Loft insulation is a layer of insulating material laid in the roof space to stop heat escaping through the top of a house, and the Energy Saving Trust recommends a minimum depth of 270mm. Around 25% of the heat in an uninsulated home is lost through the roof, per the Energy Saving Trust, and correctly installed loft insulation lasts around 40 years. Taking a gas-heated semi-detached loft from 0mm to 270mm saves about £190 a year, per Energy Saving Trust figures published by Which? in July 2026.
By The Roofing Brief Team (masthead). Last reviewed: October 2026.
What is loft insulation and how does it work?
Loft insulation is material placed either across the loft floor (between and over the ceiling joists) or against the sloping roof (at rafter level) to slow heat loss through the roof. It works by trapping still air between fibres or in closed cells. Each product declares a thermal conductivity (lambda, in W/mK): Knauf Insulation Loft Roll 44 glass mineral wool declares 0.044 W/mK, and a foil-faced PIR board such as SOPRATHERM GA4000 (formerly Celotex GA4000) declares 0.022 W/mK, so the board needs about half the depth for the same thermal resistance.
Where the insulation sits defines the whole job. Insulate at joist level and the loft itself stays cold, which is the standard low-cost approach the Energy Saving Trust describes for most homes. Insulate at rafter level and the loft becomes part of the warm envelope of the house, which the Energy Saving Trust says needs professional advice for flat roofs, lived-in lofts and homes with damp.
How thick should loft insulation be?
The Energy Saving Trust recommends a minimum of 270mm of loft insulation: with 120mm already down, top up by at least 150mm; from bare joists, lay at least 270mm. In England, Approved Document L Volume 1 (2021 edition) sets a limiting U-value of 0.16 W/m²K for all roof types in new dwellings (Table 4.1), 0.15 for a new roof on an existing dwelling (Table 4.2), and, for a renovated or retained roof, an improved value of 0.16 where the existing roof is worse than the 0.35 threshold (Table 4.3).
Depth needed depends on the material’s lambda: thermal resistance equals thickness in metres divided by lambda. A 100mm Knauf Loft Roll 44 layer is R 2.25 m²K/W and a 170mm layer is R 3.85, so the two together give R 6.10 at 270mm, per Knauf’s datasheet. In US units, 0.27 m divided by 0.044 to 0.040 W/mK is 6.1 to 6.75 m²K/W, or roughly R-35 to R-38 (multiply by 5.678); cross-check with our insulation R-value chart.
| Material | Declared lambda (W/mK) | Depth for R 6.1 m²K/W | Source |
|---|---|---|---|
| Glass mineral wool roll (Knauf Loft Roll 44) | 0.044 | About 270mm | Knauf Loft Roll datasheet |
| Glass mineral wool roll at the lower end of the Knauf range | 0.040 | About 245mm | Knauf Loft Roll datasheet (0.040 and 0.044 W/mK range) |
| PIR board (SOPRATHERM GA4000) | 0.022 | About 135mm | SOPREMA GA4000 |
| Blown cellulose, sheep’s wool | Check the product’s declared value | Thickness = 6.1 x lambda | Maker declaration of performance |
Types of loft insulation compared
Four material families cover almost every UK loft: mineral wool blanket rolls, blown loose-fill, rigid PIR boards, and natural wool. The Energy Saving Trust lists fibreglass, mineral wool, foam boards and natural fibres (such as cork or wood) as the common materials. Blanket rolls are the default for a cold loft floor; rigid boards suit a warm loft at rafter level; blown fill handles awkward or hard to reach spaces.
| Type | Named example and spec | Where it goes | Best for |
|---|---|---|---|
| Blanket roll | Knauf Loft Roll 44: 0.044 W/mK, Euroclass A1 to BS EN 13501-1, 1140mm rolls split 2×570 or 3×380 | Cold loft floor, between and over joists | DIY cold loft, joists at 400mm or 600mm centres |
| Stone wool roll or granulate | ROCKWOOL stone wool: A1 classified, withstands up to 1000°C per ROCKWOOL | Cold loft floor; granulate blown in | Fire resilience, irregular joists |
| Loose-fill (blown) | Cellulose or mineral wool fibre | Cold loft floor, blown in | Low headroom, irregular joists |
| Rigid board | SOPRATHERM GA4000 PIR: 0.022 W/mK, foil faced, 50mm to 100mm thick | Warm loft, between and under rafters | Loft conversions, room-in-roof |
| Natural wool | Sheep’s wool rolls | Cold loft floor | Low-irritant handling |
Blown loose-fill and rigid board work are usually installer jobs, both for access and for cutting waste. Blanket rolls are the one type most homeowners fit themselves: Which? puts a 100mm roll at about £20 for 8.3m². For deeper background on the loose-fill option, see the guide to blown-in loose-fill insulation, and for rafter-level foam the guide to spray foam insulation and its ventilation caveats. The Energy Saving Trust warns against firms that spray foam onto the underside of a leaking roof without fixing the leak first.
Cold loft vs warm loft: which do you need?
A cold loft is insulated at the ceiling joists, leaving the roof space cold and ventilated; a warm loft is insulated at the rafters, bringing the roof space inside the heated envelope. Choose a cold loft if the space is only for storage, and a warm loft only if the loft is, or will become, a habitable room. If you board the loft for storage, the Energy Saving Trust advises raising the floor and laying a second layer across the joists, then leaving a small ventilation gap between insulation and boards.
| Factor | Cold loft (joist level) | Warm loft (rafter level) |
|---|---|---|
| Insulation position | Loft floor, between and over joists | Under and between rafters |
| Loft usable as a room | No, stays cold | Yes, stays warm |
| Typical material | Mineral wool blanket (0.040 to 0.044 W/mK) or blown fill | PIR rigid board (0.022 W/mK), sometimes spray foam |
| Approved Document L target when renovated (England) | 0.16 W/m²K improved value | 0.16 W/m²K improved value |
| Ventilation approach | Ventilate the roof void above the insulation at the eaves | Keep a ventilation gap above the boards or use a breathable membrane |
| If your loft | Then | Threshold or rule |
|---|---|---|
| Has 0mm to 100mm of insulation | Top up to 270mm with rolls (DIY) or a pro installer | EST minimum 270mm |
| Has about 120mm | Add at least 150mm; expect a small saving | About £17 a year on a gas-heated semi (EST) |
| Already has 270mm | Leave it; fix gaps and compressed areas only | Further top-ups add little |
| Will be a habitable room | Warm loft at rafter level with PIR board | Approved Document L: 0.15 for a new roof element, 0.16 when renovating |
| Is damp, has a flat roof, or is lived in | Get professional advice before insulating | Energy Saving Trust guidance |
| Has a cold-water tank | Leave insulation off underneath; lag the sides and pipes | Stops the tank freezing |
How much does loft insulation cost in 2026?
Professional loft insulation for a semi-detached house costs around £750 to go from an uninsulated loft to 270mm, or around £600 to top up from 120mm to 270mm, per the Energy Saving Trust’s loft guide. Doing it yourself costs much less: Which? puts a 100mm roll at about £20 for 8.3m².
| Home type | Saving a year, 0mm to 270mm | Saving a year, 120mm to 270mm | Source |
|---|---|---|---|
| Mid-terrace house | £140 | £13 | EST data via Which? (July 2026) |
| Semi-detached house | £190 | £17 | EST data via Which? (July 2026) |
| Detached bungalow | £350 | £35 | EST data via Which? (July 2026) |
| Detached house | £360 | £35 | EST data via Which? (July 2026) |
Savings figures are Energy Saving Trust estimates for gas-heated homes; the Energy Saving Trust gives £20 a year in Northern Ireland for the 120mm to 270mm top-up versus £17 in Great Britain. Topping up an existing 120mm layer saves about a tenth of what insulating a bare loft saves, because most of the benefit comes from that first layer.
Is loft insulation worth it?
For an uninsulated or thinly insulated loft, yes. Worked example for a 40m² semi-detached loft floor (the area is an assumption; measure yours): 270mm needs 2.7 layers’ worth of 100mm roll at 8.3m² each, or about 13 rolls, which at Which?’s £20 a roll is about £260 of material. Against the £190 a year EST saving for a semi, that pays back in about 1.4 years, and the insulation lasts around 40 years. The professional route at about £750 pays back in about 4 years on the same saving.
The case weakens when a loft already holds 120mm or more: the 120mm to 270mm top-up on a semi saves about £17 a year, so a £600 professional top-up takes about 35 years to pay back on energy alone. The wider case for treating the roof space first is set out in the guide to the benefits of insulating the roof space.
How to insulate a loft: cold loft, step by step
Fitting mineral wool blanket in a cold loft is a manageable DIY job for most accessible lofts. Work in a mask, gloves, goggles, and long sleeves, kneel only on the joists or a crawl board, and light the space well before starting. The Energy Saving Trust says to measure the existing depth and the joist spacing first; the most common spacing is 400mm or 600mm, and Knauf’s 1140mm rolls split into 2x570mm or 3x380mm widths to fit between them.
- Clear the loft and check for old wiring, damp, or pests before laying anything. Fix any roof leak first.
- Lay the first layer of blanket between the joists, cut to a snug friction fit, filling the joist depth (often about 100mm; Knauf’s 100mm roll covers 13.885m² per pack).
- Lay the second layer across the joists at 90 degrees to reach about 270mm total (a 170mm roll covers 8.014m² per pack), so the joists themselves are covered and thermal bridging through the timber drops.
- Keep insulation clear of any recessed downlights unless they are fire-rated and rated for insulation contact, using covers or spacers where needed.
- Do not insulate directly beneath a cold-water tank, and lag the tank sides and any exposed pipes so they do not freeze.
- Stop the insulation short of the eaves so the roof void keeps its eaves ventilation: per Marley’s summary of BS 5250, that is a continuous 10mm gap for pitches over 15 degrees and 25mm for pitches of 10 to 15 degrees. Fit rafter trays where the insulation must reach the edge.
Ventilation and condensation: the mistake that causes damp
The most common loft insulation fault is smothering airflow, which traps moisture and causes condensation, damp timber, and mould. The Energy Saving Trust says not to cover any vents, grilles or airbricks during DIY insulation, and to add vents or ventilation trays if the existing ventilation is not enough. BS 5250 guidance, as summarised by Marley, adds continuous 5mm high-level ventilation where the pitch exceeds 35 degrees or the span is over 10m on a lean-to or mono-pitch roof.
Never push blanket tight into the corners where the roof meets the wall. If you plan to board part of the loft for storage, do not crush the insulation under the boards: the Energy Saving Trust warns that squashed mineral wool loses effectiveness, and it advises raising the floor and leaving a small ventilation gap between insulation and boards. Raised loft boarding on legs keeps the full 270mm intact with an air gap above it.
Can you get free or grant-funded loft insulation in 2026?
Some households can, but the 2026 routes differ from earlier years. The Great British Insulation Scheme ended on 31 March 2026 and is closed to new installations, per Ofgem. The Energy Company Obligation (ECO4) runs until 31 December 2026 after a nine-month extension, delivered by energy suppliers to low-income and vulnerable households.
In England, the Warm Homes: Local Grant can fully fund loft insulation through your council if the home is privately owned, has an EPC of D, E, F or G, and household income is usually £36,000 a year or less (some postcodes and benefit recipients qualify above that). The Energy Saving Trust says the wider Warm Homes Plan will invest close to £15 billion, with fully funded insulation for low-income households and low and zero-interest loans for other homeowners. Scotland, Wales and Northern Ireland run separate schemes, per GOV.UK.
Frequently asked questions
Is loft insulation worth it?
For a bare loft, yes. Insulating an uninsulated, gas-heated semi-detached loft to 270mm saves about £190 a year per Energy Saving Trust figures published by Which? in July 2026, and a DIY roll job costing about £260 for a 40m² loft pays back in under 2 years. A top-up from 120mm to 270mm saves only about £17 a year on a semi.
How thick should loft insulation be?
The Energy Saving Trust recommends at least 270mm. If you already have 120mm, add at least 150mm. In England, Approved Document L sets 0.16 W/m²K for roofs in new dwellings and as the improved value for a renovated roof. Lower-lambda PIR board at 0.022 W/mK reaches the same thermal resistance in about half the depth of 0.044 W/mK mineral wool.
What is the best type of loft insulation?
For a cold loft floor, mineral wool rolls such as Knauf Loft Roll 44 (0.044 W/mK, Euroclass A1) are the standard DIY option, at about £20 for a 100mm roll covering 8.3m² per Which?. Blown loose-fill suits low-headroom lofts, and PIR board (0.022 W/mK) is used at rafter level when the loft is a warm, habitable room.
Do I need ventilation with loft insulation?
Yes, a cold loft must stay ventilated above the insulation to prevent condensation and damp. BS 5250 guidance, as summarised by Marley, calls for a continuous 10mm eaves gap on pitches over 15 degrees and 25mm on pitches of 10 to 15 degrees. Do not cover vents, grilles or airbricks, and fit rafter trays where insulation reaches the eaves.
Can I install loft insulation myself?
Blanket rolls in an accessible cold loft are a common DIY job, done in a mask, gloves, goggles, and long sleeves while kneeling only on joists or a crawl board. The Energy Saving Trust advises a professional for flat roofs, lived-in lofts, or any damp or structural concern, and blown fill, rafter-level board and spray foam are installer jobs.
How long does loft insulation last?
Correctly installed loft insulation has a lifespan of around 40 years, per the Energy Saving Trust. Performance falls if it gets wet, is crushed under loft boards, or is disturbed by pests, which is why ventilation and raised boarding matter.