Insulation Calculator

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:

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:

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:

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:

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):

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 can estimate the paint needed to finish the outside face of the wall.

How Much Does Insulation Cost?

The total cost depends on the insulation type, the area to cover, whether you hire a professional, and the complexity of the installation.

Material Cost by Insulation Type

The "equivalent R-value" column shows what you'd pay to match the thermal resistance of 100mm of mineral wool using each material.

DIY vs Professional Installation Costs

Cavity wall and external wall insulation require certified professionals and specialist equipment — they are not DIY projects.

Energy Savings and Payback Period

Solid wall insulation has a long payback on cost alone, but it also improves comfort, cuts draughts, and raises property value — factors the payback figure misses.

Cost-Saving Tips

1. Start with the loft — fastest payback, easiest DIY job; a single weekend can save EUR 250+ per year.
2. Check for grants first — schemes can cover 50-100% of the cost for eligible households. Apply before buying.
3. Buy in bulk — multi-packs are 15-25% cheaper per m2 than individual rolls.
4. Top up, don't replace — if existing loft insulation is dry, add a layer on top rather than removing it.
5. Combine projects — adding insulation during other building work is far cheaper than retrofitting later.
6. Compare three quotes — professional prices vary significantly, especially for cavity and solid wall work.

Loft Insulation: Depth, Top-Up & Installation

Loft insulation is the single most cost-effective energy upgrade. Heat rises, and an under-insulated loft loses up to 25% of a home's heat through the roof.

The 270mm Two-Layer Standard

How to Top Up Existing Insulation

Do not remove sound existing insulation — top it up. To board the loft for storage, raise the boards on loft legs so you never compress the insulation: compressing 270mm down to 100mm halves its thermal performance.

Common Loft Mistakes to Avoid