Diall XPS Underlay U-Value Calculator

Manufacturer U-value calculators work out a floor, wall or roof's U-value from scratch by building up every construction layer (screed, insulation board, ground conditions, etc.) - a much bigger job aimed at Building Regs compliance calculations. This calculator answers a narrower, more practical question: given a floor U-value you already know or estimate, how much does adding a specific thickness of thin XPS underlay improve it.

Yes, but only by a small amount. At 1-5mm thick, XPS underlay adds roughly 0.03-0.14 m²K/W of thermal resistance - enough to measurably reduce heat loss (commonly a few percent to around 10% improvement on an already poor floor), but nowhere near enough on its own to bring an uninsulated floor up to Building Regulations insulation levels. Its main job is providing a smooth, moisture-resistant, sound-dampening base for the flooring above, not serious thermal insulation.

If you have an EPC (Energy Performance Certificate) for your property, its underlying assessment data (or a follow-up survey) is the most accurate starting point. Otherwise, use this calculator's typical presets as a rough guide - around 0.7 W/m²K for an uninsulated suspended timber floor, or around 1.0 W/m²K for an uninsulated solid concrete floor - but treat these as ballpark figures only, since actual ground floor U-values vary significantly with a property's specific construction and floor perimeter-to-area ratio.

That mostly depends on your flooring type and manufacturer's fitting instructions, not thermal performance: Diall's 1mm XPS foam underlay is sold for LVT (luxury vinyl tile) flooring, while the 2.2mm and 5mm foam underlays are sold for laminate and solid wood flooring, with the 5mm Aquastop variant adding an integrated moisture barrier. Check what your specific flooring manufacturer recommends before choosing a thickness for thermal reasons alone.

XPS stands for extruded polystyrene - a closed-cell rigid foam material, here manufactured as a thin, slightly compressible foam sheet designed to sit beneath laminate, solid wood or LVT flooring. It provides a smooth, level base, some sound dampening (reducing footstep noise), a degree of moisture resistance, and a small amount of thermal insulation.

Not meaningfully - Aquastop adds an integrated vapour/moisture barrier film to the same 5mm XPS foam base, aimed at protecting against damp rather than improving thermal performance. This calculator treats the standard and Aquastop 5mm products the same for U-value purposes.

Yes, thermal resistance scales roughly linearly with thickness for the same material - doubling the underlay thickness roughly doubles its own added resistance. But because the numbers involved are small (a few hundredths of a m²K/W), even a much thicker underlay than any current Diall product offers would still only make a modest dent in a poorly insulated floor's overall U-value; proper floor insulation (50mm+ of rigid insulation board under a suspended timber floor, or insulation beneath a solid slab) makes a far bigger difference.

A U-value (in W/m²K) measures how much heat, in watts, passes through one square metre of a building element for every 1°C (or 1 Kelvin) of temperature difference between the two sides. Lower U-values mean less heat escapes - a well-insulated modern floor might be around 0.13-0.18 W/m²K, while an old uninsulated floor can be well above 1.0 W/m²K.

Under Approved Document L (2021 edition), the backstop (maximum permitted) U-value for a floor in an existing dwelling or extension is 0.18 W/m²K, and the notional new-build dwelling target is 0.13 W/m²K, per gov.uk's Approved Document L guidance. A few millimetres of underlay on its own will not bring most existing floors down to either figure - proper floor insulation is needed for that.

No - this tool only models the incremental effect of adding a thin underlay layer on top of a U-value you provide or estimate. A full ground-floor U-value calculation from individual construction layers and ground geometry needs the BS EN ISO 13370 method, which manufacturer tools (Kingspan, Rockwool, Recticel and others) provide for free.

Percentage improvement is relative to the starting U-value - on a very poorly performing floor (a high starting U-value), the same small added resistance from a thin underlay makes a proportionally smaller dent than it would on a floor that's already reasonably insulated. This is a real effect of how the maths works (diminishing returns from adding resistance in series), not a calculation error.

The formula works for any XPS (extruded polystyrene) foam underlay of a known thickness, using the same 0.035 W/mK conductivity assumption - but the result will be most accurate for products with a similar published or estimated thermal conductivity to Diall's. If your product's data sheet states a different lambda value, the result here will be an approximation rather than an exact match.

No - this is an independent estimate tool and is not affiliated with, endorsed by, or produced by B&Q or Diall. Product names are used only to identify the specific underlay type being calculated for.

No. This calculator is a rough estimate tool for a homeowner or DIY fitter weighing up the thermal benefit of an underlay choice. Any Building Regulations or SAP/EPC submission needs a proper U-value calculation from an accredited assessor or a manufacturer's compliance tool, using your actual construction build-up.