Loft insulation and Building Regulations: 270mm and Part L explained

Part L and U-values explained

Part L of the Building Regulations covers the conservation of fuel and power. The practical guidance on meeting it in homes is in Approved Document L, Volume 1: Dwellings (2021 edition incorporating 2023 amendments).

A U-value measures how much heat passes through part of a building, in watts per square metre for each degree of temperature difference (W/m²K). The lower the number, the better the insulation.

Different figures apply to flat roofs and roofs insulated at rafter level. Where reaching the standard isn't technically or functionally feasible, the Planning Portal explains that the element should be upgraded to the best standard that can be achieved within a simple payback of no more than 15 years.

Why 270mm?

The Energy Saving Trust recommends a minimum of 270mm of mineral wool for a typical loft (Energy Saving Trust). It's usually laid in two layers: one between the ceiling joists, and a second at right angles across the top of them, so the joists don't act as cold bridges. That build-up generally achieves the 0.16 W/m²K figure in Approved Document L.

Many older homes still have 100mm or less – the standard when they were built – and decades later it's often compressed, patchy or damp. If your existing insulation is dry and in good condition, a top up may be all you need. If it's damp, contaminated or badly compressed, it should come out first. The Energy Saving Trust estimates topping up from 120mm to 270mm saves around £17 a year in Great Britain.

Other materials perform differently. Rigid PIR boards, for example, insulate better per millimetre than mineral wool, so a thinner layer can reach the same U-value – the manufacturer's data or a U-value calculation will confirm what's needed.

When do the Building Regulations apply?

• Topping up or replacing loft insulation: you don't usually need to make a Building Control application, but the Planning Portal says insulation installed in a loft should meet the minimum energy efficiency values in the Approved Documents.
• Renovating the roof: where insulation is installed as part of a roof renovation and more than 25% of the roof is being renewed, the Planning Portal says the insulation should meet the standards in the Approved Documents – and roof work may itself need Building Control approval.
• Loft conversions: turning a loft into a living space needs Building Regulations approval, covering structure, fire safety, stairs, sound insulation and insulation (Planning Portal).
• Planning permission: isn't normally needed for insulation where there's no change to the outside of the building, but check with your local planning authority if your home is listed or in a conservation area (Planning Portal).

If we think your project may need approval, we'll tell you and suggest you check with your local Building Control. This guide covers England – Scotland and Wales have their own building regulations.

Ventilation and condensation

Insulating at joist level keeps heat in your rooms, which means the loft above gets colder. Colder lofts are more prone to condensation if moist air can't escape, so ventilation matters. The Planning Portal advises taking care not to block ventilation at the eaves, and the Energy Saving Trust warns not to cover vents, grilles or airbricks, and to get damp problems fixed before insulating.

When we insulate a loft we keep the eaves clear and look for signs of condensation, damp or leaks as part of the job.

Storage, pipes, tanks and cables

• Storage: don't squash mineral wool – compressed insulation works less well. If you want to store things, raised loft boarding lets you keep the full depth underneath, with a small ventilation gap.
• Pipes and tanks: a colder loft means pipes and cold water tanks are more likely to freeze, so they should be insulated, leaving the area directly under the tank uninsulated so a little warmth can rise into it.
• Loft hatch: insulate and draught-proof it, or warm air will escape straight past your new insulation.
• Cables and downlights: cables are lifted clear of insulation where possible, and recessed downlights need suitable clearance or covers – ask an electrician if you're unsure.

Want your loft brought up to standard? See loft insulation removal, replacement and top ups, or check whether you could get help with the cost in our loft insulation grants guide.

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