ᐅ Bathroom Renovation: Installing a Bathtub Directly on Insulation?

Hi everyone,

We are currently renovating our older house. To install underfloor heating in the ground floor bathroom, the bathroom will receive a new base slab to avoid any steps between it and the adjacent rooms.

The floor build-up is currently planned as follows:

Soil - 30mm (1.2 inches) lightweight concrete - bitumen membrane - 140mm (5.5 inches) XPS insulation - 100mm (4 inches) cement screed with underfloor heating

Now regarding the bathtub (180x80cm (71x31 inches))—we want to install a steel bathtub with a polystyrene (Styrofoam) bathtub support frame. I would like the bathtub rim to be lower than it would be if the support frame were placed directly on the cement screed.

Since the underfloor heating doesn’t really make sense under the bathtub anyway, my question is:
Is it possible to omit the screed in the area of the bathtub (exactly the size of the support frame) and place the support frame directly on the 140mm (5.5 inches) XPS insulation, or won’t that adequately support the load? The exact XPS insulation to be used has not yet been decided. The support frame will have relatively large surface contact with the floor, but of course it won’t be fully supported everywhere due to the drain and other fittings.

This approach would allow the bathtub to sit significantly lower (if it ends up too low, I could add some XPS to raise the height again).

Best regards and thanks for your feedback

I would not completely skip the screed under the bathtub. Even though the insulation supports the weight, you still need a stable base – otherwise, over time, the floor might settle or stress the drain connection.
If you want to install the bathtub deeper, simply make the screed thinner in that area or use firmer insulation. Some people also place a construction board underneath, which works well.

Thanks for the tip.
How can it be determined how much or how little screed should go under the tub?

Alternatively, placing a construction board on the insulation instead of screed? That sounds like a straightforward option.
Similar to screed, how do I determine the sufficient thickness of the construction board? Since these usually come in only 60cm (24 inches) widths, I’ll probably need at least two boards to fit under the 180x80cm (71x31 inches) tub.

This is how you can do it – the important thing is that the bathtub is fully supported and stable. XPS alone does not reliably provide that. Construction boards are possible, but only with a load-distributing layer; otherwise, there will be movement and problems at the drain. Make sure to clearly separate the transition to the screed, otherwise it will crack there. It is better to use two glued 20mm (0.8 inch) panels with full support.

So you mean two layers of 20mm (0.8 inches) construction boards each, placed as a base with a 90-degree rotation between them? And then a separating layer for the screed, like an edge insulation strip, for example?

With your constraints, it still sounds like a potential problem area. Leaving out the screed at the bathtub location and then pouring an additional 5cm (2 inches) of screed afterward might be a safer alternative?

The idea of adding an extra layer of screed beneath the bathtub is definitely a better approach. Using two layers of construction board is possible, but it’s more of a temporary solution and the joint may later cause issues. If you’re going to pour screed anyway, it’s best to do it properly from the start.