Renovation

Electric vs Water Underfloor Heating: The Honest Answer for a Kitchen

Electric is cheap to fit and expensive to run, so people stop using it. Wet suits the 55°C standard the regulations now require. Sometimes a radiator wins.

By Vladimir Castravet·

Two systems share one name and behave nothing alike. Electric is cheap to fit and expensive to run, so people quietly stop using it. Water costs more upfront and then runs cheaply for 20 years — and it suits the 55°C maximum flow temperature that Approved Document L now requires of a new or fully replaced wet system. The honest answer is not always the expensive one. The estimator prices both, and our extensions page covers the kitchen builds where this decision usually lands.

They are not two versions of the same thing

Electric underfloor heating is a resistance mat or cable laid under the floor finish, converting electricity directly into heat. It is thin, it is simple, it needs no plumbing, and it warms up quickly.

Water (wet) underfloor heating is a circuit of pipe carrying warm water from a boiler or heat pump, embedded in screed or laid in panels. It needs a manifold, a pump and a heat source, it adds height, and it is slow to respond — a wet floor is a thermal store rather than a switch.

The important difference is not installation. It is what each one costs to run, because that cost lands every winter for as long as you own the house. Electric converts roughly one unit of electricity into one unit of heat, and electricity costs several times more per unit than gas. A wet system delivers the same warmth for far less; fed by a heat pump, it delivers several units of heat per unit of electricity.

This is why so many electric floors get switched off in their second winter. The system works perfectly. The bill is the problem.

The figures

| The rule | What it requires | Source | |---|---|---| | 55°C | Where a wet heating system is newly installed, or fully replaced in an existing building (appliance, emitters and pipework), all parts should be sized to operate effectively at a maximum flow temperature of 55°C or lower. | AD L Vol 1 §5.10 | | Downward heat loss | Ground floors and floors in contact with the outside should be insulated to limit heat loss to not more than 10 W/m². | AD L Vol 1 §6.29 | | Insulation resistance | Intermittent or cyclical operation, or over unheated rooms: at least 1.25 m²K/W. Per BS EN 1264-4: not less than 0.5 m²K/W (electric) and 0.75 m²K/W (wet). | AD L Vol 1 §6.30–6.31 | | Electric screed depths | Direct electric systems within screeds not exceeding 60mm. Night energy-storage systems within screeds of at least 65mm, with at least 20% of the dwelling floor area on fast-response systems. | AD L Vol 1 §6.33–6.34 | | Controls | All underfloor heating should have controls to adjust the operating temperature. Screed floors thicker than 65mm should automatically set back the temperature at night or when unoccupied. | AD L Vol 1 §6.28 | | Room controls | In new dwellings, and when a heat generator is replaced in an existing dwelling, each room should have thermostatic room controls. | AD L Vol 1 §5.20 | | Zoning | Wet heating in new dwellings with a floor area of 150m² or greater: a minimum of two independently controlled heating circuits. | AD L Vol 1 §5.14 | | Floor U-value | New floor in an existing dwelling: maximum 0.18 W/(m²·K). | AD L Vol 1 Table 4.2 | | Notifiable electrics | A new circuit is notifiable work — normally self-certified by a registered electrician under a competent person scheme. | reg. 12(6A) |

Approved Documents are statutory guidance to the Building Regulations 2010 for England. Compliance is judged against the Requirements in Schedule 1; your building control body has the final word on your specific building.

The insulation figure nobody mentions

A heated floor radiates in both directions. Without insulation beneath the circuit, a meaningful share of what you are paying for goes into the ground.

The requirement is that downward heat loss is limited to not more than 10 W/m², which in practice means a proper insulation layer under the pipe or mat — at least 1.25 m²K/W where the system runs intermittently or sits over an unheated space.

This is the most commonly skimped element in a retrofit, because insulation costs height and height is exactly what a retrofit does not have. A floor circuit laid straight onto an uninsulated slab will work, will feel warm, and will cost noticeably more to run than the same circuit properly insulated. If height is genuinely tight, that is an argument for a different heating strategy rather than for omitting the insulation.

When each one is right

Choose wet when:

  • The floor is being rebuilt anyway — a new extension, a new screed, a full renovation. The marginal cost of adding a circuit into a floor already open is far lower than retrofitting one.
  • The room is a main living space you heat for hours at a time.
  • You have or might have a heat pump. The 55°C requirement exists to make houses heat-pump ready, and underfloor heating meets it naturally because it emits from a very large surface.
  • You want it to be the actual heating, not a comfort layer.

Choose electric when:

  • The build-up cannot take the height. This is the honest, common reason, and it is a good one.
  • The room is small — a bathroom, an ensuite, a utility.
  • You want warmth underfoot on tile or stone rather than a heat source.
  • The room is heated occasionally rather than continuously.

And consider neither. If the budget is tight and the kitchen has large glazed doors, a trench radiator set into the floor along the glazed elevation is frequently the better answer. Cold air falls down the inside of big glass; a trench heater sits exactly where that downdraught lands and cancels it. It costs a fraction of either underfloor system, adds no height, and does not take the room out of use while a screed cures.

Nobody selling underfloor heating will tell you this, which is precisely why it is worth saying.

What to get right on site

Commission it and keep the record. A wet floor needs pressure testing before the screed goes on, and photographing while the circuit is visible. Once it is buried, a leak is a floor-up repair.

Respect the screed cure and the commissioning sequence. A screed has to dry before it is heated and before a floor finish goes on it — and a rigid floor finish laid over a screed that has not finished drying will fail from underneath. This is the same substrate discipline that governs seamless finishes.

Get the controls right. Thermostatic room controls, and automatic setback for screeds over 65mm. A wet floor responds slowly, so a control strategy of turning it on when you feel cold does not work — it wants to be run at a steady low temperature rather than boosted.

Keep the electrical certificate. A new circuit for an electric mat is notifiable work, normally self-certified by a registered electrician. The certificate is what a buyer’s solicitor asks for, years later.

The takeaway

Electric underfloor heating is cheap to fit and charged to you every winter afterwards. Wet costs more once and suits the 55°C low-temperature standard the regulations now require, which is also what makes the house heat-pump ready. And if the budget is tight, a trench radiator by the doors is not a compromise — it is often the better answer.

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