Radiant Floor Heating

Every floor covering adds thermal resistance between the tubing and the room, expressed as an R-value, and the higher that resistance, the hotter the water has to run to push the same amount of heat into the room. Warmboard's own design chart for its radiant panel system plots exactly this relationship: at a low covering resistance (around R-0.5, roughly a thin tile or stone floor) the system needs only about 80°F average water temperature to deliver around 20 Btu/h per square foot, while the higher-resistance coverings on the same chart, such as carpet with pad, push the required water temperature well past 140°F for the same output. That relationship holds for any radiant system, not just a panel one — a system designed around tile will underperform under carpet unless the water temperature or the tube spacing is adjusted for it.

Across installation types, radiant floor systems commonly run supply water well below the 160–180°F baseboard often needs, which is a large part of why radiant pairs so well with a condensing boiler: lower return-water temperatures are exactly what let a condensing boiler hit its rated efficiency.

The three common ways to install radiant tubing trade off installed cost, response time, and the water temperature the system needs.

How far apart the tubing runs also drives both output and comfort, because it depends on how well the material around the tube spreads the heat. Warmboard's own spacing comparison found that its aluminum-faced panel needs tubing only every 12 inches, while a thin concrete or gypsum slab needs it every 4 to 6 inches for comparable output — and would need it every 2 inches to fully match a Warmboard panel's heat output, because concrete and gypsum "absorb heat like a sponge" rather than spreading it. Spacing also affects how even the floor feels underfoot: the same document measured a 5–10°F difference between the warmest point (directly over a tube) and the coolest point (midway between tubes) on a thin slab with 12-inch spacing, versus a 1–3°F difference on a Warmboard panel at the same spacing.

The takeaway for a homeowner comparing quotes isn't that one spacing number is right — it's that tube spacing, floor covering, and water temperature are one connected design decision, not three separate ones. A contractor who tightens the spacing, lowers the design water temperature, or does both to compensate for a higher-resistance floor covering is doing the same job a manufacturer's own design chart is built to guide. If a quote lists a floor covering and a supply water temperature but not a tube spacing, that's a reasonable question to ask before the job starts, not after the floor is closed up.

Nearly every major radiant tubing line sold today, including Uponor's and Rehau's residential radiant systems, is a single-layer, oxygen-barrier-coated PEX-a. PEX-AL-PEX is a different, three-layer construction: a thin aluminum layer bonded between two layers of PEX. According to the technical description in Wikipedia's cross-linked polyethylene entry, that aluminum layer "serves as an oxygen barrier, stopping the oxygen diffusion through the polymer matrix" — a barrier that doesn't rely on a coating that can wear over time — and it "provides some rigidity to the tube such that when bent it retains the shape formed," unlike standard PEX tubing, which "will spring back to straight." The aluminum layer also lets the composite pipe handle higher pressure and temperature. The trade-off is that PEX-AL-PEX is stiffer to route through tight joist bays and typically costs more per foot than standard PEX.

The oxygen barrier matters most in systems that mix plastic tubing with iron or steel components — a cast-iron boiler, steel circulators, or iron fittings — because oxygen that diffuses into the water accelerates rust in anything ferrous it touches. A radiant loop feeding a modern condensing boiler with mostly plastic and brass hydronic parts leans less on the barrier for that reason, but installers still specify barrier tubing as standard practice rather than deciding case by case.