Floor Insulation Calculator
Calculate insulation boards, recommended thickness and U-value compliance for solid concrete and suspended timber floors. Includes DPM estimate and 2026 Building Regs targets.
PIR: highest performance per mm. Best for solid floors where build-up height matters. Standard board 2400×1200mm.
0.18 W/m²K is the 2026 Building Regulations Part L target for new ground floors.
Perimeter of the floor where it meets external walls. Used to calculate the P/A ratio and recommended thickness.
Only enter this if not all walls are external — e.g. mid-terrace (2 external walls), single room in a larger building, or party wall with a neighbour. Leave blank to use the full room perimeter.
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Installation Guide
How to Insulate a Ground Floor
The P/A Ratio — Why Floor Shape Matters
Heat escapes from a ground floor primarily through the edges where the floor meets the external walls, not uniformly across the surface. The Perimeter-to-Area ratio (P/A) captures this — a room with a high P/A ratio has more edge per m² of floor and therefore loses more heat.
A large open-plan kitchen (8×6m) has a P/A of 0.35. A small bathroom (2×2m) has a P/A of 2.0. The bathroom needs roughly double the insulation thickness to achieve the same U-value. This is why a single insulation thickness specified for an entire house is often wrong — each room should be calculated individually.
Solid Concrete Floor — Build-Up Order
The correct build-up for a solid concrete floor from bottom to top: concrete slab → 1200-gauge polythene DPM (lapped up edges 150mm) → PIR or EPS insulation boards → sand/cement screed or liquid screed → floor finish.
Butt insulation boards tightly together with no gaps. Stagger joints between layers if using two layers. Tape all joints with foil tape to prevent thermal bridging. The DPM must turn up at all edges and be dressed into the wall cavity — never terminate at floor level.
Suspended Timber Floor — Between Joists
For suspended timber floors, mineral wool slabs are usually preferred as they friction-fit snugly between joists without cutting. The standard approach is to staple breather membrane netting between joist lower edges to support the insulation, then push mineral wool slabs up to friction-fit from below.
A vapour control layer (VCL) must be fitted above the insulation and below the floor deck. Do not use a VCL below the insulation in a suspended timber floor — you need free air movement below to prevent condensation. Ensure airbricks in external walls remain unobstructed.
Choosing Between PIR, EPS and Mineral Wool
PIR (polyisocyanurate): highest thermal performance (λ ≈ 0.022 W/mK), smallest build-up thickness. Best where floor-to-ceiling height is critical. More expensive. Use where every mm of build-up matters.
EPS (expanded polystyrene): good performance (λ ≈ 0.038 W/mK), lower cost, easy to cut. Needs roughly 70% more thickness than PIR for the same U-value. Excellent under beam-and-block or where budget matters.
Mineral wool slabs: semi-rigid, friction-fit between joists. Excellent acoustic performance (unlike PIR). Non-combustible. Best for suspended timber floors. Not suitable under screed.
FAQ
Common Questions
This depends on your floor's P/A ratio and insulation type. For a typical semi-detached room (P/A ~0.5) using PIR board, 70mm achieves the 2026 Building Regs target of 0.18 W/m²K. Smaller or narrower rooms need more. Our calculator works this out from your actual dimensions.
Under Approved Document L (2026), new ground floors in England must achieve a U-value of 0.18 W/m²K or better. For extensions, the notional target is also 0.18. For renovation work upgrading an existing floor, 0.25 W/m²K is sometimes accepted when the full target cannot be achieved without excessive build-up.
Yes for solid concrete floors. A 1200-gauge polythene DPM is laid on the concrete slab before the insulation. Lap all joins by 300mm and tape. For suspended timber floors, a vapour control layer (VCL) is fitted above the insulation and below the floor deck — never below the insulation, as you need free airflow underneath.
Standard PIR boards are 2400×1200mm, covering 2.88m² each. Divide your floor area by 2.88 and add 10% wastage for cuts. A 20m² floor needs approximately 8 boards. Our calculator gives the count for your dimensions including wastage.
Yes. Existing concrete floors are typically insulated by laying PIR or EPS boards on top of the existing slab, then covering with a new screed or floating floor deck. This raises the floor level by the insulation thickness plus the screed depth — typically 90–130mm total. Check door heights, step heights and DPC levels before proceeding.