Concrete block under low-energy house to save costs?

Hello everyone,

we're entering the hot phase - I've looked up architects and will call them this week; but of course I have to present our rough idea to the good people for a cost estimate.

- A question for you:
I had an idea while planning, and I would like to ask the specialists among you if it is a good one.

We have a slightly sloping plot (height difference from the street at the top to the end of the building area at the bottom (i.e. down towards the garden): approx. 3-4 meters.

The lowest floor would therefore be at garden level, the floor above would be at street level, so that would be the ground floor, and above that there would be an expanded attic floor (or prepared for expansion).

It is to be a low-energy house (something between a passive house and KfW40).

Since we won't actually use the lowest floor for living in the winter anyway (except for guests), I wanted to ask you if you can approve my idea to construct the garden floor as a thermally separated - "concrete block" that, although it basically complies with the new energy saving ordinance, and can also be heated cheaply, e.g. by underfloor heating, the expensive thermal envelope (wall) which will be about 50 cm thick, should only be the ground floor and the attic floor.

Or is that a contradiction in itself?
So, if I have underfloor heating operated by the same system in ALL floors, is it still thermally separated?!

I have read that lightweight concrete also has good thermal insulation properties, so if a guest ever stayed overnight in the garden floor, they wouldn't freeze, right?

Furthermore, REAL basements, that is those which lie completely underground, which our garden floor would only do partly in the slope, are usually concrete basements, as far as I have read?

So concrete can't be that bad if you later partially turn it into a residential basement, right?

There is only seepage water in the slope, no groundwater, so no pressing water; it is said to be a very clayey silt, so water-retentive, hardly any infiltration possibility.

It would be great if you could share your experiences with me.

Because building a solid low-energy house is certainly not cheap, and such precast concrete parts or even complete precast concrete basements are said to be very inexpensive in comparison?

Best regards and thanks in advance,
Honigkuchen

Here for overview, if needed, the planned rooms:

The garden level we would only use in summer / on warm days, because from there we can access the garden.

It will have a (closed, i.e. with walls) lounge room facing the garden, which can serve as a guest room. The room next to it (open) then a kind of small kitchenette. Otherwise, the garden level mainly serves technology, storage, utility room etc.

And at the very beginning, we will also have no lounge rooms like guest room or kitchenette there, but no interior partition walls yet, for cost reasons. (Except for the walls that would be statically required.)

The ground floor will be the living floor, i.e. living/dining room/kitchen (open), bathroom, 1 multipurpose room (bedroom/guest room/possibly children’s room sometime).

Attic, with bedroom, bathroom, small studio, possibly a small library or guest room/additional children’s room.

There are two of us, planning for 1-2 children.

We ourselves would like to live on the ground floor at first, later, when the first (or only) child is there, then move into the expanded attic, with the child then on the ground floor; if a second child comes, I would need to know whether it can be nice and warm downstairs in the garden level in a concrete block with underfloor heating, if the second child had their room there.

Best regards,
Honigkuchen

So, and the third contribution, so that it doesn't become too much at once while reading.. I have different questions :)

Question regarding subsidized KfW loan with reduced interest rate:
Am I correct in assuming that for the energy balance the ENTIRE HOUSE, including the concrete block separated from the thermal envelope, will be taken into account, even if initially no habitable rooms will be designated there, but officially everything is only storage rooms/cellar rooms?

Underfloor heating:
Is it worth having the underfloor heating installed directly everywhere, including in the "concrete block"? Maybe so that the "half-basement" doesn't get damp in the first place, or so that moisture from the screed and through the hillside seepage water evaporates directly?

We will have a controlled ventilation system, so moisture removal would not be a problem.

Thanks,
Honigkuchen

Hello my goodness that is a lot all at once, unfortunately I can't tell you anything about most of the questions because I am not familiar with them. But as someone affected by water damage I can only advise you to choose water-repellent concrete. Especially on a slope you are certainly more affected by heavy rain than others.

Regards Helen

Sorry that I have to say it like this but a concrete block is just not beautiful!!! Well, it has to please you but for me it would be nothing.
No offense
Michi

Hihihi.. they are all just blocks :D

Sorry that I have to say this but a concrete block is not beautiful!!! Well, you have to like it but for me it would be nothing

Hello Michi,
You are forgetting something very important:

The entire house, soberly considered, is nothing more than a block!

So also the nice low-energy house behind or above it..

It only depends on how you furnish the blocks, which floor you choose, what color or wallpaper on the wall, etc.?

I wouldn’t leave such a concrete block grey?! For heaven’s sake :D

- Our future double garage (practically also a concrete block) will, for example, be painted green.

In the anteroom concrete block with stairs & co., I will make almost everything silver, expensive wallpaper, possibly fancy slate on the floor, silver accessories, etc. pp. – you can’t tell anymore that beneath it all is concrete, can you?

Regards
Honeycake