Radiant Heat Tubing: Choosing and Laying PEX for Floor Heat

A radiant floor lives or dies by the tubing buried inside it. Pick the wrong radiant heat tubing, space it too wide, or run one loop too long, and you get cold stripes across the floor, a boiler that short-cycles, or rust sludge clogging the system years later. Once the slab is poured or the subfloor goes down, none of that is easy to fix.

The good choices are not complicated. For almost every hydronic floor, the answer is oxygen-barrier PEX in 1/2 inch size, laid 6 to 12 inches apart, in loops no longer than about 300 feet, arranged in a counterflow spiral or serpentine pattern that puts the warmest water near outside walls. The details depend on the floor assembly, the heat loss of the room, and the finished flooring.

This piece focuses only on the tubing: material, size, spacing, loop length, and layout. System design, boilers, and manifolds are separate topics, and the final design should be checked by a heating professional.

What Radiant Heat Tubing Has to Do

Radiant heat tubing carries warm water, usually 85 to 140°F depending on the floor type, through a slab, a thin pour, or channels under the subfloor. It needs to survive decades of heating cycles without cracking, resist kinking during installation, and keep oxygen out of the water. That last job is the one homeowners most often overlook.

Plain plastic tubing is slightly permeable. Oxygen molecules migrate through the wall into the heating water, where they corrode iron and steel components: cast iron boilers, circulator pump bodies, steel expansion tanks, and valves. Over time that creates sludge, blocked heat exchangers, and premature pump failure. Closed-loop heating systems need tubing that stops it.

Oxygen Barrier PEX vs Other Options

Oxygen Barrier PEX

Oxygen barrier PEX has a thin layer, usually ethylene vinyl alcohol (EVOH), bonded to or inside the tubing wall. It looks like regular PEX with a slightly glossy coating, often in red, orange, or gray. This is the standard choice for nearly all residential radiant floors. PEX tubing for radiant heat comes in long coils of 300 to 1,000 feet so loops can be run without splices.

Barrier location matters a little. Outer-layer barriers can scuff if tubing is dragged across rough concrete or rebar, so handle the coil carefully. Embedded or inner-layer barriers are better protected. Either works when installed with care.

PEX-AL-PEX

PEX-AL-PEX sandwiches an aluminum layer between two PEX layers. The aluminum is a complete oxygen barrier, and it also makes the tube hold its shape when bent, which helps in staple-up and joist-bay installs. It expands less with heat and resists sagging. Downsides: higher cost, a tighter minimum bend radius before kinking, and fittings specific to the tubing. It is a strong choice for exposed or suspended runs and for installers who want tubing that stays put.

Non-Barrier PEX

Standard potable PEX without a barrier should not go in a closed hydronic loop with ferrous components. It is acceptable only in open systems or all-nonferrous systems with heat exchangers designed for it, and even then most designers avoid it. Do not use leftover plumbing PEX for floor heat.

PEX-A vs PEX-B for Radiant

Both are used. PEX-A is more flexible, and a kink can often be repaired with a heat gun, which is a real advantage when laying tight loops on a cold day. PEX-B is stiffer and less expensive. Oxygen barrier PEX tubing is available in both types.

Choosing Tube Size

• 3/8 inch: thin-pour overlays and low-profile panel systems where floor height is limited. Shorter loop lengths.
• 1/2 inch: the residential workhorse for slabs, thin pours, and most staple-up or plate systems.
• 5/8 inch: large open areas, garages, basements, and commercial slabs where longer loops reduce manifold ports.
• 3/4 inch: very large slabs, snowmelt, and supply mains rather than floor loops.

Larger tubing moves more water with less pressure drop, so loops can be longer. Smaller tubing fits in thinner assemblies and bends tighter. If the designer has not specified otherwise, 1/2 inch is the safe default.

Tube Spacing

Spacing, measured center to center, controls both heat output and floor surface evenness. Closer spacing delivers more heat at lower water temperature and avoids warm and cool stripes.

• 6 inches: high heat loss zones, perimeter bands under large windows, bathrooms where warm tile is the goal, and floors with thick wood or carpet.
• 9 inches: a common general spacing for well-insulated living areas in a slab or thin pour.
• 12 inches: low heat loss interior rooms, basements in mild climates, and garages.

Joist-bay systems with aluminum heat transfer plates usually run two tubes per bay, roughly 8 inches apart. Wider spacing than 12 inches in living space tends to produce noticeable stripes under thin flooring like tile.

Finished flooring changes the math. Tile and stone conduct heat well. Engineered wood and luxury vinyl are moderate. Thick carpet and pad act as insulation and may need closer spacing or warmer water. Check the flooring maker’s maximum surface temperature, often around 80 to 85°F for wood and vinyl.

Loop Length Limits

Every loop has a maximum length before pressure drop and temperature loss become excessive. Typical guidance:

• 3/8 inch: roughly 200 to 250 feet
• 1/2 inch: roughly 250 to 300 feet
• 5/8 inch: roughly 350 to 400 feet
• 3/4 inch: up to about 500 feet

Those lengths include the leader tubing running from the manifold to the heated area. Keep all loops on one manifold within about 10 to 15 percent of each other in length so flow balances without excessive valve throttling. Estimating length is simple: divide the room square footage by spacing in feet. A 200 sq ft room at 9 inch (0.75 ft) spacing needs about 267 feet plus leaders, which would push past a single 1/2 inch loop, so split it into two.

Layout Patterns

Serpentine

Tubing runs back and forth like a lawn mower pattern. Water starts hottest at one end and cools along the way, so the floor is warmer where the loop begins. Start serpentine loops at the coldest exterior wall. It is the easiest pattern to lay and works well in narrow rooms and for perimeter bands.

Counterflow Spiral

Also called a double serpentine or bifilar pattern, the tube spirals inward to the center, then spirals back out alongside itself. Hot supply runs next to cooler return all across the room, which averages out surface temperature. This is the preferred layout for most square or open rooms.

Perimeter Bands

Along large windows or slider doors, run tighter spacing in a 2 to 4 foot band, then widen spacing in the interior. You can do this within one loop or with a dedicated perimeter loop.

How to Lay Radiant Heat Tubing

Tools and materials: oxygen-barrier tubing coils, uncoiler, tube staples or clips, rebar or wire mesh ties or foam panel channels, bend supports, chalk line, tape measure, marker, PEX cutter, and a pressure test gauge.

1. Draw the plan. Map rooms, manifold location, loop routes, spacing, and lengths. Avoid where toilets, cabinets, and floor anchors will go.
2. Prepare the base. In a slab, lay rigid insulation over compacted base and vapor retarder, then mesh or rebar if specified. Edge insulation at the slab perimeter stops heat bleeding outdoors.
3. Mark the grid. Snap chalk lines at your spacing on insulation or the subfloor so loops stay consistent.
4. Uncoil without twisting. Use an uncoiler so tubing feeds flat. Twisted tubing fights you and kinks.
5. Start at the manifold. Label the supply end of each loop with its length and room. Run the leader, lay the pattern, return, and label the return end.
6. Fasten often. Secure tubing every 2 to 3 feet on straight runs and at both sides of each bend so it cannot float up during a pour.
7. Respect bend radius. Do not bend tighter than the tubing maker allows, often around 5 to 8 times the outside diameter. Use a loop-end “bulb” turn instead of a sharp U.
8. Sleeve penetrations. Where tubing crosses slab control joints or exits the slab, use protective sleeves.
9. Pressure test. Pressurize to the level the design calls for, commonly well above operating pressure, and keep it under pressure during the pour so any puncture is caught immediately.

Never splice tubing inside a slab or pour. If a loop is damaged, the fix should be a single repair coupling rated for embedding, documented on the plan, and only where unavoidable.

Troubleshooting Tubing Problems

• Cold stripes: spacing too wide for the floor covering or water too cool. Raise supply temperature within flooring limits.
• One room never warms: loop too long, air trapped, or a manifold balancing issue. Purge each loop separately.
• Sludge or rust in the system: non-barrier tubing or a barrier damaged during installation. A heat exchanger can isolate ferrous components.
• Kink found before pour: PEX-A can often be reformed with heat; PEX-B or PEX-AL-PEX usually needs replacement of that section.
• Cracks in tile over heat: slab not cured or heated too quickly. Bring new slabs up to temperature gradually over several days.

Cost Ranges

Oxygen barrier PEX is inexpensive per foot, and tubing is usually a small share of total radiant floor cost. A typical room might need 300 to 600 feet. Fasteners, foam panels or heat transfer plates, insulation, and the manifold add more. PEX-AL-PEX costs noticeably more than barrier PEX. Professional radiant floor installations, including design, heat source, and controls, commonly range from several dollars to over twenty dollars per square foot depending on the assembly and region.

When to Call a Licensed Professional

Laying radiant heat tubing is DIY-friendly, but the system around it is not. Heat loss calculations, water temperature, boiler or water heater connections, mixing valves, and any gas-fired heat source belong to a licensed HVAC or hydronic contractor, and permits are commonly required. Electrical work for circulators and controls should go to a licensed electrician. Before a slab pour, have a pro review the loop plan; mistakes are permanent once concrete covers the tubing.