My wife and I will be building a detached house next year, and I have a question about it. The house will be built of aerated concrete and will have a hipped roof. There is not much room under the roof, just some storage space. I can't stand upright there, so there will never be any living space or anything like that. Only boxes with some stuff will be stored there.
In our case, should we choose insulation directly under the roof or on the ceiling of the floor? Is there any experience here, which one is more comfortable in winter/summer? Will I notice any difference at all on the top floor? Where can I get a better price?
On the floor slab means "cold roof". Under the roof means "warm roof." It's important that warm, moist air can escape outside. If you only have a pantry upstairs but a warm roof, you should plan for windows in the gable so that there is ventilation. With a cold roof, the air should circulate enough on its own and you won't need ventilation.
I contend that a cold roof is cheaper in terms of costs (construction, energy). In storage, however, you have to consider the summer/winter temperature differences. The climate in this case is about the same as an unheated tool shed.
We just recently insulated our upstairs ceiling. Lined with foil, glued overlaps, glued strips of insulation around the edges of the floor, then laid one layer of polystyrene horizontally, one vertically and put OSB boards on top, tongue and groove glued. A total of $1,200. We had 50 square meters.
All you need are two stompboards to cut the boards, a miter saw, a pencil and a meter.
What to use depends on what you want to store there. If it's the only storage space, I'd go with a warm roof.