165mm Insulation

165mm insulation is most commonly supplied as rigid thermal insulation boards used in construction to slow heat flow. It is deeper than common thinner boards, meaning it lowers heat loss more effectively for improved internal thermal performance in buildings.

What is 165mm Thick Insulation?

165mm insulation thickness is the total depth of the board from the warm side to the cold side. Typically:

• Many 165mm PIR boards are supplied at approximately 2400mm x 1200mm x 165mm.
• Most have a PIR foam core – a high-performance material that has a closed-cell structure that restricts heat transfer.
• In PIR boards, both faces are foil-finished to reduce radiant heat transfer and improve overall thermal performance.

Using a thick 165mm insulation board helps achieve target U-values with fewer layers, saves time on site, and reduces the number of joints compared with multiple thinner layers.

A typical example is the 165mm PIR Celotex XR4165, made from polyisocyanurate with foil facings. These light, rigid panels deliver strong thermal performance in a single layer. This often eliminates the need to stack multiple thinner layers to meet energy standards.

What is Thick 165mm Insulation Used For?

This level of insulation is used where high thermal resistance is required. Its main applications include:

1. Roof insulation: Houses and commercial roofs where regulations or design require deep insulation to reduce heat loss. PIR boards fit between rafters or above decking.
2. Floor insulation: Underneath ground floors – concrete slabs, beam & block, or suspended timber. Here, it helps reduce thermal bridging and improves comfort when correctly installed.
3. Wall insulation: Within timber-frame walls or lining systems used to upgrade masonry walls. An adequate cavity or lining depth must be available to accommodate a thicker solution.
4. Other insulated structures: Steel frames or buildings that need good thermal performance, and the space is deep enough for a thicker board.

The key idea is that thicker insulation at the right places effectively reduces heat loss and helps buildings meet Part L energy performance requirements when correctly specified.

FAQ

Where to buy 165mm insulation?

Buy thick 165mm insulation boards online at Insulation Wholesale. We stock a wide range of thermal insulation materials, including high-performance 165mm PIR boards that suit roofs, walls, and floors. We sell high-quality products at competitive prices, with UK stock and fast nationwide delivery. Our store also offers trade discounts and easy ordering for bulk jobs – great for trade pros who need multiple boards.

How to cut 165mm insulation?

Cutting 165mm thick insulation is straightforward when you have the right tools:

1. Mark the cut line with a straightedge and a pencil or chalk line.
2. Use an insulation saw. It gives a cleaner edge on PIR boards.
3. Support the board on a flat surface so it does not bend or crack when you cut.
4. Take your time – firm boards like PIR resist bending, so slow, even strokes make a neater cut.

Good cutting makes fitting easier and avoids gaps where heat could leak.

How to fix 165mm insulation?

Fixing thick insulation boards depends on the location of the install:

Roofs:

• Slide boards between rafters or lay over decking.
• Use support battens, screws with washers, or adhesive where needed.

Floors: 

• Lay insulation directly on a clean, level sub-floor.
• Ensure boards butt up tightly. Seal joints if required by spec.

Walls: 

For timber frames, push boards into stud spaces. Add fixings or battens to hold them in place. Secure fixing and tight joints reduce thermal bypass and improve overall performance. Check the manufacturer’s fixings guide and building spec

Planning a bulk order of 165mm insulation? Get a fast quote and take advantage of our pallet deals for bigger orders. Our team is ready to support you on your project, from start to delivery.