In an existing home, the underfloor heating system has to suit the floor you already have. Before you compare a new screed, a dry system and milling, establish three things: what is in the floor now, how much height you can add, and how much heat each room loses.
This guide helps you collect that information. It includes a printable measurement worksheet, an illustrative floor section and questions for your installer and other specialists. It covers water-based underfloor heating, not electric mats.
Collecting measurements is not the same as assessing a building. Neither this guide nor HeatAlgo checks a floor's load capacity, damp or the condition of the existing layers. That needs an inspection on site by an appropriately qualified person. If you are still gathering plans and product details, start with the underfloor heating planning checklist.
Decide what stays and what is rebuilt
Start with a floor plan and mark each room separately. There are three options: keep the floor and add new layers on top of it, remove only the finish, or take the floor back to its structure - the slab on the ground, a concrete upper floor or the joists.
Each option leaves a different height for new layers. Taking the floor back shows what is underneath. If the floor stays, you have to find out its layers another way: from drawings or from a small opening-up. The options can differ from room to room, so record the decision for each room.
Measure the levels at doors, thresholds and stairs
The new floor has to meet what stays: the floor beyond a doorway, the threshold of the front or patio door, the first step of the stairs. Those places limit the height, not the floor's thickness in the middle of the room.
Mark one level reference line in the room, for example with a laser, and measure every dimension down from it. An old floor may not be level, so measuring from its surface in different places gives different results. For each dimension, record how you know it: measured, taken from drawings or assumed. An assumed dimension is a question for your installer, not an input for the design.
Floor section at a doorway
Illustrative, not to scale
Layers, top down
- 1
- Floor finish with its adhesive or underlay
- 2
- Heating layer: screed with pipes, or a dry system
- 3
- Insulation
- 4
- Layers that stay - to be checked
- 5
- Structure: ground slab, concrete upper floor or joists
Where to measure
- A
- Reference line - measure every dimension from it
- B
- Level to keep: the floor beyond the door, the threshold
- C
- Height available for the new layers
Height C is the margin between the highest point of what stays in the floor and the level you have to keep. Work it out from your measurements down from line A: take the distance to level B away from the distance to the highest point of what stays. Where there are several levels to keep, use the lowest one, the one furthest below line A. Record the place that limits you most, because that is where the choice of system is decided. Any new threshold or step is a separate decision to discuss with your installer.
Also record the room's height at its lowest point: every new layer reduces it.
Measurement worksheet for one room
Print the worksheet or copy it into a spreadsheet with the buttons under it, one copy per room. The print dialog also lets you save it as a PDF. In the "How do you know" column, write measured, drawing or assumed. A blank box is a question to resolve, not a zero.
| What to measure or find out | Result | How do you know | Who confirms it |
|---|---|---|---|
| What stays: the whole floor, the floor without its finish, or only the structure | |||
| A. Reference line: where it is marked | |||
| B. Finished floor level at each doorway to a room that stays as it is | |||
| Front, balcony and patio door thresholds, and the bottom edges of the doors | |||
| First and last step of the stairs | |||
| Items that stay at their current level: kitchen units, shower tray, drains, hearth | |||
| Highest and lowest point of the existing floor | |||
| Room height at its lowest point | |||
| C. Height for the new layers: from the highest point of what stays up to level B | |||
| Existing floor layers and their thicknesses, including insulation: present, absent or unknown | |||
| What is below the floor: the ground, an unheated space, a heated room or outside air | |||
| Pipes and cables in the floor, old vinyl tiles or adhesive, and signs of damp, cracks or deflection | |||
| Planned floor finish: product, thickness and thermal resistance from its data sheet |
Find out what is in the floor and beneath it
If you have drawings or a survey of the house, check the floor section in them. If you do not, or the house has been altered, ask the contractor to open up the floor in one place before you sign a contract. Record what the opening shows and mark everything else as assumed.
In older buildings, vinyl floor tiles, their adhesive and some screeds can contain asbestos (see HSE's list of where it is found). Do not break out or mill such a floor until you know what it is made of, and leave any removal to a specialist.
Also check what is below the floor: the ground, an unheated space, a heated room or outside air, for example over a passageway. That decides how much heat flows downwards. EN 1264-4 gives a minimum thermal resistance for the insulation under underfloor heating, depending on what is below it. Ask your installer which value applies to your floor and what insulation thickness that means in the chosen product.
Damp and the condition of the structure are a separate question. If you see damp, cracks or a floor that deflects, record it in the worksheet and have it assessed by a specialist before you choose a system. Neither this guide nor a software report replaces that assessment.
Check the heat loss before you choose a system
In an existing home, the floor alone does not always heat the room. A floor's output per square metre has an upper limit, set by the maximum surface temperature. A room's heat requirement depends on its walls, windows, roof, floor and ventilation, not just on its floor area.
So before you choose a system, have the heat loss worked out room by room to EN 12831-1. If you plan to add insulation or replace windows, work it out for the house after that work. Where the floor cannot cover a room's heat loss, the installer may keep or add a radiator, so the choice is not always underfloor heating or radiators.
For each room, also record the floor area that can take pipes, leaving out fixed units, a bath and a shower tray. That area and the heat loss show how much each square metre of floor has to deliver.
What goes into that calculation is explained in what a heat load calculation is and what depends on it. How the floor finish and the water temperature affect a floor's output is shown in the guide to underfloor heating flow temperature.
A new screed, a dry system or milling?
Below is a comparison of three approaches. Compare them on the same measurements and with the same floor finish. HeatAlgo calculates only floors with the pipe embedded in a screed laid over a layer of insulation (type A in EN 1264). It does not calculate dry systems or milled floors. For a dry system or milling, ask the system's manufacturer or the contractor for heat output data for the complete build-up with your floor finish.
| Approach | How it works | What to check first | Does HeatAlgo calculate it |
|---|---|---|---|
| New screed | Pipes embedded in a new screed, usually over a new layer of insulation | Height C, the insulation under the pipes, the weight of the new floor, the time before the finish can be laid | Yes, it is type A in EN 1264 |
| Dry system | Pipes in boards with metal spreader plates, covered by dry boards, with no wet screed | The full height with the finish, the floor's load capacity, the manufacturer's output data for your finish | No, you need the manufacturer's data |
| Milling | Channels cut into the existing screed, with the pipes laid in them | The screed's thickness, material and condition, what is under it, pipes and cables in the floor | No, you need the contractor's or pipe manufacturer's data |
Where the pipe sits in each approach
Illustrative, not to scale
New screed
The pipe rests on the insulation, inside a new screed.
HeatAlgo calculates this build-up (type A in EN 1264) for a cement screed and a 16×2 or 17×2 pipe.
Dry system
The pipe sits in a groove in an insulation board, under dry screed boards. A metal spreader plate carries the heat.
HeatAlgo does not calculate it. Ask the system's manufacturer for output data.
Milling
The pipe sits in a channel cut into the top of the existing screed. Adhesive or a filling compound fills the channel, often under a thin levelling layer. What lies under the screed has to be checked.
HeatAlgo does not calculate it. Ask the contractor or the pipe manufacturer for output data.
Key
- Floor finish with its adhesive or underlay
- New screed
- Levelling layer (if any)
- Existing screed
- Dry screed boards
- Thermal insulation
- Unknown layers - to be checked
- Structure
- Heating pipe
- Metal spreader plate
- Adhesive or filling compound in the channel
A new screed with the pipes in it
A new build-up goes on the structure or on the layers you keep: insulation, pipes embedded in screed, and the floor finish. It lets you improve the insulation under the pipes, but you need room for all of those layers at once. A wet screed adds weight and has to dry before the finish goes down.
Ask which screed the contractor proposes, how much the whole new floor will weigh and when the finish can be laid. Also ask who carries out the screed's initial heating, and to whose instructions. Of these approaches, this is the one HeatAlgo calculates: in the app you enter, among other things, the screed thickness over the pipe, the pipe spacing and the type of floor finish.
Dry systems
In a dry system the pipes sit in grooved insulation boards, and metal spreader plates carry the heat across the floor. Dry boards cover the lot, so there is no wet screed and no drying time. Manufacturers offer such systems for renovations, including joisted floors. The UK supplier Nu-Heat, for example, describes retrofit options for joisted floors, some fitted between the joists, and overlay systems laid over existing solid floors.
A dry system's output depends on its construction and on the floor finish. Ask the manufacturer for heat output data for the complete build-up with your chosen finish, and for the full height including the finish. Manufacturers classify such systems as a different construction type in EN 1264 (type B, with the pipes in the insulation layer rather than in a screed), which is why HeatAlgo does not calculate them. The same goes for low-profile overlay systems sold on their small build-up height: ask for the full height with the finish and for output data. HeatAlgo does not calculate those either.
Milling channels into the existing screed
With milling, the contractor cuts channels into the existing screed and lays the pipes in them, instead of breaking the floor out. So everything depends on that screed: its thickness, material and condition, what is under it, and any pipes or cables running through it.
Ask the contractor to open up the screed in one place and show you the floor's section before you sign. Also ask what they will do if old vinyl tiles or adhesive, which can contain asbestos, are still on the screed. Ask what they propose if there is no insulation under the screed. Then more heat will flow downwards, into the ground or the room below.
This guide gives no prices. A price per square metre without a list of the work does not let you compare quotes. Ask for a quote that lists milling and cleaning up, the pipes and laying them, filling the channels and levelling, the manifold and its connection, the pressure test, and making good the floor finish. You can compare the scope with the worksheet in the underfloor heating design guide.
HeatAlgo does not calculate a milled floor. Ask the contractor or the pipe manufacturer for output data for that build-up with your floor finish. If they have none, ask before you sign who will work out the floor's output, and on what basis.
Suspended timber and other joisted floors
On a timber floor, start with the structure. A structural engineer should check the joists' condition and the floor's load capacity. Tell them which system you are considering and how much the complete new floor with its finish weighs, according to the manufacturers' data. If the engineer recommends strengthening or replacing joists, ask whether that work needs approval under your local building rules.
Manufacturers offer lightweight systems for joisted floors, with no wet screed. HeatAlgo does not calculate these floors and does not assess structures. Ask the manufacturer for output data and the structural engineer for an assessment of the floor.
Also record what is below the floor: a heated room or an unheated space. That decides the insulation under the pipes. For a suspended floor over the ground, also ask about ventilation and damp in the void below it.
Flats and shared buildings
In a flat, the floor structure and any heating system shared with other flats are not yours alone to change. Before you order a design, check what your lease or the building's rules say. Then ask the building's management or owner in writing whether, and on what terms, you can install underfloor heating.
If the flat is heated from a shared system, also ask:
- What water temperature the system runs at. A shared system designed for radiators may run hotter than your floor needs, since underfloor heating uses a lower water temperature than radiators. If it does, ask whether a mixing unit that lowers the temperature for your floor may be fitted.
- How heat is billed. If heat is shared out by meters or cost allocators on the radiators, ask how a flat with loops in the floor will be billed.
Whatever heats the flat, ask whether the new floor may be heavier than the existing one, and who will assess that.
Who to ask what
Keep the answers with your measurement worksheet, so that everyone works from the same information.
- Installer or heating designer: which system suits height C and the heat loss, the full section of the new floor with the thickness of each layer from the product documentation, the insulation under the pipes, and the output of the floor with your finish.
- Structural engineer: whether the floor or its structure can carry the new layers, especially on a timber floor or where layers are removed.
- Milling contractor: the condition of the screed and what is under it, after an opening-up and before you sign a contract.
- Building management or owner: consent, the water temperature and heat billing in a flat.
- Floor finish supplier: whether the product suits underfloor heating, its thermal resistance and its thickness.
- Damp or structural specialist: where your worksheet records damp, cracks or deflection.
Once you have a design or a quote, check what an underfloor heating design should include and compare the scope of quotes with its worksheet.
What you can prepare in HeatAlgo
In HeatAlgo you can work out room-by-room heat loss to EN 12831-1 and prepare a water-based underfloor heating layout on your floor plan. It calculates floors with the pipe embedded in a screed (type A in EN 1264), with the screed thickness over the pipe that you take from your floor section (the app starts from a default value; replace it). It assumes a cement screed, a PE-X or PE-RT pipe and at least the minimum insulation under the pipes from EN 1264-4. If you plan an anhydrite screed or less insulation than that, tell the installer who reviews the report. For milling, a dry system or a low-profile system, ask the manufacturer or the contractor for output data for the complete build-up with your floor finish.
The app works from the inputs you enter. It does not assess load capacity, damp or the condition of the floor, and it does not replace a site visit. Discuss a report you prepared yourself with your installer, who should review the design for your home and the selected system.
See how to prepare your plan and reportFor the module itself, read how to use the underfloor heating module.