Another common method of hydronic floor heating uses a 1.5-inch “thin slab” poured over a wooden deck. The slab can be either concrete or poured gypsum, but should never be lightweight concrete, which uses vermiculite or polystyrene beads instead of stone aggregate, and has significantly higher thermal resistance.
Because the slab is thinner, it has somewhat poorer heat dispersion characteristics, needing a slightly higher water temperature for a given rate of heat output—but this difference is slight: A 1.5-inch concrete thin-slab with 12-inch tube spacing and covered with a finish flooring resistance of 0.5°F/hr./ft.2/Btu yields about 8% less heat output than a 4-inch-thick slab with the same tube spacing and finish flooring. To get the same efficiency, 9-inch, rather than 12-inch, tube spacing can be used.
The following guidelines are suggested for thin slabs supplied by renewable heat sources:
Walls and ceilings can also be turned into low-temperature hydronic radiant panels. These radiant walls are indistinguishable from a standard interior wall. Its low thermal mass lets it respond quickly to changing room load conditions or zone setback schedules. This fast response is especially important in homes with low heat loss or significant internal heat gain because such spaces can quickly overheat.
The panel’s rate of heat emission is approximately 0.8 Btu/hr./ft.2 for each 1°F the average water temperature in the tubing exceeds room air temperature. For example, if the average water temperature in the tubing is 110°F in a room with 70°F air temperature, each square foot of wall releases about 32 Btu per hour [0.8 × (110°F - 70°F)]. This average water temperature is well within the range of what most renewable energy heat sources can supply.
If you plan to install this system on the inside of an exterior wall, make sure the R-value of that wall is 50% higher than that of unheated exterior walls. That keeps the rate of heat loss to the outside about the same as for an unheated wall. If you’re installing this on an inside partition wall, use 3.5-inch fiberglass batt in the stud cavities behind the heated wall. Finally, radiant wall panels work best constructed no higher than 3 to 4 feet above floor level. These heights bias the radiant heat output into the occupied zone of rooms, and thus improve comfort.
Radiant ceilings use the same construction as a radiant wall. The only difference is that the materials are fastened to the ceiling framing rather than the studs. The infrared thermograph shows such a ceiling as it warms up. The red areas on the left side indicate that the aluminum heat transfer plates are dissipating heat away from the tubing and across the adjacent ceiling surfaces.
Like the radiant wall, a radiant ceiling has low thermal mass and can respond quickly to interior temperature changes. Heated ceilings also have the advantage of not being covered by rugs or furniture, and thus are likely to retain good performance over the building’s life, but can be a bit more expensive relative to a heated slab-on-grade floor.