlayment on SPC, WPC, and laminate is not a vapor barrier. Over concrete subfloors, a dedicated vapor barrier is required in addition to whatever underlayment your floor comes with.
Does your floor already have underlayment?
Check the product page and the back of your sample plank. Pre-attached underlayment feels like a thin foam or cork pad bonded directly to the underside of the plank — you can see and feel it clearly.
Portofino's SPC vinyl, WPC vinyl, and laminate all come with underlayment pre-attached. This means:
- You do not need to purchase or install separate underlayment for above-grade wood subfloor installations
- You should not add a second layer of underlayment on top of the pre-attached layer
That second point is important. Adding a separate underlayment layer on top of pre-attached underlayment creates a floor that's too soft and compressible beneath the click-lock joints. The joints are engineered to operate within specific tolerances — too much give underneath puts lateral stress on the locking system with every footstep, eventually causing joint failure, gaps, and a floor that feels bouncy and unstable. If your floor comes with pre-attached underlayment, install it directly over your subfloor or vapor barrier without adding anything else underneath.
Portofino's engineered hardwood does not come with pre-attached underlayment. For floating installation, a separate underlayment is required.
The vapor barrier question — the most important thing most buyers miss
Here is the part of underlayment guidance that most flooring content gets wrong or skips entirely: pre-attached underlayment on SPC, WPC, and laminate is not a vapor barrier.
Concrete subfloors — basement slabs, ground-level concrete, slab-on-grade construction — release moisture vapor continuously. This is a normal property of concrete, not a sign of a wet or problem basement. Even concrete that appears and tests dry releases moisture vapor at levels that, over time, can affect flooring installed directly over it.
For wood subfloors above grade, this is not a concern — wood subfloors allow moisture to dissipate and are not a significant vapor source. For concrete subfloors, it is a concern regardless of what flooring you're installing.
The solution is straightforward: install a 6 mil plastic sheeting vapor barrier over the concrete before installing your flooring. Roll it out across the entire floor area, overlap seams by at least 8 inches, and tape the seams with moisture-resistant tape. Run it up the walls slightly and trim it after the floor is installed. Then install your flooring — with its pre-attached underlayment — directly over the plastic sheeting.
This applies to all four Portofino floor types over concrete:
- SPC vinyl: pre-attached underlayment + 6 mil plastic over concrete
- WPC vinyl: pre-attached underlayment + 6 mil plastic over concrete
- Laminate: pre-attached underlayment + 6 mil plastic over concrete
- Engineered hardwood: separate underlayment with integrated moisture barrier, or separate underlayment + 6 mil plastic over concrete
A 6 mil plastic vapor barrier is inexpensive — typically $0.10–$0.20 per square foot — and available at any home improvement store. It's one of the cheapest and most important steps in a concrete subfloor installation.
How to test your concrete for moisture:
Before installing any flooring over concrete, a simple plastic sheet test gives you a baseline read on moisture levels. Tape a 24-inch square of plastic sheeting to the concrete, seal all four edges with tape, and leave it for 24–48 hours. If moisture collects on the underside of the plastic when you lift it, your concrete is releasing significant vapor and a vapor barrier is essential. If the plastic is dry, vapor levels are low — but a vapor barrier is still recommended as standard practice.
Underlayment by floor type
SPC vinyl
Pre-attached underlayment: Yes, on all Portofino SPC products. Separate underlayment needed: No, for above-grade wood subfloor installations. Vapor barrier needed: Yes, over concrete subfloors — 6 mil plastic sheeting installed before the floor.
SPC's rigid stone core is the least sensitive of all four floor types to minor subfloor imperfections — the density of the core bridges small variations better than softer floors. The pre-attached underlayment handles sound absorption and minor surface variation adequately for most installations. On concrete, the 6 mil plastic goes down first, then the SPC planks with their pre-attached underlayment install directly over it.
WPC vinyl
Pre-attached underlayment: Yes, on all Portofino WPC products. Separate underlayment needed: No, for above-grade wood subfloor installations. Vapor barrier needed: Yes, over concrete subfloors — 6 mil plastic sheeting.
WPC installs identically to SPC in terms of underlayment requirements. The pre-attached underlayment is sufficient for wood subfloor installations. Over concrete, add the 6 mil plastic before installation.
Laminate
Pre-attached underlayment: Yes, on Portofino laminate products. Separate underlayment needed: No, for above-grade wood subfloor installations. Vapor barrier needed: Yes, over concrete subfloors — 6 mil plastic sheeting.
Laminate's HDF core is more sensitive to subfloor flatness than SPC's stone core — the pre-attached underlayment compensates for minor variation but a flatter subfloor produces better results with laminate than with SPC. Over concrete, the 6 mil plastic vapor barrier is particularly important for laminate because the HDF core, while dense and moisture-resistant, is more vulnerable to sustained vapor exposure than SPC or WPC's plastic-composite cores.
Engineered hardwood
Pre-attached underlayment: No. Separate underlayment needed: Yes, for floating installation. Vapor barrier needed: Yes, over concrete subfloors.
Engineered hardwood in a floating installation requires a separate underlayment. Look for a product specifically designed for hardwood floating floors — typically a foam or cork underlayment in the 2–3mm thickness range. Some engineered hardwood underlayments include an integrated moisture barrier layer, which simplifies installation over concrete by combining the two functions in one product. If your underlayment does not include an integrated moisture barrier, install 6 mil plastic sheeting first, then the underlayment, then the flooring.
For glue-down engineered hardwood installation, underlayment is not used — the adhesive bonds the planks directly to the subfloor. A moisture barrier may still be required depending on the adhesive manufacturer's specifications.
Underlayment types: what's available if you need a separate product
For engineered hardwood floating installation, or for any situation where separate underlayment is needed, here are the main types available:
| Type | Thickness | Sound absorption | Cushioning | Moisture barrier | Best for |
|---|---|---|---|---|---|
| Foam | 2–3mm | Good | Moderate | No | Engineered hardwood floating, budget installations |
| Cork | 3–6mm | Excellent | Good | No | Engineered hardwood, premium installations |
| Rubber | 3–6mm | Excellent | Excellent | No | High sound reduction priority |
| Combination (foam + film) | 2–3mm | Good | Moderate | Yes | Over concrete where integrated barrier is preferred |
Foam underlayment is the most widely available and affordable option. It provides adequate sound absorption and cushioning for most residential installations and is the standard choice for engineered hardwood floating installations. It does not include a moisture barrier — add 6 mil plastic separately over concrete.
Cork underlayment provides superior sound absorption compared to foam and has natural antimicrobial properties. It's the premium choice for engineered hardwood in multi-story homes where sound transmission is a priority concern. It compresses slightly more than foam over time but remains effective for the floor's useful life. Does not include a moisture barrier.
Rubber underlayment offers the best sound absorption and cushioning of any underlayment type and is highly durable. It's typically specified for commercial installations or residential applications where maximum sound reduction is the priority. It is the most expensive option and does not include a moisture barrier.
Combination underlayment integrates a thin plastic film moisture barrier into the underlayment product, simplifying installation over concrete by eliminating the separate vapor barrier step. Sound absorption and cushioning performance is similar to standard foam. A practical choice for engineered hardwood floating installation directly over concrete.
What underlayment cannot fix
Setting realistic expectations before installation prevents frustration afterward.
Underlayment is not a subfloor leveler. It compensates for surface texture and minor variation — small ridges, slight unevenness in a wood subfloor, minor imperfections in concrete surface finish. It does not compensate for:
- Low spots deeper than 3/16 inch over 10 feet
- High spots that create a raised area under the floor
- Structurally compromised subfloor sections that flex or move
- Significant slope across the room
Any of these conditions needs to be addressed with subfloor preparation before installation — self-leveling compound for low spots, sanding or grinding for high spots, subfloor repair or reinforcement for structural issues. Installing flooring over an out-of-tolerance subfloor regardless of underlayment thickness results in joint stress, click-lock failure, and a floor that feels uneven and unstable underfoot.
If you're uncertain about your subfloor condition, have a flooring installer assess it before ordering materials. Subfloor preparation is much easier to budget for before the flooring arrives than to address mid-installation.
Quick reference: underlayment requirements by floor type and subfloor
| Floor type | Wood subfloor above grade | Concrete subfloor |
|---|---|---|
| SPC vinyl | Pre-attached — nothing to add | Pre-attached + 6 mil plastic vapor barrier |
| WPC vinyl | Pre-attached — nothing to add | Pre-attached + 6 mil plastic vapor barrier |
| Laminate | Pre-attached — nothing to add | Pre-attached + 6 mil plastic vapor barrier |
| Engineered hardwood (floating) | Separate underlayment required | Separate underlayment + 6 mil plastic, or combination underlayment with integrated barrier |
| Engineered hardwood (glue-down) | No underlayment — check adhesive specs | Check adhesive manufacturer's moisture barrier requirements |
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