ᐅ Installing Underfloor Heating Retrofitting: Experiences and Insights!

I am also familiar with electric underfloor heating, but my question is: can it actually heat a bathroom of, say, 6-7m² (65-75 sq ft) efficiently on its own? From what I remember, these systems have usually been promoted as supplementary heating – just for warm feet.

This is a matter of heating output, whether it concerns "underfloor heating" or general space heating.
Of course, the heating surface (including that of the bathroom) should have a sufficient size so that the heating output for the room does not only become effective at (here meant humorously) 50°C surface temperature. But the system does work.
There are not many options to consider given all the pros and cons, especially with the extent of the renovation measures involved. Therefore, the question of economic efficiency does not really apply here.
Example with purely hypothetical numbers:
If you have to pay €10 more per month for electric heating, you might prefer to save that for economic reasons and instead invest €8,000 in renovation, resulting in a system built according to standards – well, that was the question here, but with a different objective.
So: space heating with electric foil heating under tiles is certainly possible with an appropriate room size!

This is a very interesting topic (sorry for the off-topic). We will also be renovating a bathroom in the next three years. It is 6.5 m² (70 ft²) in size (3.2 x 2.1 m / 10.5 x 6.9 ft), but will be expanded to include a walk-in shower, resulting in a total area of 8 m² (86 ft²). Since some plumbing connections need to be relocated (shower and bathtub), the screed will likely be damaged. Installing underfloor heating could be done relatively easily because there is a free connection on the underfloor heating manifold located directly in the hallway beneath the bathroom. A towel warmer will definitely be installed later (there is currently a radiator), so an electric underfloor heating system might also be considered. The advantage would be a faster response time, but I think the increased heating demand for the underfloor heating due to the additional room will hardly make a difference financially. At the moment, I have no idea what the electric underfloor heating’s energy consumption would be.

The cost of installing the underfloor heating can probably only be estimated once the tiles are removed. It would, of course, be great to have the consumption figures on the table beforehand.

Sanitary fixtures are to be at least partially repositioned; the plan seems to be to handle this in the basement room below and more or less just drill through the floor. I need to ask at the next meeting what the plumbing company thinks about the whole thing.

Since I’m currently renovating my bathroom (actually the entire house), here are my two cents. During our bathroom renovation (built in 1959), it turned out that unlike the rest of the house, there was no floating screed installed; instead, the screed was poured directly onto the raw floor with a waterproof layer. So, everything had to be removed. After installing the connections, not much was left of the raw floor either. Even if much of the work in your case will be done from the basement, you will still have penetrations through the old screed at the old connections.

Given the typically manageable size of bathrooms, there are many reasons to simply replace the screed and build up the floor completely with proper insulation.

As a simple, cost-effective, and environmentally sensible alternative to milling out the screed: mill a NEW “screed.”

For this, from the raw floor upwards, lay full-surface cement-coated, fiber-reinforced XPS boards (building panels, waterproof boards) everywhere except where the shower and bathtub are placed, up to the desired build-up height/tile height, and seal the joints. Then mill slots into the boards for the underfloor heating pipes at the required diameter and about 3mm (0.1 inch) deeper, lay the pipes, cover with tile adhesive, reinforce again, and you have a tileable, waterproofed, optimally insulated, cost-efficient bathroom floor with fast-responding underfloor heating.

I discussed this approach with my plumber, and it is also recommended by Wedi. For renovations of older buildings with limited build-up height, this is the best solution I’ve found so far. It is, of course, also possible to use electric heating without milling.

If everything is going to be new anyway: have you already considered wall or ceiling heating?