Flooring Underlay: Types, UFH Compatibility, and What Goes Under Each Floor Finish
A complete UK guide to flooring underlay: foam vs rubber vs cork, tog ratings for UFH, which underlay goes under laminate or LVT, and why the wrong type voids your floor warranty.
A floor finisher installs engineered oak on a UFH screed, with a standard 3.5mm foam underlay underneath because it was what the builder had on site. Eighteen months later the homeowner reports cold spots on the floor and rising energy bills. The UFH system is working; the floor temperature sensors show the screed is reaching the set point. The problem is the underlay acting as an insulating blanket between the warm screed and the room above. The thermal resistance of that foam underlay is high enough to make the UFH economically inefficient and to void the heat-pump service warranty.
Underlay is the one flooring component where the wrong choice cannot be fixed without lifting the whole floor.
What flooring underlay is and why it matters
Flooring underlay is a thin layer of compressible or cushioning material laid between the subfloor (usually concrete screed or timber boarding) and a floating floor covering. It serves four purposes:
- Acoustic dampening: reduces impact sound (footfall noise) transmitted through the floor structure to the rooms below. Relevant for multi-storey houses and conversion projects.
- Minor subfloor irregularities: bridges small surface undulations in the screed or boarding so the floor covering is not supported on high spots only.
- Thermal insulation: adds a small amount of thermal resistance between the floor covering and the room. On a well-insulated concrete slab this has little practical effect. Over UFH it has a large and potentially damaging effect.
- Moisture barrier: some underlays incorporate a polythene vapour layer on the underside that reduces moisture vapour transfer from a green screed into the floor covering above. This is not a substitute for a damp-proof membrane at slab level.
The choice of underlay is governed primarily by the floor covering it will sit under, the substrate it sits on, and whether there is underfloor heating in the screed.
UFH compatibility: the tog limit
If the floor has a wet underfloor heating system in the screed, the combined thermal resistance of the floor covering and the underlay must stay below the limit specified by the UFH designer or the Future Homes Standard.
0.15 m²K/W combined tog limit
Thermal resistance in floor products is usually expressed in m²K/W (sometimes loosely called "tog" in retailers, where 1 tog = 0.1 m²K/W). The limit of 0.15 m²K/W covers both the floor covering and the underlay together. A typical LVT floor covering has a thermal resistance around 0.01–0.03 m²K/W. A typical 3.5mm foam underlay has a thermal resistance of 0.08–0.12 m²K/W. Together they comfortably fit within the 0.15 limit. A thicker underlay (5mm foam) may push the combined value close to or above the threshold.
For engineered wood, the floor covering itself has a higher thermal resistance (0.10–0.12 m²K/W for typical 18mm boards). Adding any underlay of meaningful thickness pushes the total over the limit. For UFH under engineered wood, use a very thin (1–1.5mm) thermally conductive underlay only if the UFH designer has confirmed the combined value is acceptable.
Warning
Using a standard foam underlay over a UFH screed is one of the most common flooring mistakes on extension projects. Check the combined thermal resistance of your chosen floor covering and underlay against the UFH designer's specification before buying. The consequence of exceeding the limit is not just reduced comfort: it can void the heat-pump or boiler warranty.
Types of underlay and what they suit
Foam underlay (polyethylene foam)
The most common type. Closed-cell PE foam, typically 2–5mm thick, sold in rolls. Available in standard and premium grades. Some grades incorporate a thin vapour layer on one face. It is inexpensive, easy to fit (cut with scissors or a knife), and gives good performance under laminate and floating engineered wood on non-UFH substrates.
Not suitable over UFH unless specified to a very thin profile and confirmed thermally compliant.
Rubber crumb underlay
Made from recycled car tyres. Higher density and more dimensionally stable under load than foam. Good acoustic performance. Suits heavy foot traffic and is often specified under luxury engineered wood and floating floors in commercial applications. More expensive than foam. Check manufacturer's thermal resistance figures if used over UFH.
Cork underlay
A natural product with good acoustic and thermal properties. 2–3mm thickness. Compressible enough to absorb minor subfloor variation. Suits floating floors on timber substrates where acoustic performance is important. Check thermal resistance against the UFH limit if applicable.
LVT-specific underlay
LVT (luxury vinyl tile) and sheet vinyl are thin, dimensionally stable, and respond badly to subfloor irregularities that compress unevenly under the product. LVT-specific underlays are typically 1–1.5mm, very dense, and designed to support the thin flooring without allowing it to flex over imperfections. Some LVT is designed to be laid without underlay (glue-down or click-lock direct on a prepared screed). Check the manufacturer's fitting instructions before specifying any underlay for LVT.
For LVT over UFH, a thin compliant underlay is usually specified. Many LVT manufacturers state that standard foam underlay is not suitable under click-lock LVT over UFH and that a purpose-made acoustic underlay with a thermal resistance below 0.03 m²K/W must be used.
Acoustic underlay (specialist)
For refurbishments and conversions where Part E acoustic separation is a performance target, specialist acoustic underlays with high mass and low resonant frequency are available. These are not general-use products but are specified by acoustic engineers for particular constructions. They are more expensive and thicker than standard underlays.
Which underlay goes under which floor
| Floor covering | Standard underlay type | UFH: use this instead | Notes |
|---|---|---|---|
| Laminate | 3–5mm PE foam with vapour layer | 1.5–2mm UFH-rated foam | Check combined tog < 0.15 m²K/W |
| Floating engineered wood | 2–3mm foam or cork | 1–1.5mm thermally conductive underlay or none | UFH: always confirm with UFH designer |
| Click-lock LVT | 1.5mm LVT acoustic underlay | 1.5mm UFH-rated LVT underlay | Do not use standard foam under LVT over UFH |
| Glue-down LVT | None: direct to screed | None required | Screed must be prepared to DIN 18202 flatness |
| Carpet | 8–10mm foam or rubber | Not recommended over UFH | Carpet + underlay typically exceeds 0.15 m²K/W |
Fitting underlay: practical points
Underlay is laid loose before the floor covering goes down. The fitting process is straightforward but the common errors matter.
Direction: roll out the underlay perpendicular to the direction of the floor boards or planks. This means the underlay joints run at 90 degrees to the floor joints, which prevents both layers of joints from concentrating stress at the same point.
Joints: butt the underlay sheets tightly together at the edges without overlapping. An overlapping underlay joint creates a ridge that the floor covering will follow, producing a visible line on the finished floor surface. Tape butted joints with general-purpose tape to stop them shifting during installation.
Walls: the underlay runs up to the wall but not up the wall face. The gap at the perimeter of a floating floor (required for expansion) sits between the floor edge and the skirting board, not under the underlay.
Subfloor prep: the subfloor must be flat to the tolerances the floor manufacturer specifies before any underlay goes down. Underlay does not bridge bumps or fill hollows in any meaningful way. A 3mm foam underlay will conform to a 3mm high spot in the screed and transmit it to the floor covering above, often visibly. Grind or fill the screed to the required tolerance first.
Vapour layer orientation: on an underlay with a polythene vapour layer on one face, that face goes down (facing the screed) not up. Check the manufacturer's instructions. An upside-down vapour layer traps moisture from above rather than blocking it from below, which is the opposite of the intended function.
Reading the manufacturer's fitting instructions
Every floor manufacturer specifies the underlay type that is acceptable under their product, and many void the product warranty if an unspecified underlay is used. Before buying underlay, find the floor manufacturer's fitting instructions and confirm the following:
- Maximum underlay thickness permitted
- Whether an integrated vapour layer is required (if laying on concrete)
- Thermal resistance limit (if over UFH)
- Whether the floor can be laid without underlay in some circumstances (glue-down LVT)
This step takes five minutes and prevents the most common cause of flooring failures that result in a voided warranty.
Moisture and vapour considerations
On a concrete screed in a new extension, the screed will still be releasing moisture vapour for weeks after pouring. A DPM under the slab controls rising ground moisture. The vapour layer on many foam underlays provides limited additional protection against vapour rising from a warm screed in the interim period.
For engineered wood, the hygrometer box test (to BS 8203/BS 8204) must confirm the screed is below the manufacturer's specified RH threshold (typically 75% RH for most engineered wood products) before the floor is installed. An underlay with a vapour barrier does not eliminate the requirement for the screed to be dry. It delays but does not prevent moisture-related damage if the floor is installed too early.
What it costs
Underlay is priced per m², with rolls typically 7–10m² for standard residential use or 15–20m² per roll for larger projects.
| Type | Thickness | Price per m² | Typical roll size |
|---|---|---|---|
| Standard PE foam (with vapour layer) | 3.5mm | £1.50 – £3.50 | 10m² |
| Premium foam acoustic | 5mm | £3.00 – £6.00 | 10m² |
| Rubber crumb | 5–8mm | £5.00 – £10.00 | 6m² |
| Cork sheet | 2–3mm | £4.00 – £8.00 | 5–7m² |
| LVT / UFH specialist | 1–1.5mm | £3.00 – £7.00 | 10m² |
Underlay for carpet
Carpet underlay follows different rules from the types covered above. A separate guide would cover carpet underlay in full, but the key points relevant to an extension project are:
- Standard carpet underlay (8–12mm foam or rubber) produces a combined thermal resistance with the carpet that typically exceeds the UFH limit. Carpet over UFH is not recommended and is excluded from many UFH system warranties.
- For non-UFH rooms, carpet underlay thickness is a matter of comfort and budget: thicker underlay makes carpet feel more luxurious underfoot and extends carpet life by reducing flex at the backing.
- Rubber crumb underlay is preferred in high-traffic hallways and stairs because it does not compress permanently under repeated point loading the way foam does.
Part E acoustic compliance
In conversions and multi-occupancy residential buildings, floor constructions may need to achieve a minimum impact sound insulation level under Building Regulations Part E. The standard is Lpn,w 65 dB or better for impact sound. Meeting this with a floating floor typically requires:
- An independent floating screed or platform deck, not just an underlay
- A high-mass, low-frequency resonant underlay as part of the build-up
- Testing by an acoustic engineer before the floor is signed off
A standard flooring underlay does not achieve Part E compliance on its own in any construction likely to be encountered on a domestic extension. If Part E compliance is a requirement of the project, involve an acoustic specialist early to specify the floor build-up. This is relevant to loft conversions and garage conversions where previously uninhabited space is being converted to habitable use above another occupied space.
The cost difference between a standard foam underlay and a UFH-rated specialist underlay is small (typically one to three pounds per m²) against the cost of re-laying a floor because the underlay was incompatible. On a 25m² kitchen extension floor, that difference is 25 to 75 pounds. On an engineered oak floor costing several thousand pounds in material and labour, the risk-adjusted saving is clear.
What to buy for a 25m² kitchen extension floor
For a typical single-storey kitchen extension with a UFH screed:
- Confirm the floor covering type and the UFH designer's thermal resistance limit.
- Read the floor covering manufacturer's installation instructions to confirm approved underlay types and thicknesses.
- For LVT or laminate with UFH: choose a 1.5mm UFH-rated underlay with a combined tog value confirmed below 0.15 m²K/W.
- For engineered wood with UFH: consult the UFH designer. Many manufacturers specify either no underlay, or a purpose-designed 1mm thermally conductive product.
- Order sufficient rolls for the area, with 5–10% waste allowance for cutting.
For a room with no UFH (a first-floor extension or a room above ground on a suspended floor), a 3.5mm PE foam underlay with an integrated vapour layer is the standard choice for laminate or engineered wood. It is widely stocked and performs reliably in that application. The guidance above is specific to UFH situations because that is where the wrong choice causes the most costly failures, but for standard non-UFH floors the key decision is simply to match the underlay to the floor covering type and to confirm it carries a vapour layer for concrete subfloors.