New construction: insulation of collar beam ceiling and/or roof?

We are currently planning the construction of a detached house with a gable roof. The attic is intended to serve as the location for a gas condensing boiler as well as storage space. Access should be via a pull-down staircase. Where should the insulation ideally be applied?

[*]Only the roof (Disadvantage: You heat the attic in winter and in summer the upper floor may become somewhat warmer)
[*]Only the collar beam ceiling (Disadvantage: No frost protection for the heating)
[*]Both (Disadvantage: Cost)

Thank you.

The question does not arise; the heat generator must be located within the insulated envelope.

Don't you get problems with mold if the roof is insulated but not ventilated? You hardly go upstairs daily through an extendable attic ladder to ventilate.

Regards
Sabine

don't you get problems with mold if the roof is insulated but not ventilated?

In new buildings, there is usually a controlled residential ventilation system that can handle that. Maybe it is even located up there?