Floor Insulation: Suspended Timber, Solid Concrete & UFH Compatibility

Summary

Floor insulation is frequently overlooked compared to loft and wall insulation, but ground floors account for 10–15% of total heat loss in an uninsulated dwelling. The challenge is that floor insulation is more disruptive than other insulation work — it typically requires either lifting floorboards (suspended timber) or breaking out and replacing a screed (solid floor), making it a significant job that is best combined with other refurbishment work.

The approach depends entirely on the floor construction type. Suspended timber floors (predominantly pre-1940s housing) have a void beneath the floor joists with air vents in the external walls — this void must remain ventilated to prevent timber decay, which constrains how the insulation is installed. Solid concrete floors (most post-war construction) have no void — insulation must go above or below the slab, both of which have implications for floor level and threshold heights.

Underfloor heating (UFH) fundamentally changes the insulation requirement. Without adequate insulation below the pipes, heat is lost downward into the ground rather than upward into the room. UFH-heated rooms require higher levels of floor insulation than conventionally heated rooms, and the insulation specification should always be confirmed by the UFH system designer.

Key Facts

• Suspended timber floor insulation — 100mm mineral wool between joists; friction-fit using netting or rigid support battens
• Suspended timber U-value — 100mm mineral wool achieves approximately 0.25 W/m²K; 150mm achieves 0.22 W/m²K
• Solid floor insulation — 70mm PIR board achieves approximately 0.22 W/m²K; 100mm achieves 0.18 W/m²K
• Building Regs Part L target — 0.22 W/m²K for new-build ground floors; 0.25 W/m²K for refurbishment threshold (renovation to existing floor)
• UFH insulation minimum — 50mm PIR minimum below water UFH (70–100mm strongly recommended); electric UFH requires 20–25mm minimum foil-faced board
• Ventilation preservation — suspended timber void must retain ventilation; never seal sub-floor air bricks
• Floor level change — solid floor insulation plus screed raises floor level; 70mm PIR + 65mm screed = 135mm rise; door thresholds and stairs must be checked
• Perimeter insulation — 25–50mm vertical insulation strip at wall/floor junction reduces heat loss to edges; particularly important for UFH
• Damp-proof membrane — solid floor insulation must incorporate or be above the existing DPM; never trap moisture between insulation and slab
• Edge loss coefficient (psi value) — floor edge heat loss is significant; treated correctly with perimeter insulation
• Electric underfloor heating — lower capital cost but significantly higher running cost than water UFH on a heat pump or gas boiler system

Quick Reference Table

Detailed Guidance

Suspended Timber Floor Insulation

Pre-war housing typically has a ground floor of suspended timber — floorboards on joists that span between sleeper walls, with a void beneath and airbricks providing ventilation.

Method: Mineral wool between joists from above

1. Lift floorboards (or access from below if crawl space is accessible)
2. Fix netting or battens below the joist bottom flange to support the mineral wool
3. Option A (netting): staple plastic mesh netting to the underside of the joists at 300mm intervals; push mineral wool up into the mesh-lined bays
4. Option B (battens): nail 50mm × 25mm counter battens along the underside of the joists; cut rigid foam or mineral wool batts to fit between the joists and rest on the counter battens
5. Ensure the insulation is butted tightly with no gaps at joist flanges
6. Do NOT seal the sub-floor void or block the airbricks
7. Refit floorboards

Critical: Sub-floor ventilation must be maintained. The void below a suspended timber floor must have adequate ventilation to prevent condensation and decay of the floor structure. Air bricks in the external walls provide this. If air bricks are below the outside ground level, they are likely blocked — clear them and consider fitting terracotta air brick covers with an internal ducted connection if the floor level inside is lower than outside.

Achieving a draught-free floor: Mineral wool between joists reduces heat loss from the room but leaves the floorboard level exposed to draughts from the ventilated void. The floorboards themselves are the air barrier. If the floorboards have gaps, fill with papier-mâché or proprietary draught-seal strips. For a new finish, tongue-and-groove boards or engineered wood flooring over the mineral wool provides a better draught seal.

Solid Concrete Floor Insulation: Above the Slab

The most common retrofit approach for solid concrete ground floors is insulation above the slab, with a floating floor finish on top.

Method: PIR board floating floor

1. Ensure the existing slab is dry; if damp, investigate and resolve the damp source before insulating
2. Install a DPM (polyethylene sheet, minimum 1200 gauge) if the existing DPM is damaged or absent — this goes under the insulation
3. Lay PIR boards (Kingspan, Celotex) over the slab; boards should be staggered and joints tightly butted; tape all joints with foil tape
4. Lay perimeter insulation strip (50mm rigid PIR) vertically against all walls, to the full height of the insulation
5. Lay the floating floor directly on the PIR boards (engineered wood, chipboard floating floor, or similar); use floor clips or adhesive as required
6. Do NOT fix the floating floor to the slab — it must be free to move thermally

Floor level implications: 70mm PIR + 22mm chipboard floating floor = 92mm rise at the perimeter of the room. This affects:

• Door thresholds: doors may need to be shortened or replaced
• Stair bottom tread: may become shorter or the first riser proportions change
• Kitchen and bathroom fittings at floor level
• Radiator pipes and skirting board heights

Solid Concrete Floor Insulation: Screed on Insulation

For bathrooms, kitchens, and other areas where a floating chipboard floor is not appropriate, insulation under a sand:cement or liquid screed is the standard method.

1. DPM on existing slab
2. PIR boards — minimum 70mm; 100mm recommended for UFH or Part L compliance
3. Perimeter insulation strip
4. Edge film (polythene) around perimeter walls to allow screed to float
5. Screed: traditional 1:4 sand:cement semi-dry screed minimum 65mm thick; or self-levelling liquid screed minimum 40mm thick
6. Allow screed to cure before finishing: sand:cement — minimum 3 weeks; liquid — 24–48 hours to foot traffic, 7 days for coverings

Total floor rise: 70mm PIR + 65mm screed = 135mm; 100mm PIR + 65mm screed = 165mm. This is a significant change and must be planned into the project from the outset. Opening thresholds, kitchen plinths, and bathroom fixtures will all be affected.

UFH Compatibility

Underfloor heating and floor insulation are inseparable design considerations. UFH pipes (or electric mats) emit heat upward, but heat will travel in all directions if the path of least resistance allows. Without adequate insulation below the UFH, heat is lost into the ground rather than upward into the room.

Water UFH in screed:

• Minimum insulation: 50mm PIR board below the screed; achieves a downward heat loss of approximately 10% (acceptable but not ideal)
• Recommended: 70–100mm PIR board; downward heat loss under 5%; significantly better system efficiency
• The UFH system designer should confirm the insulation R-value required to achieve the pipe spacing and flow temperature they are designing to
• The screed above the UFH pipes must be the minimum depth required by the pipe manufacturer to achieve good thermal contact (typically 50–65mm over pipe centre)

Electric UFH (heating mat):

• Electric mats are typically 3–6mm thick; laid in adhesive tile bed
• Insulation below: 20–25mm foil-faced PIR or proprietary electric UFH underlay board
• Without insulation, electric UFH into a concrete slab is very inefficient — the slab acts as a huge thermal mass absorbing heat downward
• Typical foil-faced insulation boards for electric UFH: Schlüter DITRA-HEAT; Wedi boards; proprietary systems

See also underfloor heating for UFH pipe spacing, loop lengths, and screed depth specification.

Insulation Under a New Slab (New Build or Major Renovation)

Where a new concrete slab is being poured (new extension, new build, or basement conversion), insulation under the slab is possible and thermally superior to above-slab methods (no floor level impact; no thermal bridges at perimeter).

• EPS (expanded polystyrene) is the standard under-slab insulation: 150–200mm EPS100 (load-bearing grade)
• EPS is impervious to moisture; can go directly on compacted hardcore below the slab
• No DPM required below the EPS; DPM goes above the EPS, below the slab
• Perimeter insulation: continue the EPS vertically around the outside of the foundation/edge beam, or use a vertical PIR strip inside the wall, at least 500mm down
• Thermal break between slab and foundation wall: if the foundation wall is cold and the slab bears on it, heat conducts from the slab edge to the outside; use a thermal break block (Schöck Thermomur, Isobloc) at the slab edge

Frequently Asked Questions

Will insulating my floor make a significant difference to my heating bills?

Yes, but it is the least impactful measure per pound spent compared to loft and wall insulation. Ground floors typically account for 10–15% of total heat loss. If your loft and walls are already insulated, floor insulation becomes proportionally more important. In a solid floor property with no insulation, adding 70mm PIR under a floating floor can save £80–£150 per year — a payback period of 10–15 years at typical material and labour costs.

Can I insulate a suspended timber floor from outside (through the air bricks)?

In theory, EPS bead can be blown into the sub-floor void through the air bricks, filling the void to joist level — similar to CWI. In practice, this is rarely recommended because: it removes the ventilated void beneath the joists, dramatically increasing the risk of joist decay from condensation; the beads are difficult to remove if problems occur; and it doesn't insulate between the joists themselves. Only specialist contractors with the correct equipment and detailed knowledge should attempt this approach.

My building control inspector wants me to insulate the floor in a ground floor extension. Can I use EPS instead of PIR?

Yes. EPS (expanded polystyrene) has a lower thermal conductivity than PIR (typically 0.038 vs 0.022 W/mK), so you need approximately 70% more thickness for the same R-value. 150mm EPS achieves approximately the same performance as 90mm PIR. EPS is significantly cheaper per m³ and is perfectly acceptable under Building Regulations as long as the overall system achieves the required U-value. It is commonly used under new concrete slabs where depth is not constrained.

How do I prevent the floating floor from creaking?

Creaking floors over PIR insulation boards are typically caused by: the floor boards rubbing on each other at the tongue-and-groove joint; the floor boards moving on the insulation boards; or the insulation boards moving on the slab. Solutions: ensure the PIR boards are stable and joints are tight; use floor adhesive between chipboard panels rather than just clips; allow the required expansion gap at all walls; and ensure the room humidity is stable (wet screed not yet fully dried can cause creaking as moisture moves through the floor during final curing).

Regulations & Standards

• Building Regulations Approved Document L (2022) — energy conservation; ground floor U-value targets (0.22 W/m²K new build, 0.25 W/m²K refurbishment)

• Building Regulations Approved Document C — site preparation; DPM requirements for solid floors

• BS EN 13162 — thermal insulation products; mineral wool

• BS EN 13163 — thermal insulation products; expanded polystyrene (EPS)

• BS EN 13165 — thermal insulation products; polyurethane rigid foam (PIR/PUR)

• BS 8203 — installation of resilient floor coverings; relevant for floating floor installation over insulation

• Approved Document L (2021/22) — current floor U-value requirements

• Kingspan: Floor Insulation Technical Guide — PIR floor insulation installation details

• Celotex: Floor Insulation Guide — practical installation guidance

• Energy Saving Trust: Floor Insulation — costs and savings estimates

• underfloor heating — UFH pipe spacing, loop lengths, and screed depth

• thermal bridging — floor perimeter thermal bridging

• loft insulation — complementary insulation measures

• foundations — insulation considerations in new foundations