Installing Underfloor Heating

Underfloor heating was once considered a lavish feature, but it’s now a common part of modern bathroom designs. Fitting underfloor heating obviously adds some time and expense to any bathroom renovation, but many clients will consider this very much worthwhile.

Setting up underfloor heating comes at the first fix stage of your bathroom project. As with almost every feature you install at this point, it’s likely you know how difficult it can be to make changes once the bathroom fixtures, fittings and flooring have been put in. For this reason, double and triple check the flooring works as it should before you move on to any other tasks.

When done right, underfloor heating is a feature that will make any bathroom seem comfortable and welcoming.

  • 1. Before you begin

    Start with establishing your measurements and working out how much underfloor heating you need. Here are a few tips to refresh your memory for this stage of the process.

    Consider the bathroom plumbing and other features

    It’s not enough just to measure the dimensions of the room. To avoid wasting money, you always take the whole bathroom suite into account. Remember, bathroom fixtures and fittings won’t need underfloor heating beneath them. If there are other large pieces of furniture likely to be permanent fixtures in the room, think about these too. Use the layout of the room to subtract the area taken up by fittings and furniture from the heating requirements.

    Remember to add extra space

    Your underfloor heating should not make contact with any walls, doors or permanent fixtures. To be safe, you should leave a gap of around six inches between the heating and the walls and bathroom furniture. Account for this during the initial measuring to help you save money when you order flooring.

  • 2. What to order

    You can get a good idea of the kinds of things you might need over at our underfloor heating supplies section.

    However, as a rule you’ll need to order these three main things:

    A mat or cable underfloor heating system

    Underfloor heating comes in both mat and cable form. If the bathroom design you are working on is very irregularly shaped, you should opt for cable form. However, if the bathroom is a standard shape, the mat type of heating will be easier to work with.

    A thermostat

    The thermostat used to control underfloor heating is usually affixed outside the bathroom door. Make sure there is a suitable place for it before you start the project.

    Insulation boards

    Installing insulation boards is an optional extra step. Many clients will choose this if you suggest it to them, as insulation boards will keep heat in their bathroom and save them money in the long run.

  • 3. Installing underfloor heating, step by step

    You know how varied different heating systems can be, so it’s always worth reading the instructions that come with the specific system you’ve ordered. The below steps give a helpful overview to remind you of the work involved.

    You can also learn more by watching Flexel’s video on how to install underfloor heating.

    Step 1: Clean up

    It may seem counterintuitive to begin a task with a clean-up, but once underfloor heating is laid down, it can be almost impossible to hoover or clean around it. To avoid ending up with dust stuck underneath your bathroom tiling, make sure the room is as clean as possible before you begin.

    Step 2: Take a resistance reading

    Taking a resistance reading from the ends of the wires attached to your underfloor heating system is the best way to make sure it’s working. Use a multimeter to take your reading, and check the figures against the ones in the instruction manual that came with the underfloor heating. Take a note of your results so you can continue to check new readings against them throughout the installation process.

    Step 3: Put down the insulation boards

    Insulation boards, also sometimes known as thermal boards, backing boards or sub-floor insulation, sit underneath the heating system to prevent its heat from escaping downward. Not all clients will choose this option, but many will find it worthwhile, particularly those who favour luxury bathroom designs.

    Lay the boards over the entire surface of the bathroom floor, cutting them to fit where necessary. Use waterproof tape to join the boards together. You can also screw them into place if there are wooden floorboards underneath.

    Step 4: Lay down the underfloor heating itself

    In the majority of cases, the underfloor heating system you’re working with will consist of a mesh mat that can be unrolled to cover the floor.

    It may seem obvious, but make sure that when you’re laying it, the two wires that will eventually be connected to the thermostat are positioned as close to the thermostat as possible.

    Once you’re happy with the position of the underfloor heating system, secure it using duct tape.

    Step 5: Take another resistance reading

    Underfloor heating can be damaged during the installation process, so take another reading and again check the results against your initial figures and those in the instruction manual.

    Step 6: Put down the bathroom flooring

    With the underfloor heating laid down, you can now go on to installing the bathroom flooring. Wooden floorboards, bathroom tiles or laminate flooring are all compatible with underfloor heating.

    Laying tile grout directly on top of the underfloor heating system will help to keep everything in place. Just remember to keep the power switched off as you are working on this job, as wet tile grout and metal trowels must never come into contact with live wires.


  • How do I measure a bathroom for underfloor heating?

    It’s obvious, but it bears keeping in mind that when measuring up, it’s important to take bathroom fixtures and fittings into account. You’ll need to disregard any floor space occupied by bathroom furniture, as this won’t need heating under it.

  • What do I need to order to lay underfloor heating?

    Along with the tools you’ll already have for use in your bathroom installation work, you’ll have to stock up on some specific supplies for underfloor heating. Namely, you’ll need a mat or cable underfloor heating system, a thermostat and insulation boards.