How the floor calculation works
Many older homes have suspended timber floors: floorboards on joists, with a ventilated void underneath. Cold air moves through that void, so an uninsulated floor loses heat and lets in draughts through the gaps between the boards. Fitting insulation between the joists deals with both.
The target
- Building Regs (Part L): U-value 0.25 W/m²K. Approved Document L, Table 4.3, gives this for an upgraded floor.
- New-build standard: U-value 0.18 W/m²K. The Table 4.2 value for a new floor in an existing home.
The formula
U = 1 ÷ (Rsi + Σ thickness/λ + Rse)
For a floor, heat flows downwards, so the inside surface resistance is higher: Rsi is 0.17 and Rse 0.04 m²K/W. The floorboards, the ventilated void and the ground add a further 1.04. That gives an uninsulated suspended floor about U-value 0.80. To reach 0.25, the total R must be 1 ÷ 0.25 = 4.00, so the insulation needs to supply R 2.75 m²K/W.
Joists bridge the insulation
All of the insulation sits between the joists, so the timber bridges all of it. The calculator assumes 150 mm joists covering 10.8% of the floor, and works out the combined performance of wood and insulation with a parallel-path calculation. A 150 mm loft roll, rated R 3.41 on its own, works out at about R 2.81 between the joists. That's just enough for 0.25, which matches the Energy Saving Trust's guidance that about 150 mm of mineral wool is needed.
Worked example: a 40 m² suspended floor
Take a 40 m² ground floor, for example 8 m × 5 m. With 10% for waste, you need to cover 44 m².
| Option | Depth | To buy | Material cost | U-value after |
|---|---|---|---|---|
| Uninsulated | – | – | – | 0.80 |
| Loft roll λ 0.044, Part L | 150 mm | 5 rolls | £437–£500 | 0.25 |
| PIR board λ 0.022, Part L | 100 mm | 16 boards | £848–£880 | 0.24 |
| Loft roll, new-build standard | 150 mm + 100 mm | 5 + 4 rolls | £573–£828 | 0.16 |
Mineral wool roll is the cheaper option and easy to fit snugly between uneven joists. PIR board is thinner for the same result but has to be cut accurately to fit tightly, or gaps around the edges let heat past.
The new-build standard needs 250 mm of roll, which is deeper than the 150 mm joists the calculator assumes. It only works if your joists are deep enough to hold it, so measure them before you plan for 0.18.
If you'd rather have it done for you, the Energy Saving Trust puts the installed cost of insulating a suspended timber floor at £1,400–£2,500.
Fitting insulation between the joists
Getting to the joists
If there's a cellar or a void deep enough to work in, you can insulate from below without disturbing the rooms. Otherwise, lift the floorboards. That's a bigger job, but it's a good chance to fix loose boards and check the joists for rot or damp first.
Support the insulation
Insulation fitted from above can sit on netting or breathable mesh stapled across the bottom of the joists, or on battens fixed to their sides. From below, staple netting across the joists to hold it in place. Either way, the insulation should fill the full depth, touch the boards above and have no gaps at the sides. Don't let it sag, which opens a gap for cold air.
Keep the airbricks clear
The void needs to stay ventilated so the timber stays dry. Never block the airbricks, and keep insulation from falling into the void and blocking the airflow under the floor.
Draught-proof the gaps
Seal the gaps between the floorboards and along the skirting boards with a flexible filler or sealant. Insulation does little if cold air blows straight past it.
Pipes and cables
Once the floor is insulated, the void under it gets colder. Lag any water pipes that run below the insulation, and keep cables clear of it where you can.
When Part L applies
If you renovate more than half of a floor, for example lifting and relaying most of the boards, Approved Document L expects it to be upgraded to 0.25. If that isn't technically feasible or wouldn't pay back within 15 years, you go as far as is feasible within 15 years. Note 8 to Table 4.3 says a lesser standard may be appropriate where adjoining floor levels would be significantly affected.
For the rest of the house, see the loft insulation calculator and the cavity wall insulation calculator. All targets are on Part L U-values, and the U-value calculator converts between R and U.
Sources
- Approved Document L, Volume 1: Dwellings (2021 edition incorporating 2023 amendments)
- Energy Saving Trust: Floor insulation
- BRE BR 443: Conventions for U-value calculations (2019)
- Knauf Insulation: Loft Roll 44 (Combi-cut)
- SOPREMA UK: SOPRATHERM GA4000 (previously Celotex GA4000)
- Wickes: loft insulation (prices)
- Toolstation: insulation (prices)