ᐅ Is additional insulation of the upper floor ceiling necessary or beneficial when there is a heated attic space?

Good evening,

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

We are constructing a pitched roof with an attic and both exterior rafter insulation and cavity (between-rafter) insulation. The cavity insulation will be done using blown-in insulation. The insulation is applied on the roof slopes, so the attic will be conditioned (warm). There is a vapor retarder both on the ceiling of the attic and below the ceiling of the upper floor.

For better understanding, here is a simplified structure from bottom to top:
Upper floor -> Vapor retarder -> Collar beam -> Wood fiber boards (floor of the attic)
Attic -> Vapor retarder -> Between-rafter insulation (blown-in insulation) -> Exterior rafter insulation -> vapor-permeable underlay membrane

Now we have two options for implementation and I would like your opinion:
1) As described above. The insulation is only applied on the roof slopes and in the conditioned attic. The ceiling between the upper floor and attic has a vapor retarder but is not insulated.
2) Same as option 1, but the floor/ceiling between the upper floor and attic is additionally filled with blown-in insulation. This would involve an extra cost.

We were told that option 2 is recommended. The reasoning was that in the "warm" but unheated attic, the temperature is significantly lower than in the living space. This can cause warm air, especially from humid rooms (e.g. bathroom), to rise, condense on the "cold" ceiling, and over time, lead to mold growth. Also, the cold ceiling can transmit cold to the rooms below. With the additional insulation, it would be cooler in summer and warmer in winter.

Now I would like to hear the forum’s opinion. Is it worthwhile to insulate the ceiling/floor between upper floor and attic and pay the additional cost (amount still unknown)?
Does it provide a noticeable and significant improvement in summer and winter? Is this ceiling really "cold" or "colder" if you don’t insulate it?

Thanks a lot.

Option 3 would be to also heat the attic space (lightly). Otherwise, I don’t really understand why you would insulate the very top if it is not going to be heated.

Tolentino schrieb:

Option 3 would be to also heat the attic space (lightly). Otherwise, I don’t see why you would insulate the very top if it’s not heated.

That’s exactly what we did. We installed two low-temperature radiators, which are integrated into the underfloor heating circuit. So far, it’s working great.

Tolentino schrieb:

Option 3 would be to also heat the attic (lightly). Otherwise, I don’t really understand why you would insulate the top floor if it’s not heated.

To be honest, I didn’t fully understand that either and unfortunately didn’t ask further. The attic height is a maximum of 1.3-1.4m (4.3-4.6 ft). So, no living space will be created there; it will be used purely for storage. The construction company told us during planning that they would insulate the exterior so that the attic doesn’t get completely cold, allowing for proper storage conditions. I initially thought that was a good idea.

Now the question has come up that with an insulated attic but no heating, the temperature probably won’t reach 20°C (68°F), but rather somewhere around halfway between the outside temperature and the upstairs interior temperature (simplified assumption). And with an uninsulated ceiling between the upper floor and attic, I usually experience a cooler ceiling with the issues described above. Or am I mistaken?

The plumbing and heating company has not planned any heating in the attic so far (the rest of the house is heated with underfloor heating) and currently doesn’t see the need for it.

This means I have several options now:
a) Insulated attic - uninsulated ceiling between floors → as currently planned, but probably not the best solution?
b) Insulated attic - ceiling between floors also insulated → additional costs, but likely a sensible solution to balance the temperature difference
c) Insulated attic - uninsulated ceiling between floors, but with a radiator in the attic → I would need to discuss with the plumbing and heating company how feasible this is. Does it make sense to heat the attic continuously if it is only used for storage?

I would appreciate your opinions. Thank you.

Of course, I cannot judge how things are on your side, but I can at least share our experience.
We have a fairly large converted attic with two rooms, each connected to the rooms below by a stair cupboard that is open at the top. The attic is insulated both above and below the roof, similar to your situation.
When we moved in at the end of January, almost exactly a year ago, we experienced similar conditions to now, with outside temperatures between 5-10°C (41-50°F).
At the time of moving in, the low-temperature radiators were not yet installed due to delivery issues. They arrived about two months later.
During that period, the underfloor heating in the upper-floor rooms had to partially heat the attic as well. In practice, the attic rooms were about 2°C (4°F) cooler.

But as I said, your situation could be quite different.
How is the connection to the attic? Is it open or closed?

If you are not actively heating, then the warmth naturally comes from the rooms below, bringing moisture with it that condenses in the cooler attic space. In this case, it makes sense to insulate the ceiling below the attic. Insulating the roof itself is more of a luxury.

I would rather keep the insulation in the ceiling and, for example, heat with low-temperature underfloor heating as @Tarnari suggested. Alternatively, electric heating with an oil-filled radiator or fan heater could work. A low-level heat pump (air conditioning unit) is another option, which also dehumidifies.