2026/09/28
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Most people planning radiant floor heating start by comparing boilers. That is roughly two decisions too early.
The floor decides first. How much height it can give up, how much weight it can carry, and whether anything can reach it from underneath — those three answers determine which systems are even available to you.
Get them in the wrong order and the discovery arrives late and expensively: a quote for a system the joists cannot hold, or a finished floor that no longer clears its own doors.
The Three Layers, and What Each One Takes
A hydronic radiant floor is not tubing. It is a sandwich, and every layer has a thickness.
Insulation goes underneath, so heat travels up rather than down. PEX tubing sits above it. Screed — mortar or a self-levelling compound — encases the tubing and becomes the thermal mass that does the actual radiating.
The screed is the layer that sets the budget for everything else. Conventional sand-cement screed over tubing is generally specified at no more than 65 mm. A liquid or flowing screed, which self-compacts around the pipe, can come down to around 40 mm and still perform.
Add insulation below and a finish floor above, and a full new-construction hydronic assembly lands near 100 mm — about four inches of total build-up.
That figure is a convention rather than a law, and the rest of this article is largely about the methods that get underneath it. But it is the number a designer starts from, and it explains at a glance why radiant heat is far more common in new builds than retrofits.
The Roman hypocaust ran on the same constraint two thousand years ago. The floor had to be built around the heating system, raised on stacks of tile with the heat passing beneath, because there was no way to introduce it afterwards.
Designed In, or Squeezed In
In a new build, four inches is simply a line in the drawings.
The structural engineer specifies the slab with the assembly already in it. Ceiling heights get set higher to compensate, or the overall building height absorbs the difference. Nothing is fighting anything.
An existing house has no such slack.
An overpour retrofit — tubing laid over the existing subfloor, then a poured layer on top — raises the finished floor by roughly half an inch to one and three-quarter inches. Every door in the work area may need trimming or reframing. Every doorway into an untouched room becomes a threshold detail somebody has to design.
An inch sounds like nothing until you list what it touches: the undercut on the doors, the step into the hallway, the toe kick under the kitchen cabinets, the bottom stair riser that is now shorter than the fifteen above it. Every dimension in a house is tied to every other dimension.
Above the Joists, or Underneath Them
If there is a crawl space or unfinished basement under the room, a different option opens — and it is the one most homeowners have never heard of.
Staple-up installs the tubing against the underside of the floor joists, from below, pressed into aluminium heat-transfer plates that spread the warmth across the subfloor above.
The finished floor is never touched. Height gain is zero. Dead load added to the structure is negligible.
The trade-off is output, and it is worth quantifying rather than hand-waving. A slab or overpour system can deliver on the order of 50 BTU/hr per square foot; a joist-bay system lands nearer 35. That sounds decisive until you check demand: a well-insulated house needs roughly 20–25 BTU/hr per square foot even at 0°F outside. For most modern homes, staple-up is sufficient. For a leaky old house with big glazing, it may not be.
One thing staple-up does better: because there is no slab to charge, it responds faster than a wet system. Less mass, less inertia, less waiting.
The plates are not optional. Bare tubing stapled into a joist bay with air around it is the weakest way to install radiant heat. Aluminium transfer plates are what turn it into a working system, and skipping them to save money is the most common way staple-up installations disappoint.
The limitation is access. Staple-up needs open space beneath, so it works on a ground floor over a basement or crawl space and nowhere else. A second-floor bedroom or a slab-on-grade room has no underside to reach.
For those, there is a third route: low-profile dry panels. Thin pre-grooved boards laid over the subfloor with channels routed for the tubing, at roughly five-eighths of an inch total. Often that clears existing door bottoms without cutting anything. The cost is thermal mass — less of it means less stored heat and a somewhat lower ceiling on output than a wet screed.
The Weight Question Ends More Projects Than Height
Height is a design problem. Weight is a structural one, and it is the faster project-killer.
Industry guidance puts the added dead load of a poured underlayment system at 12 to 14 pounds per square foot.
That number means more than it looks like. Residential floor framing is typically designed around a dead load allowance in the region of 10–15 psf, so an overpour can roughly double the permanent load the joists were sized for. The binding issue is usually not collapse — it is deflection. A floor that sags slightly under the new weight cracks the screed it is supposed to be supporting.
Which is why the standard guidance is to use deeper joists than you otherwise would, and why older framing is the problem case: undersized members, long spans, and lumber cut to historical rather than current standards.
In some retrofits the load calculation simply rules the wet system out, and the hydronic options reduce to a dry panel or staple-up. That is a perfectly good outcome — it is just one you want to learn before ordering materials.
If there is any doubt, the structural engineer is the first phone call, not the last.
Korean construction never has this argument, for the reason this whole series keeps returning to. Traditional 구들 (gudeul) built the flue structure and its 구들장 (gudeuljang, capping stones) into the building's skeleton. Modern Korean apartments carry the logic forward — the floor assembly is engineered for hydronic tubing before any wall exists. Retrofitting a Western timber-framed house inverts that sequence, and the structural review exists to tell you whether the inversion is possible.
How Hot the Surface Is Allowed to Get
Clear the structure and the next constraint is thermal: how warm the floor may be before warm becomes unpleasant.
ASHRAE Standard 55 puts the range for occupied spaces at 19°C to 29°C (66.2°F to 84.2°F) at the floor surface.
One scope note that matters here more than almost anywhere. That criterion assumes occupants in lightweight indoor shoes, and the standard explicitly does not cover people sitting directly on the floor. Which is to say the Western thermal comfort standard does not describe the Korean way of using a heated room at all — sitting, eating and sleeping at floor level puts far more of the body in contact with the surface than a shoe ever does, and the comfortable band narrows accordingly.
The 29°C ceiling is not just a comfort figure, though. It works backwards into the whole system design.
To hold a surface in that band, most residential hydronic systems run supply water at 90–120°F (32–49°C). Korean practice tends toward the upper end of it, because Korean floors use wider tubing spacing under a thicker mortar layer than a typical European screed.
Compare that with a conventional baseboard radiator or a forced-air coil, which generally wants water at 140–180°F to move the same heat into a room. Radiant runs cooler because it warms the surfaces you touch instead of warming air and waiting for the air to do the rest.
That low operating temperature has a consequence that matters increasingly. Heat pumps get more efficient as the gap between source and output narrows. A floor loop at 90–120°F is a natural partner for one; a baseboard system demanding 180°F is not, without an expensive workaround.
Electric Doesn't Solve This — It Sidesteps It
Faced with the load calculation, the obvious thought is to skip water entirely.
Electric mats and cables are millimetres thick and usually set straight into the tile adhesive. Essentially no height gain, no dead load worth calculating, no structural engineer. For a bathroom refit it is often the cleanest answer available.
The constraint is running cost, and it is worth stating the reason precisely rather than vaguely. Electric resistance heating is 100 percent efficient at the point of use — every watt becomes heat. That sounds excellent until you compare it with the alternatives: a gas boiler buys its heat from a cheaper fuel, and a heat pump delivers three or four units of heat per unit of electricity. Against either, one-for-one is a poor exchange rate.
So electric radiant is well matched to one bathroom or an entry hall, where the point is comfort underfoot rather than heating the house. Running it as the primary heat source for a kitchen, family room or finished basement through a cold winter is expensive in most markets.
The two are not substitutes. They answer different questions at different scales.
Structural Requirements by Method
| Method | Height gain | Added dead load | Output | Key constraint |
|---|---|---|---|---|
| Hydronic — slab embed (new build) | In the design | In the slab spec | Highest (~50) | Must be decided at design stage |
| Hydronic — overpour (retrofit) | ½–1¾ in. | 12–14 lbs/sq ft | Highest (~50) | Structural review required |
| Hydronic — low-profile dry panel | ~⅝ in. | Minimal | Moderate | Less mass, lower output ceiling |
| Hydronic — staple-up under joists | None | None | Lower (~35) | Needs access below; plates essential |
| Electric mat or cable | Negligible | None | Local only | Running cost rules out whole-house use |
Output figures are BTU/hr per square foot and are indicative. Against a typical well-insulated demand of 20–25, every hydronic row above clears the bar; the differences matter most in poorly insulated rooms and cold climates.
Why a Radiator Never Asks These Questions
It is worth pausing on the contrast, because it is the whole explanation for radiant heat's premium reputation in the West.
A radiator bolts to a wall. Ductwork threads through ceiling voids and wall chases. Neither touches the floor structure, neither costs you a single millimetre of ceiling height, and neither has ever required anyone to ask what the joists can carry.
The structural complexity is the entry fee for what radiant delivers: warmth arriving from the surface you stand on, a gradient that keeps feet warm and head cool, no fan noise, and no mechanical air movement pushing dust around the house. (Air still moves — warm floor, natural convection — just without a fan driving it.)
Traditional Korean rooms were organised around a different gradient worth distinguishing carefully. The vertical one — warm floor, cooler air above — is the comfort principle ASHRAE eventually quantified. The horizontal one, from the hot 아랫목 (araetmok) nearest the firebox to the cooler 윗목 (witmok) at the far end, was an artefact of heating a room with a single fire at one end. Modern manifolds and loop design exist precisely to eliminate that second gradient while keeping the first.
Western construction inherited the opposite default: heat the air, move it around, leave the floor alone. Changing that default in a finished building means rethinking the floor structurally before you can rethink it thermally.
Three Questions Before the Catalogue
Choosing a system before answering these is the wrong order of operations.
Access. Is there open space beneath the floor? If yes, staple-up is on the table and the whole height problem disappears. If no, the tubing comes from above and everything below follows from that.
Load. Can the joists and their spans take 12–14 pounds per square foot of additional dead load within deflection limits? If not, the wet system becomes a dry one by necessity — and that is a decision the structure makes, not you.
Height. How much finished floor rise can the house absorb before doors need cutting, transitions need ramping and built-ins need rebuilding? Measure the tightest door in the work area; that is your real budget.
Answer those three and the interesting questions become answerable: tubing spacing, screed depth, manifold placement, boiler or heat pump. Answer them afterwards and they become expensive corrections.
The floor decides what the heating system can be. That is where every radiant project actually begins — whether the room is a new 온돌방 (ondolbang, a Korean room built around its heated floor) or a hundred-year-old bungalow with joists nobody has looked at since 1925.
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