What U-value does my floor need to achieve?

UK Building Regulations Part L requires a U-value of 0.25 W/m²K or better for new-build ground floors. For renovations and extensions, the same target applies when the floor is being replaced or significantly altered. Some energy schemes like Passivhaus target 0.15 W/m²K for ultra-low energy performance.

How do I insulate a suspended timber floor?

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.

What thickness of insulation do I need for a floor?

To achieve the Building Regs target of 0.25 W/m²K: PIR board (Celotex/Kingspan) needs approximately 75-100mm, mineral wool needs approximately 120-150mm, and EPS needs approximately 130-150mm. The exact thickness depends on the floor construction and other thermal elements.

Do I need a vapour control layer under floor insulation?

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.

Is PIR or mineral wool better for floor insulation?

PIR boards (Celotex, Kingspan) achieve the required U-value with less thickness due to their lower thermal conductivity (0.022 vs 0.035 W/mK). However, mineral wool is cheaper, easier to cut around pipes and services, and provides better acoustic insulation. For limited depth between joists, PIR is often the better choice.

Floor Insulation Calculator — Thickness & Quantities

Calculate floor insulation thickness to meet UK Building Regulations Part L. Mineral wool, PIR board and EPS quantities.

Floor Length

Longest dimension of the area

Floor Width

Shortest dimension of the area

Floor Construction

Select the floor construction for your calculation

Insulation Type

Each type has different thermal performance

Target U-Value

0.25 W/m²K meets Building Regs Part L

Include wastage allowance

Adds 10% for cutting and fitting around obstacles

Wastage Percentage

10% standard, 15% if many pipes/obstacles

Include vapour control layer (VCL)

Polythene membrane for warm side of insulation

Insulation Price per m² (optional)

Enter your supplier price for a cost estimate

How We Calculate This

This calculator determines the insulation thickness needed to achieve your target U-value, then calculates the quantity of insulation material required including wastage allowance.

The formula

R_insulation = (1 / U_target) - R_floor - R_si - R_se
Thickness = R_insulation × Lambda × 1000 (in mm)

Where:

R_si = 0.17 m²K/W (internal surface resistance)
R_se = 0.04 m²K/W (ground surface resistance)
R_floor = varies by construction type

Insulation lambda values

Mineral Wool: 0.035 W/mK
PIR Board (Celotex/Kingspan): 0.022 W/mK
EPS: 0.038 W/mK
Spray Foam: 0.025 W/mK

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.