Wet versus dry systems
Hydronic underfloor heating in Polish construction falls into two broad categories. Wet systems embed plastic pipes — typically cross-linked polyethylene (PEX) or PE-RT — in a screed layer poured over an insulation board. Dry systems clip the pipes into a pre-formed panel and cover them with a dry board (usually gypsum fibreboard), with no screed required.
Wet systems dominate in new construction because they are less expensive per square metre, tolerate heavier floor loads, and have lower thermal mass (which makes the floor slower to heat up but more stable once at temperature). Dry systems are more often used in renovation work where structural load limits or ceiling height constraints rule out a full screed build-up.
Step 1: Structural substrate preparation
The concrete slab or structural floor must be clean, level, and dry before anything is laid on it. Maximum permissible moisture content varies by screed type but is typically below 2.0% for cement-based screeds, checked with a carbide pressure meter (CM device). Starting on a damp slab leads to insulation compression, pipe movement during screed pour, and — in worst cases — screed cracking or delamination months later.
A perimeter expansion strip (Styrofoam or PE foam, 8–10 mm thick) is fixed around all walls and columns before insulation goes down. This accommodates thermal expansion of the screed and prevents the slab from cracking at wall junctions.
Step 2: Insulation board
The insulation layer serves two purposes: it reduces downward heat loss into the structural slab or the ground below, and it provides a surface to which the pipe clips or studs attach. In new construction on a ground floor slab, the insulation layer is typically 100–150 mm of expanded polystyrene (EPS), with a thermal conductivity of 0.031–0.038 W/mK.
Compressive strength matters. Standard EPS for underfloor use is rated EPS 100 (compressive stress at 10% deformation: 100 kPa), which is adequate for residential use without heavy point loads. Under kitchens with heavy stone worktops or load-bearing partitions, EPS 150 or EPS 200 is more appropriate.
Nop panels — insulation boards with moulded studs that retain the pipe without clips — simplify installation considerably and are now standard on most Polish residential sites.
PEX pipes fixed to nop insulation board before screed. Pipe spacing here is 150 mm. Photo: Wikimedia Commons / CC BY-SA
Step 3: Pipe layout and spacing
Pipe diameter is most commonly 16 mm or 17 mm for residential systems. Spacing is typically 100–200 mm, with tighter spacing (100–125 mm) near exterior walls and wider spacing (150–200 mm) in room centres. The relationship is direct: halving the pipe spacing roughly doubles heat output per square metre, up to the limit imposed by flow temperature and floor surface temperature regulations.
Polish and EU standards limit maximum floor surface temperature to 29°C for occupied rooms and 35°C in bathrooms. Exceeding these limits can cause discomfort and — in wooden floors — structural damage to the material. The design flow temperature of the circuit should be calculated to respect this constraint, typically placing it at 35–45°C flow and 28–32°C return for a standard residential system.
Maximum floor surface temperature: 29°C for occupied rooms, 35°C in bathrooms. This determines the upper limit of flow temperature and pipe spacing.
Circuit length
Each circuit (loop) should not exceed approximately 80–100 metres for a 16 mm pipe to keep pressure drop manageable without a high-power pump. Larger rooms are divided into multiple circuits, all connected at a manifold (distributor) with individual flow balancing valves.
Step 4: Screed
Cement screed is the most common choice in Poland. Standard formulations are based on sand and Portland cement at a ratio of approximately 1:4.5 by volume, laid at 65–70 mm minimum thickness above the pipe for residential loads. Flowing screed (anhydrite/calcium sulphate) is used increasingly in newer projects: it self-levels, is poured at 30–40 mm minimum cover, and has better thermal conductivity (0.8–1.2 W/mK versus 0.9–1.4 W/mK for cement).
Anhydrite screed cannot tolerate moisture during or after curing and must not be used in bathrooms or rooms with high humidity risk without a fully sealed DPM. It also reacts with cement-based tile adhesives, requiring a specific primer coat.
Step 5: Curing, testing, and commissioning
Before the heating system is pressurised, the screed must cure. Cement screed requires a minimum of 21 days at standard temperature before heating can begin. Anhydrite screed requires 7 days, after which the heating should be brought up gradually — starting at 20°C flow temperature and increasing by 5°C per day — to allow residual moisture to escape without cracking.
The hydraulic circuit should be pressure-tested (typically at 1.5× working pressure, held for 24 hours) before screed is poured. Identifying a leak after the floor is finished is significantly more expensive than catching one at this stage.
Thermostats and zoning
Each room or zone is typically controlled by a room thermostat connected to an actuator on the manifold. Modern systems use wireless thermostats connected to a central controller, which modulates flow temperature based on outdoor temperature (weather compensation) for maximum efficiency.
Weather compensation is particularly effective with heat pumps: as outdoor temperature rises, flow temperature is reduced automatically, allowing the pump to run at higher COP rather than modulating on and off.
Further reading
- How Heat Pumps Actually Work in Polish Homes
- Thermal Insulation Standards for Polish Residential Buildings
- REHAU technical guidance on underfloor systems