When installing underfloor heating (UFH) within a concrete slab, insulation beneath the system is essential. It ensures the heat rises into the room rather than being lost into the ground or subfloor. Whether you’re working on a new build, extension, or retrofit project, a properly insulated base will improve efficiency, reduce heat-up times, and lower long-term energy costs. This guide explains how to correctly insulate beneath a heated concrete floor — from base prep to insulation board placement — for a high-performance result.
Begin by excavating to the required depth — typically to allow for hardcore, blinding, insulation, and the full concrete slab build-up. The sub-base should be compacted Type 1 MOT or similar hardcore to a minimum of 100–150 mm, ensuring it’s firm and level. Add a 25–50 mm sand blinding layer over the hardcore to protect the insulation from sharp edges and help with final levelling. This forms the stable foundation on which all subsequent layers are built.
Depending on site conditions and insulation type, a damp-proof membrane (DPM) may be laid before or after insulation. In most builds, it's placed below the insulation, directly over the sand blinding. This prevents ground moisture from affecting the insulation or entering the slab. Use 1200-gauge polythene, carefully taping joints and running up walls to create a full seal. Some high-spec insulation boards come with integrated foil layers that serve as vapour barriers, which may influence placement.
Lay rigid insulation boards across the floor area, covering the entire slab footprint. PIR or XPS boards (such as Celotex or Kingspan) are typically used, with thicknesses ranging from 75 mm to 150 mm depending on U-value requirements. Boards should be laid tightly together with staggered joints, avoiding gaps that can cause thermal bridging. Trim as needed to fit around penetrations. Tape all joins and perimeter edges to maintain insulation integrity and prevent slab leakage.
Install perimeter insulation along all walls and upstands to prevent thermal bridging and allow for slab expansion. This insulation is usually a flexible strip or pre-formed edge board, typically 25 mm thick, running full height of the slab depth. It provides both thermal and structural decoupling between the heated floor and cold masonry walls. This step is crucial to prevent heat loss at slab edges — one of the most common failure points in poorly insulated floors.
Once insulation is laid, fix UFH pipework directly on top. Most systems use clip rails, staples, or fixing systems that penetrate slightly into the insulation to secure the pipe layout. This stage should follow a pre-drawn heating design with loop lengths, zones, and spacing already planned. Pipes are usually spaced between 100–200 mm depending on heat load. Ensure all pipe runs are fixed flat and evenly spaced before moving on to pouring the slab.
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