How thick does loft insulation need to be in the UK?

Current UK Building Regulations (Part L, 2021) require loft insulation to achieve a U-value of 0.16 W/m²K or better. For mineral wool (glass or rock wool), this means a total thickness of approximately 270mm. Many properties only have 100-150mm — topping up to 270mm can reduce heat loss through the loft by 50% or more.

What is a U-value and why does it matter?

A U-value measures how quickly heat passes through a building element (wall, roof, floor) in watts per square metre per degree temperature difference (W/m²K). Lower U-values mean better insulation. UK Building Regulations set maximum U-values: walls 0.30, floors 0.25, roofs 0.16, windows 1.6. Improving your U-values reduces energy bills and carbon emissions.

What type of insulation should I use?

For loft spaces (between and over joists), mineral wool rolls (glass wool or rock wool) are the most cost-effective at £5-10 per roll. For walls and where space is limited, PIR boards (such as Celotex or Kingspan) offer the best insulation per millimetre thickness. For cavity walls, blown mineral fibre or bead insulation installed by a registered installer is the standard choice.

Can I get a grant for insulation in the UK?

Yes, several UK government schemes help with insulation costs. The Great British Insulation Scheme (GBIS) provides free or subsidised cavity and loft insulation for eligible households. The ECO4 scheme helps low-income and vulnerable households. The Boiler Upgrade Scheme offers £7,500 towards heat pumps which work best with good insulation. Check the government's Simple Energy Advice website for eligibility.

Insulation Calculator — Rolls, Boards & U-Value Guide

Complete Guide to Home Insulation in the UK

Insulation is the single most effective way to reduce energy bills and improve comfort in a UK home. With heating accounting for approximately 55% of household energy costs, and the UK's housing stock being among the least energy-efficient in Europe, improving insulation offers substantial financial and environmental benefits.

UK Building Regulations Part L — U-Value Requirements

The UK Building Regulations Part L (Conservation of Fuel and Power) set maximum U-values for different building elements. These were updated in June 2022 and apply to all new-build and renovation projects requiring Building Regulations approval:

Building Element	Maximum U-value (W/m²K)	What This Means
Pitched roof (between/over rafters)	0.16	~270mm mineral wool or ~80mm PIR
Flat roof	0.18	~100mm PIR board
External walls (new build)	0.26	Full fill cavity + insulated inner leaf
External walls (renovation)	0.30	~70-90mm PIR board internally
Ground floor	0.18 (new) / 0.25 (reno)	~75-100mm PIR under screed or between joists
Windows	1.6	Double-glazed minimum, triple preferred
Doors	1.6	Insulated composite or equivalent

Energy saving estimate: Upgrading loft insulation from 100mm to 270mm saves approximately £200-£300 per year on heating bills for a typical 3-bed semi-detached house. The payback period for DIY loft insulation is typically 1-2 years.

Insulation Types Compared

Each insulation type has different thermal properties, expressed as thermal conductivity (lambda value, λ, in W/mK). Lower lambda values mean better insulation per millimetre of thickness:

Type	λ (W/mK)	Form	Cost per m²	Best Application
Glass mineral wool	0.044	Rolls	£2-£5	Loft, between joists
Rock wool (Rockwool)	0.035-0.038	Rolls/batts	£4-£8	Loft, fire-rated areas, acoustic
PIR (Celotex, Kingspan)	0.022-0.023	Rigid boards	£8-£20	Walls, floors, flat roofs
EPS (expanded polystyrene)	0.035-0.040	Rigid boards	£3-£8	Cavity walls, under floor slabs
XPS (extruded polystyrene)	0.030-0.036	Rigid boards	£6-£12	Below ground, high moisture areas
Sheep wool	0.035-0.040	Rolls/batts	£8-£15	Loft, eco-builds, breathable walls
Blown cellulose	0.035-0.040	Loose fill	£5-£10	Loft, cavity walls (pro install)
Phenolic foam	0.018-0.021	Rigid boards	£12-£25	Space-critical applications

Loft Insulation — The UK's Best Value Upgrade

Loft insulation is the most cost-effective energy improvement for the majority of UK homes. Heat rises, and an uninsulated or poorly insulated loft can lose 25-30% of a home's heat. The recommended approach for a standard cold loft (where the insulation sits at ceiling level) is:

First layer (100mm): Lay mineral wool rolls between the ceiling joists. Cut to fit snugly without compressing.
Second layer (170mm): Lay a second layer perpendicular across the tops of the joists. This eliminates cold bridges through the timber joists.
Total: 270mm — this achieves approximately 0.16 W/m²K, meeting current Building Regulations.

Loft ventilation: When insulating a cold loft, DO NOT block the eaves ventilation. Lofts need airflow above the insulation to prevent condensation. Ensure a minimum 25mm gap between the insulation and the underside of the roof tiles/felt at the eaves. If your loft has no eaves ventilation, it should be added before insulating.

Wall Insulation Options

Walls account for approximately 35% of heat loss in a typical UK home. The approach depends on your wall construction:

Cavity wall insulation: Most UK homes built after 1930 have cavity walls (two layers of brick/block with a gap between). This cavity can be filled with blown mineral fibre, bead insulation or foam. Cavity wall insulation must be installed by a registered installer (CIGA-guaranteed) and typically costs £500-£1,500 for a 3-bed semi. Many households qualify for free installation under the Great British Insulation Scheme.

Internal wall insulation (IWI): For solid-walled properties (pre-1930), PIR insulated plasterboard (such as Celotex PL4000 or Kingspan Kooltherm K17/K18) is fixed to internal walls. Typical thickness: 50-90mm including plasterboard. Cost: £50-£100/m² installed. Reduces room size slightly but dramatically improves thermal performance.

External wall insulation (EWI): An insulation layer is fixed to the outside of the building and rendered over. More expensive (£8,000-£22,000 for a whole house) but does not reduce room sizes and can transform the appearance of a property. Usually requires planning permission and scaffold access.

Floor Insulation

Ground floors lose approximately 10-15% of a home's heat. The approach depends on floor type:

Suspended timber floors: Mineral wool batts or PIR boards fitted between the joists from below (if accessible via a cellar) or by lifting floorboards. Target: 0.25 W/m²K or better.
Solid concrete floors: PIR board insulation laid on top of the existing slab with a new floating floor or screed over it. More disruptive but highly effective.
For new builds: High-performance insulation (100mm+ PIR or EPS) below the concrete slab, achieving the Part L target of 0.18 W/m²K.

Government Grants for Insulation (2026)

Several UK government schemes can help with insulation costs:

Great British Insulation Scheme (GBIS): Free cavity and loft insulation for eligible households in Council Tax bands A-D (England) or A-E (other nations).
ECO4: Free insulation measures for low-income and fuel-poor households. Delivered through energy suppliers.
Boiler Upgrade Scheme: £7,500 towards air-source heat pumps (which perform best with well-insulated homes).
Home Upgrade Grant (HUG): For off-gas-grid homes with EPC ratings D-G. Can fund insulation, heating upgrades and renewables.

Check the Simple Energy Advice website (simpleenergyadvice.org.uk) or call 0800 444 202 to find out what grants you qualify for.

Insulation Calculator — Rolls, Boards & U-Value Guide