25mm Loft Insulation

25mm loft insulation is a thin thermal layer specifically designed for tight spaces where overall depth is restricted.

This insulation depth, on its own, is not installed to reduce heat loss and improve overall energy efficiency. It is more about fine-tuning a detail, lining a junction, or adding controlled thermal resistance without raising levels too much.

A 25mm loft insulation thickness will not transform a cold roof by itself, but in the right place, it addresses common site detailing constraints.

What is 25mm thick loft insulation?

25mm loft insulation thickness means a board or slab manufactured to a nominal thickness of 25mm deep across its full face. The thickness directly affects the thermal resistance it provides.

In rigid PIR form, 25mm thick loft insulation is a closed-cell foam board faced with foil. The foil can contribute to a vapour control layer when foil faces, joints, and penetrations are fully sealed and correctly positioned within the build-up. Typical thermal conductivity (lambda) for PIR is 0.022–0.023 W/m.K, depending on product.

In mineral wool form, such as rigid and semi-rigid stone wool products, the fibres trap air. Lambda is typically around 0.035–0.040 W/m·K, depending on density.

• Thinner board = lower total thermal resistance of roof insulation.
• Better lambda = more performance per millimetre of high-performance insulation.

So a thin 2400mm x 1200mm x 25mm PIR loft insulation board gives noticeably more resistance than 25mm mineral wool, simply because the high-performance foam conducts less heat.

But 25mm loft thin insulation is still a shallow layer. It is usually used as part of a build-up, not as the only insulation solution in a loft, to meet the Part L requirements of the UK Building Regulations.

What is 25mm loft insulation thickness used for?

25mm loft insulation comes in handy in a variety of applications:

Floor applications

In loft conversions or raised deck details, 25mm PIR can sit above joists before chipboard goes down. Why use it?

• You need a thin thermal layer within a restricted floor build-up.
• To correct thermal bridging at joist lines.
• You cannot afford to lose ceiling height below.
• It improves comfort underfoot.

It does not replace the deeper floor insulation required between or below joists to achieve regulatory U-values.

Framing walls

In timber or metal stud partitions, a 25mm layer can be added internally when:

• You’ve already insulated between studs.
• You want a continuous thermal layer across the face.
• You’re improving SAP calculation results.

Boards are fixed over the frame, joints taped, and then plasterboard is applied for excellent thermal insulation.

Internal masonry walls

On solid brick or block, 25mm PIR can be fixed using insulation adhesive (dot-and-dab) on specific systems or mechanical fixings before plasterboard. It is a common wall insulation solution where:

• Room size is limited.
• You are upgrading a single cold wall.
• Full internal wall systems are too thick.

Again, it improves thermal comfort in your living space, but it will not achieve deep retrofit targets when used alone.

Pitched roof

In a rafter roof, 25mm PIR is often used:

• As a thermal layer beneath rafters.
• To maintain ventilation space above thicker insulation.
• As a correcting layer of roof insulation to reduce cold bridging.

For example, 100mm between rafters + 25mm across the underside reduces overall U-value by reducing thermal bridging through the rafters.

Types of 25mm loft insulation

We stock two main categories of loft insulation materials at 25mm thick. What you choose depends on the problem you hope to solve: fire performance, thermal efficiency, or acoustic insulation.

PIR (rigid foam board)

High-performance rigid boards for insulating the loft have the following features:

• Lambda ≈ 0.022 W/mK
• High compressive strength
• Foil-faced
• Precise, easy to cut using a fine-toothed saw or sharp insulation knife with a straight edge

Best for tight spaces where exceptional thermal output per millimetre is desired.

Rock mineral wool (rigid or semi-rigid slab)

• Lambda ≈ 0.035–0.040 W/mK
• Non-combustible (Euroclass A1)
• Excellent acoustic absorption
• Vapour permeable

Best where fire resistance and sound control are priorities over maximum thermal performance per depth.

Benefits of thin 25mm loft insulation thickness

Installing 25mm loft insulation is useful in these ways:

• Minimal build-up height
• Useful for thermal detailing and junctions
• Reduces thermal bridging
• Lightweight and easy to install
• Works well in layered systems
• Adaptable for targeted retrofit upgrades