50mm Loft Insulation

50mm loft insulation is a practical working depth used when space is limited or when you are layering materials to reach a calculated U-value. It helps you adjust thermal performance without excessively raising floor levels or rafter depth.

What is 50mm thick loft insulation?

50mm thick loft insulation is a thermal layer measuring 50mm in depth, supplied either as rigid boards (typically PIR) or as flexible mineral wool slabs or rolls. 

Materials like PIR insulation board trap gas within closed cells. The trapped gas within the material is a poor thermal conductor, which means heat transfer is slowed significantly. 

Mineral wool performs differently. It traps still air between fibres, and because air is a poor heat conductor, the fibres disrupt heat flow (typically 0.035–0.040 W/mK). 

Because of those conductivity values, 50mm insulation loft boards made from PIR provide more resistance per millimetre than mineral wool. 

But a single layer of 50mm loft insulation solution will not meet the typical Part L new-build cold loft roof target of around 0.16 W/m²K on its own. It is usually combined with other layers to reach compliance.

Insulation 50mm loft thickness is specified where height is controlled, or where you are improving rather than rebuilding.

What is 50mm loft insulation used for?

Thin 50mm loft insulation is commonly installed on:

Floor applications

Floor build-up matters a lot in loft conversions. You may lay 50mm PIR between or over joists to add thermal performance without increasing floor height. It is often paired with acoustic insulation and a structural deck for enhanced energy efficiency.

A 50mm loft insulation roll is not suitable as a structural floor layer, but mineral wool at this depth can reduce sound transfer between floors when installed between joists.

Roof (cold loft ceiling level)

You may be topping up an older property with, say, 100mm already in place. Adding loft insulation 50mm thick perpendicular to the first layer reduces thermal bridging across joists. It improves performance but still falls short of full 270mm guidance unless additional insulation layers are added.

Pitched roof

Between rafters, 50mm PIR is common where rafter depth is shallow. It is normally combined with additional insulation under the rafters. Ventilation gaps above insulation must be maintained in cold roof designs to prevent condensation risk.

• Ventilation requirements control should be designed in line with document Part C and BS 5250.
• Warm roof constructions do not require ventilation above insulation, subject to condensation risk assessment.
• Cold-pitched roofs do require maintained airflow.

Walls and framing walls

In timber frame partitions within a loft conversion, 50mm mineral wool slabs help with sound and some thermal control. PIR at 50mm can also line internal studwork to limit heat loss from conditioned rooms into cold voids.

Internal masonry walls

On solid brick gable walls inside a loft space, 50mm PIR boards fixed mechanically or with adhesive can significantly reduce heat loss. A vapour control layer may be required depending on the build-up to manage interstitial condensation.

Types of 50mm loft insulation

At Insulation Wholesale, we stock two main categories:

PIR Insulation (rigid boards)

These are rigid 50mm loft insulation boards.

• Usually foil-faced
• Thermal conductivity around 0.022 W/mK
• High compressive strength
• Suitable for floors, pitched roofs, and internal wall lining

PIR boards provide strong thermal resistance at low thickness. They must be tightly fitted to avoid gaps, as air leakage reduces performance.

Mineral wool insulation (rolls or slabs)

These are often specified where fire resistance and sound absorption matter more than maximum thermal value per millimetre, especially in roof insulation.

• Thermal conductivity typically 0.035–0.040 W/mK
• Non-combustible (Euroclass A1 reaction to fire)
• Good acoustic absorption
• Flexible and easy to friction-fit, glass mineral wool rolls are ideal for various applications.

A loft insulation roll 50mm thick, such as glass mineral wool, is easier to fit around irregular shapes.