25mm Insulation SR/UF

25mm insulation is 25mm thick (just under 3cm). It reduces heat transfer and helps to moderate internal surface temperatures. In most cases, this type of thin insulation is specified when space is limited.

What is 25mm thick insulation?

25mm thick insulation is a sheet or layer of insulating material with a depth of 25mm insulation thickness.

Unlike the thicker quilt or roll insulation used in lofts, 25mm insulation is made for tight spaces. It slips neatly into shallow voids or sits on surfaces where thicker products would not fit.

On its own, it is unlikely to meet full heating demand. Its performance depends on its composition.

The key difference between materials lies in how well they conduct heat within that limited depth. Some materials slow heat transfer better than others. By placing insulation in its path – even a thin 25mm layer – you reduce how easily that transfer happens.

What is 25mm insulation thickness used for?

Thin 25mm insulation is useful when:

You do not have much space to add deep boards, such as on interior stud walls or external wall insulation (EWI) render systems.
You want to improve thermal performance without rebuilding, for example, before plasterboard or as a “top-up” in retrofit work.
It is effective at reducing thermal bridging, preventing heat from leaking through frames or junctions.
You can use it to underpin airtightness in assemblies where larger boards would interfere with finishes. The insulation is part of a sealed layer here.
In some floors or ceilings, a 25mm insulation layer helps stabilise temperature swings.

It is not usually sufficient on its own to meet current Part L Building Regulations for new builds, but it helps bring assemblies closer when combined with other materials.

Types of 25mm insulation

Common 25mm insulation materials include:

25mm EPS (Expanded Polystyrene)

EPS is a white foam made of tiny beads that are expanded and fused together.

It is light, cost-effective, and easy to cut. EPS gives reasonable thermal performance, though not as good as PIR or phenolic at the same 25mm insulation thickness.

Used in floor screeds or lightweight infill panels where cost matters.

Glass mineral wool

This insulation material is made from spun glass fibres. It is breathable (allows moisture to pass through), soft to cut, and effective at thermal and sound control.

Thin 25mm glass wool sheets are used behind plasterboard or in partition ceilings to provide additional insulation and acoustic performance.

Phenolic foam

Phenolic’s closed-cell structure gives it a higher insulating ability per millimetre compared with many others. A thin 25mm insulation layer of phenolic offers better performance than many thicker, lower-performance materials.

PIR Insulation 25mm (Polyisocyanurate)

PIR boards are rigid foam with foil or laminate facings. They have a higher thermal resistance per millimetre (low λ-value) than EPS or standard mineral wool, making 25mm thin insulation boards useful where space is at a premium.

PIR is common in walls, floors, and ceilings where good thermal control is required with limited depth.

25mm Rock mineral wool

Rock wool works best in thicker layers. So 25mm rock wool insulation is mostly chosen to enhance acoustic or fire performance, not for full thermal insulation.

25mm VIP (Vacuum Insulated Panels)

VIPs have a core sealed in a vacuum pouch, which dramatically reduces heat flow relative to their thickness. A VIP at 25mm insulation thickness can outperform almost any other material the same size.

They are pricier and more delicate to handle, but highly effective where maximum performance in minimal space is required.

Benefits of thin 25mm insulation thickness

Even though it is slim, 25mm thick insulation brings meaningful gains when used right:

Space saver

In tight spaces such as interior stud walls, slim cavity tops, or retrofits, 25mm will not take up much space. You can still install plasterboard or finishes without making the setup bulky.

Ease of installation

Thin sheets are lighter and easier to handle than deep boards. Cutting and fitting them takes less effort.

Improved thermal comfort

Heat moves more slowly through any insulation. A thin 25mm insulation layer makes internal spaces warmer by reducing cold spots on walls and ceilings.

Moisture management

Depending on the material (like breathable glass mineral wool), thin insulation can complement vapour control strategies without trapping moisture when used with appropriate vapour control layers.

Acoustic help

Some 25mm options – especially mineral wool – add noticeable sound damping across partitions and ceilings.

Fire resistance

Mineral wool and phenolic boards offer excellent fire performance even in thinner builds, subject to manufacturer classification.

Fewer fixing issues

Thin boards do not bow or squeeze in narrow cavities. There is less risk of them pushing finishes or creating gaps when correctly specified and installed.

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