Exterior wall insulation or perimeter insulation? Change by general contractor

Hello,

we are currently in the process of planning our new building and are now about to finalize the contract for work. However, in the offer presented to us and amended according to our wishes, the main contractor changed the type of insulation. He wanted to explain the change to us by phone, but due to the weekend I have not yet had the opportunity to discuss it with the main contractor.

I will also consult our KfW appraiser about this. Nevertheless, I think that multiple opinions cannot hurt and would like to ask you this question as well.

So far, the following was planned:
Solidly built single-family house to Kfw 40 standard
Exterior walls made of 150 mm aerated concrete
200 mm core insulation boards KD2 WLG 035
10 mm finger joint gap
115 mm facing bricks (clinker)

The perimeter insulation was to be made of 160 mm extruded rigid foam boards WLG 038 / WLZ (Id) 038.

Now, the perimeter insulation has been reduced to 80 mm and, in return, the exterior wall insulation has been changed from WLG 035 to WLG 032. Each without additional or reduced cost.

I have no idea what effects this has and whether the improvement in exterior wall insulation can actually compensate for the reduced perimeter insulation. From my (layman’s) point of view, good perimeter insulation is a great advantage with underfloor heating, as not all the heat is lost directly through the floor. Am I right?

As I said, I cannot say anything about the reasons for the change yet. I will ask about this on Monday.

By the way, the house is being built in Hamburg on sandy soil without clay or similar, if that matters.

I would be very grateful for your help!

I would be surprised if a real perimeter insulation is laid under the slab in northern Germany. The building standard here, besides clinker brick, is a strip foundation reinforced with steel, with the strips connected by an equally reinforced thinner concrete slab. On top of that, Ytong blocks are built, by the way 150 is a bit stingy, I would consider 240 normal!, and then it is sealed with a bitumen membrane. On top of that, electrical wiring and water pipes are laid. The next step is interior plaster. Now come the screed guys, they lay Styrofoam all over and cover it with foil. On top of that go the pipes for the underfloor heating, then the screed. At the wall, it is decoupled by a blue foam layer, which looks like foamed garbage bag. Now to your question. If that’s the case, 160 Styrodur means a pretty high floor, a high structural height, you’re supposed to keep about 2.5 m interior height. Maybe the builder wants to save something here and compensate that with more stuff in the wall. My advice would be to open up the whole package, forget about kfw 40 for now, build in the classic way with 240 and less insulation, and spend the saved money on a nice front garden. Karsten

Is it really about the insulation under the floor slab or the base area?

Exactly, that's what I mean. Is it really about something like that?

It is actually about insulation below the floor slab. I have attached a picture for you.

As Nordlys already wrote, a strip foundation (64 cm) and the floor slab (16 cm) above it will be poured. The perimeter insulation is to be laid below the floor slab.

I have no idea what effects this has and whether the improvement of the exterior wall insulation can actually compensate for the reduced perimeter insulation.

Whether it is clear: yes. To what extent, i.e. which change "here" is equivalent to which change "there," is a complex calculation and absolutely not my area of expertise.

From my (lay) perspective, good perimeter insulation is a great advantage with underfloor heating, as not all the heat is lost directly through the floor. Am I right?

From my also lay perspective ;-) it is like this: the underfloor heating heats the room, and part of its heat is lost again through the wall. The perimeter insulation limits the heat radiation of the underfloor heating downwards, which saves energy relative to the same amount of heat emitted upwards. At this point, effectiveness increases. The exterior wall insulation, on the other hand, has the task of keeping more of the already upward-radiated heating power trapped inside the heated room. Then less replenishment must be produced. Ultimately, both pull together in this sense.

Ytong is built on this, by the way 150 is stingy, 240 I would consider normal!,

This strange (in my opinion unfortunate) ratio of building material thickness to insulation thickness in this wall construction strikes me like a liver sausage sandwich with crispbread :)