Roof insulation with heat pump - Disadvantages if the attic is warm?

Hello everyone,

I have already used the search function but did not find the relevant information.

We are planning the construction of a single-family house. A friend (roofer) recommended an external roof insulation. We want to operate our single-family house with a heat pump (brine / water). The construction company now said that this does not make much sense because the heating costs per year are so low that although you save some through the external roof insulation, the payback period is very long. Furthermore, the construction manager said that with external roof insulation we would be heating the attic, since with external roof insulation the insulation of the top floor ceiling is omitted.

What do you think, does it make sense to have external roof insulation with a heat pump?

Thanks for your help!!!

... A friend (roofer) recommended an external roof insulation to me.

The decisive factor is the use (system boundary)! If the attic is heated, i.e. inside the thermal envelope, insulation is required. The minimum insulation thickness is then determined by the Energy Saving Ordinance. This can be external roof, insulation between rafters, or insulation beneath rafters. Sometimes also combined. In a new building, foregoing insulation between the rafters would be wasted space. If the attic is not used for residential purposes, the external roof insulation forms the system boundary. It must then be insulated according to the requirements.

... Furthermore, the construction manager said that with external roof insulation, we heat the attic because with external roof insulation the insulation of the upper floor ceiling is omitted.

That you can heat with insulation would be news to me ;-) Also, the insulation is not omitted but is a bit more economical ;-)

... What do you think, does it make sense to have external roof insulation with a heat pump?

This detailed insulation has no immediate, direct connection with the intended heat generator. A brine heat pump is however a good choice, provided the entire system is sufficiently sized. Everything must be coordinated; otherwise, you end up with a money pit. Special attention must be given to the source design. For this, among other things, the required extraction power and energy must be known.

Best regards