25mm Kingspan Kooltherm K103 Floorboard is a rigid thermoset phenolic floor insulation board, supplied here as single sheets measuring 2.4m x 1.2m. It is the thinnest board in the K103 range, and at 25mm it is typically specified where floor depth is limited: as a thin upgrade or topping-up layer, in garage conversions, as part of a multi-layer build-up, or cut down to form the perimeter upstand strips that reduce heat loss at the floor edge.

Each sheet measures 2400mm x 1200mm and covers 2.88m². The phenolic core has a thermal conductivity of 0.019 W/mK, one of the lowest figures available in rigid floor insulation, giving the 25mm board a thermal resistance of 1.30 m²K/W. Its compressive strength of 120 kPa allows it to carry screed and normal floor loads, and it is faced on both sides with a glass tissue based facing, autohesively bonded to the core during manufacture. Because phenolic reaches a given thermal resistance in less thickness than higher-lambda materials, the 25mm board adds insulation where there is little room to spare.

The glass tissue facing keeps the board straightforward to handle and to cut cleanly, either with a fine-toothed saw or by scoring and snapping. Single sheets can therefore be trimmed accurately to fit between joists or sized into perimeter strips with minimal waste, which matters most on smaller areas and finishing work where offcuts add up.

25mm Kooltherm K103 is used in beam and dense block ground floors, suspended timber ground floors and solid concrete ground floors, installed below the screed, below the slab or between the joists depending on the construction. It should be laid over a damp proof membrane, installed in line with Kingspan's guidance and the project specification, and is not fitted directly over timber joists. It suits new build and refurbishment across domestic, commercial and industrial floors, and is compatible with underfloor heating systems.

Key Benefits of Kingspan Kooltherm K103:

• 25mm rigid thermoset phenolic floor insulation board, the thinnest board in the K103 range.
• Thermal conductivity of 0.019 W/mK, one of the lowest available for rigid floor insulation.
• Thermal resistance (R-value) of 1.30 m²K/W for the 25mm board.
• Compressive strength of 120 kPa, allowing it to carry screed and normal floor loads.
• Glass tissue facing on both sides for clean cutting and straightforward handling.
• Single sheets of 2.4m x 1.2m covering 2.88m² each, so you order the exact quantity the job needs.
• Suitable for beam and block, suspended timber and solid concrete floors, below slab or screed.
• Adds thermal performance where floor build-up height is limited, including garage conversions and topping-up layers.
• Cuts down to form the minimum 25mm perimeter upstand strips that limit cold bridging at floor edges.
• Compatible with underfloor heating systems and suited to both new build and refurbishment.

Good to know:

• Kingspan Kooltherm K103 must be installed over a damp proof membrane (minimum 1200 gauge polythene) and is not recommended in direct contact with subsoil.
• The board resists the passage of water vapour but is not a vapour control layer; below a screed it should be overlaid with a separate polythene sheet, lapped and taped, to act as the VCL.
• It is not suitable for installation directly over timber joists; in suspended floors it sits between the joists on battens, saddle clips or nails.
• At 25mm the board's 1.30 m²K/W resistance suits upgrade, top-up, multi-layer and perimeter-strip use; check the full floor build-up against your target U-value before ordering.

Buying for a whole floor? This board is also available by the pack (12 sheets, 34.56m²): view the K103 pack.

When floor depth is the constraint, the 25mm board adds thermal performance in the slimmest layer in the range, sheet by sheet.