Short definition
Underfloor heating (UFH) circulates warm water at 95–125°F through PEX pipework laid in a concrete slab or under suspended timber floors, heating the room from the floor up. Compared with radiators or baseboard, it delivers more even comfort at lower thermostat settings and pairs well with heat pumps because it accepts low supply temperatures. Common in WA bath/kitchen retrofits and full-house custom builds.
What it is
A hydronic UFH system has a manifold (the central distribution point), multiple PEX pipe loops (one per zone or room), an actuator at the manifold for each loop (electrothermal valves controlled by room thermostats), and a heat source (boiler or heat pump) supplying warm water.
Two common install patterns:
- In-screed (solid floor). PEX is laid on insulation, tied to a mesh or rail, and embedded in a concrete or gypsum screed pour. Pipe spacing is typically 100, 200, or 300 mm (4, 8, or 12 inches) on center, depending on heat density target. Output: roughly 100 W/m² (32 BTU/h per square foot). Best for new construction or major renovations where the floor is being rebuilt anyway.
- Suspended timber (joist-bay). PEX runs in joist bays with aluminum heat-spreader plates pressed against the underside of the subfloor. No screed; lighter weight; lower output. Output: roughly 70 W/m² (22 BTU/h per square foot). Used on retrofits where ripping up the floor isn’t feasible.
Design supply temperatures are intentionally low — 40–45°C (104–113°F) for in-screed installs, 50–60°C (122–140°F) for joist-bay. The lower supply temp is what makes UFH a natural pairing with mod-con boilers and heat pumps.
Maximum loop length per circuit is typically 80–120 meters (260–390 feet); longer loops have unacceptable pressure drop and uneven heat output along the run.
A pressure test is required after install before any screed is poured — typically 6 bar (90 psi) for one hour. Catching a pinhole or fitting failure before pouring concrete over the pipe is non-negotiable.
Why it matters to a homeowner
UFH delivers the most-comfortable hydronic heat available — even temperature distribution, no cold spots, no visible emitters, no cold floors. Homeowners who experience properly designed UFH in a custom WA home rarely want to go back to radiators.
Three things to plan for at design time.
Floor covering matters. UFH heat output depends on how easily heat passes through the floor finish. Carpet, especially with thick padding, blocks too much heat. UFH covering should be under about 2.5 TOG (a UK insulation rating; roughly under R-2 in US terms). Tile and engineered hardwood are excellent. Thick carpet defeats the system.
Response time is slow. In-screed UFH heats up over hours, not minutes. Smart thermostats with optimum-start logic help schedule warmup. The system rewards a steady setpoint over aggressive setbacks.
Retrofit cost. Whole-house UFH retrofit on an existing home (hardwood floors up, insulation down, pipe in joist bays, floors back) runs $8–$15 per square foot installed in WA. Worth it on a deep renovation; rarely justified as a standalone retrofit. New construction is much cheaper — UFH on a new slab can be cost-competitive with conventional emitters once you factor in the heat-pump synergy.
For WA bathroom retrofits specifically, the more common path is an electric mat under tile rather than full hydronic. A 30 sq ft master-bath mat with a dedicated thermostat runs $400–$1,200 installed and delivers warm tile underfoot without tying into the heating loop. Different product, same goal — see Common variants below.
When you’ll encounter this term
- A custom new-construction quote in WA.
- A whole-floor or major-room renovation with the floor being rebuilt.
- A bath remodel deciding between electric mat and full hydronic.
- A heat-pump replacement on a hydronic home where the existing emitters can’t handle low supply temps.
Common variants and disambiguation
- Hydronic UFH vs. electric UFH mat. Hydronic = water-loop, full-room heat. Electric mat = under-tile resistance, comfort-warming only. Different products, different scope.
- Solid-floor (in-screed) vs. suspended-timber (joist-bay). In-screed has higher output and thermal mass; joist-bay is faster to install on retrofits but lower output.
- UFH vs. panel radiator vs. baseboard. UFH is invisible and even, with low supply temps and slow response. Radiators and baseboard are visible, faster-responding, designed for higher supply temps.
Common failure modes
- Pipe puncture during install. A nail through PEX during a follow-up trade — flooring, tile, baseboard installer — destroys a buried loop. Catastrophic when in screed; requires slab cut. Mitigation: clear marking of pipe routes and a pre-pour pressure test plus during-construction protection.
- Manifold actuator failure. Zone won’t heat. Replace the electrothermal head; cheap part.
- Air-lock in a loop. Zone runs cold. Bleed at the manifold.
- Cold floor over a slab leak. Pinhole in pipe inside slab. Thermal imaging locates; repair is invasive — slab cut, fitting splice, screed patch.
Washington note
UFH is increasingly common in WA custom builds and high-end remodels, especially in master baths and kitchens. The pairing with heat pumps is what’s driving adoption — air-to-water and ground-source heat pumps deliver supply temperatures (95–125°F) that work natively with UFH. A WA new-construction custom home with heat pump + whole-floor UFH delivers heating costs in the $300–$700/year range with excellent comfort.
WAC 51-52 (mechanical) governs UFH install. WSEC residential R403 favors low-temp emitters like UFH because they enable mod-con boilers and heat pumps to run at high efficiency. Permit and inspection requirements typically include the pre-pour pressure test as a hold point — the inspector verifies the test before the screed is poured.
For WA bath retrofits where full hydronic isn’t practical, electric UFH mats under tile are the common alternative. A 240V mat with a dedicated thermostat and GFCI breaker runs $400–$1,200 installed. Most jurisdictions require an electrical permit for the dedicated circuit.
FAQ
Can I add underfloor heating to an existing house in Washington?
Yes, but the cost depends on the floor type. Suspended-timber retrofit (PEX in joist bays from below, accessed from a basement or crawlspace) is the cheapest path — roughly $8–$15/sq ft. In-screed retrofit (rip up flooring, pour new screed) is much more expensive and only makes sense as part of a deep renovation. Bathroom-only electric mats are an inexpensive comfort upgrade ($400–$1,200) without the hydronic complexity.
Does underfloor heating work with a heat pump?
Yes — extremely well. UFH is the canonical low-temperature emitter, designed for 95–125°F supply, which is exactly the range an air-to-water or ground-source heat pump delivers. The pairing is one of the most efficient residential heating combinations available, and PSE and SCL rebates apply to the heat pump side.
What’s the difference between hydronic and electric underfloor heating?
Hydronic UFH circulates warm water through PEX, runs off a boiler or heat pump, and is appropriate for full-room or whole-house heat. Electric UFH (typically a thin mat under tile) uses 120V or 240V resistance elements, runs off a wall thermostat, and is sized for comfort-warming a bathroom or kitchen — not as a primary heat source. Different products for different jobs.