Can a strip foundation be insulated afterwards?

Hello house construction forum,

unfortunately, I have not yet found a proper solution to my problem and hope to get some advice, thoughts, or considerations from you.

At the moment, we are still building our new single-family house (Kfw 60). Unfortunately, I dealt with the topic of thermal bridges too late. I got myself a thermal bridge catalog and mainly focused on the components involved. I know, one should have already thought about that during the planning phase. That's just how it is. But since we are self-builders, I can/could still minimize some thermal bridges myself (e.g., windows, ring beam, floor battens). Our single-family house stands on a strip foundation with plinth insulation. The walls are made of aerated concrete (thermal conductivity 0.090) and are mortared directly onto the floor slab (16 cm). As floor insulation, we will install 10 cm of Kingspan rigid foam (thermal conductivity 0.021).

Now I have seen in my thermal bridge catalog that the strip foundations represent a thermal bridge. One could/should have insulated the floor slab underneath and the strips on both sides. But since I can hardly dig out my house again, I am now considering whether it makes sense to at least insulate the full strips from the outside. Since we still have to bury the geothermal heat exchanger anyway, the strips will be completely exposed. In this context, I could apply insulation (e.g., Styrodur). A 6 cm insulation would cost me about 300 EUR. However, 16 cm are specified in the thermal bridge catalog. If the thing makes sense, I would do it. What thickness would you take? What do you think about it?

Thank you very much and best regards

Fello’w.

Hello Fellow

I would process Neopor (11 cm).
Expose the foundation to about 80-90 cm and glue the boards to the concrete, making sure that the boards fit together nicely.

Hello Fellow

It depends on whether you have to meet legal values. Depending on the canton, strict requirements must be met regarding the overall U-value.
For example, if you insulate the exterior facade more heavily, you can theoretically use thinner insulation for the base and thus compensate the worse insulation value in the base area with the better U-value of the facade. Ultimately, the energy certificate (energy regulations) provides information on the minimum insulation you would need.

If you want to be sure, you have to compare the lambda value of the insulation. The lower it is, the thinner the insulation, the more expensive the insulation. It might be cheaper if you choose thicker base insulation with a worse lambda value. If you want, I can send you an Excel file for the U-value calculation.

Where exactly are you building?

Regards