ᐅ 6.5 cm Floor Construction with 3 cm Anhydrite Screed F5 – Which System Is Most Suitable?
Created on: 18 Nov 2025 13:49
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Olli1989Hello everyone,
Since I’m new here and this is my first post, I’ll briefly introduce myself and my project.
I’m Olli, 36 years old, and I bought a house from the 1970s. It has a basement of 120 sqm (partly heated—4 rooms fully heated, 3 not), a ground floor of 120 sqm, and an unfinished attic of 120 sqm (with sloped ceilings, etc.). The roof is 14 years old and insulated. The windows are triple-glazed and 10 years old. The walls are made of 24 cm (9.5 inches) Durisol blocks with no insulation, then brick cladding.
Now I want to install underfloor heating (only on the ground floor). The basement and attic will have radiators later.
My first idea was to use the dry construction system from Zweotherm, but it turned out to be too expensive. The floor is relatively stable but still has some issues across the entire surface (100 mm (4 inches) crack, uneven height from 106.5 to 109 cm (42 to 43 inches)). If I level the floor completely, I won’t have enough height clearance.
My second idea was the Schlüter BEKOTEC-EN system. The screed installer came and said, “We must apply a 3 cm (1.2 inch) calcium sulfate screed with F5 strength” (Schlüter recommends 0.8 to 2.5 cm (0.3 to 1 inch)). That makes it so tight in height that he says I must install it without insulation, which I want to avoid. Why he won’t go with the 0.8 to 2.5 cm (0.3 to 1 inch) layer, I don’t know—because then it would be possible. The flooring will be click vinyl, hybrid flooring, or Kiwi flooring, all about 0.5 cm (0.2 inch) thick.
The third idea was the Zweotherm XPS 500 option, but the floor height is still a problem.
The fourth idea was the Zweotherm noppen (stud) system, which fits perfectly according to the specs, including at least 1 cm (0.4 inch) insulation. It has a good pipe spacing for 16 mm (0.6 inch) tubing, everything looks great, but Zweotherm requires a 4.5 cm (1.8 inch) screed on top, so this option is off the table.
The last idea before asking for your suggestions is that I install the insulation myself and then put BEKOTEC-EN-F over it.
The Zweotherm noppen system for commercial buildings is the only one that ticks almost all boxes: 16 mm (0.6 inch) pipe, good pipe spacing, insulation underneath—but it demands a quite thick screed.
I hope someone here has an idea that will finally get me to the finish line.
Thanks in advance for your help.
Best regards,
Olli
Since I’m new here and this is my first post, I’ll briefly introduce myself and my project.
I’m Olli, 36 years old, and I bought a house from the 1970s. It has a basement of 120 sqm (partly heated—4 rooms fully heated, 3 not), a ground floor of 120 sqm, and an unfinished attic of 120 sqm (with sloped ceilings, etc.). The roof is 14 years old and insulated. The windows are triple-glazed and 10 years old. The walls are made of 24 cm (9.5 inches) Durisol blocks with no insulation, then brick cladding.
Now I want to install underfloor heating (only on the ground floor). The basement and attic will have radiators later.
My first idea was to use the dry construction system from Zweotherm, but it turned out to be too expensive. The floor is relatively stable but still has some issues across the entire surface (100 mm (4 inches) crack, uneven height from 106.5 to 109 cm (42 to 43 inches)). If I level the floor completely, I won’t have enough height clearance.
My second idea was the Schlüter BEKOTEC-EN system. The screed installer came and said, “We must apply a 3 cm (1.2 inch) calcium sulfate screed with F5 strength” (Schlüter recommends 0.8 to 2.5 cm (0.3 to 1 inch)). That makes it so tight in height that he says I must install it without insulation, which I want to avoid. Why he won’t go with the 0.8 to 2.5 cm (0.3 to 1 inch) layer, I don’t know—because then it would be possible. The flooring will be click vinyl, hybrid flooring, or Kiwi flooring, all about 0.5 cm (0.2 inch) thick.
The third idea was the Zweotherm XPS 500 option, but the floor height is still a problem.
The fourth idea was the Zweotherm noppen (stud) system, which fits perfectly according to the specs, including at least 1 cm (0.4 inch) insulation. It has a good pipe spacing for 16 mm (0.6 inch) tubing, everything looks great, but Zweotherm requires a 4.5 cm (1.8 inch) screed on top, so this option is off the table.
The last idea before asking for your suggestions is that I install the insulation myself and then put BEKOTEC-EN-F over it.
The Zweotherm noppen system for commercial buildings is the only one that ticks almost all boxes: 16 mm (0.6 inch) pipe, good pipe spacing, insulation underneath—but it demands a quite thick screed.
I hope someone here has an idea that will finally get me to the finish line.
Thanks in advance for your help.
Best regards,
Olli
From practical experience: 30mm (1.2 inches) NORIT-TE 30 Therm GF dry screed panels with milled installation grooves for 12mm (1/2 inch) Norit pipes + 3mm (1/8 inch) cement-based Norit grout + adhesive/tiles or glued 3-layer floorboards 15mm (5/8 inch) = total build-up height of 48mm (1 7/8 inches). This leaves 17mm (11/16 inch) thickness available for, for example, 12mm (1/2 inch) Isover Akustic EP3 impact sound insulation + 5mm (3/16 inch) leveling compound or for 17mm (11/16 inch) bonded screed/cement-bound lightweight screed such as Thermozell 250, but then without impact sound insulation.
Prior to installation, the structural load-bearing capacity of the ceiling must be checked to ensure it can handle the additional loads. Ceilings from the 1970s may have been built quite thin, around 12-14cm (5-5 1/2 inches). If a full energy retrofit is not being carried out, a room-by-room heating load calculation according to DIN 12831 must be performed, and based on this, the underfloor heating and radiators must be properly sized.
Prior to installation, the structural load-bearing capacity of the ceiling must be checked to ensure it can handle the additional loads. Ceilings from the 1970s may have been built quite thin, around 12-14cm (5-5 1/2 inches). If a full energy retrofit is not being carried out, a room-by-room heating load calculation according to DIN 12831 must be performed, and based on this, the underfloor heating and radiators must be properly sized.
Hi Olli,
interesting project, and it’s clear you’ve already put a lot of effort into researching the details. A classic challenge with many renovations from the 1970s: limited build-up height combined with varying subfloor conditions, and in the end, you want to get it just right – technically sound, but without having to cut door frames.
Your screed installer seems quite fixed on using 3 cm (1¼ inches) of anhydrite, even though BEKOTEC explicitly allows thinner layers within the system structure. The specification of 0.8 – 2.5 cm (⅓ – 1 inch) refers to the screed thickness above the stud edge, not the total height, and this often causes misunderstandings. Have you asked him if he has actually read the system approval and BEKOTEC processing guidelines, or is he just going by feel? Because there are manufacturer approvals that cover exactly what you need – but the screed installer is responsible, so many prefer to play it safe.
The Zewotherm-XPS system is technically solid, but given the unevenness of your subfloor, you risk voids if you don’t level extremely carefully. That’s probably where the issue lies: before choosing any system, you should know exactly how large the unevenness is. Are there localized 5 mm (⅕ inch) spots, or waves of 2 cm (¾ inch) across the surface? If you record this precisely (laser level, straight edge, measuring points), you can make much better decisions for the next steps.
The idea to insulate yourself and then lay BEKOTEC-EN-F on top is realistic, as long as you control the flatness. Just don’t use insulation that’s too soft – otherwise, the screed will push it down when heated, causing micro-cracks. For 1 cm (⅜ inch) of insulation, I would rather use a more compressive material than EPS, otherwise the load distribution won’t be even.
Slightly ironically: sometimes it might be easier to reroof than to argue about every millimeter of the floor build-up, but that’s how old building renovations go. Maybe you could post the entire planned layer structure including screed and finish flooring as a cross-section diagram – that would allow for a more targeted evaluation.
interesting project, and it’s clear you’ve already put a lot of effort into researching the details. A classic challenge with many renovations from the 1970s: limited build-up height combined with varying subfloor conditions, and in the end, you want to get it just right – technically sound, but without having to cut door frames.
Your screed installer seems quite fixed on using 3 cm (1¼ inches) of anhydrite, even though BEKOTEC explicitly allows thinner layers within the system structure. The specification of 0.8 – 2.5 cm (⅓ – 1 inch) refers to the screed thickness above the stud edge, not the total height, and this often causes misunderstandings. Have you asked him if he has actually read the system approval and BEKOTEC processing guidelines, or is he just going by feel? Because there are manufacturer approvals that cover exactly what you need – but the screed installer is responsible, so many prefer to play it safe.
The Zewotherm-XPS system is technically solid, but given the unevenness of your subfloor, you risk voids if you don’t level extremely carefully. That’s probably where the issue lies: before choosing any system, you should know exactly how large the unevenness is. Are there localized 5 mm (⅕ inch) spots, or waves of 2 cm (¾ inch) across the surface? If you record this precisely (laser level, straight edge, measuring points), you can make much better decisions for the next steps.
The idea to insulate yourself and then lay BEKOTEC-EN-F on top is realistic, as long as you control the flatness. Just don’t use insulation that’s too soft – otherwise, the screed will push it down when heated, causing micro-cracks. For 1 cm (⅜ inch) of insulation, I would rather use a more compressive material than EPS, otherwise the load distribution won’t be even.
Slightly ironically: sometimes it might be easier to reroof than to argue about every millimeter of the floor build-up, but that’s how old building renovations go. Maybe you could post the entire planned layer structure including screed and finish flooring as a cross-section diagram – that would allow for a more targeted evaluation.
The topic just doesn’t seem to end.
I just spoke with Schlüter. They said the BEKOTEC-EN system has to be fully supported by the subfloor. With this floor and plan, that would sometimes work and sometimes not. They told me it would be better to level the floor so that there is still 6.5 cm (2.6 inches) left above the lowest point across the entire house. After that, the underfloor heating can be installed. In the end, there would be only about 1 mm (0.04 inches) of tolerance left. That is so little that I’m not willing to take the risk with the BEKOTEC-EN system.
Then I remembered XPS 500. If the floor is flat, even, and at the specified height, I could install 2 cm (0.8 inches) of insulation plus 2.5 cm (1 inch) of XPS 500. That would give me a total height of 4.5 cm (1.8 inches), leaving 2 cm (0.8 inches) for the rest. However, I see a problem with the heat transfer coefficient (VA) of 15 in the undamped house.
Switching back to radiators now would feel like going under the ground—I just can’t come to terms with the idea that everything I’ve done so far would be for nothing 🙂 🙂
I just spoke with Schlüter. They said the BEKOTEC-EN system has to be fully supported by the subfloor. With this floor and plan, that would sometimes work and sometimes not. They told me it would be better to level the floor so that there is still 6.5 cm (2.6 inches) left above the lowest point across the entire house. After that, the underfloor heating can be installed. In the end, there would be only about 1 mm (0.04 inches) of tolerance left. That is so little that I’m not willing to take the risk with the BEKOTEC-EN system.
Then I remembered XPS 500. If the floor is flat, even, and at the specified height, I could install 2 cm (0.8 inches) of insulation plus 2.5 cm (1 inch) of XPS 500. That would give me a total height of 4.5 cm (1.8 inches), leaving 2 cm (0.8 inches) for the rest. However, I see a problem with the heat transfer coefficient (VA) of 15 in the undamped house.
Switching back to radiators now would feel like going under the ground—I just can’t come to terms with the idea that everything I’ve done so far would be for nothing 🙂 🙂
N
nordanney24 Nov 2025 16:17Maybe XPS 500 and mill it yourself? However, the bending radius of the 16mm (5/8 inch) pipes would then be a problem. A small, consistent radius won’t work. I would rather switch to the 14mm (9/16 inch) pipes and 12.5cm (5 inches).
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