ᐅ Floor construction height: 150 mm, including KfW55 standard insulation, ventilation ducts, and underfloor heating

Dear forum members,

Unfortunately, I couldn’t find any suitable information using the search function. If this was due to user error and there are already existing entries on this topic, I would appreciate a brief pointer.

Otherwise, I have the following question:

For our new build with KfW55 standard, the general contractor suggests a floor construction thickness of 190mm (7.5 inches). An independent architect confirmed that in the past he has planned and built floor structures with a total thickness of 150-160mm (6-6.3 inches). His calculation is as follows:

• Concrete slab
• 50mm (2 inches) insulation, with recessed space for ventilation ducts (50mm / 2 inches height), supply and return pipes, fresh water, and electrical wiring
• Fill layer with perlite granulate
• 20-30mm (0.8-1.2 inches) impact sound insulation
• 20mm (0.8 inches) for underfloor heating
• 45mm (1.8 inches) screed
• 15mm (0.6 inches) floor covering
• totaling 150-160mm (6-6.3 inches)

As we are limited by the maximum building height, we want to save every centimeter in the floor construction to gain more room height.

Wastewater pipes will not be routed under the floor in our plan; instead, they will be installed directly in the drywall partitions one story below or in the basement.

Can you verify or dispute the above floor thickness calculation based on your experience?

Thank you very much for a brief assessment from the knowledgeable members of this forum!

Have a nice Sunday,
Best regards,
Stephan

StephanM schrieb:

Can you follow the above calculation of the floor thickness and confirm or refute it based on your experience?

Technically, it can certainly be done as you described. However, whether it is sufficient for KFW 55 standard is another question. You can install inexpensive 040 EPS insulation or high-performance 022 PUR insulation. In the end, the energy consultant has to make the decision.

What kind of basement is beneath the described floor? Insulated or not insulated?

If it is not insulated, you would need to use high-quality insulation for 50mm (2 inches) thickness to be sufficient.

Tolentino schrieb:

What kind of basement is under the described floor? Insulated or not insulated?
If not insulated, you would need to use good insulation for 50mm (2 inches) thickness to be sufficient.

The basement under the ground floor is also insulated and heated.

Regarding the planned screed thickness of 45mm (1.8 inches), this will not meet the standards for a heated screed, as it is too thin.

Heating elements in a hydronic underfloor heating system typically have a diameter of 12mm (0.5 inches). Allowing for some tolerances, we round this up to 15mm (0.6 inches). This thickness (15mm) plus the standard screed thickness of 45mm (1.8 inches) required for cement screed Ct-C25-F4 in residential construction—which totals 60mm (2.4 inches)—must be maintained for a cement-based screed.

If only 45mm (1.8 inches) is planned in total, subtracting the heating element thickness of 12mm (0.5 inches) leaves just 33mm (1.3 inches) of screed above the heating elements.

This would be acceptable only with cast asphalt.

So please discuss this again with the general contractor or other responsible parties and refer to DIN 18560 Part 2. That should be sufficient as an argument.

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Best regards, KlaRa

KlaRa schrieb:

Regarding the planned screed thickness of 45mm (1.8 inches), this will not meet standards for heated screed, as it is too thin.

It states 20mm (0.8 inches) for underfloor heating + 45mm (1.8 inches) screed (covering?) = 65mm (2.6 inches) total. That would work.

P.S. A 12mm (0.5 inches) underfloor heating pipe is no longer up to date. Nowadays, 16mm (0.6 inches) or 17mm (0.7 inches) pipes are more common. These also fit much better with the 20mm (0.8 inches) build-up height of the underfloor heating.