Additional insulation for the ceiling of the upper floor with a warm attic: useful/necessary?

Good evening,

we are currently building a solid house and I have a question regarding the insulation of the roof.

We are building a gable roof with an attic and both above-rafter insulation as well as between-rafter insulation. The between-rafter insulation will be done as blown-in insulation. The insulation is applied to the roof slopes, so that it is a warm attic. There is a vapor retarder both on the ceiling in the attic and below the ceiling of the upper floor (OG).

For better understanding, the simplified structure from bottom to top:
OG -> vapor retarder -> collar beam -> wood fiber boards (attic floor)
attic -> vapor retarder -> between-rafter insulation (blown-in insulation) -> above-rafter insulation -> vapor diffusion open underlay membrane

Now we have two options for execution and I would need your opinion:
1) As described above. The insulation is applied exclusively to the roof slopes and in the warm attic. The ceiling between the upper floor (OG) and attic has a vapor retarder but is not insulated.
2) Like option 1, only that the intermediate ceiling between the upper floor and attic is additionally filled with blown-in insulation. However, we would have to pay an extra cost for this.

We were told that option 2 is recommended. The statement was that in the "warm" but unheated attic the temperature is significantly lower than in the living area. This can lead, especially in warm rooms (e.g. bathroom), to warm air rising, condensing on the "cold" ceiling, and over time mold can form as a result. Additionally, the cold ceiling can transfer cold to the rooms below. With the additional insulation it would be cooler in summer and warmer in winter.

Now I would need the opinion of the forum. Is it sensible to additionally insulate the intermediate ceiling and pay the extra costs (€ amount is still pending)?
Does this bring a noticeable clear advantage in summer and winter? Is the intermediate ceiling really "cold" / "colder" if you do not do it?

Thank you.

Option 3 would be to also heat the attic (lightly). Otherwise, I wouldn’t understand why you insulate the very top if you don’t heat it.

Option 3 would be to also heat the attic (slightly). Otherwise, I wouldn’t understand why you would insulate at the very top if you don’t heat it.

That’s exactly what we did. We have two low-temperature radiators integrated into the underfloor heating circuit. It works great so far.

Option 3 would be to also (lightly) heat the attic. Otherwise, I also wouldn’t understand why you would insulate at the very top if you don’t heat it.

To be honest, I didn’t fully understand that either and unfortunately didn’t ask further. The attic has a height of max. 1.3-1.4m. That means no living space will be created there, but it will be used purely as storage space. The construction company told us during planning that they would insulate the outside so that the attic wouldn’t be completely cold and that everything could be stored there accordingly. At first, I thought that idea was good.

Now the question arose that with an insulated attic without heating, it won’t be 20°C either, but probably somewhere halfway between the outside temperature and the upper floor indoor temperature (simplified assumption). And with an uninsulated intermediate ceiling on the upper floor, I always have a somewhat cold ceiling with the described problems from above. Or am I seeing that wrong?

The plumbing company has so far not planned any heating in the attic (the rest of the house has underfloor heating) and so far does not see the necessity.

That means I now have several options:
a) Insulated attic - no intermediate ceiling insulation -> as currently planned, but probably not the best solution?
b) Insulated attic - intermediate ceiling additionally insulated -> incurs additional costs, but probably a sensible solution to compensate for the temperature difference
c) Insulated attic - intermediate ceiling not insulated, but radiator in the attic -> I would have to clarify with the plumbing company to what extent this is still feasible. Does it make sense to heat the attic continuously if it is only used for storage?

I would be interested in your assessment. Thank you.

I obviously cannot judge how it looks for you, but I can at least contribute our experience. We have a fairly large converted roof with two rooms, each connected to the rooms below by a stair cupboard open at the top. The roof is insulated both above and below, as is the case with you. When we moved in at the end of January, almost exactly one year ago, we had similar conditions as now. Outside temperatures were between 5-10 degrees. At the time of moving in, the low-temperature radiators were not yet available due to delivery problems. They arrived about two months later. During this time, the underfloor heating of the rooms on the upper floor had to "co-heat" this attic. In practice, the rooms in the roof were about two degrees colder. But as I said, it can be quite different for you. How is the connection to the roof? Open or closed?

So if you don’t actively heat, then the warmth naturally comes from the rooms below and with it also moisture, which then condenses in the colder attic. Therefore, it also makes sense to insulate the ceiling to the attic. Insulating the roof as well is more of a luxury. I would rather keep the insulation in the ceiling and heat with e.g. low-temperature radiators like . Alternatively, electric heating with an oil radiator or fan heater. Heat pump (air conditioning unit) would also be an option, which even dehumidifies.