PIR Insulation for Floors: Specifications for Solid and Suspended Floor Applications

PIR insulation for floors requires minimum 120 kPa compressive strength to resist loading without compression beneath screed or floor finishes. For solid concrete floors, 100mm PIR above the slab beneath screed achieves 0.17-0.18 W/m²K U-value meeting Part L requirements, while suspended timber floors need 100-150mm PIR between or above joists depending on joist depth and target U-value. Floor insulation sits directly in the load path, making compressive strength specification more critical than wall or roof applications where insulation doesn’t carry structural loads.

Floor Insulation Requirements Under Part L

Part L 2021 sets a maximum U-value of 0.18 W/m²K for ground floors in new builds and extensions. This applies to both solid concrete floors and suspended timber floors, though the installation methods differ significantly between the two.

What counts as a ground floor for Part L:

Ground floors include any floor in contact with the ground or exposed to outside air beneath. This covers:

• Ground-bearing concrete slabs on new builds
• Beam and block ground floors
• Suspended timber ground floors over ventilated voids
• Floors over unheated garages or undercrofts
• Floors over crawl spaces

Upper floors between heated spaces don’t need insulation (though they often get acoustic insulation). Floors separating heated from unheated spaces need insulation meeting Part L requirements.

Scotland Building Standards:

Scotland requires 0.15 W/m²K for ground floors, tighter than England’s 0.18 W/m²K. This typically means 120mm PIR instead of 100mm for solid floor constructions.

Renovation requirements:

When renovating floors where more than 50% of the floor area is being replaced or upgraded, reasonable provision must be made to improve thermal performance. The target is typically 0.25 W/m²K where achievable, recognising that retrofit situations face constraints.

The “where achievable” caveat matters for floor retrofits more than any other building element. Adding insulation raises floor level, potentially creating:

• Steps at doorways requiring threshold modifications
• Door trimming to maintain clearance
• Conflicts with damp-proof course levels
• Issues with service entries and penetrations

If achieving 0.25 W/m²K creates these problems, a lower performance standard is acceptable with proper justification to Building Control.

Why floor insulation matters:

Uninsulated ground floors lose significant heat. A typical 50m² ground floor with U-value 0.70 W/m²K (poorly insulated older floor) loses approximately 4,000 kWh annually compared to a floor achieving 0.18 W/m²K. At current energy prices, that’s £400-500 per year in wasted heating.

Beyond energy costs, uninsulated floors feel cold underfoot, reducing comfort even when room air temperature is adequate. This cold floor perception often leads occupants to turn up heating, increasing energy use further.

PIR Insulation for Concrete Floors

Solid concrete ground floors are the most straightforward application for PIR floor insulation. The standard build-up places insulation above the concrete slab, beneath the screed.

Standard Solid Floor Build-Up (Bottom to Top)

Layer 1: Ground preparation and sub-base Typically 100-150mm hardcore or MOT Type 1 aggregate, compacted in layers. This provides stable base and drainage path for ground water. Blinding layer (sand or weak concrete) over hardcore creates smooth surface for DPM installation.

Layer 2: Concrete ground-bearing slab Minimum 100mm thick concrete slab (typically 150mm for residential). The slab provides structural base and distributes loads to the ground. Must achieve adequate strength before insulation installation.

Layer 3: Damp-proof membrane (DPM) Polythene DPM (minimum 300 micron, preferably 1200 gauge) over the concrete slab. The DPM prevents moisture rising from the slab into the insulation and screed. Lap minimum 150mm at joints, sealed with DPM tape. Lap with wall DPC at perimeters, sealed to prevent moisture bypass.

Some specifications place DPM beneath the slab instead of above. This protects the slab from ground moisture but puts the DPM at risk during slab pouring. Above-slab positioning is more common and generally more reliable.

Layer 4: PIR insulation 100mm PIR boards (or thickness determined by U-value calculation) laid loose over DPM. Boards fit tightly together with staggered joints. All joints taped with aluminium foil tape to prevent air infiltration and improve airtightness.

Layer 5: Perimeter edge insulation strip 10-20mm thick compressible foam strip around the room perimeter, full screed depth. This provides movement joint allowing screed to expand/contract without cracking and prevents acoustic flanking transmission to walls.

Layer 6: Screed Minimum 65mm unbonded sand-cement screed (or minimum thickness per screed manufacturer specification for proprietary screeds). The screed distributes loads across the insulation and provides base for floor finishes.

Layer 7: Floor finish Tiles, timber, carpet, or other floor finish per specification.

Why this sequence works:

Placing insulation above the slab keeps it accessible if problems occur. If you need to add service penetrations later, you can cut through screed and insulation without breaking into the structural slab.

The unbonded screed (separated from insulation by the screed’s own surface) allows the screed to move slightly without cracking. Screed bonded directly to insulation can crack as it dries and shrinks.

The DPM beneath insulation prevents moisture affecting the PIR boards. While PIR’s closed-cell structure resists water absorption, keeping moisture away from the boards entirely is better practice.

U-value performance for solid floors:

*Assumes standard build-up: 150mm concrete slab, DPM, PIR insulation, 65mm screed. Actual U-values vary with specific constructions – always run proper calculations.

These U-values account for edge effects (heat loss at floor perimeters) which increase the overall floor U-value above the centre-of-panel calculation. A floor using 100mm PIR might calculate as 0.16 W/m²K for the main area but achieves 0.17-0.18 W/m²K overall when edge losses are included.

PIR Insulation for Suspended Timber Floors

Suspended timber floors need different insulation approaches compared to solid floors because the floor structure creates a void beneath the floor deck.

Suspended Floor Construction Types

Modern suspended timber floors typically use engineered joists (I-joists or metal web joists) spanning between foundations or beam and block construction. Older properties use traditional timber joists spanning between sleeper walls or external walls.

The floor void beneath may be:

• Ventilated to outside air (traditional approach requiring underfloor ventilation bricks)
• Unventilated with ground cover (modern approach using ground gas membrane and perimeter sealing)

Three PIR Installation Methods for Suspended Floors

Method 1: Between Joists

PIR boards fit between floor joists, supported on timber battens fixed to joist sides:

1. Fix 50x25mm timber battens to joist sides, approximately 50mm below floor deck level
2. Cut PIR boards to fit tightly between joists, resting on battens
3. Ensure boards fit full joist depth without gaps
4. Tape board joints where boards meet
5. Install floor deck over joists

This method requires adequate joist depth. For 100mm PIR insulation, you need minimum 150mm joist depth (100mm insulation plus 50mm for battens and air gap).

Advantages:

• Doesn’t increase floor build-up height
• Joists remain accessible from above for services
• Straightforward installation

Disadvantages:

• Joists create thermal bridging (heat loss through timber structure)
• Requires adequate joist depth
• Difficult to install retrofit without removing floor deck

Method 2: Above Joists

PIR boards lay over the top of joists, beneath the floor deck:

1. Lay PIR boards across joists with joints staggered
2. Tape all board joints
3. Install floor deck over insulation, fixing through insulation into joists

This method works with any joist depth and eliminates thermal bridging through joists. The downside is losing floor-to-ceiling height (insulation thickness plus floor deck thickness).

Advantages:

• No joist thermal bridging
• Works with shallow joists
• Better airtightness than between-joist method

Disadvantages:

• Reduces ceiling height
• Services in joists become harder to access
• Fixing deck through insulation requires longer fixings

Method 3: Below Joists

PIR boards fix beneath joists in the underfloor void:

1. Fix PIR boards to underside of joists using mechanical fixings or battens
2. Seal board joints with tape
3. Install breathable membrane beneath insulation if void is ventilated

This method keeps joists and floor deck as-is, making it suitable for retrofit. However, it puts insulation in a potentially damp environment and complicates services installation in the void.

Advantages:

• No impact on floor or ceiling levels
• Suitable for retrofit without disturbing existing floor
• Joists remain accessible

Disadvantages:

• Insulation in potentially damp environment
• More complex installation in restricted void
• Services in void must route below insulation
• Joists become cold, affecting floor temperature

Which method to use?

New builds: Use between-joist or above-joist methods. Between-joist works if you specify adequate joist depth. Above-joist works better for shallow joists or maximum thermal performance.

Renovations with access from above: Above-joist method works well. Accept the ceiling height loss in exchange for better thermal performance and straightforward installation.

Renovations preserving existing floor: Below-joist method if void access is adequate and void conditions are suitable (dry, accessible, adequate height).

U-value performance for suspended floors:

U-values for suspended floors are complex because they depend on floor construction, void ventilation status, and edge details. As a rough guide:

These are approximate values. Run proper calculations using approved software for accurate U-values accounting for joist thermal bridging and edge effects.

Above Slab vs Below Slab Insulation

For solid concrete floors, insulation can theoretically go above or below the structural slab. In practice, above-slab insulation dominates UK construction.

Above Slab Insulation (Standard Approach)

Build-up: Concrete slab → DPM → PIR insulation → Screed → Floor finish

This places insulation between the structural slab and floor finish, creating a floating floor where screed and finish sit on insulation without bonding to the structural slab.

Advantages of above-slab insulation:

Thermal continuity: The insulation forms an unbroken layer across the entire floor area without structural elements penetrating it. This eliminates thermal bridging through the floor structure.

Damp protection: The DPM sits between slab and insulation, protecting insulation from any residual moisture in the slab. Even though PIR resists water absorption, keeping moisture away entirely is better.

Service routing: Services can penetrate the screed and insulation without compromising the structural slab. This makes alterations easier and allows service routing without affecting structure.

Acoustic benefits: The floating screed, isolated from the structural slab by insulation, provides useful impact sound insulation for upper floors.

Below Slab Insulation (Uncommon in UK)

Build-up: Hardcore → Insulation → DPM → Concrete slab → Floor finish

This places insulation beneath the structural slab, requiring the insulation to carry the slab weight.

Why below-slab insulation is rare in UK practice:

Compressive strength requirements: The insulation must carry the full slab weight plus any additional loading. This typically requires specialist high-strength insulation products (200+ kPa compressive strength) rather than standard PIR boards.

Installation risk: Insulation laid before slab pouring is vulnerable to damage during construction. Wet concrete can damage insulation if placement isn’t carefully controlled.

Moisture exposure: Insulation beneath the slab sits in direct contact with ground moisture (even with ground gas membrane beneath). This requires insulation with very high moisture resistance.

Thermal bridging: Ground beams, strip footings, and other foundation elements penetrate the insulation layer, creating significant thermal bridging that’s difficult to detail properly.

When below-slab insulation gets used:

Some industrial or commercial applications use below-slab insulation when the floor finish requires direct bonding to a smooth concrete surface (certain resin flooring systems, for example). In these situations, specialist high-strength insulation products (typically XPS or cellular glass) sit beneath the slab.

For UK residential construction, above-slab insulation is standard practice. Don’t specify below-slab insulation unless you have specific requirements that make above-slab unsuitable.

Floor U-Value Calculations with PIR

Calculating floor U-values involves more complexity than walls or roofs because heat flow paths differ and edge effects matter significantly.

Centre-of-Floor vs Overall Floor U-Value

The centre of a floor (away from perimeter walls) has different heat flow than the floor edges. Heat escapes more readily at perimeters through:

• The exposed floor edge at external walls
• Gaps between insulation and perimeter walls
• Thermal bridging through wall foundations
• Joist ends at external walls (suspended floors)

U-value calculations must account for both centre-of-floor performance and edge effects. The overall floor U-value (what matters for Part L compliance) represents the average across the entire floor area.

For a typical residential room, edge effects increase the floor U-value by approximately 0.01-0.03 W/m²K compared to centre-of-floor calculations. Small rooms have worse edge effects (higher proportion of floor area is near edges). Large rooms have less significant edge effects.

Calculation Methods

Method 1: Use manufacturer U-value calculators Celotex, Kingspan, and other manufacturers provide online U-value calculators. Input your floor construction details (slab thickness, insulation thickness, screed thickness, floor dimensions) and the calculator returns overall floor U-value accounting for edge effects.

These calculators are free, easy to use, and acceptable for Building Regulations submissions. They’re the practical choice for most projects.

Method 2: Use approved calculation software Software like BuildDesk U or similar approved tools provides detailed U-value calculations with full control over all parameters. This suits complex situations or when you need calculations that go beyond standard constructions.

Method 3: Manual calculation per BS EN ISO 13370 The British Standard sets out the calculation methodology for floor U-values. Manual calculation is time-consuming and error-prone. Use software instead unless you need to understand the calculation methodology in detail.

Example U-Value Calculation

For a solid floor construction:

• 150mm concrete slab
• Damp-proof membrane
• 100mm PIR insulation (0.022 W/mK)
• 65mm screed
• Floor dimensions: 8m x 6m (48m² area)

Step 1: Calculate centre-of-floor R-value

• Concrete slab: R = 0.150m / 1.35 W/mK = 0.11 m²K/W
• DPM: negligible R-value
• PIR insulation: R = 0.100m / 0.022 W/mK = 4.55 m²K/W
• Screed: R = 0.065m / 1.20 W/mK = 0.05 m²K/W
• Surface resistances: 0.17 m²K/W (internal) + 0.04 m²K/W (ground)
• Total R-value: 0.11 + 4.55 + 0.05 + 0.17 + 0.04 = 4.92 m²K/W

Centre-of-floor U-value = 1 / 4.92 = 0.20 W/m²K

Step 2: Account for edge effects For a floor 48m² with perimeter 28m, edge effects typically add 0.02-0.03 W/m²K.

Overall floor U-value ≈ 0.17-0.18 W/m²K

This achieves Part L compliance (0.18 W/m²K maximum). For more accurate calculations, use manufacturer tools or approved software.

Impact of Floor Dimensions

Floor U-values improve (get lower) as floor area increases because edge effects become proportionally less significant:

*Approximate values for 100mm PIR in standard solid floor construction

Small rooms need slightly thicker insulation to achieve the same U-value as large rooms. For a small utility room or WC, you might need 110mm PIR where 100mm would suffice in a large living room.

Selecting PIR Thickness for Floor Applications

Choosing appropriate insulation thickness balances thermal performance requirements with practical constraints.

Thickness Selection for Part L Compliance

For solid concrete floors achieving 0.18 W/m²K (England & Wales Part L requirement):

• Small rooms (under 25m²): 100-110mm PIR
• Medium rooms (25-50m²): 100mm PIR
• Large rooms (over 50m²): 90-100mm PIR

For Scotland (0.15 W/m²K requirement):

• Small rooms: 120-130mm PIR
• Medium rooms: 120mm PIR
• Large rooms: 110-120mm PIR

These thicknesses assume standard build-up with 150mm concrete slab and 65mm screed. Different constructions require recalculation.

Beyond Minimum Compliance

Consider specifying thickness beyond Part L minimums for:

Future-proofing: Building Regulations will continue tightening. Specifying 120mm instead of 100mm now costs relatively little but extends compliance lifetime. When regulations change in 5-10 years requiring 0.15 W/m²K, your floor already complies.

Energy Performance Certificates: Better U-values improve EPC ratings. For rental properties, EPC ratings affect rental value and legal ability to rent (properties below EPC E rating face restrictions). The difference between 100mm and 120mm insulation might move a property from EPC D to C.

Comfort: Better-insulated floors feel warmer underfoot, improving occupant comfort. The difference between 0.18 W/m²K and 0.15 W/m²K is noticeable, particularly for properties with tile or stone floors that conduct heat readily.

Running costs: Thicker insulation reduces heating costs. For a 50m² floor, improving from 0.18 W/m²K to 0.15 W/m²K saves approximately 150-200 kWh annually. Over 30 years, this represents significant savings that typically exceed the additional material cost for extra insulation thickness.

Constraints Limiting Thickness

Practical constraints sometimes limit insulation thickness:

Floor-to-ceiling height: In extensions matching existing internal floor levels, total floor build-up (insulation plus screed) must fit within available depth. If you have 150mm available and need 65mm screed, maximum insulation thickness is 85mm.

Door clearance: Adding floor insulation raises finished floor level. Existing doors need adequate clearance. If raising floor level by 120mm (insulation plus screed) means doors require trimming or replacement, this affects project cost and viability.

Step heights: Where insulated new floor meets existing uninsulated floor, steps occur. Regulations require step height management for accessibility. Sometimes limiting insulation thickness reduces step height to acceptable levels.

DPC levels: Floor level must remain below damp-proof course level (typically 150mm below DPC). In situations with limited height between existing ground level and DPC, total floor build-up is constrained.

When constraints limit thickness below what’s needed for Part L compliance, document the constraint and discuss with Building Control. They may accept lower performance if you can demonstrate achieving the target creates disproportionate cost or technical problems.

Compressive Strength Requirements for Floor Insulation

Floor insulation sits directly in the loading path, making compressive strength the critical specification parameter.

What Compressive Strength Means

Compressive strength (measured in kPa – kilopascals) indicates how much loading insulation can carry before permanently compressing. 120 kPa means the insulation can carry 120 kilonewtons per square metre, equivalent to 12 tonnes per square metre.

When insulation compresses, two problems occur:

1. Thickness reduces, decreasing thermal resistance and worsening U-value
2. Screed can crack due to differential settlement

Minimum Strength Requirements by Application

Residential floors: 120 kPa minimum Standard Celotex GA4000 or equivalent achieves 120 kPa, adequate for residential loading including screed weight (approximately 130 kg/m² for 65mm thick), floor finish, furniture, and occupant loads.

Light commercial floors: 120-150 kPa Shops, offices, and similar applications need minimum 120 kPa, preferably 150 kPa for safety margin.

Commercial kitchens: 150-200 kPa Heavy equipment loading (fridges, freezers, cooking equipment) and regular heavy trolley traffic requires enhanced strength boards.

Industrial applications: 200+ kPa Warehouse conversions, plant rooms, and areas with vehicle access need specialist high-strength products.

Why Wall-Grade PIR Doesn’t Work for Floors

Some PIR boards intended for walls achieve only 100 kPa compressive strength. These boards cost less than floor-grade boards, creating temptation to use them for floors to save money.

Don’t. Wall-grade PIR beneath floors compresses under loading. The compression is often gradual – floors seem fine initially, then screed cracks develop after months or years as insulation slowly compresses. Repair requires removing and replacing screed and insulation, costing far more than the initial saving.

Always specify minimum 120 kPa compressive strength for residential floors. Check the technical datasheet – if it doesn’t explicitly state 120 kPa minimum, the product isn’t suitable for beneath-screed applications.

Loading Calculations

For standard residential floors, loading consists of:

Screed: 2,000 kg/m³ density × 0.065m thickness = 130 kg/m²

Floor finish:

• Ceramic tiles: 20-30 kg/m²
• Timber flooring: 10-15 kg/m²
• Carpet: 5-10 kg/m²

Imposed loading: 1.5 kN/m² per Building Regulations

Total loading: Approximately 2.0-2.5 kN/m² (200-250 kg/m²)

This is well within 120 kPa capability (12,000 kg/m²). The high safety factor (120 kPa capacity vs 2.5 kN/m² actual loading) accounts for concentrated loads (furniture legs, appliances) and provides long-term performance security.

Suspended Floor Considerations

For suspended timber floors with insulation between or above joists, compressive strength becomes irrelevant. The insulation doesn’t carry floor loading – joists carry all loads.

In between-joist installations, boards rest on support battens without loading. In above-joist installations, floor deck fixings penetrate insulation, but the fixings carry floor loads, not the insulation itself.

For suspended floors, standard wall-grade PIR (100 kPa) works fine because compressive strength isn’t needed. This provides cost saving compared to specifying floor-grade boards unnecessarily.

Celotex and Kingspan Floor Insulation Products

Major manufacturers produce PIR ranges specifically for floor applications, though general application boards work for most residential floors.

Celotex Products for Floors

Celotex GA4000: General application range with 120 kPa compressive strength. Suitable for most residential and light commercial floor applications. Available 25mm-200mm thickness. The most commonly specified product for solid floors.

Celotex FL4000: Floor-specific range with enhanced 150 kPa compressive strength. For commercial applications or situations requiring extra loading capacity. Available 75mm-150mm thickness. Costs more than GA4000 – only specify if you actually need the extra strength.

Celotex for suspended floors: Standard GA4000 works between joists or above joists. The 120 kPa compressive strength exceeds requirements (no loading on insulation in suspended floors), but GA4000 is readily available making it the practical choice.

Kingspan Products for Floors

Kingspan Therma TF70: Standard floor insulation product with 120 kPa compressive strength. Suitable for residential and light commercial beneath-screed installations. Available 50mm-150mm thickness.

Kingspan Therma TW55: Enhanced strength product achieving 180 kPa. For heavy commercial applications. Available 60mm-150mm thickness.

Kingspan for suspended floors: Kingspan doesn’t produce specific suspended floor products. Standard Therma range or general application boards work fine for between-joist or above-joist installations.

Product Selection Guide

Always check manufacturer technical datasheets. Products and specifications change over time. Verify current compressive strength values before specifying.

When Brand Doesn’t Matter

For standard residential floors achieving Part L compliance, any reputable PIR brand at correct thickness and compressive strength performs identically. Celotex, Kingspan, Recticel, Ecotherm, and IKO all produce suitable products.

Choice often comes down to:

• What your regular merchant stocks
• Which brand offers better trade pricing
• Delivery lead times for required quantities

PIR insulation is a standardised product. Once you specify correct thickness (100mm or 120mm) and compressive strength (120 kPa minimum), any major brand works well.

Installation Guidance for Floor Applications

Getting designed thermal performance from floor insulation requires proper installation across all build-up layers.

Solid Floor Installation Sequence

Step 1: Verify concrete slab condition The slab must be dry, level, and free from defects before starting insulation installation. Use a moisture meter to check slab moisture content – it should be below 75% relative humidity before laying DPM. Wet slabs trap moisture beneath the DPM causing long-term problems.

Check slab level with a straight edge. High spots exceeding 5mm should be ground down. Low spots exceeding 5mm should be filled with self-levelling compound. Level slabs prevent insulation rocking on high spots and voids beneath low spots.

Step 2: Install damp-proof membrane Lay DPM over the slab, starting from one edge and working across the room. Overlap joints minimum 150mm, sealed with DPM tape. Press tape firmly for good adhesion – poor tape sealing allows moisture bypass.

Lap the DPM up perimeter walls minimum 150mm above finished floor level. This lap ties into the wall DPC preventing moisture bypass at the junction. Don’t trim the DPM flush with floor level – you need the upstand to seal properly.

Seal around all penetrations (pipes, drains, services) with appropriate DPM collars or mastic. Any gaps allow moisture through.

Step 3: Lay PIR insulation boards Start from one wall and work across the room systematically. Lay first row with tight joints and square to the wall. This establishes pattern for subsequent rows.

Stagger joints between rows minimum 300mm. This prevents continuous joints that could allow air circulation. Cut boards with fine-toothed saw or sharp knife to maintain stagger pattern.

Tape all board joints with aluminium foil tape. Some installers skip taping for floors, reasoning that screed will seal joints. This is poor practice. Taping improves airtightness significantly and takes minimal time.

Fit boards tightly to perimeter walls without gaps. Cut boards to fit rather than leaving gaps and attempting to fill with offcuts. Perimeter gaps create thermal bridging and allow air circulation.

Step 4: Install perimeter edge insulation strip Fix 10-20mm thick compressible foam strip around room perimeter, full screed depth. This strip sits between screed and wall, creating movement joint.

The strip serves two purposes:

• Allows screed to expand/contract without cracking (screeds move as they dry and with temperature changes)
• Breaks acoustic transmission path from screed to walls (important for upper floors)

Don’t skip the edge strip. Screed poured directly against walls will crack as it dries.

Step 5: Lay screed Pour screed to minimum 65mm thickness for unbonded screeds (or manufacturer minimum for proprietary screeds). Level screed with proper screeding rails ensuring consistent thickness.

Work screed thoroughly to eliminate voids. Pay particular attention to perimeters and around penetrations where voids commonly occur.

Protect screed during curing. Cover with polythene sheets to slow moisture loss and prevent rapid drying that causes cracking. Keep the room temperature stable – rapid temperature changes during curing increase cracking risk.

Suspended Floor Installation Tips

For between-joist installations:

• Fix support battens accurately at consistent depth below floor deck
• Cut boards to fit snugly between joists – friction holds them in place
• Don’t compress boards into spaces that are too narrow – this reduces thickness and thermal performance
• Tape joints between board sections where boards meet
• Check boards are fully supported along their length – boards that sag create air gaps

For above-joist installations:

• Lay boards across joists with staggered joints like a brick pattern
• Tape all joints thoroughly – this is critical for above-joist method
• Fix floor deck through insulation into joists using fixings long enough to achieve adequate penetration (typically 50mm into joist)
• Walk on boards carefully during installation – unsecured boards can slide creating gaps

Common Installation Mistakes

Wet slabs: Installing DPM and insulation over wet slabs traps moisture. The moisture has nowhere to go and remains trapped in the floor construction indefinitely. Always verify slab dryness before proceeding.

Inadequate joint sealing: Gaps between boards allow air circulation that reduces thermal performance by 20-30%. Tape joints or ensure extremely tight fits throughout.

No edge strip: Screed without perimeter edge strip cracks as it dries. The cracks start at corners (highest stress concentration) and propagate across the floor. Cracked screed requires removal and replacement.

Insufficient screed thickness: Screed thinner than 65mm (for unbonded screeds) cracks under normal use. The screed must be thick enough to carry loading and distribute it across the insulation surface.

Wrong compressive strength: Using wall-grade PIR (100 kPa) beneath screed causes gradual compression and eventual screed cracking. Always verify 120 kPa minimum compressive strength for floor applications.

DPM damaged during installation: Tears or punctures in the DPM allow moisture through. Walk carefully on DPM during insulation installation. Repair any damage with DPM tape before covering with insulation.

Quality Checks During Installation

Stop and verify at intervals during installation:

• Is the DPM continuous without tears or punctures?
• Are DPM joints overlapped and sealed properly?
• Are board joints tight without gaps?
• Is the stagger pattern maintained?
• Are boards flat without rocking on high spots?
• Is perimeter edge strip installed continuously?
• Have all penetrations been sealed?

Catching problems during installation is straightforward. Correcting problems after screed is poured requires removing screed and starting over.Need PIR insulation for your floor project? Online Insulation stocks the complete range of floor-suitable PIR boards from Celotex and Kingspan in all thicknesses, with fast UK delivery for trade professionals.