Summary

Floor insulation is one of the most under-quoted retrofit jobs in UK domestic energy work. The materials cost is straightforward — £8–£35 per m² depending on insulation type and thickness — but the labour is where the variance lives. A suspended timber floor with a 600mm-deep accessible underfloor crawl space is a 30–50 m² per day job; the same floor with a 200mm void requiring board lift from above can take a day per room.

The retrofit market is driven by two things: rising energy costs (a typical 90m² Victorian terrace loses 10–15% of its heat through an uninsulated suspended floor) and Building Regulations Approved Document L requirements when any element is renovated. Most domestic floor insulation work is unregulated (purely customer-funded), but where it forms part of a wider refurbishment (extension, kitchen rebuild, structural alterations) the Approved Document L thermal element rules apply: if you're renovating more than 50% of the floor area, you must bring the floor up to the renovation U-value target (currently 0.25 W/m²K).

The pricing rule that catches out new entrants to the market: never quote per m² without confirming access. A "100m² floor" job from below (crawl space) and from above (board lift) are completely different jobs at completely different prices. Walk the floor, look in every corner, and price honestly.

Key Facts

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Floor Type Method Typical Cost per m² (S&F) Notes
Suspended timber, accessible from below Mineral wool between joists, netting underside £35–£60 Fastest, lowest cost
Suspended timber, lift from above Lift boards, fit insulation, refit £60–£120 Doubles labour
Suspended timber, PIR between joists PIR cut to fit, taped joints £70–£140 Best U-value/depth ratio
Suspended timber, sheep wool / natural Wool/hemp/cellulose, breather membrane £55–£100 Vapour-open construction
Solid floor, over existing slab Insulation + chipboard floating £80–£140 Adds height
Solid floor, lift + reinstate slab Demo + DPM + insulation + new slab £180–£350 Major work
Solid floor, new build under slab EPS/PIR before pour £45–£100 Easy in new build
Insulation Material Thermal Conductivity λ (W/mK) Thickness for U=0.18 (typical) Cost per m² @ 100mm (trade)
PIR (Celotex, Kingspan PIR) 0.022 100–120mm £20–£35
Phenolic (Kingspan Kooltherm) 0.018 90–105mm £30–£45
Mineral wool batt (Knauf, Rockwool) 0.034–0.044 150–180mm £8–£15
EPS (Jablite, Polyfoam) 0.034 130–150mm £6–£12
Wood fibre (Steico, Pavatex) 0.038–0.045 175–200mm £15–£30
Sheep wool (Thermafleece) 0.038 160–180mm £18–£35
Hemp (UK Hempcrete) 0.040 175–200mm £20–£40
Variable Effect on Price
Underfloor crawl space depth 400mm+ accessible: easy; 150–300mm: hard; <150mm: lift boards from above
Joist condition Rot/damp: stop, dry out and treat before insulating
Existing services in void Each diversion adds £40–£150
Floor finish reinstatement Carpet relay £8–£15/m²; engineered timber £25–£40/m² re-fit
Furniture move + return £100–£300 per room
Building Regs notification Required if >50% renovation; Building Control £200–£600
Air-tightness detailing Tape + seal: +£8–£18 per m²

Detailed Guidance

Suspended Timber — Accessible from Below

The optimal job: a Victorian/Edwardian house with a 400–800mm-deep crawl space accessed from a coal-hole or external hatch. Insulation goes in from below; the existing floorboards are untouched; the customer has a warmer floor by lunch the next day.

Procedure (mineral wool):

  1. Clear underfloor void of debris, rubble, old insulation
  2. Check joist condition (probe with bradawl; replace any with rot/insect damage)
  3. Confirm sub-floor ventilation airbricks are clear and number per Approved Document C
  4. Install nylon netting (e.g. Hodgson Sealants underfloor netting) stapled to the joist undersides
  5. Fit 150–180mm mineral wool batts between joists, full depth, tight to joist sides
  6. Where pipes/cables pass through, cut around tightly and seal with foam
  7. Optional: 50mm rigid PIR underside as a continuous layer, taped joints, to break thermal bridging at joists
  8. Verify ventilation airbricks unaffected; clear any that became blocked during the work

Labour: 30–60 m² per day depending on crawl space height and ease.

Procedure (PIR cut between joists):

  1. As above for prep
  2. Cut PIR boards to exact joist spacing (typically 350–500mm) and full insulation thickness
  3. Push up tight between joists; foam-seal any gaps; tape joints
  4. Optional cross-batten + second layer of PIR underside

Labour: 20–40 m² per day; slower than mineral wool due to cutting time.

Suspended Timber — Lift from Above

Where the underfloor void is shallow (<300mm) or inaccessible (no hatch, solid floor over original brick, concrete oversite), insulation must go in from above by lifting the floorboards.

Procedure:

  1. Strip carpet, move furniture (often to other rooms; charge for the inconvenience)
  2. Lift floorboards in sections — number them for reinstatement
  3. Insulation as before
  4. Refit boards; replace any split or split-prone with new
  5. Reinstate finish

Labour: 12–25 m² per day; the lift-and-relay-floor stage is the time killer. Often a two-person job.

This is where retrofit floor insulation prices double over the from-below version. The customer sees the same m², but it's a different job. Quote per m² with a clear note that "above-floor access" is included.

Solid Floor — Over Existing Slab

A 1960s+ house with a concrete oversite slab is harder to retrofit because removing the slab is destructive. The compromise: insulation laid over the existing slab, with a new floor finish above.

Procedure (floating chipboard system):

  1. Check existing slab is sound, level (±5mm in 2m), and has a functioning DPM
  2. Lay a vapour control layer over the slab
  3. Lay 80–120mm PIR (foil-faced) with taped joints
  4. Floating chipboard (P5 grade T&G, 22mm) over the insulation with expansion gaps at perimeter
  5. Edge strip (e.g. perimeter foam) to allow expansion
  6. Floor finish over

Build-up: typically 100–150mm. Will affect door clearance — doors need to be removed, bottoms planed, and rehung. Fire doors must NOT be planed beyond manufacturer's allowance (typically 6mm max from bottom).

Labour: 25–40 m² per day; relatively fast install but the door + threshold rework is hidden labour.

Solid Floor — Lift and Reinstate

For full thermal upgrade or where the existing slab has no DPM, the slab is broken out and rebuilt:

  1. Strip floor finish, kitchen units, bathroom fittings as required
  2. Break out slab (skip + waste disposal cost significant)
  3. Excavate to design level
  4. Hardcore + sand blinding + DPM
  5. EPS/PIR insulation (typically 100–150mm)
  6. New concrete slab or screed
  7. Floor finish

This is major work — £180–£350 per m². Specified during whole-house renovation, not as a standalone insulation upgrade.

Air-Tightness — The Hidden Cost

A perfectly insulated floor with leaky perimeter detailing performs poorly. Cold air bleeds in at the wall/floor junction; insulation is bypassed. Air-tightness detailing means:

Add 10–20% to insulation labour for proper air-tightness — and it is what makes the difference between a "performs as designed" and "still feels draughty" outcome.

Building Regulations Triggers

For floor insulation:

In practice the standalone upgrade is the common case and doesn't need Building Control. Document the work for the customer's records (specification, photos, U-value calc); future buyers will ask.

Worked Example: Whole-House Suspended Floor

Job: 1930s 3-bed semi, 85m² of suspended timber ground floor. Accessible from a coal-hole hatch. Insulation to 175mm mineral wool with netting and breather membrane on underside. Air-bricks checked and replaced as needed.

Item Cost
Knauf Earthwool RS45 175mm × 85m² £680
Hodgson underfloor netting × 100m² £85
Pro Clima breather membrane (under insulation) £180
Foam, tape, fixings £55
4× replacement air-bricks + grilles £24
Materials subtotal £1,024
Labour 14 hours @ £55/hr × 2 men £1,540
Setup, hand-over, certification £80
Labour subtotal £1,620
Subtotal £2,644
Markup 20% £529
Net price £3,173
VAT 20% (if registered) £635
Customer price (VAT-reg) £3,808

Per m²: £37 trade, £45 customer (VAT-reg). Within the £35–£70 band quoted at the top — sits at the lower end because access is good.

For the same job lifting from above (no underfloor access), the same materials but 5 days for two men (£3,000 labour) takes the total to £5,500–£6,500 — within the £60–£120/m² band.

Frequently Asked Questions

Do I need Building Control for retrofit floor insulation?

For a standalone insulation upgrade (no other works), generally no. Building Control involvement is triggered by larger renovation (>50% floor area being renovated) or by works requiring structural notification (extension, material alteration). Document the work for the customer's records regardless.

Will insulating my suspended floor cause condensation?

No, if done correctly. The underfloor void must retain its ventilation airbricks; the insulation must not span across them. Add a vapour control layer on the warm side if required by the manufacturer. Condensation problems arise where ventilation is accidentally blocked during installation or where insulation is fitted without a proper vapour layer in a high-moisture house.

What's the cheapest insulation for a suspended floor?

Mineral wool is the lowest cost per m² (£8–£15 per m² at 100mm trade). PIR is more expensive per board but gets to a target U-value in less thickness, so per finished m² with thinner build-up it can be comparable. Cellulose (blown-in) is competitive on labour for large areas if you have the equipment.

Can I lay insulation directly under the floorboards without netting?

For mineral wool, no — it slumps and creates an air gap over time. For PIR boards cut tight between joists, yes — the boards are rigid enough to stay in place. Netting/breather membrane on the underside provides the wind-tightness that stops cold air blowing through the insulation in the underfloor void.

What U-value should I aim for in a retrofit?

Approved Document L's renovation target is 0.25 W/m²K and the minimum is 0.30 W/m²K (where reasonable practicability prevents achieving the target). For new build it's 0.18 W/m²K. Aim for 0.20–0.25 W/m²K in retrofits; the marginal cost of better insulation is small relative to the lifetime energy saving.

Regulations & Standards