How thick should loft insulation be? 270 millimetres of mineral wool is the minimum recommendation for UK lofts, backed by current building regulations and the Energy Saving Trust. If your loft already has insulation, the figure that matters is the total depth, not what you add on top of it. Measure what is there, top it up to reach the 270mm mark, and you have done one of the most cost-effective winter upgrades a British home can get.

Lofts are the great overlooked heat escape route of British housing. The Energy Saving Trust estimates that an uninsulated home loses around a quarter of its heat through the roof, and a properly installed layer of loft insulation can last around 40 years. It is cheap, it is quick to fit, and it pays for itself many times over. Yet survey after survey finds lofts under-insulated, and most of the confusion comes down to one question: how much is enough?

How thick should loft insulation be? The 270mm rule

For the commonest material, mineral wool insulation in blanket or roll form, the answer is at least 270mm. The Energy Saving Trust states that current building regulations recommend a minimum depth of 270mm for mineral wool insulation in a loft. GOV.UK publishes a handy ruler homeowners can use to check the depth of insulation already installed.

The figure is not arbitrary. It is the depth of standard mineral wool needed to reach the thermal performance building regulations expect of an existing home’s roof, commonly expressed as a U-value target of 0.16 W/m²K. Thicker or more thermally efficient materials can reach the same performance with less depth, which is why the number changes depending on what you lay.

Why the material changes the number

270mm is the benchmark for mineral wool and fibreglass rolls. It does not apply universally to every product on the market:

• Mineral wool and fibreglass rolls: 270mm minimum. The standard recommendation.
• Rigid PIR (polyisocyanurate) boards: significantly thinner layers can reach the same U-value, because the material is more thermally efficient per millimetre. This is useful where headroom or floor height is limited.
• Blown or sprayed insulation: installed to whatever depth achieves the required U-value, typically similar in total to rolled products for mineral-based materials.
• Natural fibres (cork, wood fibre, sheep’s wool): depths vary by product; follow the manufacturer’s declared thermal performance rather than the 270mm figure.

The principle to remember: the target is thermal performance, and 270mm of mineral wool is simply the most quoted way to get there.

How to measure what you already have

Before buying a single roll, check what is already in your loft. Most British lofts have some insulation, often laid when the house was built or topped up once decades ago. Here is the Energy Saving Trust’s approach:

• Pick an accessible, clear spot in the loft, away from stored boxes and wiring.
• Push a ruler or tape measure gently through the insulation until it touches the loft floor, without pressing the material down.
• Measure at several points, because depth varies across the loft, and note the lowest reading.

The maths then follows the simple top-up rule the Energy Saving Trust publishes: if you have 120mm of insulation already installed, you need to top it up by at least 150mm to reach the 270mm minimum. If you are starting from zero, you need the full 270mm.

The right way to lay two layers

Reaching 270mm is not just about piling material up. The Energy Saving Trust’s loft insulation guide sets out the correct method for DIY installations:

• Lay the first layer between the joists, the horizontal beams that form the floor of the loft.
• Lay the second layer at right angles across the joists, so the joists themselves are covered and thermal bridging is reduced.
• Match the roll width to your joist spacing. The most common spacings are 400mm or 600mm, and most rolls are pre-cut so they separate into the right strips.

Two things wreck an otherwise good installation. Compression is the first: insulation works by trapping air in tiny pockets, and squashing it, by walking on it or cramming boxes onto it, destroys that trapped air and cuts performance. If you want to use the loft for storage, fit raised loft legs and a new floor above the insulation rather than boarding straight onto the joists. The second is gaps and cold spots: insulation needs to be continuous, so butt joints tightly and keep it clear of the eaves vents, which the loft still needs to breathe through.

When you should remove the old insulation first

Topping up works when what is already there is in decent shape. The Energy Saving Trust advises removing the old insulation and starting fresh if it is very old, has been damaged by weather, has become compressed, or has been fouled by vermin. A tired, matted layer will not perform properly no matter how much you add on top, and if you have added extra layers over years, the original one may have compressed under the weight.

Also treat the loft like an inspection opportunity. Check for signs of damp or condensation before you start, and make sure downlights, cables and the loft hatch are in safe order. Damp problems need a professional assessment and repair before any insulation goes in.

Boarded lofts and room-in-roof: the special cases

Not every loft suits the standard 270mm-over-the-floor approach:

• Lofts you walk on or store heavy items in: raise the floor on loft legs above the insulation so the full 270mm depth stays uncompressed.
• Room-in-roof conversions: insulation goes between and under the roof slope rather than across the loft floor, and the depth depends on the materials and build-up. The Energy Saving Trust says these situations need specialist techniques and are not DIY jobs.
• Flat roofs: not suitable for DIY insulation at all; a professional installer is needed.
• Damp or structurally suspect lofts: fix the underlying problem first, then insulate.

What it costs and what you get back

The Energy Saving Trust’s installation cost estimates for a typical home in Great Britain put the full 270mm job at around £750 when starting from nothing, or £600 when topping up from 120mm. In Northern Ireland the equivalents are £1,100 and £700.

The payoff from a straight top-up is modest but real: the Trust estimates topping up from 120mm to 270mm saves around £17 a year in Great Britain and £20 in Northern Ireland on energy bills. The bigger win comes from the loft that has almost nothing in it, where moving to the recommended depth stops a substantial slice of that quarter of heat escaping through the roof. With a lifespan of around 40 years, loft insulation is one of the longest-serving measures a homeowner can buy.

If the upfront cost is the barrier, several schemes cover loft insulation for qualifying households. The ECO4 scheme, the Great British Insulation Scheme, and the Warm Homes: Local Grant are the three live routes; our guide to free insulation grants in the UK walks through who qualifies for each. The government’s Warm Homes Plan, investing close to £15 billion in home improvements, also promises fully funded support for insulation for people on low incomes, so keep an eye on the details as they are published.

DIY or professional: how to decide

The Energy Saving Trust says loft insulation is a reasonable DIY job only if your loft is easy to access, has no damp or condensation problems, and does not have a flat roof. If those conditions hold and your joists are regular, rolls of mineral wool are straightforward to fit in an afternoon.

For anything else, hire a professional, and hire a good one. The Trust recommends choosing an installer registered with TrustMark, the government-endorsed quality scheme for home improvements, checking which Competent Persons Scheme they belong to on GOV.UK, and getting at least three quotes. Useful questions to ask before hiring include how much experience they have with loft insulation, what guarantee they offer, and whether they will assess and address ventilation needs as part of the job.

And pair the job with the rest of the winter-prep list. Loft insulation is only part of the picture: our draught-proofing guide covers the cheap window and door fixes that complement insulation, and if you are planning any heating changes, proper insulation is also part of what makes a home ready for a heat pump.

How loft insulation helps your EPC rating

Insulation shows up in your home’s Energy Performance Certificate, and with new multi-metric EPC ratings rolling out, the fabric of the building, how well walls, roofs and floors hold heat, is getting its own dedicated metric. An insulated loft scores directly on that fabric metric, which matters for the 2030 rental rules and for anyone selling a home. It is worth keeping receipts and photos of the installation, so you can show an assessor what was done.

Frequently asked questions

How thick should loft insulation be?

At least 270mm of mineral wool or fibreglass, which is the minimum depth the Energy Saving Trust and current building regulations recommend. The figure refers to total depth: if you already have 120mm, you only need to add about 150mm. More thermally efficient materials such as rigid PIR boards can reach the required U-value with a thinner layer.

Can I just pile new insulation on top of the old?

Yes, if the existing insulation is in good condition. The Energy Saving Trust advises removing it only if it is very old, damaged by weather, compressed, or fouled by vermin. Topping up a sound existing layer is the cheapest route to 270mm.

Is it worth topping up from 120mm to 270mm?

The Energy Saving Trust estimates the top-up saves around £17 a year on energy bills in Great Britain, against a typical installation cost of £600. It will not make you rich, but combined with insulation’s 40-year lifespan and the fact that grant schemes often cover it, it is an easy win.

Can I board over loft insulation and still use it for storage?

Not directly onto the joists. Boarding crushes the insulation and destroys its trapped-air performance. Use raised loft legs to lift a new floor above the full 270mm depth, keeping the insulation uncompressed and continuous.

Does loft insulation need building regulations approval?

Straightforward top-up loft insulation in a normal home does not usually need a building control application, but the work still needs to meet the thermal performance the regulations expect. If the work is part of a larger project, such as a loft conversion, different rules apply, and your installer or surveyor can confirm what is needed.

Can I get free loft insulation in the UK?

Possibly, depending on your home and circumstances. ECO4, the Great British Insulation Scheme and the Warm Homes: Local Grant are the three live schemes covering insulation. Our free insulation grants guide explains the eligibility for each.

Sources

• Energy Saving Trust, Roof and loft insulation guide: 270mm minimum depth for mineral wool under current building regulations; topping 120mm to 270mm saves around £17 a year in GB and £20 in NI; around 25% of heat lost through an uninsulated roof; installation costs around £750 (0 to 270mm) or £600 (120 to 270mm) in GB; 40-year lifespan. energysavingtrust.org.uk
• Energy Saving Trust, How to insulate a loft: lay the first layer between the joists and the second at right angles; remove old insulation if it is damaged by weather, vermin or compression; DIY only where access is easy with no damp and no flat roof. energysavingtrust.org.uk
• GOV.UK loft insulation ruler for checking existing depth, via the Energy Saving Trust guide. energysavingtrust.org.uk
• GreenMatch UK, How much insulation do you need for your loft or roof: U-value 0.16 W/m²K target for existing homes; PIR boards reach the same performance with less depth. greenmatch.co.uk