Insulation Calculator
Loft roll, PIR board, mineral wool, insulated plasterboard, loose fill, and pipe lagging
10% is typical; increase to 15-20% for awkward shapes or many joists/rafters to work around.
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Frequently Asked Questions
What thickness of insulation do I need to meet Building Regulations?
Approved Document L U-value targets for new build: external walls 0.18 W/m²K; roofs 0.13 W/m²K; ground floors 0.13 W/m²K. Required thickness = lambda / U-value. A PIR board (lambda 0.022 W/mK) needs approximately 120 mm to achieve 0.18 W/m²K. Mineral wool (lambda 0.035-0.044 W/mK) needs greater thickness for the same result.
What is the difference between PIR, mineral wool, and EPS insulation?
PIR (lambda ~0.022-0.028 W/mK): thinnest boards, best U-values - ideal where space is limited (floors, roofs, cavities). Mineral wool (lambda ~0.035-0.044 W/mK): non-combustible (Class A1), excellent acoustic performance - used in lofts, stud walls, party walls. EPS (lambda ~0.033-0.038 W/mK): used for EWI, floor slabs, and below-DPC applications.
How much loft insulation do I need?
Recommended total depth in a cold loft: 270 mm of mineral wool - typically 100 mm between joists + 170 mm across joists at 90°. For a 50 m² loft floor: approximately 100 m² of roll total (50 m² each layer). Always leave a 25 mm ventilated air gap between insulation and roof felt at the eaves.
Can I insulate a solid concrete floor?
Yes - insulate from above by laying rigid PIR or EPS boards on the existing slab, then a new screed or floating timber floor on top. 100 mm PIR (lambda 0.022 W/mK) achieves approximately 0.21 W/m²K. Note: insulating raises the finished floor level by the board thickness plus screed - allow for this at doorways and thresholds.
What insulation should I use for a flat roof?
Warm roof (preferred): PIR boards above the structural deck, below the waterproofing - typically 100-150 mm to achieve 0.13-0.18 W/m²K. Keeps the deck above the dew point, avoids condensation. Inverted roof: insulation above the waterproofing - use XPS (extruded polystyrene) which is moisture-resistant. Cold roof: insulation between and below rafters with ventilated air gap above - more condensation risk, less common in new work.
What is a U-value and how is it calculated?
A U-value (W/m²K) measures heat loss through a building element per m² per degree of temperature difference. Lower = better insulation. Calculated by summing R-values of all layers (R = thickness in metres ÷ lambda) plus surface resistances, then U = 1 ÷ total R. Example: 120 mm PIR (R = 0.12 / 0.022 = 5.45) + blockwork + plasterboard + surface resistances gives total R ~6.1, so U ~0.16 W/m²K.