The Radiant Floor
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Problem
When heat enters a room from registers in the ceiling or high on the wall, the warm air rises and pools where no one can use it — two meters above the floor, stratifying into a hot band at head height and a cold layer at your feet. Furniture blocks baseboard heaters; forced-air ducts lose heat through unconditioned attics. The occupants turn up the thermostat to compensate, burning more energy to achieve less comfort. Yet burying tubes in concrete is expensive, irreversible, and slow to respond — a system that cannot easily be undone or adjusted once the slab is poured.
Evidence and Discussion
The human body loses heat primarily through the feet when standing or sitting. Fanger's thermal comfort research, conducted at the Technical University of Denmark beginning in the 1970s, established that a floor surface temperature of 19–26°C produces the highest comfort ratings, while cold floors below 17°C trigger discomfort even when air temperature is adequate. Radiant floors address the body where it contacts the building.
In a forced-air system, the temperature differential between floor and ceiling can reach 5–8°C in a room with 2.7-meter ceilings. The thermostat, mounted at 1.5 meters, reads an average that leaves feet cold and heads warm. A radiant floor inverts this gradient: the warmest surface is at ankle height, cooling slightly as heat rises. Scandinavian housing standards have long favored hydronic radiant systems for this reason — in Sweden and Norway, radiant floors are standard in new residential construction, particularly in bathrooms, entryways, and ground-floor rooms over unheated space.
The pairing of radiant floors with heat pumps is thermodynamically elegant. Heat pumps achieve their highest coefficient of performance when the supply temperature is low — a ground-source system delivering 35°C water to a radiant floor operates at a COP of 4 or higher, compared to a COP of 2.5 when delivering 50°C water to baseboard radiators. THE HEAT PUMP SYSTEM (194) generates low-grade heat efficiently; radiant floors use low-grade heat effectively. The thermal mass of the concrete slab — see THERMAL MASS (45) — stores hours of heat, allowing the system to run during off-peak electrical hours or when solar electricity is abundant, then coast through the evening without cycling on. When connected to THE THERMAL STORAGE TANK (195), the floor becomes a slow-release battery, drawing warm water when available and radiating steadily through cold nights.
In Edmonton's climate, where January temperatures regularly fall below -25°C, the ground-floor slab is often the coldest surface in the house — particularly over an unheated garage or crawlspace. Radiant tubing in this slab transforms a liability into an asset. The 100 kg/m² of thermal mass recommended for PASSIVE SOLAR DESIGN (126) performs double duty: absorbing solar gain through south windows during the day, and accepting heat from hydronic tubing at night. The same slab, the same mass, serving two purposes.
Installation requires planning before the pour. PEX tubing (typically 16mm or 20mm diameter) is laid in serpentine loops at 150–300mm spacing, fastened to the reinforcing mesh or insulation, and pressure-tested before concrete is placed. The slab must be insulated below — minimum R-10 rigid foam over gravel — or heat migrates into the ground rather than into the room. For frost-protected shallow foundations, as described in FROST-PROTECTED FOUNDATION (64), the perimeter wing insulation must connect to the under-slab layer, creating a continuous thermal break that keeps heat in the living space.
Therefore
embed PEX hydronic tubing in any concrete floor slab that will serve as finished floor — entryways, mudrooms, kitchens, bathrooms, and basements. Space tubes at 200mm on center in primary living areas, 300mm in secondary zones. Insulate below the slab with a minimum R-10 rigid foam board. Supply water temperature should not exceed 40°C when connected to a heat pump system; target floor surface temperature of 24–28°C in occupied areas. Test: place a thermometer on the floor surface at three points across the room during steady-state heating — all readings should fall within 3°C of each other, and none should exceed 29°C.