In an existing house, the radiant floor system has to suit the floor you already have. In most older US houses that floor is wood joists with a subfloor, sometimes over a basement or crawlspace, and the usual retrofits are tubing in the joist space from below or low-profile panels on top. Before you compare them, 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 measurement worksheet, the retrofit options with what HeatAlgo does and does not calculate, a section on cutting grooves in an existing slab, and questions for your contractor. It covers hydronic (water) radiant floor 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, moisture 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 radiant floor heating quote checklist.
The retrofit options, and which ones HeatAlgo calculates
HeatAlgo calculates only floors with the tubing embedded in a slab or thin slab laid over insulation (type A in EN 1264). It does not calculate joist-space, staple-up, plate or panel systems. For those, ask the system's manufacturer for heat output data for the complete build-up with your floor covering.
| Option | Where the tubing sits | What to check first | Does HeatAlgo calculate it |
|---|---|---|---|
| Staple-up or plates in the joist space | Under the subfloor, between the joists, stapled up or held in aluminum heat transfer plates, with insulation below | Access to the joists from below, the insulation under the tubing, the floor covering's thermal resistance | No, you need the system manufacturer's data |
| Low-profile panels on top of the subfloor | In grooved panels, often aluminum-faced, under the finished floor | The full height with the finish at every door and stair, the manufacturer's output data for your finish | No, you need the system manufacturer's data |
| New slab or thin slab | Embedded in a new concrete slab or thin slab, usually over a new layer of insulation | Height, the weight of the new floor, the time before the finish can go down | Yes, it is type A in EN 1264 |
| Grooves cut into an existing slab | In grooves cut into the top of a basement or slab-on-grade floor | The slab's thickness and condition, what is under it, pipes and cables in it | No, you need the contractor's or tubing manufacturer's data |
Staple-up and plates from the basement
This is one of the most common US retrofits where the floor is joists over a basement or crawlspace. The tubing goes up into each joist bay from below, either stapled straight to the underside of the subfloor or held in aluminum plates that spread the heat across it, with insulation under the tubing so the heat goes up. The finished floor does not rise, and the rooms above stay in use while the work is done. A homeowner on HeatingHelp planning a phased whole-house job weighed exactly this against a "sandwich" system on top.
Its output depends on the plates, the subfloor, the floor covering and the water temperature. HeatAlgo does not calculate it. Ask the manufacturer for output data with your floor covering, and for the supply water temperature that output needs.
Low-profile panels on top of the subfloor
Where you cannot reach the joists from below, for example on a second floor over a finished ceiling, panels with grooves for the tubing go on top of the existing subfloor and the new floor goes over them. On Green Building Advisor, an owner asked whether second-floor loops can go in from above or whether the ceiling below must come down: panels on top are the usual answer to the first half. They are sold on their small build-up height, so measure carefully (the next section) and ask for the full height with the finish. HeatAlgo does not calculate panel systems either.
A new slab or thin slab
On a basement or slab-on-grade floor, or where the subfloor can carry it, tubing can go into a new slab or thin slab. It lets you add insulation under the tubing, but you need height for all the layers at once, the new floor adds weight, and a poured slab has to cure before the finish goes down. This is the option HeatAlgo calculates: in the app you enter, among other things, the slab thickness over the tubing (the engine takes 30 to 100 mm, 1.2 to 3.9 in), the tubing spacing and the floor covering. It assumes a cement-based slab; if you plan a gypsum thin slab, tell the contractor who reviews the report.
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, the joists or the subfloor.
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, from the basement or crawlspace, or from a small opening. The options can differ from room to room, so record the decision for each room.
Measure the heights 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 stair tread. Those places limit the height, not the floor's thickness in the middle of the room. This matters most for panels on top and a thin slab; staple-up from below leaves the floor height as it is.
Mark one level reference line in the room, for example with a laser, and measure every dimension down from it. An old floor is rarely 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 contractor, not an input for the design.
The height available for new layers is the margin between the highest point of what stays in the floor and the level you have to keep. Where there are several levels to keep, use the lowest one. Record the place that limits you most, because that is where the choice of system is decided. Also record the room's height at its lowest point: every new layer reduces it.
Measurement worksheet for one room
Copy the table into a spreadsheet or print the page, one copy per room. 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 | |||
| Reference line: where it is marked | |||
| Finished floor level at each doorway to a room that stays as it is | |||
| Front, patio and deck door thresholds, and the bottom edges of the doors | |||
| First and last stair tread | |||
| Items that stay at their current level: cabinets, shower pan, drains, hearth | |||
| Highest and lowest point of the existing floor | |||
| Room height at its lowest point | |||
| Height for the new layers: from the highest point of what stays up to the level to keep | |||
| Existing floor layers and their thicknesses, including insulation: present, absent or unknown | |||
| Joists: size, spacing, direction, and whether they are open from below | |||
| What is below the floor: the ground, a basement, a crawlspace, a heated room or outside air | |||
| Pipes, ducts and wiring in the floor or joist bays, old vinyl tile or mastic, and signs of moisture, cracks or sag | |||
| Planned floor covering: product, thickness and thermal resistance from its data sheet |
Find out what is in the floor and beneath it
If you have drawings of the house, check the floor section in them. If you do not, or the house has been altered, look from the basement or crawlspace where you can, or ask the contractor to open the floor in one place before you sign a contract. Record what you find and mark everything else as assumed.
In older houses, vinyl floor tiles, their mastic and some other floor materials can contain asbestos. Do not break out, sand or cut 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, a basement, a crawlspace, a heated room or outside air, for example over a garage or porch. That decides how much heat flows downward. EN 1264-4 gives a minimum thermal resistance for the insulation under radiant floor heating, depending on what is below it. Ask your contractor which value applies to your floor and what insulation that means in the chosen product.
Moisture and the condition of the structure are a separate question. If you see moisture, cracks or a floor that sags, record it in the worksheet and have it assessed before you choose a system. On a wood floor, a structural engineer should check the joists and the floor's capacity for the system you are considering, with the weight of the complete new floor from the manufacturers' data.
Check the heat loss before you choose a system
In an existing house, the floor alone does not always heat the room. A floor's output per square foot has an upper limit, set by the maximum surface temperature. A room's heat requirement depends on its walls, windows, roof, floor and air leakage, not just on its floor area. A homeowner on HeatingHelp asked whether radiant in a 200-year-old, drafty house was wise; the honest answer starts with the room-by-room numbers, not the floor.
So before you choose a system, have the heat loss worked out room by room, as the contractor's room-by-room load calculation does. If you plan to air-seal, 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 contractor may keep a baseboard or add supplemental heat, so the choice is not always radiant or nothing. The answer can radiant floor heat be the only heat source shows that comparison room by room.
For each room, also record the floor area that can take tubing, leaving out cabinets, a tub and a shower pan. That area and the heat loss show how much each square foot of floor has to deliver. How the floor covering and the water temperature affect a floor's output is in the guide to radiant floor heating water temperature.
Cutting grooves in an existing slab
On a basement floor or a slab-on-grade house, there is one more way to avoid raising the floor: a contractor cuts grooves into the existing slab, lays the tubing in them and fills them in. It is less common in the US than staple-up or a new thin slab, and everything depends on the slab you already have.
- The slab. The method is more common in the UK, where it is called milling, and the UK specialists whose pages we read describe sound concrete or a sand and cement topping. They turn down weak, damp or cracked floors, and floors that are too thin. One UK guide to the method gives roughly 75 to 100 mm (3 to 4 in) of slab and topping combined as the depth needed. These are the contractors' own rules, not a standard, so ask yours what they measured and what they need.
- What is under it. Cutting grooves leaves whatever is under the slab as it is. If there is no insulation, more of the heat goes down into the ground instead of into the room. Ask what the contractor proposes if there is none.
- What is in it. Water lines, drains and conduit may run through the slab. Find out where they are before any cutting.
- What is on it. Old vinyl tile or mastic on the slab can contain asbestos. Do not cut until you know what it is.
- The steps. The contractor checks the slab, cuts the grooves with dust extraction, lays the tubing and connects it to the manifold, pressure-tests it before it is covered, fills the grooves back to floor level and lays the finish once the filler has cured. Every loop has to reach the manifold, so grooves also run through halls and doorways; see where to put a radiant floor heating manifold.
Ask for the slab to be opened or test-cut in one place before you sign, and note the result in the worksheet. Ask for a quote that lists every part of the job: cutting and cleanup, the tubing and laying it, the manifold and its connection to your heat source, filling and leveling, the pressure test, and the floor finish.
HeatAlgo does not calculate tubing in cut grooves: the layer over the tubing is much thinner than the slab cover the engine takes, and old concrete lies under the tubing. You can still work out each room's heat loss in HeatAlgo, which is the output the grooved floor has to deliver. Ask the contractor or the tubing manufacturer for output data for that build-up with your floor covering. If they have none, ask before you sign who will work out the floor's output, and on what basis.
Who to ask what
Keep the answers with your measurement worksheet, so that everyone works from the same information.
- Radiant contractor or designer: which system suits the height you have 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 tubing, and the output of the floor with your covering.
- Structural engineer: whether the joists or the slab can carry the new layers, especially for a thin slab on a wood floor or where layers are removed.
- Slab-cutting contractor: the condition of the slab and what is under it, after a test cut and before you sign.
- Flooring supplier: whether the product suits radiant heat, its thermal resistance and its thickness.
- Moisture or structural specialist: where your worksheet records moisture, cracks or sag.
- In a condo or shared building: ask the HOA or building management before planning anything.
Once you have a design or a quote, check it against the radiant floor heating design guide.
What you can prepare in HeatAlgo
In HeatAlgo you can work out room-by-room heat loss to EN 12831-1 and prepare a hydronic radiant floor layout on your floor plan. It calculates floors with the tubing embedded in a slab or thin slab (type A in EN 1264), with the slab thickness over the tubing that you take from your floor section (the app starts from a default value; replace it). It assumes a cement-based slab, PEX or PE-RT tubing and at least the minimum insulation under the tubing from EN 1264-4. If you plan a gypsum thin slab or less insulation than that, tell the contractor who reviews the report. For staple-up, plates, panels or cut grooves, ask the manufacturer or the contractor for output data for the complete build-up with your floor covering.
The app works from the inputs you enter. It does not assess load capacity, moisture or the condition of the floor, and it does not replace a site visit. Discuss a report you prepared yourself with your contractor, who should review the design for your home and the selected system. For an overview of the tool, see radiant floor heating in HeatAlgo.
Work out each room's heat loss firstA HeatAlgo report follows EN 12831-1 and EN 1264, not ACCA Manual J, and is not accepted for permits or rebates.