125mm Insulation

125mm insulation is a commonly specified thickness in the UK construction industry. It provides reliable thermal performance without making walls or roofs difficult to build around. It is often specified for applications requiring good thermal performance, particularly in modern residential construction.

What is the insulation 125mm thickness?

125mm insulation thickness means the insulating material is 125 millimetres deep.

A layer of 125mm thick insulation creates a longer path for heat to travel, reducing how quickly warmth escapes and passes through the building fabric.

A 125mm PIR provides high thermal resistance by efficiently resisting heat flow. In comparison, fibrous materials like glass mineral wool rely on trapped air within the fibres. So 125mm thickness offers slightly lower thermal performance but better sound absorption and noncombustible fire performance compared with PIR at the same thickness.

What is 125mm insulation used for?

Insulation at 125mm thickness is commonly used in areas where building regulations or design require solid thermal performance without oversizing the wall or roof.

Its uses include:

• Cavity wall infill – Between the skins of a brick wall to stop heat escaping.
• Floor insulation – Under screeds or between joists to cut heat loss downwards.
• Roof or ceiling insulation – Between rafters or joists in pitched and flat roofs.
• Partition and separating walls – Primarily for acoustic and fire performance rather than thermal separation.

Types of 125mm thick insulation

Several materials are available in 125mm insulation thickness in the UK:

Glass wool insulation

Products in this category include cavity wall slabs, such as 125mm Knauf Dritherm and Superglass Superwall 125mm thickness. They are:

• Non-combustible, so excellent for fire safety.
• Soft to semi-rigid – Easy to cut and fit between ties or joists.
• Water-repellent – Designed to resist moisture ingress when correctly installed.
• Good for sound and thermal protection.

Glass mineral wool slabs are often used to fill the whole cavity or the space between timber or steel framing.

PIR Insulation (Polyisocyanurate) Boards

PIR is available in rigid foam boards. It has lower thermal conductivity than mineral wool, meaning greater heat resistance per millimetre. A 125mm PIR insulation panel gives very good thermal performance in a relatively thin thickness. Applications include:

• Between rafters in roofs.
• Underfloor insulation beneath screeds or timber floors.
• Internal and external wall insulation where boards are bonded or fixed mechanically.

PIR boards are often faced with foil or composite facings for added protection.

Other materials

Expanded polystyrene (EPS) and rock mineral wool may also be made to this depth, but they are less commonly stocked at this thickness.

Benefits of 125mm insulation thickness

A thick 125mm insulation layer offers:

• Better thermal resistance than thinner layers – greater thickness generally increases thermal resistance.
• Works well with UK typical cavity widths and joist depths.
• Good compliance with building regulations when paired with the right materials and details.
• Often easier to fit than very thick insulation because it is easier to handle in many site conditions.

A product with 125mm insulation typically performs notably better than 100mm options in slowing heat flow.