The Short Answer
The Energy Saving Trust puts loft insulation at £750 to take a bare loft to 270mm and £600 to top up from 120mm, for a typical home in Great Britain. A top-up saves only about £17 a year, so it's mostly about comfort. A bare loft is where the saving is.
Our view: if your loft is bare or close to it, insulate it, and lay it yourself if you can do it safely, since that costs less. If it already has 100mm or more, a professional top-up won't pay back for decades on the bill alone.
How Thick Should It Be?
The Energy Saving Trust uses 270mm, which it calls the minimum recommendation for new builds, and says loft insulation fitted correctly lasts around 40 years. It says homes can lose around 25% of their heat through an uninsulated roof.
What It Costs
For professional installation in Great Britain, the Energy Saving Trust puts a bare loft taken to 270mm at £750 and a top-up from 120mm at £600. In Northern Ireland its figures are £1,100 and £700.
What It Saves
For a top-up from 120mm to 270mm, the Energy Saving Trust puts the saving at about £17 a year in Great Britain and £20 in Northern Ireland. It no longer publishes a saving for a bare loft. DESNZ, measuring real gas meters in England and Wales, found a median saving of 3.2% of gas across all loft jobs, top-ups included: about £27 a year on a semi at today's price cap. A bare loft should save more than that; the Energy Saving Trust's older figure for a semi was £190 a year, at earlier prices. So a bare loft pays back somewhere between about 4 and 28 years.
How Many Rolls, and Is It Worth It?
Put in your loft's floor area and what's there now for the number of rolls, or switch tabs for the cost and saving:
Loft insulation calculator
How many rolls do you need?
Length times width of the loft floor. On a house it's roughly the ground floor footprint.
What's up there now?
7 rolls
of 170mm (6.5m² each), laid across the joists
That takes you from 100mm to 270mm, including 10% for offcuts and the awkward bits by the eaves. A top-up from around 120mm saves about £17 a year on a semi (Energy Saving Trust), so it's mostly about comfort.
Coverage per roll is Knauf's published figure for Loft Roll 44; other brands differ, so check the pack. Keep the eaves clear so air can still get into the loft, and don't board straight onto the joists over 270mm.
Doing It Yourself
The Energy Saving Trust says you should speak to a professional installer if you have a flat roof, use your loft as a living space, or have concerns about damp or structural problems. Otherwise, from its advice:
- Between and then across the joists. Insulation between the joists alone may not reach modern levels; a second layer laid across the joists makes up the depth.
- Don't squash mineral wool, which makes it less effective.
- Storage needs boards. If you want to store things up there, raise the floor and fit boards above the insulation, leaving a small gap for ventilation to prevent condensation. Our loft boarding cost guide has fitted and DIY prices.
- Watch for damp. Insulation makes the loft colder, which makes condensation more likely; if there's damp already, get professional advice first.
- The hatch counts too. Draught strips around the loft hatch, as on a door.
Spray Foam in the Roof
Spray foam sprayed onto the underside of the roof is a different job from insulating the loft floor, and it can cause trouble with lenders and surveyors. Gov.uk's guidance of 22 October 2026 names 8 lenders that will consider homes with spray foam, but says only a specialist surveyor can tell you whether yours should come out. Our view: insulate the loft floor instead. If you already have foam, our spray foam insulation guide has what to do, with a step-by-step tool.
Grants
- ECO4 (England, Scotland and Wales, until 31 December 2026): for people on certain benefits or in social housing.
- Warm Homes: Local Grant (England): loft insulation is on the list, for privately owned homes rated D, E, F or G with household income usually £36,000 or less (or certain benefits or postcodes), where your council has funding.
- Scotland: Home Energy Scotland grants of up to £1,500 for loft, cavity or underfloor insulation, up to 75% of the combined cost.
- Wales: Nest can fund insulation for eligible households.
- Northern Ireland: NISEP fully funds insulation under £33,000 (single) or £40,000 (couple) household income, and gives 40% off up to £650 a measure at any income.
The Great British Insulation Scheme closed on 31 March 2026.
Loft Insulation and Heat Pumps
If a heat pump is coming, do the loft before the heat loss survey, so the heat pump is sized for the house you'll have. Our insulate before a heat pump guide runs the numbers, and our insulation costs guide compares the loft with every other job.
Frequently Asked Questions
Everything You Need for DIY Loft Insulation
If you're laying it yourself, here's what you need. Check the depth you're aiming for and the roll coverage on the pack.
Knauf Loft Roll 44: 170mm
£50.99Top up existing loft insulation to reach the recommended 270mm, or lay it in a bare loft, where the saving is bigger.
170mm / 0.044 W/mKFind on Amazon
Knauf Loft Insulation Roll: 100mm
£52.99Ideal for topping up thin existing insulation. Lay between joists before adding a 170mm cross-layer on top.
100mm thicknessFind on AmazonLoft Leg Insulation Spacers (12-Pack)
£23.79Board your loft for storage without squashing insulation. Compressed insulation loses effectiveness.
Raises boards above 270mm insulationFind on Amazon
Loft Hatch Insulation Cover
Check priceCovers the loft hatch, which is easy to miss when the loft is insulated.
Foldable, fits standard hatchesFind on AmazonAs an Amazon Associate we earn from qualifying purchases. Prices and stock are checked against Amazon every few hours and may have changed since.
