Loft Insulation By Thickness

Loft insulation thickness allows you to choose the appropriate depth for the job.

Many older UK homes already have 75mm–100mm of insulation installed. Current UK and Wales guidance on loft insulation thickness usually recommends 270mm of mineral wool in a cold loft to meet regulatory targets.

That figure is based on the target U-value required for your specific project under Approved Document L. Loft insulation thick enough for today’s performance levels balances thickness, material type, and proper installation.

When choosing loft insulation thickness, think about:

• Joist height
• Airflow at the eaves
• Existing insulation
• Storage plans
• Target U-value under current regulations to ensure your roof insulation is effective.

What is loft insulation by thickness?

Loft insulation by thickness refers to insulation products categorised and sold by depth – usually mineral wool rolls, rigid boards, or multi-layer systems.

Let’s focus on mineral wool roll, as that’s the most common for cold lofts.

It’s made from molten glass or stone spun into fibres. Those fibres trap millions of tiny air pockets. Air does not move much inside those pockets, so heat transfer slows. That’s how insulation works – by resisting heat flow and effectively reducing energy bills.

Performance depends on:

• Thermal conductivity (lambda value, W/mK)
• Installed thickness
• Installation quality (no gaps, no compression)

So when someone says they need “loft insulation thick enough for regulatory compliance,” they mean thick enough, with the right lambda, to achieve the required U-value.

Thickness categories make it easier for builders to:

• Match existing joist height
• Add a cross-layer
• Upgrade older insulation
• Price jobs accurately

What are the different loft insulation thicknesses available?

15mm–40mm insulation listed under loft insulation by thickness are usually rigid PIR boards or specialist thermal boards. These thinner products are precision tools used where space is tight, detailing is important, or you’re improving performance without rebuilding the roof, especially with mineral wool insulation.

15mm loft insulation

15mm is a detail layer that improves surface temperature and reduces condensation risk in hard-to-insulate areas. It is used for:

• Insulating loft hatches
• Backing boards behind cold water tanks
• Thermal lining to small service cupboards within loft spaces
• Reducing cold bridging around steel beams (goalposts, ridge steel ends)

In refurbishment projects, thin boards are sometimes laid over existing insulation before raised loft legs are installed to add performance without lifting the finished floor height too much.

25mm loft insulation 

25mm PIR will not meet current minimum loft insulation thickness standards on its own in a cold loft. But paired with additional PIR insulation layers, it enhances overall performance.

It improves thermal resistance, especially when used as a cross-layer of insulation to reduce thermal bridging through timber joists. Its main applications include:

• Upgrading shallow existing insulation where you cannot add 170mm or more
• Lining rafters in small warm loft conversions

30mm loft insulation 

30mm gives stronger thermal resistance while still keeping height under control. It’s common in upgrade jobs where increasing full loft insulation thickness is not possible due to eaves clearance or ventilation limits.  

Usually used for:

• Insulating party wall junctions within loft voids
• Thermal lining to sloping ceilings in older properties
• Hybrid systems combining rigid board + mineral wool – Not for DIY projects. This option should only be used when specified.

40mm loft insulation 

40mm makes a difference without interfering with head height or airflow, allowing you to effectively insulate your loft. In warm loft designs, subject to condensation risk assessment, 40mm PIR under rafters can help achieve target U-values without excessively increasing rafter depth when combined with thicker insulation between rafters. It has to be a combined build-up.

 Typical uses include:

• Warm roof build-ups at rafter level
• Continuous insulation under rafters to reduce cold bridging
• Thermal improvement of dormer cheeks

50mm–140mm

Mineral Wool insulation (Cold Loft Floors) 

50–140mm of mineral wool on its own would not meet current UK upgrade targets under Approved Document L.

140mm mineral wool (λ 0.035 W/mK) = ~0.24 W/m²K.

The current recommended upgrade level for loft floors is typically around 0.16 W/m²K.

Because of this, 50–140mm is normally used as:

• A first layer between joists
• A top-up to older insulation
• Part of a multi-layer build-up with an additional cross-layer

It is not usually sufficient as a standalone upgrade thickness.

PIR insulation (Rigid Board Insulation) 

With PIR insulation (λ typically between 0.022–0.026 W/mK), performance per millimetre is significantly higher.

At 140mm thickness, PIR can achieve a much lower U-value than mineral wool at the same depth. In loft conversions or warm roof systems, this thickness may form part of a compliant roof build-up, depending on the overall design and detailing.

PIR at 50–140mm is typically used in:

• Loft conversions
• Between or below rafters
• Warm roof constructions

Instead of loose-laid floor insulation.

150mm–180mm loft insulation

This creates a solid base layer. On its own, it’s still below current regulatory targets, but it fills the joist depth before adding a cross-layer. Often fitted between 150mm–170mm joists.

200mm Loft insulation

200mm of mineral wool significantly improves performance but falls below the typical 0.16 W/m²K upgrade target unless combined with an additional layer. It is used in refurbishment work where:

• Joist depth limits height
• Partial upgrades are being done to reduce energy bills by enhancing insulation performance.
• Budget constraints apply

Benefits of loft insulation thicknesses

Different thickness options are available to meet the thermal requirements of varying loft designs.

• Older properties may only have 75mm installed.
• Refurbishment work often means building up gradually with additional layers of insulation.
• New builds aim straight for regulatory targets.

Regulated or assessed projects require improved SAP scores. Others focus purely on lowering heating demand. Having a range of thicknesses means:

• You can meet the minimum loft insulation thickness requirement without overloading joists.
• You can top up poor existing layers instead of stripping everything out.
• You can adapt around pipes, tanks, and services.
• You can upgrade older houses without altering the roof structure by adding a layer of mineral wool insulation – applicable only to mineral wool insulation.
• In short, no wasted material and no guesswork.