ᐅ Exterior wall insulation or perimeter insulation? Change required by the construction supervisor

Hello,

we are currently planning our new build and are about to finalize the construction contract. In the revised offer presented to us—adjusted according to our wishes—the general contractor has changed the type of insulation. He wanted to explain this to us by phone, but due to the weekend, I haven’t had the chance to discuss it with him yet.

I will also consult our KfW (German Development Bank) energy consultant about this. Nevertheless, I believe it’s helpful to get several opinions, so I would like to ask you as well.

So far, the plan was as follows:
A solidly built single-family house to KfW 40 standard
Exterior walls of 150 mm (6 inches) aerated concrete
200 mm (8 inches) cavity insulation boards KD2 with thermal conductivity 0.035 W/(m·K) WLG 035
10 mm (0.4 inches) ventilated cavity
115 mm (4.5 inches) facing bricks (cladding)

The perimeter insulation was to be made of 160 mm (6.3 inches) extruded rigid foam boards with thermal conductivity 0.038 W/(m·K) WLG 038.

Now, the perimeter insulation has been reduced to 80 mm (3 inches), and instead, the exterior wall insulation’s thermal conductivity has been improved from WLG 035 to WLG 032. Both changes are without any price difference.

I have no idea about the effects this might have or whether the better exterior wall insulation actually compensates for the reduced perimeter insulation. From my (non-expert) perspective, good perimeter insulation is very beneficial with underfloor heating, as it prevents heat loss through the foundation. Is that correct?

As mentioned, I don’t yet know the contractor’s reasons for the change. I will ask about this on Monday.

By the way, the house will be built in Hamburg on sandy soil without clay or similar material, if that’s relevant.

I would be very grateful for your help!