PIR Insulation Thickness for Floors

Choosing the right PIR insulation thickness for floors is crucial for achieving strong thermal performance, meeting Building Regulations and ensuring long-term comfort and energy efficiency. Floor insulation plays a huge role in reducing heat loss because ground floors can account for a significant proportion of total energy loss in an uninsulated home. Getting the specification right — especially the correct PIR thickness — makes all the difference.

Whether you are insulating a concrete slab, floating floor, or suspended timber floor, this guide explains the most suitable PIR thicknesses, how to choose between them, and what regulations typically require. For a full range of options, visit our most popular thickness categories: 75mm PIR Insulation Boards, 100mm PIR Insulation Boards and 150mm PIR Insulation Boards.

Why PIR Is Ideal for Floor Insulation

PIR (polyisocyanurate) boards are one of the most thermally efficient rigid insulation materials available, offering a typical λ-value of around 0.022 W/m·K. This means you can achieve excellent U-values without the excessive thickness or build-up required by mineral wool or EPS boards.

Because floor build-ups often have height restrictions — especially in renovations — PIR is the most practical way to reach modern insulation standards while still maintaining usable floor heights.

Typical PIR Thicknesses for Floors

• 75mm PIR
• 100mm PIR
• 150mm PIR

Best PIR Thickness for Different Floor Types

1. Concrete Slab Floors (New Builds)

For new-build concrete slabs aiming to meet or exceed current Part L U-values (often around 0.18 W/m²K or better), the typical PIR thickness specification is:

• 100–150mm PIR

Thicker insulation is usually placed above the slab with a screed or floating chipboard finish. Many specifiers now choose 150mm PIR boards to future-proof the building against potential tightening of standards.

2. Floating Floors (Chipboard Over PIR)

Floating floors are common in renovations and timber-frame builds. Because the insulation sits directly beneath the chipboard, you need sufficient thickness to avoid cold bridging and meet U-value requirements.

• 100mm PIR — the most common choice for solid U-value improvement.
• 75mm PIR — suitable if space is constrained and the existing floor is being upgraded rather than rebuilt.

3. Suspended Timber Floors

Suspended floors often allow air movement beneath the joists, increasing heat loss. Because of this, a thicker PIR insulation is typically needed compared to slab floors to compensate.

• 100–150mm PIR

Many installers specify 150mm PIR boards either between joists or layered beneath the joists to provide a thermal break and achieve the required performance.

How Building Regulations Influence PIR Thickness

• ~0.18 W/m²K for new build floors
• ~0.25 W/m²K for renovation or refurbishment floors

PIR insulation boards make it easier to meet these values within a manageable thickness. As a general rule:

• 75mm PIR — often used for upgrades and renovations where floor height is limited.
• 100mm PIR — standard for many renovation and new-build applications.
• 150mm PIR — recommended for new build floors or high-performance, low-energy homes.

Quick Comparison Table

Choosing the Correct PIR Thickness

• Renovations: aim for 100mm PIR (or more if possible)
• New builds: target 150mm PIR where floor depth allows
• Space-restricted upgrades: go for 75mm PIR and supplement other elements if needed

PIR insulation remains one of the best solutions for achieving warm, energy-efficient floors while minimising build-up depth. By choosing the right thickness early, you improve comfort, reduce heating bills and meet or exceed modern insulation standards.

Note: Always consult manufacturer datasheets, U-value calculators and Building Regulations to confirm the correct PIR thickness for your specific floor construction.