Insulation Calculator
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How Much Insulation Do I Need?
Calculating the right amount of insulation before you buy prevents costly shortfalls and wasteful over-ordering. The amount you need depends on three factors: the area to cover, the required thickness, and the roll or batt dimensions of your chosen product. After fitting wall insulation, you will need to cover it with plasterboard — use our drywall calculator to estimate sheets, screws, and compound.
Area Calculation
For most insulation projects, the area calculation is straightforward:
| Location | How to Measure | Typical Size (UK/EU 3-bed semi) |
|---|---|---|
| Loft | House footprint (length x width) | 35-50 m2 |
| External walls (cavity) | Wall height x perimeter, minus windows/doors | 80-120 m2 |
| Internal solid walls | Wall height x wall length per wall | 6-15 m2 per wall |
| Ground floor | Room length x width for each room | 40-70 m2 |
| Suspended timber floor | Same as ground floor | 40-70 m2 |
For irregular shapes, break the area into rectangles, calculate each one, and add the totals together. Always add 5-10% extra to account for cutting waste around obstacles, pipes, and wiring.
Recommended Thickness by Location
Building regulations specify minimum insulation levels. These are the current recommended thicknesses for new installations:
| Location | Mineral Wool | PIR Board (Celotex/Kingspan) | Target U-value |
|---|---|---|---|
| Loft (between joists) | 100mm first layer | Not typical for lofts | — |
| Loft (over joists) | 170mm second layer | Not typical for lofts | 0.16 W/m2K |
| Loft (total depth) | 270mm | — | 0.16 W/m2K |
| Cavity wall | 100mm batts | 50-75mm boards | 0.30 W/m2K |
| Solid wall (internal) | 60-100mm + plasterboard | 50-70mm + plasterboard | 0.30 W/m2K |
| Ground floor | 100-150mm | 75-100mm | 0.25 W/m2K |
| Flat roof | 150-200mm | 100-140mm | 0.18 W/m2K |
For loft and roof insulation projects, our roofing calculator can help you estimate tiles, battens, and felt if the roof covering also needs replacing.
Standard Roll and Batt Sizes
Insulation products come in standard sizes. Knowing these helps you convert your area into a shopping list:
| Product | Width | Length | Thickness Options | Coverage per Pack |
|---|---|---|---|---|
| Mineral wool roll | 1200mm (fits between 600mm joists x2) | 4.2-8.4m | 100, 150, 170, 200mm | 5.0-10.1 m2 |
| Mineral wool roll (narrow) | 600mm (fits between joists) | 4.2-8.4m | 100, 150, 170, 200mm | 2.5-5.0 m2 |
| Mineral wool batt (wall) | 455mm (fits between studs) | 1200mm | 50, 75, 100mm | 0.55 m2 per batt |
| PIR insulation board | 1200mm | 2400mm | 25, 50, 75, 100mm | 2.88 m2 per board |
| Sheep wool roll | 570mm | 4000-6000mm | 100, 140mm | 2.3-3.4 m2 |
R-Value Reference Table
R-value (thermal resistance) varies by material and thickness. Use this table to check that your chosen insulation meets building regulation requirements:
| Material | Thermal Conductivity (W/mK) | R-value at 50mm | R-value at 100mm | R-value at 150mm | R-value at 200mm |
|---|---|---|---|---|---|
| Mineral wool (glass/rock) | 0.044 | 1.14 | 2.27 | 3.41 | 4.55 |
| PIR board (Celotex/Kingspan) | 0.022 | 2.27 | 4.55 | 6.82 | 9.09 |
| Expanded polystyrene (EPS) | 0.034 | 1.47 | 2.94 | 4.41 | 5.88 |
| Sheep wool | 0.038 | 1.32 | 2.63 | 3.95 | 5.26 |
| Cellulose (blown) | 0.040 | 1.25 | 2.50 | 3.75 | 5.00 |
| Spray foam (closed cell) | 0.025 | 2.00 | 4.00 | 6.00 | 8.00 |
R-value is calculated as thickness (in metres) divided by thermal conductivity. A higher R-value means better insulation performance.
Quick Estimation: Rolls Needed for Common Loft Sizes
For 270mm loft insulation (100mm between joists + 170mm over joists):
| Loft Size | Area | 100mm Rolls (between joists) | 170mm Rolls (over joists) |
|---|---|---|---|
| Small terrace | 25 m2 | 3-4 rolls | 3-4 rolls |
| Semi-detached | 35 m2 | 4-5 rolls | 4-5 rolls |
| Detached (3-bed) | 50 m2 | 6-7 rolls | 6-7 rolls |
| Large detached | 70 m2 | 8-9 rolls | 8-9 rolls |
Based on rolls covering approximately 8 m2 each. Always round up and buy one extra roll per layer for cutting waste around pipes, wires, and awkward corners.
Tips for Accurate Calculation
- Measure twice — Double-check your length and width measurements before ordering. A 10cm error over a 10m loft adds 1 m2 of missing coverage.
- Account for obstacles — Lofts have pipes, cables, water tanks, and hatches. Add 10% waste to your calculated area.
- Check existing insulation — If you already have some insulation, measure its depth. You may only need a top-up layer to reach the recommended 270mm.
- Buy from one batch — Insulation rolls from the same batch have consistent thickness and density. Mixing batches can create uneven coverage.
- Do not compress insulation — Squashing mineral wool reduces its R-value. If it is 100mm thick, it needs 100mm of space to perform correctly.
If you are insulating exterior walls as part of a broader renovation, our exterior paint calculator can estimate the paint needed to finish the outside face of the wall.
These calculations are estimates only. Actual requirements may vary depending on surface conditions, product specifications, and installation methods. Always consult a qualified professional for precise measurements.
Prices updated: 2026-03Frequently Asked Questions
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