Loft Insulation Calculator

Check insulation thickness and rolls per layer for a worked project comparison and purchasing advice.

How to budget for loft insulation

Loft insulation is sold by roll coverage at a stated thickness. Reaching the target depth often means one layer between joists and another across them. Record existing depth, measure only the insulated footprint and avoid blocking ventilation.

Use the calculator for a quantity-led starting point, then check it against the chosen manufacturer's technical information and a supplier quote. Product dimensions, coverage and pack sizes change, and a low advertised unit rate can be offset by delivery, minimum orders or accessories. Keep the measurements, quantity calculation and cost assumptions together so they can be updated without remeasuring the project.

Typical UK loft insulation prices

The figures below are illustrative UK ranges as at September 2026, including VAT where normally charged. Supply-only means the main named material and does not automatically include preparation, delivery, accessories or waste. Supplied-and-fitted ranges are broad consumer budgeting figures, not quotations; London, difficult access and unusually small jobs can cost more.

Prices are shown per m² or roll as appropriate. Compare like with like: clarify VAT, delivery, removal of old material, preparation, finishing and guarantees. A contractor rate may include several of these, while a retailer's headline price generally does not. Obtain current local prices before ordering or agreeing work.

Worked example: calculation step by step

An 8m × 6m loft has a footprint of 48m². For a full two-layer mineral-wool installation, the calculator treats this as 48 × 2 = 96m² of layer coverage. If each roll covers 8.3m² at its stated thickness, 96 ÷ 8.3 = 11.57, so buy 12 rolls.

At £24.50 per roll, the insulation is 12 × £24.50 = £294. Add £45 for protective equipment, a hatch seal and small ventilation accessories, making a DIY material allowance of £339. If installers quote £21 per footprint square metre supplied and fitted, the broad comparison is 48 × £21 = £1,008.

Do not automatically double area if one suitable layer already exists. Measure its depth and condition. Damp, compressed or contaminated insulation may require investigation or replacement rather than simply covering it.

What drives the cost?

• Existing insulation depth and condition. Ask whether this is included in a price and what assumption has been made. A change here can affect material quantity, labour time and the supporting work, so it deserves its own budget line rather than being hidden in a general contingency.
• Required thermal performance and material type. Ask whether this is included in a price and what assumption has been made. A change here can affect material quantity, labour time and the supporting work, so it deserves its own budget line rather than being hidden in a general contingency.
• Roll coverage at the specified thickness. Ask whether this is included in a price and what assumption has been made. A change here can affect material quantity, labour time and the supporting work, so it deserves its own budget line rather than being hidden in a general contingency.
• Access, eaves detail and water tanks or services. Ask whether this is included in a price and what assumption has been made. A change here can affect material quantity, labour time and the supporting work, so it deserves its own budget line rather than being hidden in a general contingency.
• Raised boarding, hatch work and electrical precautions. Ask whether this is included in a price and what assumption has been made. A change here can affect material quantity, labour time and the supporting work, so it deserves its own budget line rather than being hidden in a general contingency.

Project scale also changes the unit cost. A contractor still has travel, setup and administration on a small job, while larger orders may attract delivery efficiencies. Geography matters too: labour and parking are commonly higher in London and parts of the South East, while remote deliveries can carry their own premium. Seasonal demand may change lead times even where the quoted rate is stable.

Wastage allowance

Allow 5–10% where rolls must be cut around trusses, tanks and awkward eaves. A clear rectangular loft with joist spacing matched to the roll may sit near 5%; complex roofs can need 10–15%. Offcuts can sometimes fill small bays, but insulation must not be compressed, stuffed into ventilation gaps or used where a fire-safe clearance is required.

Apply waste to quantity before rounding to whole packs, boards, bags or rolls. Rounding first and then adding a percentage can either overbuy or leave an unusable fraction. If unopened stock is returnable, check the return window and batch restrictions. Do not assume every surplus item can be returned: special orders, mixed products and cut lengths are commonly excluded.

Measure and plan before buying

Draw the area and label every dimension instead of relying on one length and width from memory. Split L-shaped or stepped spaces into rectangles, calculate each one, and add the results. Measure in the same unit throughout. Recheck the narrowest access, floor levels, corners and fixed obstacles because these can determine product size, installation sequence or whether specialist equipment is needed.

Read the technical sheet before substituting one product for another. Two items sold for the same purpose may have different coverage, fitting centres, thicknesses or base requirements. Check compatibility between the main material and its fixings, adhesive, membrane or substrate. Manufacturer instructions normally take priority over a general calculator and may be required to preserve a warranty.

DIY budget versus a fitted quote

A DIY total should include consumables, personal protective equipment, tool hire, delivery and disposal, not only the finish material. It is sensible to keep a separate contingency of around 5–10% of the complete budget for discoveries and price movement. That contingency is different from material waste: wastage buys enough physical product, while contingency protects the cash budget.

For professional work, seek an itemised written quote. It should identify preparation, material specification, quantity, waste ownership, access assumptions, protection, cleanup, disposal, VAT, payment stages and guarantee. Ask what would count as a variation. Comparing itemised scopes is more reliable than choosing the lowest total where important work may be omitted.

Using this estimate responsibly

The result is a planning estimate, not a structural design or site survey. Hidden defects, moisture, unsuitable ground and regulated work cannot be diagnosed from dimensions alone. Confirm structural, fire, electrical, ventilation and planning requirements with a suitably competent person where they apply. Stop and investigate if the existing surface is damp, unstable or contaminated.

Save the exact product details used in the calculation, including pack coverage and unit price. When a quote or retailer price changes, update that input rather than altering the measured area. This makes the estimate auditable and helps distinguish a genuine price change from a quantity error.

Frequently asked questions