Loft insulation is usually worth considering if your loft is bare or has only a thin, patchy layer. Energy Saving Trust estimates that insulating a previously uninsulated loft to around 270mm could save about £190 a year in Great Britain and £230 in Northern Ireland. The estimated savings from topping up 120mm to 270mm are much smaller: £17 a year in Great Britain and £20 in Northern Ireland. These are examples, not guaranteed savings for your home. Energy Saving Trust’s published insulation savings explain why the amount already in your loft matters.
The practical answer depends on your starting point. A bare loft can offer a relatively quick bill saving. A top-up from 120mm may be worthwhile for comfort or as part of other work, but it is less compelling on bill savings alone. Check the insulation depth and condition before you pay for more.
Loft insulation costs and savings
The table combines Energy Saving Trust’s installation cost examples with its published annual bill-saving estimates. The figures are useful for comparing scenarios, but they are not a quote or a household-specific forecast.
| Starting point | Estimated installation cost | Estimated annual bill saving | Simple payback from those figures |
|---|---|---|---|
| No insulation to about 270mm | £750 in Great Britain; £1,100 in Northern Ireland | £190 in Great Britain; £230 in Northern Ireland | About 4 years in Great Britain; about 5 years in Northern Ireland |
| 120mm to about 270mm | £600 in Great Britain; £700 in Northern Ireland | £17 in Great Britain; £20 in Northern Ireland | About 35 years in either case |
Simple payback is the estimated installation cost divided by the estimated annual saving. It does not account for changing energy prices, the price you are quoted, finance costs, DIY costs or the value you place on comfort. The actual cost also depends on the size and layout of the loft, access, material and any repairs needed.
Published estimates can differ. A UK Government Clean Energy campaign page gives an estimate of around £40 a year for adding the recommended 270mm to 300mm where some insulation already exists. Its footnote describes a gas-heated semi-detached home in EPC band D and says the estimate uses the October 2025 price cap. It does not give the same starting depth as Energy Saving Trust’s 120mm example, so the figures are not direct alternatives. Treat both as illustrations, not a promise of what you will save.
What does 270mm of loft insulation mean?
For mineral wool rolls, around 270mm is a commonly used benchmark in Energy Saving Trust guidance. If you find 120mm of mineral wool, adding about 150mm would bring it to that depth. Other insulation materials have different performance, so thickness alone is not a fair way to compare them. Check the product specification or ask the installer to explain the proposed material and depth.
In a typical cold loft that is not used as a room, insulation is laid at ceiling level, between and above the joists. Insulating the roof slopes instead can keep a loft room warmer, but it needs a suitable design and professional installation. Flat roofs, converted lofts, damp or condensation problems, and difficult access also call for professional advice.
When is DIY loft insulation reasonable?
DIY may be suitable when the loft is easy to access, dry, and has a straightforward layout. Before starting, read a full guide such as Energy Saving Trust’s loft insulation instructions and follow the product manufacturer’s guidance. If there is no safe, suitable way to reach the area you need to work on, use an installer.
Do not cover electrical or lighting cables with insulation. Energy Saving Trust warns that this can create a fire risk, and advises speaking to a qualified electrician if you are unsure. Check recessed lights and any insulation guards before work begins.
Do not cover eaves, tile or wall vents. Insulation can make the loft space colder, and obstructing ventilation can contribute to damp or condensation. In England, GOV.UK’s existing-home ventilation guide says energy-efficiency work must not make home ventilation worse. The guide applies to England; if you live elsewhere in the UK, ask your installer about the requirements that apply where you live.
Checks to make before you pay or start
- Measure what is already there. Check the depth at several accessible points. Uneven, compressed or missing areas may need attention even if one section looks deep enough.
- Resolve roof leaks and damp first. Do not cover a problem that has not been identified. Ask a professional to investigate persistent condensation, damp, roof leaks or damaged materials.
- Plan for storage. Boards placed directly over mineral wool can squash it and reduce its insulating value. Raised boarding may be needed to preserve the insulation depth and a ventilation gap.
- Check pipes and tanks. Insulation at ceiling level makes the loft colder. Any water pipes or tanks there may need suitable insulation to reduce the risk of freezing. Energy Saving Trust’s advice on tanks and pipes explains the related work.
- Check who can authorise the work. If you rent, speak to your landlord before arranging changes. In a flat, check with the landlord, freeholder or managing agent who controls the loft or roof space.
- Compare like-for-like quotes. Ask what material and depth will be installed, how ventilation will be maintained, what happens to existing insulation and whether the loft hatch, pipes and water tank are included.
If the loft is already dry and has roughly 270mm of suitable insulation, adding more may not be the best next spend. Draughts around a loft hatch or elsewhere may be more relevant; see our guide to draught-proofing a UK home. For a wider list of home checks, use our home resilience checklist.
Loft insulation resources
- Energy Saving Trust’s roof and loft insulation guide covers insulation types, depth, costs, savings and installation choices.
- Energy Saving Trust’s guide to insulating a loft explains the steps and flags ventilation, wiring and storage considerations.
- Energy Saving Trust’s winter home preparation advice includes current example savings for loft insulation and other measures.
- The UK Government Clean Energy campaign’s insulation page gives example savings and explains the assumptions behind its estimates.
- GOV.UK’s existing-home ventilation guide explains ventilation in homes in England, including how energy-efficiency work can affect it.
- Energy Saving Trust’s advice on insulating tanks and radiators includes guidance on insulating pipes in cold spaces.
