Choosing the right insulation material is crucial for energy efficiency, comfort, and long-term building performance. Among the most popular rigid insulation boards on the market are PIR (Polyisocyanurate) and Phenolic insulation boards. While both provide excellent thermal performance, they have distinct characteristics, applications, and benefits that homeowners and contractors should understand.

What is PIR Insulation?

PIR insulation is a type of rigid foam board made from polyisocyanurate with foil or facings on one or both sides. PIR boards, such as Celotex, are widely used in walls, roofs, floors, and loft conversions due to their high thermal efficiency, lightweight nature, and ease of installation.

The typical thermal conductivity of PIR insulation is around 0.022 W/m·K, which allows builders to achieve low U-values with thinner boards. PIR is also known for its structural rigidity, making it suitable for underfloor heating systems, flat roofs, and even some external applications.

Advantages of PIR Boards

• High thermal performance in thin board thickness
• Lightweight and easy to handle on site
• Good compressive strength for floors and load-bearing applications
• Foil facings can act as a vapour control layer

What is Phenolic Insulation?

Phenolic insulation boards are made from a rigid phenolic foam core, often with foil or fleece facings. These boards are prized for their extremely low thermal conductivity — typically around 0.019 W/m·K — making them slightly more efficient than PIR for the same thickness. Popular phenolic options include Kingspan K106 and Kingspan K103 cavity and wall boards.

Phenolic boards are rigid and stable, with excellent dimensional stability over time. They are commonly used in cavity walls, flat roofs, and floors where space-saving and high efficiency are critical.

Advantages of Phenolic Boards

• Lower thermal conductivity than PIR, allowing thinner insulation layers
• Excellent fire performance compared with some PIR boards
• High compressive strength and dimensional stability
• Durable and long-lasting performance in both retrofit and new builds

Key Differences Between PIR and Phenolic Insulation

While both PIR and phenolic insulation deliver high thermal performance, the main differences lie in conductivity, fire performance, and moisture behaviour:

• Thermal Efficiency: Phenolic boards typically achieve slightly lower thermal conductivity (≈0.019 W/m·K) compared with PIR (≈0.022 W/m·K), allowing thinner build-ups to meet building regulations.
• Fire Performance: Phenolic insulation often has a better reaction-to-fire classification than standard PIR boards, which can be critical in multi-storey or highly regulated applications.
• Moisture Resistance: Both PIR and phenolic boards are closed-cell, but PIR may require additional vapour control in high humidity areas, whereas phenolic boards with foil or fleece facings can manage moisture more effectively.
• Applications: PIR boards are versatile and widely used in lofts, floors, and walls, while phenolic boards are often preferred for cavity walls, flat roofs, and situations where minimal thickness is desired.

Which Should You Choose?

Choosing between PIR and phenolic insulation depends on your project requirements. If you need ultra-thin boards with maximum thermal performance for walls or roof cavities, phenolic insulation like K106 is ideal. If you require a more versatile solution for floors, lofts, or walls with moderate space constraints, PIR boards like K107 may be the better option.

Ultimately, both insulation types help improve energy efficiency, reduce heating bills, and contribute to sustainable building practices. Consult with your installer or building control to ensure your choice meets U-value targets and building regulations.

For more options on Kingspan insulation, check out the full collections: K103 Boards, K107 Boards, and individual products like K106 Cavity Board.