# Insulation Calculator

> Calculate exactly how much insulation you need for loft, walls, or floor. Get roll quantities, thickness recommendations, and R-value guidance for your project.

Canonical: https://renoquant.com/insulation-calculator
Prices updated: 2026-03

## At a glance

Current EU building regulations target 270 mm depth in lofts, 100 mm in cavity walls, and 50–100 mm of PIR board on internal solid walls or floors. A 50 m² loft at 270 mm needs about 14 rolls of mineral wool (~EUR 400–800 in materials), and is the single highest-ROI insulation upgrade — typical heating-bill savings of 15–25%. R-value scales with thickness: 100 mm of mineral wool gives R≈2.5, vs R≈4.5 for 100 mm of PIR.

## 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](/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](/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

1. **Measure twice** — Double-check your length and width measurements before ordering. A 10cm error over a 10m loft adds 1 m2 of missing coverage.
2. **Account for obstacles** — Lofts have pipes, cables, water tanks, and hatches. Add 10% waste to your calculated area.
3. **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.
4. **Buy from one batch** — Insulation rolls from the same batch have consistent thickness and density. Mixing batches can create uneven coverage.
5. **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](/exterior-paint-calculator) can estimate the paint needed to finish the outside face of the wall.

## Frequently Asked Questions

### How much insulation do I need?

Measure the area to insulate (length x width) and multiply by the recommended thickness for that location. For lofts, the recommended depth is 270mm. For cavity walls, 100mm is standard. Our calculator does this automatically and tells you how many rolls or batts to buy.

### What thickness of insulation should I use?

The recommended thickness depends on where you are insulating. Lofts need 270mm total depth to meet current building regulations. Cavity walls typically use 100mm batts. Solid wall insulation (internal) uses 50-100mm PIR board. Floors need at least 100mm of mineral wool or 50-70mm of PIR board.

### What is R-value and why does it matter?

R-value measures thermal resistance — how well a material resists heat flow. Higher R-values mean better insulation. R-value depends on both the material type and thickness. For example, 100mm of mineral wool gives an R-value of about 2.5, while 100mm of PIR board gives about 4.5. Building regulations specify minimum R-values for each part of the building.

### Do I need a vapour barrier with insulation?

A vapour barrier is recommended on the warm side of insulation in walls and ceilings to prevent moisture from condensing inside the insulation. In lofts with cold ventilated roof spaces, a vapour barrier is not usually needed if the loft is well-ventilated. Always check local building regulations for your specific situation.

### Can I install insulation myself?

Loft insulation is one of the easiest DIY jobs — simply roll out mineral wool between and over joists. Wall insulation (cavity or solid wall) should be done by certified professionals as it requires specialist equipment and incorrect installation can cause damp problems. Floor insulation is a moderate DIY task if you have access from below.

### How do I calculate the area I need to insulate?

For rectangular spaces, multiply the length by the width in metres. For L-shaped rooms, split the area into two rectangles and add them together. For lofts, measure the footprint of the house at ground level — the loft area is approximately the same. Deduct any areas you do not need to insulate, such as loft hatches.

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Interactive version with calculator: https://renoquant.com/insulation-calculator. Figures are estimates; final quotes vary by site conditions, materials, and region.