How to Install Radiant Tubing in a Concrete Slab

Radiant floor heating delivers even warmth because the concrete slab itself becomes the heat emitter. The slab stores energy and releases it slowly, so rooms stay comfortable at lower air temperatures than forced-air systems need. The timing is the catch: tubing has to go in during the pour, and there is no second chance to add it after the concrete sets. Planning the full sequence before the ready-mix truck arrives decides whether the job goes smoothly. The same care that goes into slab foundation design and construction carries over to the heating layer, because the slab has to carry building loads and house the heating system at the same time.

How Radiant Slab Heating Works

Two families of radiant systems exist. Hydronic systems circulate heated water through sealed tubing loops, while electric systems use resistance cable or mesh mats. In slab applications, hydronic is the standard choice for whole-house heating, and electric mats fit small bathrooms and additions. A hydronic system starts with a heat source, usually a boiler or an air-to-water heat pump, that raises water to a moderate temperature between 100°F and 130°F. A circulation pump pushes that water to a manifold, which splits the flow into individual loops of PEX tubing embedded in the concrete.

• Heat source: boiler, air-to-water heat pump, or solar thermal array sized to the total heat load
• Manifold: the distribution hub with a balancing valve and flow meter for every loop
• PEX tubing: 1/2-inch or 5/8-inch cross-linked polyethylene rated for continuous service at 200°F
• Circulation pump: sized for the friction loss of the longest loop at the design flow rate
• Thermostats and controls: room sensors that adjust supply water temperature to match demand

Concrete earns its place in the system through thermal mass. A 4-inch slab weighs about 50 pounds per square foot and acts as a heat battery, smoothing out the on-off cycles of the heat source. Because the supply temperature stays low, the system pairs well with heat pumps and condensing boilers, both of which gain efficiency at lower water temperatures. Before ordering materials, run the numbers with a concrete calculator for slabs, beams, columns, and footings so the ordered yardage matches the pad and the specified slab thickness.

Designing the Tubing Layout

Layout work starts with a room-by-room heat loss calculation, not with a roll of pipe. Every exterior wall, window, and door leaks heat at a different rate, and the tubing spacing has to respond to that. The design goal is to deliver the required output while keeping the floor surface below 85°F, the point where a floor starts to feel uncomfortably hot to bare feet.

Spacing and Water Temperature

Tighter spacing delivers more heat per square foot, but it also means more pipe, higher pressure drop, and a longer installation. The table shows typical output values for 1/2-inch PEX at common spacings and supply temperatures, assuming 2 inches of concrete cover above the tubing and edge insulation at the perimeter.

Two rules keep the layout honest. Keep individual loops under 300 feet for 1/2-inch PEX so the pump does not fight excessive friction loss. Use tighter spacing in high-loss zones: 6 inches along exterior walls and near large windows, 9 or 12 inches in open interior areas.

Zone Planning at the Manifold

Each loop returns to the manifold, and each manifold position becomes a zone with its own balancing valve and thermostat. A common arrangement puts the manifold in a mechanical room or utility closet and runs loops outward in a serpentine pattern. Counterflow layouts, where the hot supply leg runs next to the cooler return leg, even out the surface temperature across the room.

1. Calculate the heat loss for each room and total the system load
2. Select the supply water temperature, typically 110 to 120°F for most homes
3. Lay out loop paths with spacing matched to exterior exposure
4. Size the manifold, pump, and heat source to the total flow requirement
5. Mark loop lengths and label each circuit before the pour

The plan is only as good as the forms that hold it. Forming a concrete slab correctly sets the grade, squareness, and thickness that the tubing layout depends on; forms that sag or bow move the finished floor plane and change the concrete cover over the pipe.

Choosing the Concrete Mix

The mix has three jobs: protect the tubing, transfer heat, and carry the floor load. A standard 4,000 psi mix with a 4 to 5 inch slump handles most residential slabs, but aggregate size matters more than strength. Coarse aggregate should stay at 3/4 inch or smaller, because larger stone crowds the space between tubing rows and makes consolidation harder.

• Compressive strength: 3,500 to 4,500 psi for typical residential slabs
• Maximum coarse aggregate: 3/4 inch, well graded and washed
• Slump: 4 to 5 inches so the mix is pumpable and workable around tubing
• Air entrainment: 4 to 6 percent where the slab will face freeze-thaw cycles
• Concrete cover over tubing: minimum 1.5 inches, 2 inches preferred

On upper floors and remodels where dead load is a concern, lightweight concrete for radiant slab installations cuts the weight by roughly 25 to 30 percent while keeping enough thermal conductivity for even heat delivery. Lightweight mixes cost more per yard and need extra curing attention, so the tradeoff pays off mainly where the structure demands it.

Placement technique decides whether the mix performs. Dump concrete near the tubing rather than dropping it from height, and screed from the high side so workers are not standing on finished pipe. Keep vibrators at least 6 inches from tubing, and wait for the surface to support foot traffic before running a power trowel.

Insulating Below and Around the Slab

Skip the below-slab insulation and a slice of the heat output heads into the ground instead of the room. Field experience puts ground losses at 15 to 30 percent of total output for uninsulated slabs, which adds up over a heating season. Insulation below the slab and at the slab edge turns the concrete into a one-way heat emitter.

Choosing the right foam board for below-slab thermal performance comes down to R-value, moisture resistance, and compressive strength. EPS and XPS both work, but XPS holds its R-value better in wet soil, while EPS costs less and dense grades carry slab loads without crushing.

• R-10 below the slab in climate zones 4 and colder, R-5 to R-7 in mild climates
• 2-inch vertical edge insulation around the full slab perimeter
• 6-mil polyethylene vapor barrier laid below the insulation
• Foam with a compressive strength of at least 25 psi under structural slabs

Installation Steps and Pour Day

1. Compact the subgrade to the specified density and screed it to grade
2. Lay the vapor barrier with seams lapped 12 inches and taped
3. Place rigid foam panels tight against each other and tape the joints
4. Set reinforcement on chairs, positioned in the middle third of the slab
5. Unroll tubing from the manifold outward, fastening every 24 to 36 inches
6. Pressure test the tubing at 1.5 times the working pressure and hold it during the pour
7. Pour and finish the concrete while watching the pressure gauge
8. Cure at least 7 days before foot traffic and 28 days before full operating heat

The pressure test is the non-negotiable step. Fill the loops with water, pressurize to about 60 to 75 psi for a typical 50 psi system, and leave the gauge connected through pour day. If a screed or a wheelbarrow nicks the pipe, the gauge drops and the crew knows before the concrete sets.

The structural side of the pour should follow slab-on-grade foundation best practices for reinforcement placement, control joints, and curing, because a heating slab that cracks wastes the investment in the tubing below.

Commissioning and Long-Term Care

Commissioning starts after the concrete cures. Bring the system up slowly: fill the loops, purge the air through the manifold bleed valves, and run the pump for a day at a supply temperature of 80 to 90°F before raising the water to the design temperature. A ramp of 10 to 15°F per day avoids thermal shock to both the concrete and the tubing.

Flow balancing is the last construction task. With the system running, check each loop’s flow meter and adjust the balancing valves so every circuit moves its share of the design flow. Rooms that never warm up usually trace back to a loop with too little flow or an air pocket that bleeding missed.

Crews pouring through steel forms will find that slab shuttering methods with steel formwork keep edges straight and grades exact while tubing is being tied off, which protects the pipe and speeds the pour. Once the system is balanced, a radiant slab needs little attention beyond an annual check of the manifold, the expansion tank pressure, and the antifreeze concentration in closed loops that use it.