How to Correctly Lay Pipes in a System Panel Without Thermal Bridges
How to Correctly Lay Pipes into a System Board Without Thermal Bridges
Laying underfloor heating pipes looks simple at first glance – you take a pipe, insert it into the studs of the system board, and you're done. In practice, however, things are different. Over the past years, we have seen dozens of projects where, after handover, homeowners started dealing with uneven heating, cold spots on the floor, higher energy bills, or cracks in the flooring layer. In most cases, closer inspection revealed the same root cause: imprecise, inconsistent, or technically incorrect pipe installation into the system board, along with the associated thermal bridges.
This article is a detailed technical guide that will take you through the entire process – from selecting the right board, through substrate preparation, the actual pipe installation, work around walls and penetrations, up to final checks before the screed is poured. Here you will find specific dimensions, values, real-life scenarios, and answers to questions that keep coming back to us.
Why Correct Pipe Installation Is Key to the Whole System
Underfloor heating is a low-temperature radiant system. It works on the principle that a large floor area radiates heat upward into the room at a relatively low coolant temperature – typically 35 to 45 °C in the boiler circuit, while the floor surface temperature should not permanently exceed 29 °C in the living zone. The whole efficiency of this system depends on one condition: heat must go upward, not sideways and not downward into the structure.
If the pipe is not properly installed in the system board, spots are created where heat spreads in the wrong direction. These spots are called thermal bridges. In practice, we can divide them into:
- Structural thermal bridges – occur where the insulation layer is interrupted (e.g. at penetrations, at wall connections, at partitions)
- Contact thermal bridges – occur when the pipe is installed incorrectly, when the pipe lies directly on the concrete substrate instead of on the insulation
- Point thermal bridges – occur with insufficient or faulty connection of boards, gaps between boards, or incorrectly cut sections
Each of these types has a direct impact on heat loss toward the base structure. According to the STN EN ISO 10211 standard, even a relatively small thermal bridge with an area of 0.1 m² can increase the heat loss of a given detail by tens of percent. In practice, this means that a system with thermal bridges must run longer, at a higher temperature, or with a higher boiler output – which directly translates into the building's energy consumption and higher operating costs.
Choosing a System Board Before Pipe Installation
Correct pipe installation does not begin with the pipe itself, but with choosing the right system board. Different types of boards have different stud geometry, different heights, different materials, and different intended uses. Mismatched pipe and board is a source of problems right from the start.
Our range includes several proven solutions. The UHP55 system insulation board (STIROTERMAL BASIC) is designed for standard wet systems with anhydrite or cement screed. Its studs are designed for pipes with an outer diameter of 16 mm and 17 mm – which covers the vast majority of commonly used PE-RT and PEX pipes. The stud spacing allows pipe laying on a 50 mm grid, which is the basis for correct loop routing.
For combining two layers of insulation in one step, the UHP51 system insulation board (STIROTERMAL DUO 11) is suitable, as it has an integrated bottom layer and solves some insulation details more efficiently. For a dry system without a wet screed – for example under laminate, wood, or large-format tiles without a screed layer – it is worth considering the UHPD polystyrene for dry underfloor heating (STIROTERMAL DRY), where the pipe is laid in grooves and covered with aluminum distribution plates.
A more detailed comparison of individual board types can be found in the article Comparison of System Boards: STIROTERMAL BASIC vs DUO vs DRY vs SOLO or in the article How to Choose the Right System Insulation Board for Underfloor Heating.
Substrate Preparation Before Installing Boards and Pipes
A thermal bridge in pipe installation does not necessarily arise from a mistake in laying the pipe itself. Very often it originates in the substrate onto which the boards are laid. The substrate must meet several conditions, without which good work with the boards is compromised from the start.
Substrate Levelness and Cleanliness
The maximum unevenness of the substrate under the system boards should be up to 5 mm per 2 m straightedge. If the unevenness is greater, the boards follow the unevenness of the substrate and air gaps form between them, or the boards buckle and do not sit tightly against each other. This directly leads to thermal bridges in places where the boards do not touch.
Before laying, the substrate must be thoroughly swept and all debris, coarse grains, nails, and protrusions removed. Dust and fine impurities can be left, but larger particles can cause a local lifting of the board and its subsequent cracking under the pressure of wet mortar or anhydrite.
Separation Foil Under the System Boards
A separation foil is laid on the concrete substrate before installing the polystyrene boards. Its function is twofold: it prevents moisture from wicking from the concrete slab into the polystyrene and also prevents direct contact of the polystyrene with the concrete, thus preventing chemical degradation of the material. The Separation foil 601001H is a suitable solution for this layer – it is a polyethylene foil with sufficient thickness, laid with an overlap of at least 100 mm at joints and at least 100 mm at walls.
The foil must be taped at the edges to prevent it from lifting during board installation. If the foil is left loose and wavy, the boards "float" on it, making it harder to lay them precisely and tightly against each other.
Edge Expansion Tape
Before starting to lay the boards, edge expansion tape must be applied around the entire perimeter of the room. This tape serves to expand the screed away from the walls and also prevents heat transfer through the wall-floor corner, which is one of the most common places where thermal bridges occur. The tape must extend above the top edge of the finished floor, and after the flooring layer is laid, its protruding part is trimmed off.
Laying the System Board Without Thermal Bridges
Laying the boards is the first phase where thermal bridges arise, which are difficult to discuss during a later complaint – they are covered by several layers of material. Therefore, it is important to pay appropriate attention to this phase.
Laying Direction and Board Bonding
Boards are laid with staggered joints – this means that vertical seams in adjacent rows must not be in a single line. The minimum stagger is half the length of the board, though in practice a third is sufficient. Without staggering, when pouring concrete or anhydrite, there is a risk of seepage into the seams, and above all, the rigidity of the entire layer decreases – the boards can shift, leaving thermal bridges between them.
When laying, each board must be pressed tightly against the neighboring one. The side tongue and groove (if the board has them) must fit together without forcing – if you need to hit the board, it means something is wrong and the substrate should be checked. Excessive force will break the tongue, and the connection will become only nominal without real tightness.
Cutting Boards at Walls and Corners
Boards at walls must be adapted to the actual dimensions of the room. Many problems arise here when workers cut a board with a coarse knife or chisel and leave large gaps at the wall. The maximum permissible gap between the board and the wall (or edge tape) is 5 mm. Larger gaps must be filled with polystyrene offcuts.
A sharp knife, a polystyrene saw, or a hot cutting wire is used to cut polystyrene. A hot wire gives the cleanest cut without crumbling the material. At corners and cutouts around partitions, posts, or installation sleeves, it is recommended to cut the shape according to a cardboard template. A thermal bridge at a post, where the board is cut incorrectly, can have a diameter of up to 300–400 mm and causes a visible cold spot on the floor in winter.
Installation Penetrations Through the Insulation
Every penetration – a sewer pipe, a drain, an installation conduit, a support post – interrupts the insulation and can become a thermal bridge. Correct procedure: the opening in the board is cut to follow the shape of the penetrating element with minimal clearance (up to 10 mm). The remaining gap is filled with low-expansion PUR foam (not standard installation foam, which could lift the board). After the foam has hardened, the excess is trimmed flush with the surface of the board.
Sewer drains passing through the board are a special case. If the drain is located where the underfloor heating pipe should run, the loop must be rerouted around the drain. The loop must not pass closer than 50 mm to the drain, otherwise there is a risk of local overheating of the PVC sewer pipe.
Laying Pipes into the Studs of the System Board – Detailed Procedure
This is the heart of the entire technology. A correctly installed pipe holds firmly in the system board, raised above the concrete substrate, in a geometrically precise grid, and without tension that could cause it to move when the screed is poured.
Spacing and Laying Patterns
System boards with studs allow pipe spacing in modules defined by the board – most often 50 mm. This means that the actual pipe spacing is a multiple of 50 mm: 100 mm, 150 mm, 200 mm, 250 mm, 300 mm. When designing the system, the rule applies: the smaller the spacing, the more even the floor surface temperature, but the more pipe, fittings, and work required.
In the living zone (where you walk barefoot, where children sit), a spacing of 100–150 mm is used. In the zone near windows and outer walls (perimeter), 100 mm or even 75 mm is used to cover greater heat loss. In utility rooms (WC, hallway, storage), 200–250 mm is sufficient. These spacings are the result of hydraulic calculation, not improvisation.
There are two standard loop routing patterns:
- Meander (serpentine) – the pipe runs parallel back and forth. Simple to calculate, but causes a temperature gradient – the supply side of the loop is warmer than the return side.
- Spiral (counter-flow) pattern – the pipe spirals from the outer side of the room to the center and back in another row. It achieves a more even temperature distribution because the supply and return branches alternate next to each other.
Technical Procedure for Inserting the Pipe into the Studs
The pipe is unwound from the coil smoothly, without sharp kinks. The minimum bending radius of a PE-RT 17x2 mm pipe is 5 times the outer diameter, i.e. at least 85 mm. In practice, this means that the loop turn (the semicircle at the end of the meander) must have a radius of at least 85–100 mm. If the radius is smaller, the pipe either will not bend at all, or bends with a local constriction (kink), which increases hydraulic resistance and can lead to cracking.
When inserting into the studs, push the pipe into the studs in sections of 30–50 cm. Do not bend the entire loop in advance and then insert it all at once – this creates internal tension that pulls the pipe out of the studs after release. Correct procedure: fix the beginning of the loop at the supply manifold, stretch the pipe along the first row of studs, press it into each stud with your thumb or a special plastic pin, and only then move on to the bend and the next row.
Every stud must actually hold the pipe – meaning the pipe must "snap" into the space between the studs and can only be pulled out with a considerable force (approx. 15–25 N). If the pipe falls out on its own, the studs are damaged or the pipe is the wrong diameter. This must be addressed immediately, not after the concrete has been poured.
Fixing the Pipe at Atypical Locations
At passages between rooms, at door frames, and at atypical corners, the pipe cannot simply be inserted into the studs. Here, plastic clips are used, which are nailed or screwed into the insulation board and hold the pipe in the correct position. Clip spacing on a straight section: every 500–600 mm. On curves and bends: every 200–300 mm, depending on the bend radius.
Clips must not compress or deform the pipe. When properly tightened, the outer diameter of the pipe at the clip point must not be smaller than the nominal outer diameter. An overly tightened clip causes a local constriction and increased hydraulic resistance of the loop – which manifests as a "cold zone" in that part of the floor.
Thermal Bridges in Pipe Routing – Specific Risk Areas
After years of inspecting underfloor heating systems, several standard "problem areas" can be identified where thermal bridges most often form. Increased attention should be paid to them on every project.
Loop Inlet and Outlet Point at the Manifold
At this point, the pipe passes from the thermal insulation into the perimeter zone at the wall and further into the manifold cabinet. Here, the pipe is not routed through the studs but runs freely. If this entire section is not covered by a conduit (usually a corrugated plastic tube) and if the space around it is not insulated, a direct thermal bridge is created: a pipe at 35–45 °C passes through an uninsulated section directly through the concrete wall. This spot should always be insulated with PUR foam or EPS offcuts.
Screed Expansion Joints
Unprotected pipe must not be located in screed expansion joints. If a pipe must cross an expansion joint, it must be protected by a conduit for a section of at least 300 mm on each side of the joint, to compensate for concrete movement without mechanically stressing the pipe. At the same time, insulation must not be missing under the pipe at the location of the expansion joint.
Pipe Routing Through Partitions and Walls
At passages through interior partition walls (where there is no underfloor heating, e.g. under the partition), the insulation must be continuous – even under the partition, if structurally possible. If the builder does not want insulation under the partition, it must at least be ensured that the pipe does not run under the partition without a conduit and without interrupting the thermal protection.
System Board Connection at the Entrance Threshold
The door threshold is a critical spot. Here, the floor layers in different rooms may differ, the flooring layer may change, and the insulation may be interrupted. The system board must extend under the threshold. If the adjacent room has a different floor height or no underfloor heating, the transition must be handled with an angled polystyrene offcut so that there is no jump in the insulation layer.
Dry Systems and Aluminum Foils – Special Installation Requirements
Dry underfloor heating systems, where there is no wet screed, work on a different heat distribution principle. Here, the pipe lies in grooves in the polystyrene, and heat is transferred using aluminum distribution plates (foils), which spread the heat from the pipe over a larger area.
In dry systems, the Aluminum foil for dry underfloor heating is important – this plate must be in full contact with both the pipe and the floor board above it. Any air gap between the aluminum and the pipe, or between the aluminum and the flooring layer, dramatically reduces heat transfer and causes uneven heating.
A typical mistake in dry systems: the aluminum plates do not appear to be important, the worker omits them or does not have them, and the pipe remains covered only by a wooden board without a heat distributor. As a result, areas above the pipe are significantly warmer than areas between the pipes – the floor heats up in stripes, which is noticeable even through shoes.
In dry systems, the grooves in the polystyrene must not be interrupted; crooked or insufficiently deep cuts in the board result in the pipe not fitting into the groove or not lying flat in it. Polystyrene for dry systems (STIROTERMAL DRY) has factory pre-milled grooves, which significantly simplifies installation and reduces the risk of errors compared to manual milling on site.
Inspection Before and After Screed Pouring
An inspection of the entire system is mandatory before pouring or casting anhydrite. Skipping this inspection is one of the most expensive savings a builder can make. If a defect appears after pouring, it is usually irreparable without demolishing the layers.
Pressure Test of the Piping
Before pouring the screed, a pressure test of the entire distribution system is carried out. The test pressure is 1.5 times the maximum operating pressure, but at least 6 bar. The test lasts at least 2 hours, during which the pressure must not drop by more than 0.2 bar. If the pressure drops, the leak location must be found – every connection, every clip, every bend must be checked. A leaking pipe under anhydrite is a direct path to an emergency situation after years of operation.
Visual Inspection of Pipe Installation
In addition to the pressure test, a visual inspection of the entire area is necessary. Check:
- Whether each pipe lies in the system studs or clips – no loose sections
- Whether minimum distances from walls are maintained (min. 100 mm from an outer wall, min. 150 mm from a wall adjacent to an unheated space)
- Whether no pipe lies directly on concrete without insulation underneath it
- Whether all penetration and manifold locations are properly covered
- Whether the edge expansion tape is undamaged around the entire perimeter
During Screed Pouring
When pouring liquid anhydrite screed, the system must be pressurized – filled with water at a pressure of 3–4 bar. This prevents the pipe from being compressed by the weight of the anhydrite and keeps it in the correct position. After the screed has hardened (at least 48 hours), the pressure can be reduced, but the system must not be drained or heated up earlier than specified by the manufacturer of the anhydrite or cement screed – usually at least 21 days for anhydrite.
Dependence of Installation Quality on Pipe Type
Not all pipes behave the same way when installed into the studs of a system board. Key parameters that affect adhesion to the studs and pipe stiffness when bending:
- PE-RT (Polyethylene of Raised Temperature resistance) – flexible, easy to lay into studs, holds shape well in bends. Standard for wet systems.
- PEX-a (Engel cross-linked polyethylene) – stiffer, but "remembers" its shape after heating. When laid in cold conditions in winter, it can be too stiff and fall out of the studs. Solution: warm the pipe coil to 15–20 °C before laying.
- PEX-b / PEX-c – stiffer than PE-RT, less flexible when bending, the bend radius is larger. Can be problematic in tight spaces.
- Multilayer (Al-PEX) – has an inserted aluminum layer, is shaped once and holds its shape. Excellent for dry systems and straight runs, less suitable for frequent bends.
For more details on choosing pipes for individual systems, see the article Dry vs Wet Underfloor Heating: Which Polystyrene and Foil Are Suitable.
Frequently Asked Questions (FAQ)
Can I lay pipes directly on concrete without a system board?
Technically, you can get the pipe into the concrete, but such a system will have very low efficiency and high heat losses downward. Most of the heat will spread into the concrete slab and further into the ground or the space below, not upward into the room. At the same time, when renovating old buildings, there is a risk of the screed cracking without sufficient elastic insulation underneath it. A proper system board under the pipe is a technically essential condition.
What should I do if the pipe bulges out of the studs during installation and won't stay in them?
First: check whether the pipe has the correct outer diameter for the given board. Board UHP55 is primarily designed for a 17 mm diameter – a 16 mm pipe may be looser, an 18 mm pipe will not fit at all. If the diameter is correct and the pipe still falls out, the reason is either damaged studs (pipe too stiff in the cold), or too much tension in the pipe when unwinding. Solution: leave the pipe in the room at a temperature of at least 15 °C for at least an hour before laying, lay it in shorter sections, and confirm attachment with your thumb at each stud.
What is the minimum cover of pipe by anhydrite or concrete screed?
The minimum cover above the top edge of the pipe is 25 mm for anhydrite and 45 mm for cement screed. The total screed thickness above the system board is therefore, for a 17 mm pipe (outer diameter): 17 + 25 = 42 mm for anhydrite, 17 + 45 = 62 mm for cement screed. These values are minimums – with greater thickness, the system's response time increases (the screed accumulates more heat), but surface evenness improves.
How can I tell if I have a thermal bridge in the floor once the floor is already finished?
The simplest method without special equipment: heat the system to operating temperature and wait at least 2 hours. Then walk across the floor barefoot and observe temperature differences. Cold stripes or areas indicate either missing pipe or a spot with a downward thermal bridge. More precise diagnostics are done with a thermal camera – it shows the temperature distribution on the floor surface and reveals even smaller problems.
Do I need to use a separation foil both between the board and the screed, and under the insulation layer, or just under the insulation layer?
The separation foil primarily belongs under the insulation board – between the concrete substrate and the EPS. A different type of foil is used between the insulation board and the anhydrite screed – a PE foil, which prevents anhydrite from flowing under the boards and into the joints. This layer is thinner (0.1–0.2 mm) and is laid on the top surface of the boards right before pouring. More about the functions of the individual foils and their correct use can be found in the article Separation and Aluminum Foil in Underfloor Heating: When and How to Use Them.
Can I lay the system boards and pipes myself as a DIY project?
Laying boards and pipes is technically manageable even for an experienced DIYer, but under several conditions. You must have project documentation with designed loops, spacings, and outputs – improvisation has no place here. The pressure test before pouring the screed should be documented. It is advisable to entrust the actual screed pouring to a professional company – the evenness of pouring and adherence to thickness are crucial. We also describe how to avoid mistakes when laying boards and pipes in the article Common Mistakes When Laying Polystyrene and System Boards for Underfloor Heating.
Conclusion: Precision Pays Off
Laying underfloor heating pipes into a system board is work that deserves the same respect as electrical wiring or sanitary installations. Once the pipe is cast in, it cannot be repaired without demolition – and this applies equally to thermal bridges and to actual leaks. Every minute spent on proper substrate preparation, precise board installation, thorough pressing of the pipe into the studs, and visual inspection before pouring will pay off for the investor in the form of lower energy consumption, even heating comfort, and trouble-free system life for decades.
If you are unsure when choosing a board, we recommend looking at the comparison in the article How to Choose the Right System Insulation Board for Underfloor Heating or What Polystyrene Thickness Is Needed Under Underfloor Heating. For a step-by-step procedure for laying the boards themselves, see the article Installing a System Insulation Board for Underfloor Heating Step by Step.
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