UK insulation questions, answered

λ (lambda) is a material's thermal conductivity in W/m·K; lower is better. An R-value is a layer's resistance to heat flow: thickness in metres ÷ λ, in m²K/W; higher is better. A U-value is the heat flow through a whole roof, wall or floor in W/m²K; lower is better. Building Regulations targets are U-values.

The Energy Saving Trust recommends a minimum of 270 mm of mineral wool. In our model, 100 mm of λ 0.044 loft roll between the joists plus 170 mm across them gives U-value 0.16 W/m²K, the Part L standard for upgrading a loft. Use the loft insulation calculator for your own loft.

Usually, yes. Laying loft roll between and across the joists is a realistic DIY job if the loft is easy to get into. Work from boards laid across the joists, wear an FFP2 mask, goggles and gloves, and keep the eaves ventilation clear. For awkward lofts, blown insulation is an installer job.

Measure the wall thickness at a window or door. The Energy Saving Trust says a brick wall 260–350 mm thick is likely to have a cavity, and a thinner one is probably solid. An installer can confirm it.

The Energy Saving Trust says the cavity should be at least 50 mm wide and clear of rubble. The brickwork should be sound, and walls exposed to driving rain need careful assessment. A registered installer should survey first and offer a guarantee.

No. It's blown in through holes drilled in the outer wall, using specialist equipment, after a survey. The Energy Saving Trust puts the cost at about £2,200 for a typical home.

Internal wall insulation can be done room by room but takes some floor space and needs careful vapour control. External insulation keeps your rooms the same size but changes how the house looks and may need planning permission. The Energy Saving Trust puts them at about £12,000 and £15,000 for a whole house.

Fit mineral wool or rigid board between the joists, supported by netting stapled to the joists, working from below or by lifting the boards. Keep the airbricks clear and seal the gaps between boards. About 150 mm of mineral wool reaches U-value 0.25.

Sometimes. Government and energy company schemes change, and eligibility often depends on your income, your home and where you live. Ask your energy supplier and your local council what's currently available. We don't list amounts because they change.

It depends on the work. Approved Document L counts things like dry-lining, re-rendering and cavity wall insulation as renovation, and renovating more than half of an element triggers an upgrade. Ask your installer or local building control whether your job needs to be notified.

Leave it alone. Vermiculite is a pebbly loose fill that may contain asbestos; the HSE lists loose-fill insulation among the places asbestos can be found. Have it checked by a professional before any work that would disturb it.

No. Our targets are England's, from Approved Document L. Wales and Scotland set their own, with tighter values for some new elements, so check your nation's guidance.

For rolls and boards, no: it's the material cost at Wickes, Toolstation and Travis Perkins prices from 14 October 2026, plus 10% waste. Cavity fill and blown loft insulation show estimated installed prices based on Energy Saving Trust figures.

It follows BS EN ISO 6946 with BR 443 conventions and is calibrated to published results, such as 270 mm of loft roll giving U 0.16. It's a planning estimate, not a SAP or RdSAP assessment, and it doesn't check condensation risk. Our methodology lists every assumption.