Concrete slabs are one of the biggest sources of heat loss in UK construction. For trade professionals, choosing the right floor insulation isn’t just about thermal performance, it’s about meeting Building Regulations, managing costs, and ensuring long-term durability on site.
This 2026 trade guide breaks down the best insulation options for concrete floors, including PIR boards, mineral wool, and rigid foam, plus practical installation advice.
Why Floor Insulation Matters in Concrete Slabs
Uninsulated concrete floors can account for up to 10-15% of total heat loss in a building. With stricter Part L Building Regulations, proper insulation is no longer optional it’s essential.
Key benefits:
- Improved U-values (critical for compliance)
- Lower energy bills for end users
- Reduced condensation risk
- Better thermal comfort
Best Types of Floor Insulation for Concrete Slabs
1. PIR Insulation Boards (Most Popular for Trade)
PIR (polyisocyanurate) boards are the go-to solution for most contractors.
Why trade prefers PIR:
- High thermal performance (low lambda value ~0.022 W/mK)
- Thinner boards achieve required U-values
- Easy to cut and install on site
- Widely available in sizes like 2400 x 1200
Best use cases:
- New build concrete slabs
- Underfloor heating systems
- Projects with tight floor build-ups
Tip: Common brands include Celotex, Kingspan, Recticel
2. Rigid EPS / XPS Insulation
Expanded (EPS) and extruded (XPS) polystyrene are cost-effective alternatives.
Advantages:
- Lower cost than PIR
- High compressive strength (ideal for load-bearing floors)
- Moisture resistance (especially XPS)
Limitations:
- Thicker boards required vs PIR
- Slightly lower thermal performance
Best use cases:
- Budget-driven projects
- Heavily loaded floors (e.g. garages, warehouses)
3. Mineral Wool (Rockwool) Floor Insulation
Mineral wool is less common under slabs but still relevant in specific builds.
Advantages:
- Excellent acoustic performance
- Non-combustible (A1 fire rating)
- Good for separating floors
Limitations:
- Lower compressive strength
- Not ideal directly under concrete without proper system design
Best use cases:
- Acoustic floors
- Suspended floor systems (rather than slab-on-grade)
Recommended Insulation Thickness (UK 2026)
To meet Part L requirements, typical thicknesses are:
- PIR boards: 100-150mm
- EPS/XPS: 120-200mm
- Mineral wool: project-specific
Exact thickness depends on:
- Target U-value (typically ~0.13-0.18 W/m²K)
- Floor construction build-up
- Whether underfloor heating is installed
Floor Insulation Build-Up (Typical Detail)
A standard insulated concrete slab build-up looks like:
- Compacted sub-base
- Damp proof membrane (DPM)
- Insulation layer (PIR / EPS / XPS)
- Vapour control layer (if required)
- Concrete slab or screed
- Floor finish
Pro tip: Always ensure tight board joints to prevent thermal bridging.
Best Insulation for Underfloor Heating
If you’re installing UFH, PIR boards are usually the best option because:
- They reflect heat upward efficiently
- Reduce heat loss into the ground
- Allow thinner build-ups
Look for:
- High compressive strength
- Foil-faced boards for improved performance
Cost Comparison (Trade Perspective)
Typical UK trade pricing trends (2026):
- PIR boards: £££ (premium, best performance)
- EPS: ££ (budget-friendly)
- XPS: £££ (strong + moisture resistant)
- Mineral wool: ££ (niche use)
For most jobs, PIR offers the best balance of cost vs performance, especially where space is limited.
Common Mistakes to Avoid on Site
- Using incorrect thickness (fails Building Regs)
- Poor board fitting (thermal gaps reduce performance)
- Skipping edge insulation (major heat loss point)
- Not accounting for load-bearing requirements
- Installing insulation without proper DPM
How to Choose the Right Floor Insulation
For most trade projects:
- Use PIR → when performance + space matters
- Use EPS/XPS → when cost or load strength is priority
- Use mineral wool → for acoustic or fire-rated builds
Conclusion
For concrete slabs in 2026, PIR insulation remains the industry standard for trade professionals, offering the best thermal performance with minimal thickness.
However, the right choice always depends on:
- Project type
- Budget
- Load requirements
- Compliance targets
Working with a trusted supplier like Wray-Tech Supplies ensures you get the right materials, in stock, and ready for site.
FAQs
1. What is the best insulation for concrete floors in the UK?
PIR insulation boards are generally the best option due to their high thermal performance and thinner profile, making them ideal for meeting Building Regulations.
2. How thick should floor insulation be for concrete slabs?
Typically:
- PIR: 100-150mm
- EPS/XPS: 120-200mm
Exact thickness depends on the required U-value and build-up.
3. Can you put insulation directly under a concrete slab?
Yes. Insulation is commonly installed beneath or above the slab with a damp proof membrane to prevent moisture issues.
4. Is PIR better than EPS for floor insulation?
PIR offers better thermal performance, meaning thinner boards can be used. EPS is cheaper but requires greater thickness.
5. Do I need insulation under a concrete floor?
Yes. UK Building Regulations (Part L) require floor insulation to meet energy efficiency standards in most new builds and renovations.
Wray-Tech Supplies is a leading supplier of floor insulation, wall insulation, roof insulation and loft insulation needs, contact us at 0203 929 3999 or sales@wray-techsupplies.com for fast UK delivery and competitive trade prices.