Insulation of 1960s flat roof bungalow

Dear community,

we have bought a partially basemented (on a slope), single-story flat-roof bungalow in our favorite city and now want to renovate it. It is not a prefabricated house but was individually designed at the time by an architect for the builders. We now need decision support or criteria for decision-making on the subject of façade insulation (the roof was newly insulated and foil-covered in 1999). When searching for the above-mentioned criteria, one often reads about avoiding a WDVS insulation of the entire façade due to the risk of mold and instead limiting oneself to insulating the basement ceiling or room ceiling. A friend who is a construction contractor advised insulating everything possible if you also want to renew windows and shutters. What do you think about these two viewpoints? Perhaps someone can report from their own experiences or projects. We would be very grateful for help!

Thank you very much, Tobi

You often read about avoiding a WVDS insulation of the entire facade due to the risk of mold

... and the fools still believe in it.

A friend who is a building contractor advised that you should insulate everything possible if you also want to replace windows and shutters.

The question is, what is your goal? First of all, old windows are real energy wasters. You can insulate the basement ceiling very well and cheaply by yourself (sticking PUR aluminum-laminated WLG 23 is no rocket science – 10 cm is good if the ceiling height allows).

Roof was newly insulated and foil-wrapped in 1999

Well insulated? How?

To limit it to the room ceiling

Looks great. But with calcium silicate boards, it’s also a real bargain. Why should one do that?

What do you think of these two viewpoints?

Do everything. I think it’s good if the budget allows. For KfW 55 you even get a 40% subsidy for the measures (I am planning this myself, otherwise at least 20%) and you have a house with the latest standard (although your roof probably ruins much of it, theoretically).
Only basement ceiling and interior corners. Nonsense! Basement ceiling definitely in connection with windows/front door. That already helps a lot. If there is money for exterior insulation, I would definitely do it (have it done). Alone the new look and of course also some energy savings make it worthwhile. And with the bungalow the labor is extremely low.

Thank you for the response! First of all: our goal is to improve the current energy efficiency. Not necessarily to perfect it according to high-end standards but to improve it. Of course, unlimited financial resources are not available, so we want to find a balanced approach and bring the 1960s house with its now 60-year-old building fabric to an adequate "improved" condition, with which one can (hopefully) be satisfied until retirement without further extensive construction measures. In other words: we don’t need the ultimate result, but want to live in a future-oriented, modern way. You don’t seem enthusiastic about the calcium silicate boards; you talk about "a little energy savings" with external insulation. I thought it was basically "the" method to become more energy-efficient, and internal insulation can never achieve what external insulation could. If it’s only "a little," with our goal it might possibly be omitted... Regarding the roof: cross-section drawing is attached. I look forward to further input!

They don't seem very enthusiastic about the calcium silicate boards,

That is always the last option, as interior insulation a) always has to be very well calculated b) requires very clean workmanship c) costs living space and d) costs multiple times more per square meter compared to exterior insulation. It is usually only done in case of moisture in the apartment or with houses where exterior insulation is not possible (e.g. due to monument protection).

you talk about "a little energy savings" with exterior insulation, I thought it was basically "the" means to become more energy efficient and interior insulation can never achieve what exterior insulation could.

Interior insulation can be just as effective as exterior insulation. However, with limited resources, it is not the preferred method. The cost-performance ratio is much better for windows, cellar ceilings, and the top floor ceiling. Exactly how much can be saved must be calculated, but energy-efficient renovations do not start with exterior insulation, but with the measures I mentioned.

I cannot judge what the 16cm insulation in the roof amounts to. That depends on the material (WLG?).

Definitely consult an energy advisor (which is funded by KfW) and not just the contractor (whose assessment is probably biased). That way you will also get funding and make the renovation more attractive.