50mm Floor Insulation

50mm floor insulation is thick enough to make a noticeable thermal difference, but not so deep as to completely reshape your floor build-up.

In many jobs, 50mm thick floor insulation is a practical compromise. It improves thermal performance without pushing build-up height too far.

What is 50mm floor insulation thickness?

Floor insulation 50mm thick is manufactured to a nominal thickness of 50mm. The dimension refers to the insulation layer itself and not the overall floor build-up.

50mm floor insulation formats

Rigid board formats

Most 50mm insulation boards for flooring are supplied in sheet form.

Common board lengths include:

• 2400mm
• 1250mm
• 1200mm
• 300–1200mm (modular panels such as Kingspan OPTIM-R)

Typical board widths include:

• 1200mm
• 600mm
• 300–600mm (OPTIM-R panel formats)

Common rigid 50mm insulation boards are square-edged for tight butt joints. One exception in this category is Kingspan GreenGuard GG300 R, which has rebated edges to reduce heat loss at board junctions.

Mineral wool roll formats (50mm thickness)

Where mineral wool is used mainly in suspended timber floors, it is commonly supplied as rolls rather than rigid boards. Roll widths are designed to friction-fit standard joist centres, for example, 400mm or 600mm centres.

Typical roll lengths are:

• 10m
• 13.5m

Roll length affects handling, coverage per pack, and labour time.

50mm floor insulation performance

At this depth, thermal performance varies widely depending on material:

• 0.007 W/mK – Kingspan OPTIM-R (VIP)
• 0.019 W/mK – Kingspan Kooltherm K103
• 0.022 W/mK – 50mm PIR boards such as 2400mm x 1200mm Celotex GA4050, Ecotherm Eco-Versal, Mannok PIR, Recticel Eurothane GP, IKO Enertherm ALU PIR, and Unilin Thin-R PIR
• 0.033 W/mK – Kingspan GreenGuard GG300 (XPS)
• 0.034–0.036 W/mK – EPS grades like the Jablite EPS range
• 0.035–0.038 W/mK – Mineral wool products, such as Rockwool and Knauf slabs and rolls

A lower λ-value indicates lower thermal conductivity and, therefore, greater thermal resistance per millimetre.

Under Approved Document L, the new-build ground floor U-value targets are typically 0.18 W/m²K or lower, as defined by the applicable notional specification and compliance route. This covers the full floor build-up comprising:

• The concrete slab
• Screed
• Surface resistances
• Perimeter heat loss

In most standard ground-bearing slab designs, 50mm insulation alone will not achieve this target. Confirm compliance with a full U-value calculation that includes the entire floor area, perimeter-to-area ratio, and complete build-up specification.

What is 50mm floor insulation used for?

50mm solid floor insulation is appropriate for:

• Refurbishments improving existing floors
• Extensions with tight threshold heights
• Floors combined with perimeter upstands
• Layered insulation strategies
• Projects using very high-performance boards, such as VIP and PIR rigid insulation board

50mm Celotex floor insulation and similar 50mm PIR insulation boards are commonly used on:

• Ground-bearing concrete slabs – laid above a damp-proof membrane and below screed. Boards must be tightly butted with no gaps.
• Beam-and-block floors – positioned above the structural deck and beneath the screed layer.
• Suspended timber floors – fitted between joists. Air movement must be controlled to avoid performance loss.
• Retrofit overlays – where digging out is not possible, and a thin 50mm floor insulation layer provides an upgrade without major structural alteration.

Correct installation matters as much as board choice. Gaps, compression, and unsupported edges reduce performance.

Types of 50mm thick floor insulation

At Insulation Wholesale, we carry these 50mm floor insulation materials:

PIR Insulation

Examples of the products we stock include Celotex GA4050, Ecotherm Eco-Versal, Mannok PIR, Recticel Eurothane GP, IKO Enertherm ALU PIR, and Unilin Thin-R PIR.

Typical thermal conductivity of 0.022 W/mK

Typical compressive strength – 120–150 kPa

Tested to BS EN 826, with compressive stress at 10% deformation classifications such as CS(10/Y)120 or CS(10/Y)150.

For many domestic floors under screed, 120 kPa may be suitable, subject to load assessment and manufacturer guidance.

Phenolic foam

Kingspan 50mm floor insulation in phenolic format further improves thermal efficiency, with Kooltherm K103 achieving a thermal conductivity of approximately 0.019 W/mK.

It provides higher thermal resistance at the same 50mm depth. Compressive strength is generally similar to PIR, depending on the product. Suitable beneath screed when specified correctly.

XPS Insulation (Extruded polystyrene)

We stock the 50mm Kingspan GreenGuard GG300.

• Thermal conductivity is approximately 0.033 W/mK.
• Declared compressive strength approximately – 300 kPa at 10% deformation.

• Higher load tolerance
• Low water absorption
• Strong long-term dimensional stability

Best for heavier loads and high moisture environments.

EPS Insulation (Expanded polystyrene)

Typical lambda – 0.034 W/mK–0.038 W/mK

EPS grades vary:

• EPS70 – 70 kPa
• EPS100 – 100 kPa
• EPS150 – 150 kPa
• EPS200/250 – 200–250 kPa

EPS70 may be suitable for lightly loaded floors, provided load calculations confirm compliance. For screeded domestic floors, EPS100 is commonly specified as the minimum grade, subject to design calculations. Higher grades provide a better safety margin under load.

Mineral wool insulation (Rock & Glass)

Products include:

• Rockwool RWA45
• Knauf Rocksilk RS45
• Acoustic rolls from Knauf and ODE

Typical lambda: 0.035–0.038 W/mK

Fixed between timber joists for acoustic and thermal control. Standard mineral wool slabs are not normally used directly beneath screed unless specifically rated for load-bearing.

Ablative coated batt

Rockwool Ablative Coated Batt (thermal conductivity approximately 0.037 W/mK).

Used mainly around service penetrations within floor voids to maintain fire resistance.

Kingspan Optim-R (Vacuum Insulated Panels)

Kingspan OPTIM-R – 0.007 W/mK.

Provides extremely high thermal resistance at 50mm.

However:

• It cannot be cut
• Must not be punctured
• Requires precise layout
• Needs full, level support

VIP suits specialist situations where depth cannot be increased, but U-values must improve significantly.

Before choosing any 50mm floor insulation board, confirm:

• Compressive strength class (kPa)
• Suitability under the screed
• Manufacturer approval for floor use

Most domestic screeded floors use insulation rated at 120 kPa or higher, while heavier loads may require 150 kPa or higher.

Industrial floors often require higher compressive strength classes, such as 300 kPa or higher, which are commonly found in XPS products.

Do not assume structural suitability based on thickness alone.