A large concrete slab under construction, criss-crossed with reinforcement bar and green service conduit before the pour

Part 2 · Learn

11 min read

The engineering, for people who want the engineering

This page goes past the pitch. Which element types exist and how they differ, what the thermostat is really doing, where the sensor goes and why, and what each layer of the floor build-up is for. If you only want the idea rather than the detail, the explainer page covers it in half the words.

Read on

Element types

Three ways to put heat into a floor

Almost every system you will be quoted is one of these three. Establishing which one is on the table should be the first question of any conversation.

Resistive cable

A single continuous heating cable, set out by hand.

A conductor with a deliberately high resistance, sheathed and fitted with an earthed metallic screen along its whole length. Current through the conductor produces heat; the screen is there so that any damage trips the protective device rather than energising your floor.

Strengths

Spacing is chosen on site, so output can be tuned room by room — closer runs near an external wall, wider in the middle.
Works around alcoves, columns and fixed furniture without waste.
The cheapest element per square foot of the three.

Limits

Output depends entirely on the installer getting the spacing right. Sloppy setting-out gives you warm and cold stripes you can feel through the floor.
Slower to lay than a mat, so labour cost is higher.

Suits: Irregular rooms, and any job where output needs varying across one floor.

Pre-spaced mat

The same cable, factory-fixed to a mesh at a set spacing.

The cable arrives already woven onto a self-adhesive mesh at a fixed pitch, sold by the square metre at a rated output. You roll it out, cut the mesh — never the cable — and turn the run.

Strengths

Spacing is set in a factory, which removes the most common source of uneven floors.
Very fast to lay, and thin: this is what makes a 3–4 mm low-profile retrofit possible.
Easy to verify against the drawing, because the coverage is visible and regular.

Limits

Output is fixed. You cannot concentrate heat along a cold external wall without changing product.
Wasteful in rooms full of awkward corners, because the mat comes in fixed widths.

Suits: Rectangular rooms, bathrooms, and almost every retrofit.

Fluid-filled pipe

Warm water or glycol circulated through loops in the floor.

Cross-linked polyethylene pipe is laid in loops and connected to a manifold. A boiler, heat pump or other source warms the fluid; a pump circulates it. Strictly this is hydronic heating rather than electric, but the floor behaves the same way.

Strengths

Running cost per square foot is the lowest at large areas, which is why villas and hotels use it.
The heat source is decoupled from the grid — it can be a boiler, a heat pump, or anything else you can heat water with.
Very long pipe life when installed correctly.

Limits

Highest installed cost, and it needs real depth — this is not a retrofit technology.
Pumps, manifolds and a heat source are moving parts that need servicing. There is more to own.
In a freezing climate the fluid needs the right glycol concentration, and somebody has to check it.

Suits: New construction at scale, and properties with a non-grid heat source.

Why the order matters

Insulation, bed, element, cover, finish

The sequence is not arbitrary and it is not stylistic. Change the order or delete a layer and you change how much of your money reaches the room.

New construction

Structural slab or sub-baseexisting
Your RCC slab, or compacted earth and PCC in an older ground-floor room.
Insulation board25–50 mm
Sends heat up instead of down into the earth. The layer most often cut to save money, and the one that costs you every winter afterwards.
Levelling bed25–40 mm
A flat, sound bed to fix the element to. Cable laid on an uneven base is cable that gets damaged.
Heating element3–8 mm
Resistive cable, a pre-spaced mat, or fluid-filled pipe — plus the floor sensor that governs it.
Screed over the element35–50 mm
Protects the cable and spreads its heat sideways so you get a warm floor, not warm stripes.
Floor finish10–20 mm
Tile, stone or engineered wood. Denser finishes conduct heat better; thick carpet fights it.

Insulation decides your bill

Heat leaves the element in every direction. Insulation underneath is what makes the upward direction the easy one. Without it, a ground-floor room over earth loses a serious fraction of its output downward, permanently, and no thermostat can recover it.

A new build should carry 25–50 mm of rigid board. A low-profile retrofit gets 6–10 mm of dense board, which is meaningfully worse and is the price of not breaking your floor. Both are vastly better than nothing.

Screed decides how it feels

The cover over the element does two jobs. It protects the cable from everything that will later happen to your floor, and it spreads heat sideways so the runs blur into one even surface rather than a set of warm lines.

It also stores heat, which is why a full screed build-up coasts through a power cut far better than a thin retrofit does. Mass is the feature. It is the same property that makes a stone hamam work.

The other build-up

What changes when you cannot break the floor

A retrofit compresses the same five ideas into a fifth of the depth. It works — most Srinagar flats are done this way — but you should understand exactly what is being traded.

Retrofit build-up

Existing floor, left in placeunchanged
The point of a retrofit: nothing is broken out, so there is no debris, no dust and no re-laying of the slab.
Insulation board6–10 mm
Thin, dense board. Less than a new build gets, but it still decides how fast the floor warms up.
Low-profile mat or cable3–4 mm
A thinner element chosen precisely so the floor level barely moves.
Self-levelling compound5–10 mm
A thin skim that buries the cable and gives the finish a flat bed. No wet screed, no long cure.
Floor finish10–15 mm
Relaid over the new build-up — often the same tile or wood, lifted and put back.

Stand-in photograph. The control that matters is a programmable floor-sensing thermostat, not necessarily an app.

Control

The thermostat is not an accessory

The element only knows how to be on. Everything that makes an electric hamam affordable rather than alarming happens in the control, which is why a quote that mentions the cable brand for a paragraph and the thermostat in three words deserves a second look.

Floor sensor, not just air sensor

A floor sensor sits in a conduit between two element runs and reports the actual floor temperature. This is what stops the surface exceeding safe limits for your finish — critical under engineered wood — and what makes the control accurate. An air-only thermostat overshoots and undershoots, because a heavy floor responds slowly to a sensor measuring something else. Insist on a floor probe, and insist it sits in a conduit so it can be replaced without opening the floor.

A weekly schedule

The single largest running-cost saving available to you is not running the floor when nobody is on it. A programmable thermostat with a weekly schedule pays for itself in one season. Better units add adaptive start: you say “warm by six” and the thermostat works backwards from how long your particular floor takes.

Zoning

One thermostat per room, each on its own circuit. This is what lets a family heat the room they are in rather than the house they own, and it is where the difference between a sensible bill and a frightening one actually lives.

Protection

Each heating circuit on its own way in the distribution board, with a residual current device, properly earthed. In a bathroom and absolutely on a houseboat this is specification, not upgrade. The verification is an insulation-resistance reading on paper — before the screed, after the screed, and at handover.

Almost every failed underfloor installation is a construction failure, not an electrical one. The cable rarely goes wrong. The floor around it does.

Still weighing it up?

Talk it through with someone who has installed these in the Valley

Cable, mat or fluid-filled — the right answer depends on your room's shape, its finish and how much height you can give up. Describe the room and we will tell you which of the three we would specify, and why.

Talk to our expert +91 194 000 0000

Part 2 of 10

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