30mm Loft Insulation

30mm loft insulation is a thin, rigid or semi-rigid layer used to enhance thermal control in tight spaces.

It is not the depth you would choose to insulate an entire cold loft on its own, but it plays an important supporting role in many roof and floor build-ups.

It serves as a precision layer designed to improve performance, reduce cold bridging, and add controlled thermal resistance where thicker products will not fit.

What is 30mm thick loft insulation?

Thick 30mm loft insulation refers to boards or slabs manufactured to a uniform nominal thickness of 30mm. This usually means either rigid PIR boards (polyisocyanurate foam) or high-density rock mineral wool slabs.

PIR boards are:

• Closed-cell in structure, with foil facings on both sides
• Rigid, with good compressive resistance for internal building applications
• Lightweight

They are used when you need thermal performance from minimal depth.

30mm rock mineral wool (stone wool) offers:

• Non-combustible fire resistance
• Good acoustic absorption in your living space
• Vapour permeability
• Flexibility that allows a close friction fit between timber or metal framing

A 30mm mineral wool loft insulation solution is often chosen for fire and sound reasons rather than for maximum thermal efficiency.

What is thick 30mm loft insulation used for?

A 30mm thick loft insulation board or slab is rarely used across the whole loft floor in a cold roof. Instead, it is suitable for use in targeted areas where build-up depth is restricted or detailing needs tightening.

In typical site applications, it is commonly used in the following areas:

1. Floor applications

30mm PIR can be laid over structural decking or between battens as part of a layered floor build-up in loft conversions.

Typical example:

• 100mm between joists
• 30mm over joists to reduce thermal bridging
• 18mm chipboard above

The continuous insulation layer above the joists improves thermal continuity and comfort underfoot. It also helps reduce cold stripes caused by timber joists interrupting insulation.

30mm enhances the system, but does not replace deeper insulation where Building Regulations require lower U-values.

2. Framing walls

In dormer cheeks or stud partitions within a loft conversion, 30mm boards may be used:

• Between studs where cavity depth is limited
• As an internal overboard layer
• To reduce thermal bridging through timber studs

Mineral wool versions fit tightly between studs and improve sound separation. PIR boards give stronger thermal performance when space is tight.

3. Internal masonry walls

Where loft spaces meet gable or party walls, a 30mm layer can be fixed internally to reduce heat loss and cold bridging at junctions.

PIR boards are mechanically fixed or adhesively bonded, with joints taped. Mineral wool slabs are friction-fitted within a battened lining system.

This application is common in retrofit loft upgrades where full cavity insulation is not feasible.

4. Pitched roof build-ups

Within a pitched roof, 30mm loft insulation is often used:

• As an internal under-rafter layer
• Between rafters where depth is limited
• As part of a layered build-up that allows services to be routed without penetrating the primary insulation layer

For example:

• 100mm between rafters
• 30mm across rafters internally
• Plasterboard beneath

The cross-layer reduces thermal bridging from timber rafters and improves overall U-value consistency.

Some multifoil products, such as SuperFOIL insulation 30mm thick loft variants, are also used in pitched roof systems where reflective layers are specified, though performance depends heavily on correct air gaps and installation details.

Types of 30mm loft insulation

We stock two main types:

PIR insulation boards 30mm

2400mm x 1200mm x 30mm PIR insulation board is best for:

• Maximum thermal resistance per millimetre
• Low-depth build-ups
• Over-boarding

Key features:

• Lambda around 0.022 W/mK
• High compressive strength
• Foil-faced
• Light and easy to handle

This is usually the go-to option when someone specifies 30mm loft insulation thickness for thermal efficiency reasons.

Rock mineral wool slabs 30mm

Best for:

• Fire protection
• Acoustic improvement
• Breathable constructions

Key features:

• Non-combustible (Euroclass A1)
• Flexible fit
• Vapour permeable
• Heavier than PIR

If the priority is fire rating or sound insulation, mineral wool is often a better choice than rigid foam.