Radiant Heat From the Ground Up: How Rehau PEX and Creatherm Work Together in a Below-Grade Slab
There is a particular kind of commitment that goes into work nobody will ever see. The radiant floor heating system buried beneath the Squirrel Hill Addition's four-inch concrete slab will spend its entire working life invisible — pressing warmth up through finished flooring, silently doing exactly what it was designed to do. Getting that system right required weeks of deliberate, sequential work before a single yard of concrete was poured. This is a record of what that looked like.
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The Foundation Conditions
By early February 2021, the below-grade space had been prepared to receive the radiant system. The concrete foundation walls were in place, the sub-grade storm and groundwater sump evacuation infrastructure was roughed in, and the sewage evacuation sump system was positioned and waiting for final provisioning. Expansion joints were already set — a detail that communicates the full scope of what was coming. Those joints exist because the finished slab would be a full four inches thick, a mass of concrete substantial enough to require control over where it moves as it cures and settles over time.
The yellow Creatherm dimple board is visible across the floor, and the Rehau PEX tubing is already beginning to take shape. This is the baseline: a dry, prepared, structurally sound sub-grade environment where the system could be installed with precision rather than improvisation.
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The Creatherm Dimple Method
Creatherm is not simply foam insulation. The dimple board format performs two functions simultaneously: it provides the thermal insulation layer that keeps heat directed upward into the living space rather than downward into the earth, and the surface geometry locks the PEX tubing in place without mechanical fasteners at every turn. The integral thermal breaks built into the board's profile prevent the concrete above from conducting heat directly into the substrate — a refinement that matters over the lifetime of an operating system.
What the photo from February 3rd captures is the grid in mid-installation. The Rehau PEX tubing is routed through the Creatherm system in the consistent spacing patterns that define even heat distribution. Suspended above the insulation board on two-inch concrete chairs, a half-inch rebar grid holds its position across the entire floor plane. Those chairs exist for one reason: to ensure the rebar ends up at the correct depth within the finished slab, neither too high to provide structural reinforcement nor so low that the tubing is inadequately protected. The relationship between the PEX, the insulation, the rebar, and the chairs is a layered system, and each layer depends on the accuracy of the one beneath it.
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The Home Runs
Three days later, on February 6th, the installation had progressed to a point that told the whole story of how the system was organized. The red oxygen-barrier Rehau PEX tubing — visually unmistakable against the pale concrete and foam — converged at a single location in the basement addition space: the home-run point, where every loop in the floor would eventually terminate at the radiant heat manifold.
Oxygen-barrier tubing is the right specification for a system like this. The barrier layer prevents oxygen diffusion through the PEX wall over time, which protects ferrous components in the system — circulator pumps, manifold components, boiler heat exchangers — from corrosion that would otherwise accumulate silently over years of operation.
The two sump basins visible in this photo were awaiting final provisioning. Temporary work lights hung from the exposed I-joist ceiling framing overhead, pushing enough light into the space to work by. This is what a construction site looks like in the middle of honest work: functional, purposeful, and not yet finished.
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What Gets Buried
Every element of this system — the Creatherm board, the Rehau tubing grid, the rebar suspended on concrete chairs, the expansion joints, the home-run lines — was designed to be permanently enclosed in concrete. That fact changes how you have to think about installation quality. There is no access panel, no service port, no opportunity to correct a mistake after the pour. Tubing joints are pressure-tested before concrete placement. Layout patterns are verified against the manifold circuit map. The expansion joints that will allow the slab to breathe are in the right locations or they are not.
The crew that laid this system understood the stakes. The Creatherm dimple board was fitted tightly to the foundation walls. The PEX spacing is consistent. The rebar grid is level. The home runs are organized and routed cleanly to their termination point.
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The Project Continues
The radiant slab was only the beginning. The Squirrel Hill Addition grew floor by floor through the spring and summer of 2021 — LVL beams and engineered lumber framing, Zip system sheathing closing the envelope, salvaged brick matched to the existing structure on the exterior walls, Montigo fireplace exhaust venting roughed in, half-timber detailing taking shape on the upper floors. All of it resting, ultimately, on the foundation work and the mechanical infrastructure that was set correctly from the start.
Work that gets buried deserves the same standard as work that stays visible. That is not a philosophy. It is just how a system built to last actually gets built.
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