Key Takeaways
- A 6 mm insulation board is the minimum recommended thickness for electric underfloor heating laid over an existing timber or concrete mid-floor with adequate insulation already present below.
- 10–20 mm boards are standard for ground-floor concrete slabs where cold ground can absorb heat before it reaches the room above.
- Insulation boards with a thermal resistance (R-value) of at least 0.5 m²K/W are required to satisfy Part L of the UK Building Regulations for electric UFH retrofits.
- Adding a 10 mm insulation board rather than a 6 mm board can improve system efficiency by up to 20%, based on Eco Friendly Heating's observations across more than 60 UK retrofit installations.
- Boards typically cost between £8 and £25 per m², with foil-faced XPS boards at the premium end.
- Every 10 mm of added insulation board thickness raises the finished floor level by 10 mm — a critical constraint in doorways and at floor-to-stair junctions.
What insulation board thicknesses are available for electric underfloor heating?
Insulation boards for electric UFH are sold in a standard range of thicknesses: 3 mm, 6 mm, 10 mm, 15 mm, and 20 mm. A small number of specialist products reach 25–50 mm for use in new-build ground floors before a screed is poured, but those are outside the scope of a typical electric mat or cable retrofit. The three thicknesses used in the vast majority of UK domestic retrofits are 6 mm, 10 mm, and 20 mm. Boards below 6 mm are generally sold only as crack-decoupling underlays rather than true thermal insulation products and should not be relied upon to meet Part L requirements on their own.
Insulation board types compared: which should you choose?
PIR boards deliver the highest R-value per millimetre, making them the preferred choice when floor-height gain must be minimised. XPS boards are more moisture-resistant and better suited to ground-floor slabs where residual dampness is a risk.
| Board Type | Best For | Typical Cost | R-value | Limitation |
|---|---|---|---|---|
| XPS (extruded polystyrene) | Ground-floor retrofits, damp-proof slabs | £10–£16 | 0.29 m²K/W | Lower R-value per mm than PIR |
| PIR (polyisocyanurate foil-faced) | Timber floors, tight budgets | £14–£25 | 0.45–0.50 m²K/W | Brittle if point-loaded without tile backer |
| EPS (expanded polystyrene) | Low-traffic areas, budget retrofits | £8–£12 | 0.22–0.25 m²K/W | Compresses under heavy loads |
| Tile backer / cement-faced boards | Tiled floors, wet rooms | £12–£20 | 0.15–0.20 m²K/W | Needs extra insulation layer |
What thickness do I need for a mid-floor or first-floor installation?
For a mid-floor installation — such as a first-floor bathroom with a heated room below — a 6 mm insulation board is normally adequate. The substrate above an already-warm room loses very little heat downward; the primary purpose of the board is to prevent the heating element from bonding directly to a flexible or uneven subfloor, and to provide a small measure of decoupling. However, if the room below is unheated (for example, a garage ceiling forming the floor of a room above), treat that installation as a ground-floor scenario and use at least a 10 mm board.
How do I calculate the right R-value for my floor?
To calculate the R-value of a proposed insulation board, divide its thickness in metres by its thermal conductivity (λ value). The λ value is printed on the board's datasheet or packaging.
Example calculation:
- Board: 10 mm PIR, λ = 0.022 W/mK
- R-value = 0.010 ÷ 0.022 = 0.45 m²K/W
For a ground-floor retrofit to meet Part L requirements, you need the total floor R-value (including any existing screed, tile adhesive, and floor covering) to reach 0.75 m²K/W. Tile adhesive and the tile itself typically add only 0.05–0.10 m²K/W combined, so the insulation board must carry most of the load.
If a single board does not achieve the required R-value, two thinner boards can be bonded together, or a thicker board specified.
How to choose and install insulation board under electric underfloor heating: step by step
Measure the existing floor height
at the doorway threshold, the stair nosing, and against any fixed cabinetry. Note the maximum floor-height gain you can accommodate — typically 10–25 mm in a retrofit.
Identify your substrate type
ground-floor concrete slab, mid-floor timber, mid-floor concrete, or suspended timber over an unheated void — as this determines the minimum thickness you need.
Calculate your required R-value
using your local authority's building control guidance or the Part L threshold of 0.75 m²K/W for ground floors.
Select a board type and thickness
that meets the R-value requirement within your height allowance. Use PIR where height is tight; use XPS where moisture resistance matters.
Check the board's compressive strength rating
Boards under tiled floors should carry a compressive strength of at least 200 kPa; under stone or heavy format tiles, 300 kPa or above.
Prepare the substrate
sweep clean, fill cracks larger than 3 mm, and prime if required by the board manufacturer.
Lay the insulation boards
in a brick-bond pattern, butting edges tightly. Tape all joints with foil tape to prevent thermal bridging.
Lay the electric heating mat or cable
directly on top of the insulation board surface according to the manufacturer's layout plan.
Apply flexible tile adhesive or self-levelling compound
to encapsulate the heating element before laying the floor covering.
Test the heating element with a resistance meter
before and after encapsulation to confirm no damage occurred during installation.
Frequently Asked Questions
A 3 mm board provides negligible thermal resistance (typically around 0.09 m²K/W for XPS) and is not suitable as the sole insulation layer under electric UFH. It can be used as a surface-levelling layer on top of a thicker board, but should never be the primary thermal barrier, particularly on ground floors.
The floor covering itself has little effect on the choice of insulation board thickness. However, some floor coverings — such as thick carpet with a dense underlay — add enough insulation above the heating element to slow heat output to the room, which is a separate consideration from the insulation below.
Yes, materially so. Moving from a 6 mm to a 20 mm XPS board on a ground-floor slab can reduce heat lost downward by 60–70%, cutting warm-up times and reducing the minutes per hour the thermostat calls for heat. Over a full heating season, this typically saves £20–£50 per year per room at 2026 electricity tariff rates.
In most cases, yes. Adding electric underfloor heating to a room in England and Wales is notifiable work under Part P (electrical) and must comply with Part L (thermal). A registered electrician should complete a Part P certificate, and you should confirm with your local authority whether a full building notice is required.
A foil-faced PIR board of 10 mm, such as Wedi Board or similar cement-faced PIR alternatives, is the standard choice for bathrooms. These boards combine moisture resistance with a high R-value per millimetre and are compatible with flexible tile adhesive systems. Avoid EPS in wet rooms, as it can absorb moisture over time.
Yes. Two 10 mm boards laid in a staggered, brick-bond pattern with taped joints will perform similarly to a single 20 mm board and can be easier to handle in tight spaces. Ensure the adhesive or fixing method specified by the board manufacturer is compatible with this approach.
Check your building's construction drawings or original specification if available. As a practical test, if the floor feels cold underfoot in winter before heating is switched on, and the substrate is a ground-bearing slab, assume the existing insulation is insufficient and specify at least a 20 mm board.