ᐅ Experiences with Retrofitting Underfloor Heating by Cutting into the Floor

Hello everyone,

Does anyone have experience with installing underfloor heating in an older building after construction? I just read an article (Abendblatt) where they described milling into the floor instead of building it up. It sounded smart – but how does it work in practice?
This would actually be quite interesting for me, as we are currently renovating an older building. The company Nordwaerme, mentioned in the article, is actually located only 50 km (31 miles) from us.

Maybe someone here already has some practical experience?
Best regards, Josephine

josephinee schrieb:

Does anyone here have experience with installing underfloor heating in an older building afterward?

Yes, but not milled, since I don’t find the cost-effectiveness convincing in many cases. Removing the old screed, installing traditional underfloor heating (after proper calculation and design—more flexible in pipe spacing), and then pouring new screed is often similarly priced but "better."

Therefore, each case must be considered individually.

Hello Josephine,

After an early construction phase failure caused by "personal error" in an industrial hall (with underfloor hydronic heating), where the heating elements were destroyed by frost in the concrete, I recommended the method of creating recesses for new heating elements by groove cutting.

The difference, however, between screed and concrete is that the concrete is installed not only in greater thickness but also with higher strength. Therefore, no disadvantages of the method are apparent here.

With screed, it is somewhat different.
With typical installation thicknesses around 45mm (1.8 inches), the groove cutting for new heating elements (usually up to max. 20mm (0.8 inches) deep) creates a kind of "weak point."
Or at least, it can be like that.
For older screeds, the question is what "quality" (referring back to the compaction and structural quality at the time) the screed was installed with.
It is possible that the screed (which, in residential construction, is installed on insulation layers) could fail structurally just from the mechanical stress of the cutting.
I do not want to cause you unnecessary concern!
This method is outside standard normative procedures because the screed thickness must lose load-bearing capacity from the groove cutting.
It may work fine but might not. And then a complete replacement of the cut screed becomes inevitable and unrecoverable!
This risk cannot be reliably assessed. It follows the principle of "either or."
And in such a case, the risk is borne solely by the (contracting) client.

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That was part 1 of the assessment.
Now we come to the further build-up (after cutting).
Since we have introduced weak points through the groove cuts, these need to be neutralized if possible.
That means:
The new heating elements must be embedded immediately in a high-strength and load-bearing mortar or an epoxy resin mortar.
Over this, to prevent the grooves from showing through the final surface covering later, load-distributing mesh fabric should be embedded into the (flat) leveling compound layer.

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What we recognize so far:
a) This method lies outside any standards.
b) All risks are borne by the client.
c) The overall process is associated with considerable costs.
d) The core question is whether applying an approximately 3mm (0.1 inches) thick electrical foil heating mat on the old screed, then installing a ceramic floor covering on top, might be the better option…
(I think: yes, it probably is.)

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Good luck and make a wise decision: KlaRa

KlaRa schrieb:

d) The key question is whether applying an approximately 3mm (0.1 inch) thick electric foil heating element on the old screed, followed by a ceramic floor covering, might be the better option....
(I think: yes, probably)

Please also mention that the heating costs directly correspond to the electricity consumption. For example, compared to a heat pump with a conventional underfloor heating system (the original poster is currently renovating their house and might be considering this option), the heating expenses can easily be 3.5 to 4 times higher (and naturally significantly higher than if the old gas heating system were still in use). Therefore, I believe this might be the gentlest solution for the old screed, but in reality, it is the worst (and most expensive) option for heating a single-family home.

josephinee schrieb:

Does anyone have experience with installing underfloor heating later in an older building?

What type of property are we actually talking about? Laypeople often say "older building" when they really mean an existing building, which might actually be a used new build. And where is the heating supposed to go: ground floor fully/partially/not on a basement, apartment upstairs, ...?

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I had underfloor heating installed in the upper floor of my new build. It covered about 55 m² (590 ft²) including 3 bedrooms, a bathroom, and a corridor, and it cost 3300 euros including VAT, plus leveling afterwards. I think it was definitely worth it. This way, I got rid of the bulky radiators that would have otherwise taken up space.

Overall, I prefer "regular houses" with radiators over underfloor heating, but in this new build by the developer, underfloor heating was the standard option.