Loft insulation is consistently the highest-return energy-efficiency upgrade a homeowner can make. The roof accounts for roughly 25% of a poorly insulated home’s heat loss, and a full 270 mm of mineral wool insulation can cut that to near zero at a cost well below what the energy savings recoup in five to seven years. This guide covers every material option, the right depth to specify, how to check your U-value, what it will cost installed or DIY, and which grants might reduce your bill.
Why Loft Insulation Matters and What the Regulations Say
Building Regulations Part L requires a minimum 0.16 W/m²K U-value for loft insulation installed as part of a renovation or new build. For an accessible cold loft (the most common type in UK housing stock), that equates to approximately 270 mm of standard mineral wool. Older homes built before 1990 often have 100 mm or less — enough to meet the regulations of the day but less than half of what current guidance recommends.
The Energy Saving Trust estimates that a semi-detached house with no loft insulation loses around £350 per year more in heating costs than an equivalent home with 270 mm installed. The payback on a professional installation costing £500 is therefore roughly 18 months after the energy saving is netted against any grant received.
Types of Loft Insulation
Mineral Wool Rolls (Glass Wool and Rock Wool)
Mineral wool is the most widely used and lowest-cost option. It comes in rolls, typically 100 mm or 170 mm thick, and is laid in two perpendicular layers to reach the 270 mm target — first between the joists, then across them. Glass wool (e.g. Knauf Earthwool, Isover) is slightly lighter and cheaper; rock wool (e.g. Rockwool) is denser and marginally better at acoustic performance.
- Thermal conductivity (lambda): 0.035–0.044 W/mK
- Cost per roll (typically 5.5 m × 600 mm): £8–£16 depending on thickness
- Health note: wear gloves, goggles and a dust mask when installing
Sheep Wool
Natural sheep wool insulation performs comparably to mineral wool and handles moisture better — absorbing and releasing humidity without losing performance. It costs roughly twice as much per m² of coverage but suits homeowners who prefer natural materials or are insulating a breathable older building.
- Lambda: 0.036–0.040 W/mK
- Cost per m²: £6–£11
Blown Fibre (Loose Fill)
Cellulose or glasswool loose fill is blown in by machine — useful for topping up hard-to-reach areas or lofts with irregular joist spacings. Not a DIY option; requires a specialist contractor. Typically used when access is difficult or boarding is already in place at a lower depth.
- Lambda: 0.032–0.040 W/mK
- Installed cost: £5–£9 per m²
Rigid Insulation Boards
PIR (polyisocyanurate) boards such as Celotex or Kingspan have the highest thermal performance by thickness (lambda ~0.022 W/mK). This matters when headroom is tight — for example, where boarding is planned and you need to achieve a good U-value without raising the floor level significantly. Boards cost more than rolls and are less forgiving to fit around joists.
- Cost per m²: £12–£22 depending on thickness
- Note: PIR boards must be covered (fire rating) — typically with a minimum 12.5 mm plasterboard or OSB in any accessible space
Multifoil Insulation
Multifoil products (multiple layers of reflective film and wadding) are sometimes marketed for lofts but their real-world thermal performance in pitched roofs falls well short of the claimed values unless airflow is carefully controlled. Building control often rejects them as the sole insulation measure to achieve Part L compliance. If used, they must be combined with conventional insulation to reach the required U-value. See the article on best loft insulation materials for a fuller comparison.
Recommended Depth: Why 270 mm?
The 270 mm figure comes from the Standard Assessment Procedure (SAP) and Part L guidance. It assumes standard glass wool at around 0.040 W/mK. With a better-performing product (lambda 0.032 W/mK) you can achieve the same U-value at around 200 mm — useful where headroom above boarding is constrained.
To check whether your existing insulation is adequate, measure its depth in multiple spots. If it is compacted, add the compressed thickness conservatively. A common UK scenario is 100 mm between joists with nothing on top — adding a 170 mm roll across the joists brings the total to 270 mm.
Rule of thumb: if your insulation is below 150 mm anywhere, top it up.
U-Values Explained
The U-value is the rate of heat transfer through a building element in W/m²K. Lower is better. The table below shows approximate U-values for a typical cold pitched loft with mineral wool insulation:
| Insulation Depth | Approximate U-Value (W/m²K) | Meets Part L? |
|---|---|---|
| 100 mm | ~0.40 | No |
| 150 mm | ~0.27 | No |
| 200 mm | ~0.20 | Marginal |
| 270 mm | ~0.16 | Yes |
| 300 mm | ~0.14 | Yes (exceeds) |
| 400 mm | ~0.10 | Yes (best practice) |
Note: U-values vary slightly depending on the insulation product and joist depth. A structural engineer or energy assessor can calculate precise values for your specification.
DIY vs Professional Installation
DIY
Standard mineral wool rolls are genuinely DIY-friendly. The main requirements are:
- Ensure the loft hatch and any ventilation gaps at the eaves remain clear (maintain at least 50 mm airflow above the insulation at the eaves — do not block ventilation strips)
- Work on a dry day and avoid disturbing dust near the hatch
- Leave a 100 mm gap around any recessed downlighters (which must also be fire-rated) or use proprietary insulation covers over them
A typical semi-detached house (approximately 40 m² of loft floor) can be insulated in a day by two people.
Professional Fitting
For blown-fibre top-ups, large lofts or properties where access is difficult, a professional installer is the practical choice. The Government’s Great British Insulation Scheme and ECO4 obligation fund professional installation for eligible households (broadly, those on means-tested benefits or with homes rated EPC D or below). Check the government’s Simple Energy Advice service for eligibility — as of 2026, qualifying households may receive insulation at no cost.
Loft Insulation Cost
| Scenario | DIY Materials Only | Professionally Installed |
|---|---|---|
| Top-up to 270 mm, 30 m² | £120–£220 | £300–£500 |
| Top-up to 270 mm, 50 m² | £180–£350 | £400–£700 |
| Full installation from zero, 50 m² | £300–£550 | £600–£950 |
| Sheep wool, 50 m², DIY | £500–£700 | £900–£1,300 |
| Blown-fibre top-up, 50 m² | Not DIY | £350–£650 |
Prices are 2026 estimates for England, excluding scaffolding or unusual access requirements. VAT is charged at 0% on insulation materials and installation (an energy-saving measure) — confirm this with your contractor.
Grants Available in 2026
Two main routes exist for subsidised or free loft insulation:
1. Great British Insulation Scheme (GBIS): Funded by energy suppliers; targets properties in EPC bands D–G. Eligible households can receive free loft insulation regardless of income in some cases. Check via your energy supplier or Simple Energy Advice.
2. ECO4 Obligation: Energy companies fund insulation for fuel-poor households. Eligibility is means-tested (certain benefits, income below a threshold). Contact your energy supplier directly.
3. Local Authority Flex schemes: Some councils run their own retrofit grant programmes, often funded via the Warm Homes: Local Grant. Check your local council website.
Insulating Over Boarding
If your loft is already boarded at joist level with no raised legs, removing the boards to add insulation and reinstall on raised loft legs is the correct approach. Compressing insulation under boards dramatically reduces its thermal performance — a 100 mm board pressed down onto 100 mm of glass wool effectively reduces the insulation value by 30–50%. See the guide on raised loft boarding for the correct method.
Common Mistakes to Avoid
- Blocking eaves ventilation: Insulation must not be pushed into the eaves. Use eaves baffles to maintain a clear 50 mm gap.
- Insulating a warm roof instead of a cold one: If you are converting the loft to a habitable room, insulation goes between the rafters, not at joist level. These are different calculations entirely.
- Leaving the loft hatch uninsulated: A standard timber hatch loses heat at the same rate as an uninsulated section of ceiling. Fit a proprietary insulated hatch cover, or attach a 100 mm PIR board to the hatch leaf.
- Covering recessed lights without fire protection: Recessed downlighters must be IC-rated (insulation contact-rated) or covered with a proprietary fire-rated cover before insulation is laid over them.