To calculate how much electric underfloor heating you need, measure the total floor area and subtract the space occupied by permanent fixtures and other areas where heating cannot be installed. The remaining usable floor area determines the size of heating mat or cable system required, although room heat loss, floor construction and required heat output should also be considered.
Electric underfloor heating is an increasingly popular way to add warmth and comfort to tiled bathrooms, kitchens, ensuites and other living spaces. Rather than relying on radiators around the perimeter of a room, the heating system sits beneath the finished floor and distributes heat across the heated area.
However, choosing the right system starts long before installation. Accurately measuring the room, identifying the usable heated area and choosing an appropriate heat output are essential to achieving an effective system.
Ordering too much electric underfloor heating can also cause installation problems because heating cables cannot simply be shortened to make them fit.
In this guide, we explain how to measure for electric underfloor heating, what areas should be excluded from your calculations, how heating mats and loose cable systems differ, and some of the ThermoSphere underfloor heating options available to Quantum Group retailers.
How Do You Calculate How Much Underfloor Heating You Need?
The basic underfloor heating calculation is: total floor area minus the area occupied by fixed units and other non-heated spaces equals the available heated floor area.
Start by drawing a simple plan of the room and adding the measurements in metres.
For a straightforward rectangular room:
Length × Width = Total Floor Area (m²)
For example:
2.5m × 2m = 5m² total floor area
You then need to identify areas where the heating system should not be installed.
In a bathroom, this could include the footprint of:
- A bath
- A shower tray
- A toilet
- A floor-standing vanity unit
- Permanent bathroom furniture
- Other fixed units which prevent heat from dissipating from the floor
If these areas total 1.2m²:
5m² – 1.2m² = 3.8m² available floor area
Your heating system therefore needs to fit within a maximum usable area of approximately 3.8m².
It is generally better to choose a heating mat that is slightly smaller than the available area rather than trying to squeeze an oversized mat into the room.
Should Underfloor Heating Go Under Baths, Toilets and Fitted Furniture?
Electric underfloor heating should generally not be installed beneath permanent fixtures or flat-bottomed furniture that prevents heat from dissipating properly.
This is an important part of measuring a room.
A heating cable produces heat continuously while energised. Installing it beneath a fixture that traps that heat can create thermal blocking, where heat cannot disperse efficiently into the room.
When measuring a bathroom, for example, subtract the footprint of permanent fixtures before selecting the heating system.
The same principle should be considered in other rooms where permanent cabinetry, fitted units or flat-bottomed furniture will cover the floor.
It is therefore worth planning the finished layout of the room before ordering the heating system rather than measuring an empty floor and assuming the entire area can be heated.
What Size Underfloor Heating Mat Should You Order?
Choose an underfloor heating mat that fits comfortably within the usable heated floor area without overlapping, bunching or requiring the heating cable itself to be cut.
Using our previous example, if you calculate that approximately 3.8m² of floor is available, you would normally move down to an appropriate standard mat size that fits within that space.
Do not select a mat that is larger than the available area in the hope that the excess cable can simply be removed.
The mesh supporting a heating mat can usually be cut and turned to change direction during installation, but the heating cable itself must never be cut.
Cutting the heating element alters its electrical resistance and can result in system failure.
This is one of the reasons accurate measuring before ordering is so important.
How Do You Measure an Irregular-Shaped Room for Underfloor Heating?
Divide an irregular room into smaller rectangles, calculate each area separately and then add the figures together before subtracting permanent fixtures.
Most bathrooms and kitchens are not perfect rectangles.
There may be:
- Alcoves
- Recesses
- Shower areas
- Chimney breasts
- Kitchen islands
- Built-in cabinetry
- Angled walls
Rather than trying to calculate the entire shape at once, break the floor plan into simple sections.
For example:
Area A: 2m × 2m = 4m²
Area B: 1m × 0.8m = 0.8m²
Total floor area = 4.8m²
You can then subtract the footprint of any fixed items.
Creating a simple scale drawing is also useful because it allows you to visualise where the mat or cable will actually run before installation begins.
What Wattage Underfloor Heating Do You Need?
The correct underfloor heating output depends on the room’s heat loss, insulation, floor construction and whether the system is being used for comfortable floor warming or as a significant source of room heating.
This is an area where it is important not to simply assume that a higher wattage is always better.
A well-insulated room retains heat much more effectively than an older property with significant heat loss through walls, glazing or floors.
The current ThermoSphere systems available through Quantum offer several options to suit different applications.
ThermoSphere Underfloor Heating Mesh
TheroSphere’s self-adhesive mesh system provides a straightforward solution beneath tiled and stone floors.
Within the Quantum range, 150W/m² is the main standard option, with 100W/m² and 200W/m² options available by special order.
The 150W/m² option is suited to many typical tiled floor applications, while other outputs can be specified depending on the room and heating requirements.
ThermoSphere Vario Membrane Heating System
For tiled floors where both heating and movement management are required, the ThermoSphere Vario Pro Membrane and Vario Heating Cable provide another solution.
The studded membrane allows the cable spacing to be varied to provide different outputs:
- 130W/m² using a three-stud spacing
- 195W/m² using a two-stud spacing
The higher output can be useful for applications such as wet rooms, while 130W/m² can suit well-insulated spaces.
For projects where the electric underfloor heating will be relied upon as a primary source of room heating, the system should be specified against the calculated heat loss of the room rather than selecting wattage on floor area alone.
Do You Need to Cover the Entire Floor With Underfloor Heating?
No. Electric underfloor heating normally covers the usable floor area rather than the entire room because areas beneath permanent fixtures should be excluded.
The important consideration is whether the available heated area can deliver enough heat for the intended purpose.
If underfloor heating is primarily being installed to provide a comfortably warm tiled floor, partial coverage may be perfectly appropriate.
If it is expected to provide a significant proportion of the room’s heating requirement, the relationship between:
- Heated floor area
- System output
- Room insulation
- Heat loss
- Desired temperature
becomes much more important.
For this reason, percentage-of-floor rules should be treated as guidance rather than a substitute for properly specifying a system.
Should You Choose an Underfloor Heating Mat or Loose Cable?
Heating mats are particularly convenient for regular-shaped rooms, while loose cable or membrane cable systems provide greater flexibility around obstacles and irregular layouts.
Both approaches generate electric radiant floor heating, but they differ significantly when it comes to installation.
Heating Mats
A heating mat has cable already spaced across a mesh backing.
It is particularly useful for:
- Square or rectangular bathrooms
- Kitchens
- Ensuites
- Open floor areas
- Rooms with relatively few obstacles
Because cable spacing has already been determined, installation can be quicker and easier.
The ThermoSphere Mesh system supplied by Quantum uses a self-adhesive mesh which is installed wire-side-down, helping to protect the heating cable during installation. It also has a low-profile construction, making it well suited to refurbishment projects where floor build-up is an important consideration.
Heating Cables and Membranes
Loose cable systems allow installers greater control over the route taken around the floor.
They can be particularly useful for:
- Irregular bathrooms
- Wet rooms
- Rooms with several fixed obstacles
- Complex layouts
- Areas where variable heating output is required
With the ThermoSphere Vario Pro Membrane, the cable follows the studs in the membrane, allowing different spacing patterns to be used while providing a structured installation layout.
This also combines underfloor heating with the benefits of an uncoupling membrane.
What Is a Heating and Decoupling Membrane?
A heating and decoupling membrane combines a cable fixing system with movement-management properties designed to help reduce stresses transferred from the substrate into the tiled floor.
This can be particularly useful because tiled floors and their substrates can respond differently to movement and temperature changes.
The ThermoSphere Vario Pro Membrane available through Quantum is designed for tiled and stone floors and combines electric underfloor heating preparation with an uncoupling and crack-isolation function.
The membrane has a build-up of approximately 5.5mm and provides a flexible layout for the associated Vario heating cable.
For tile retailers, this provides an opportunity to offer customers more than simply a heating cable. Heating, substrate preparation and the tiled floor build-up can be considered as part of one overall system.
Why Does Floor Insulation Matter With Electric Underfloor Heating?
Insulation beneath electric underfloor heating helps direct more of the generated heat upwards towards the finished floor rather than allowing it to be lost into the substrate below.
This can improve responsiveness and reduce unnecessary downward heat loss.
The importance of insulation depends on the existing floor construction. A ground floor above a cold concrete slab, for example, will behave differently from an intermediate floor within a well-insulated building.
This is another reason why specifying underfloor heating should consider the complete floor construction rather than simply the dimensions of the room.
Where Should the Thermostat and Floor Sensor Be Positioned?
The thermostat and floor sensor should be planned before the heating system is laid so that the sensor can monitor the heated floor correctly and the electrical connections can be made safely.
The floor sensor should be positioned according to the manufacturer’s installation instructions, typically between two runs of heating cable and away from other sources that could distort the temperature reading.
The thermostat then controls when the system operates and can help users manage floor temperature, schedules and energy use.
Quantum currently supplies the ThermoSphere T-MO Thermostat, which includes a touch display as well as Wi-Fi and Bluetooth connectivity through the T-MO app.
It also provides real-time energy monitoring, allowing users to better understand and manage system operation.
Can More Than One Heating Mat Be Connected to One Thermostat?
Multiple heating mats can potentially be connected to one thermostat, provided the total electrical load does not exceed the thermostat’s rated capacity and the system has been correctly designed.
This is particularly relevant in larger rooms where several mats may be required to cover the available floor area.
Electrical load needs to be calculated as part of system design rather than assuming an unlimited number of mats can simply be connected together.
All electrical work and final connections should be carried out in accordance with applicable electrical regulations by a suitably qualified electrician.
Can Underfloor Heating Cable Be Cut if It Is Too Long?
No. Electric underfloor heating cable must never be shortened by cutting the heating element to make it fit the room.
This is arguably the most important rule when measuring for electric underfloor heating.
The cable has a predetermined electrical resistance based on its length. Cutting it changes those characteristics and can cause overheating or system failure.
If the available floor area falls between standard mat sizes, choose the smaller suitable size rather than oversizing the system.
This is why measuring permanent fixtures and drawing the proposed cable or mat layout before purchasing is so important.
Underfloor Heating Measuring Checklist
Before ordering electric underfloor heating, check the usable heated area, room layout, required output, system type and thermostat requirements.
Use this checklist before placing an order:
- Measure the full length and width of the room.
- Draw an approximate floor plan.
- Calculate the total area in square metres.
- Mark baths, shower trays, toilets, cabinets and other permanent fixtures.
- Subtract these areas from the total floor area.
- Identify any flat-bottomed furniture which could cause thermal blocking.
- Decide whether a heating mat or cable/membrane system is best suited to the layout.
- Consider the heat output required.
- Check the floor construction and insulation.
- Plan thermostat and floor sensor positions.
- Select a system that fits comfortably within the available floor area.
- Never order on the assumption that excess heating cable can be cut off.
Taking a little extra time at this stage can prevent far more difficult problems once installation begins.
Which ThermoSphere Underfloor Heating Products Does Quantum Group Supply?
Quantum Group supplies ThermoSphere electric underfloor heating systems to tile retailers and trade customers throughout the UK and Ireland, including heating mesh, membrane systems, heating cables and thermostatic controls.
Our current ThermoSphere range includes:
ThermoSphere Underfloor Heating Mesh
A low-profile self-adhesive heating mat designed to make installation beneath tiled and stone floors straightforward. The 150W/m² system forms the core of our stocked range, with other outputs available for different specifications.
ThermoSphere Vario Pro Membrane
A studded membrane which combines the installation of electric underfloor heating with uncoupling and crack-isolation properties, making it particularly useful beneath tiled floors.
ThermoSphere Vario Heating Cables
Designed to work with the Vario Pro Membrane, the spacing of the heating cable can be adjusted to provide either 130W/m² or 195W/m² output.
A modern underfloor heating controller with touch operation, Wi-Fi and Bluetooth connectivity, app control and real-time energy monitoring.
Together, these products enable retailers to offer solutions for everything from straightforward bathroom floor warming to more complex tiled heating installations.
ThermoSphere Underfloor Heating from Quantum Group
Quantum Group supplies ThermoSphere underfloor heating products on a wholesale basis to tile shops and hardware retailers throughout the UK and Ireland.
Underfloor heating is a natural complementary product for tiled bathrooms, kitchens, wet rooms and living areas, giving retailers an opportunity to offer customers a more complete tiling solution.
Alongside ThermoSphere heating products, Quantum Group supplies tiles, tile backer boards, waterproofing systems, uncoupling matting, adhesives, grouts, trims, levelling systems and professional tiling accessories.
If you are interested in stocking ThermoSphere electric underfloor heating products, expanding your existing heating range or finding out which products could best suit your customers, contact the Quantum Group sales team for further information.