20mm Insulation

20mm insulation refers primarily to rigid insulation boards or panels that are 20mm thick. Despite their slim profile, they still help slow heat transfer through walls, floors, or ceilings. It is used when there is insufficient room for thicker layers, but you still need performance and compliance with UK thermal standards (e.g., Part L).

What is 20mm insulation thickness?

Insulation 20mm thickness materials have a consistent depth of 20 millimetres across the full depth of the board.

A 20mm insulation layer is thin and does not provide high thermal resistance on its own (compared to thicker boards). However, modern materials like PIR and VIP achieve strong thermal performance in their slim profiles by having much better insulating performance per millimetre than older options.

20mm boards are chosen to deliver good thermal control when space is limited. They are a practical option when upgrading a refurbishment, adding insulation within stud wall build-ups or as an internal lining, or adding a thermal layer under finishes where depth is tight.

What is 20mm insulation used for?

Common applications of 20mm thick insulation include:

• Space-restricted upgrades: In renovations or extensions where you cannot add thick insulation, 20mm boards help raise performance without losing head height, floor clearance, or cabinet depth. 
• Layered systems: Sometimes it is part of a multi-layer build – for example, combining with thicker insulation elsewhere to achieve target U-values or reduce cold bridging.
• Thermal bridging control: Placed around window reveals, service penetrations, structural steel, and plasterboard backings to minimise heat loss paths. 
• Lightweight installs: Adds minimal dead load to the structure, making handling easier in ceilings or partitions.

Types of 20mm thick insulation

There are several kinds of thin 20mm insulation boards on the market today:

PIR (Polyisocyanurate) boards

These are rigid foam boards with foil facings.

• Offers very high thermal performance for the thickness– excellent for tight spaces.
• Often used in walls, floors, and roofs when you need to meet target U-values with limited depth.
• Lightweight and relatively straightforward to install.
• Best for refurb walls, internal lining, and installing under a finish.

EPS (Expanded Polystyrene) boards

EPS is classic rigid foam made from fused beads.

• Cheaper than many PIR options.
• Lower performance per millimetre than PIR.
• Budget-friendly and easy to cut.
• Ideal for floor insulation layers, cavity detailing, and low-demand thermal zones.

VIP (Vacuum Insulated Panels)

These are a specialist solution where 20mm insulation offers exceptionally high thermal performance. They are built with a vacuum-sealed core that vastly cuts heat flow, giving much higher resistance per millimetre than foam or fibre.

• VIPs outperform other insulation materials in thermal performance by a wide margin – sometimes five to ten times better per millimetre.
• Best for projects where space is very limited, like tight retrofit walls or floors, but where performance must still be high.

Because of the vacuum core, you cannot cut VIP boards. Doing so destroys the vacuum and ruins the performance.