Floor Insulation Calculator — Thickness & Quantities

UK Building Regulations Part L 2021 requires a U-value of 0.18 W/m²K or better for new-build ground floors. For renovation of existing ground floors (threshold trigger 0.70 W/m²K), the improved target is 0.25 W/m²K. Some energy schemes like Passivhaus target 0.15 W/m²K or better.

The most common method is to fit mineral wool batts or rigid PIR boards between the joists from below (if accessible via a cellar or crawl space) or from above after lifting floorboards. Support the insulation with netting stapled to the joists. A vapour control layer should be fitted on the warm (room) side.

To achieve the Part L 2021 new-build target of 0.18 W/m²K on a suspended timber floor: PIR board (Celotex GA4000-grade, λ 0.022) needs about 110mm (a 120mm board), phenolic board (Kingspan Kooltherm K103, λ 0.019) about 95mm (a 100mm board), mineral-wool slab (Rockwool Flexi, λ 0.035 at ≥140mm) about 175-180mm (a 180mm slab, the thickest single Flexi slab), and EPS 70 (λ 0.038) about 190mm (a 200mm board). For the renovation target of 0.25 W/m²K, PIR needs about 75-80mm. These figures use the simple layer-sum method, which is correct for suspended floors; solid concrete and beam-and-block floors must be sized with the BS EN ISO 13370 ground-floor method instead.

For suspended timber floors, yes — a VCL (polythene membrane) should be installed on the warm side of the insulation to prevent moisture from the room condensing within the insulation. For solid concrete floors, the DPM (damp proof membrane) serves this function. Overlap all VCL joints by 150mm and tape all seams.

Split the floor into rectangles and enter each one as its own section using the Add section button; the calculator adds the areas together for you. Rigid boards and mineral-wool packs are worked out per section and rounded up before being added, which covers the extra cutting where the sections meet and never under-orders. The VCL membrane is priced on the combined floor area, while the VCL tape estimate uses each section's full perimeter, which slightly over-counts at the join, so treat it as a safe upper figure. The insulation thickness itself does not change with room shape. Use three sections for a T or U shaped floor.

Rigid PIR boards (Celotex GA4000-grade, λ 0.022 W/mK) and phenolic boards (Kingspan Kooltherm K103, λ 0.019 W/mK) achieve the required U-value with much less thickness than mineral-wool slab (Rockwool Flexi, λ 0.038 W/mK at 50-120mm, improving to 0.035 W/mK at 140mm and above). However, mineral wool is cheaper, easier to cut around pipes and services, and provides better acoustic insulation. Where depth between joists is limited, a rigid board is often the better choice.

Unlike a suspended floor, a ground-bearing floor cannot be sized by simply adding up the resistances of its layers. Heat is lost to the ground at the edges, so the U-value depends on the floor's perimeter-to-area ratio as well as its construction. The governing standard is BS EN ISO 13370 (with the conventions in BRE BR 443), which is referenced by Approved Document L. The same insulation thickness gives a different U-value for a small floor than for a large one. The thickness this calculator shows for solid concrete or beam-and-block floors uses the simpler layer-sum method, which ignores the ground's resistance and therefore over-states the insulation needed — treat it as a conservative starting point and confirm the final specification with a full ISO 13370 calculation.