Radiant floor, slab & staple-up.

What this scope actually is

Hydronic radiant floor heating delivers heat to a room from below by circulating warm water through cross-linked polyethylene tubing embedded in the floor assembly. The tubing forms a loop that returns to a manifold, where multiple loops are gathered, balanced, and connected to the supply and return mains running back to the boiler. The tubing is not the heating element — the floor assembly is. The tubing's job is to deliver the energy from the plant to the floor; the floor releases that energy to the room.

We install two assemblies, depending on what the building permits. Slab assemblies have the tubing laid in a poured concrete slab — typically a 4″ gypsum or concrete pour over 2″ of XPS rigid foam over the subgrade or subfloor. The slab itself stores and emits the heat; supply water temperatures stay modest (95–125°F) and the loop spacing is typically 9″ or 12″ on-center. Staple-up assemblies attach the tubing to the underside of an existing subfloor with stapled aluminum heat-transfer plates and reflective insulation below; supply temperatures run higher (110–140°F), spacing is tighter (6″ or 9″), and the assembly emits heat through the subfloor and finish floor above. Slab is more efficient and the comfort distribution is better; staple-up is what's possible when the floor cannot be opened up.

How we design it

The design starts with an ACCA Manual J load calculation, room by room, against the actual envelope of the actual building. Once we know the load in each room, we know the required heat output of the floor in that room, in BTU per square foot. Different floor finishes have different output ceilings — bare concrete tops out around 40 BTU/sf at typical supply temperatures, hardwood at 25–30 BTU/sf, dense carpet substantially less. If the load in a room exceeds the floor's output ceiling at a sensible supply temperature, we either tighten the loop spacing, supplement with a panel radiator or hydronic toe-kick, or — most often — we tell the architect what the ceiling is and let them and the homeowner choose the finish accordingly.

Loop length is bounded by friction. We hold loops to 250 feet of ½″ tubing or 300 feet of ⅝″ tubing as a practical ceiling, so the pressure drop across the loop stays under 6 psi at the design flow and a Δp circulator can serve the manifold without stalling. The Uponor design manual and Caleffi Idronics issue 7 both publish the friction tables we work from. Loop spacing follows from the floor's required output and the supply water temperature we have committed to. Manifold layout follows from the building plan: one manifold per logical zone, mounted in a mechanical closet or on a wall in the mechanical room, with the loops fanning out from there.

How we install it

Slab assemblies are installed on the rough framer's schedule. We lay 2″ of XPS rigid insulation across the prepared subgrade or subfloor; we lay 6″ × 6″ W2.9 × W2.9 welded wire reinforcement; we lay out and tie the tubing to the wire mesh with zip ties or clips at 12″ spacing along the loop, then make the manifold connections, pressure-test the system at 60 psi for 24 hours, photograph the layout, and turn the slab over to the concrete crew. The slab is poured around our work; we re-test after the pour cures. The tubing-in-slab is not patched: if a kink or a punctured loop is discovered after the pour, we cut the slab. Pressure-testing before the pour is non-negotiable.

Staple-up assemblies are installed after the framing is complete and the mechanical, electrical, and plumbing rough-ins are coordinated. Heat-transfer plates (we use Uponor's omega-shaped extruded aluminum plates) are stapled to the underside of the subfloor at the design spacing; the tubing is run through the plates and stapled at every joist bay; reflective insulation (R-13 minimum) is installed below. The manifold mounts in the mechanical room with the loops dropping into the joist bay through ⅞″ bored holes.

How we commission it

Commissioning happens on a single day, with the building enclosed and the boiler installed. We pressurize the entire hydronic loop to 60 psi with water (no glycol unless the system has a snowmelt branch), purge each loop one at a time at the manifold using the integral purge valves, balance the manifold by setting the actuator preset based on the loop length difference, set the outdoor-reset curve at the boiler control (typical residential curve in this climate: 130°F supply at 0°F outside, 105°F supply at 50°F outside, fixed return-water minimum 95°F to protect the heat exchanger), bring the building to setpoint, measure supply/return Δt at each manifold against design, and document everything in the commissioning page of the binder. The outside temperature on commissioning day is stamped on the page.

What it costs, plainly

A complete radiant floor install in southwest Montana lands in the $8 to $18 per square foot range installed, all-in. The single biggest variable is the floor assembly: a slab pour is at the bottom of the band; a staple-up retrofit with the ceiling open below is in the middle; a heat-transfer-plate retrofit on a finished assembly is at the top. The second-biggest variable is the manifold count, which scales with the zone complexity. The third is the boiler — see the mod-con boiler scope for that pricing band.

When this is the wrong scope

Radiant floor is not the right scope for guest rooms that will be used twice a year and want fast wake-up time — the slab's thermal mass makes radiant slow to come up to temperature, and a low-temperature baseboard run off the same plant gives the homeowner a thermostat-responsive room without the wait. It is not the right scope for a finished basement where the slab cannot be opened up and the ceiling above is also finished. It is sometimes not the right scope for very small spaces (under 100 sf) where the manifold and pump capital outweighs the energy savings.

Read across

See the snowmelt scope for the closely-related glycol loop; the mod-con boiler scope for the plant that drives both; the manifold component sheet in the atlas for what we hang on the wall; and our 2024 Bridger Canyon residence for a worked example.