What is PIR insulation?

PIR (Polyisocyanurate) is a high-performance rigid foam insulation. In the UK, it is most commonly sold as foil-faced boards (like Celotex, Kingspan, or Recticel) in standard 2.4m x 1.2m sheets. It offers roughly double the thermal performance of mineral wool for the same thickness.

How many boards are in a pack?

PIR boards are usually sold individually, but larger projects can order in bulk packs. A standard pack of 100mm PIR usually contains 3 boards (8.64m²), while thinner 25mm boards might come in packs of 12.

Which thickness should I use for a floor?

For a standard ground floor extension in the UK, Building Regulations usually require enough insulation to achieve a U-value of 0.18 W/m²K. This typically requires 100mm to 150mm of high-performance PIR board over a concrete slab.

Can I cut PIR boards easily?

Yes, PIR boards can be cut easily with a fine-toothed saw or even a sharp utility knife for thinner boards. Always wear a mask and eye protection, as the dust can be irritating.

What is PIR insulation?

PIR (Polyisocyanurate) is a high-performance rigid foam insulation. In the UK, it is most commonly sold as foil-faced boards (like Celotex, Kingspan, or Recticel) in standard 2.4m x 1.2m sheets. It offers roughly double the thermal performance of mineral wool for the same thickness.

How many boards are in a pack?

PIR boards are usually sold individually, but larger projects can order in bulk packs. A standard pack of 100mm PIR usually contains 3 boards (8.64m²), while thinner 25mm boards might come in packs of 12.

Which thickness should I use for a floor?

For a standard ground floor extension in the UK, Building Regulations usually require enough insulation to achieve a U-value of 0.18 W/m²K. This typically requires 100mm to 150mm of high-performance PIR board over a concrete slab.

Can I cut PIR boards easily?

Yes, PIR boards can be cut easily with a fine-toothed saw or even a sharp utility knife for thinner boards. Always wear a mask and eye protection, as the dust can be irritating.

PIR Insulation Calculator

Professional board count estimator for rigid foam insulation.

Length (m)

Width (m)

Board Thickness (mm)

Waste Allowance10%

Board Count

Fitting Tip: When installing PIR boards (Celotex/Kingspan), ensure all joints are tightly butted together and sealed with **aluminium foil tape**. This creates a continuous vapour barrier and prevents 'thermal bridging' where heat can escape through the gaps.

Rigid Insulation: The Professional Guide

Maximum Efficiency

PIR boards are the industry standard for high-performance thermal insulation in the UK. Their rigid structure and foil facing make them ideal for floor slabs, pitched roofs, and internal wall lining.

By using our **PIR board calculator**, you can minimize off-cut waste and ensure you order the correct number of 2400mm x 1200mm sheets for your project.

Board Specifications

Standard Dimensions2400 x 1200mm

Standard Area2.88m² per board

Thermal Conductivity0.022 W/mK

Installation Best Practices

For the best thermal performance, PIR boards should be installed with a tight fit. Any gaps larger than 5mm should be filled with **expanding insulation foam** to prevent air leakage.

In roof applications, it is common to use two layers: one between the rafters and one across the rafters (under-drawing) to eliminate thermal bridging through the timber.

Foil Tape

Always use high-quality aluminium foil tape on all joints. This maintains the integrity of the vapour control layer and prevents interstitial condensation within the structure.

Storage

PIR boards are waterproof but the foil facing can delaminate if they are left in standing water for long periods. Store them flat and off the ground, covered with a tarp.

Pro Tip: Measuring into Voids

When measuring for insulation between rafters or joists, calculate the total area and then **add 15% to 20%** for waste. Cutting boards to fit specific widths between timbers results in more off-cuts than a flat floor or wall application.

PIR Insulation FAQ

Building Control Disclaimer

Insulation requirements vary based on the age of your property and the specific element being insulated (e.g., roof vs floor). Always consult with your local Building Control officer or a qualified SAP assessor to ensure your insulation thickness meets current Part L Building Regulations.