Is it possible to effectively insulate the basement floor afterwards?

Hello everyone,

we have already involved an energy consultant and are waiting for an offer. However, we still want to inform ourselves in advance.

We want to completely renovate our house. During the renovation, the exterior facade is to receive full thermal insulation. The energy consultant said in the initial conversation that the basement ceiling should also be insulated with about 8 cm.

For the complete renovation, we are considering whether to increase the room height in some basement rooms. The architect said that underpinning would be necessary here (1 or 2 meters each). A construction manager, on the other hand, said that one can simply remove the floor slab and lower it by about 20-30 cm. The final decision is, however, up to the structural engineer.

My question now: Is it possible to apply insulation under the floor slab without problems if it is removed and lowered anyway, see sketch? Won’t thermal bridges arise as indicated? Or will the insulation be useless because of the thermal bridge since the strip foundations are not provided with full thermal insulation?

Info: In the sketch, I have not yet shifted the floor slab downwards.

Attached is the forgotten sketch.

Insulation under the slab in connection with strip foundations always leads to exactly this weak point in my opinion. But in practice, the insulation under the slab is much thicker, so it shouldn't be that dramatic.

Apart from that: how old is your house and is it possible that there is a misunderstanding here? I can't imagine that the term "slab" was really used here. It rather seems that your basement walls stand on strip foundations and the basement rooms have a floor independent of that. That floor is supposed to be removed, excavated, and a new floor installed deeper again. Do you really want to put yourself through that? For a complete single-family house, that easily costs... I suppose... six figures?

Stupid choice of words on my part. I also see that I did not push the basement walls under the soil in the sketch, but only the foundation. Sorry for the incorrect representation. Attached is a small correction.

Of course, the basement walls rest on the strip foundation. And the term "floor slab" should be replaced by a normal floor (possibly directly on the soil).
Remove the floor, dig out and new floor deeper = six-figure? Six-figure because of the foundation project?

Now the sketch looks good, I think it is very close to reality. As already said: the thermal weak point cannot be completely avoided in practice, nowadays one might possibly use a series of insulation bricks or something similar. But in your situation, you will have to live with the fact that there is a slight thermal bridge there. A possible solution would be to cover the interior basement walls with combined plasterboard-polystyrene panels, which also results in nice smooth walls and an additional insulation layer.

The cost driver for your project is the labor costs. Everything has to be more or less removed manually (Hilti yes / excavator no) and then transported outside by the bucketload. Assuming a 20cm thick floor is to be removed and then another 30cm excavated. That would be 0.5m³ of waste material per m² of basement without air. On a house footprint of 100m²? and including air in the debris, that’s easily 75m³ that somehow have to be carried outside via the basement stairs. Even the semi-legal Bulgarian construction crew would need a few weeks for that and no one of them works for 5€/hr anymore.

But feel free to get some offers first, since I don’t know the situation on site.

In the sketch, I have marked the insulation in orange. Where would the insulation bricks go? With additional interior wall insulation, there is always the fear of mold behind it. You also cannot see what is happening behind it, meaning whether the wall is beginning to crumble or become sandy. With the additional interior wall insulation (basement), the thermal bridge would be reduced (it would not be completely eliminated, but still better).

Regarding the project: Hilti is not a problem. The material from the basement can be very easily transported and directly thrown onto the trailer. (Several trips would have to be made and many helpers would need to pitch in). The more difficult task would be the removal of the foundation (assuming that is necessary).