Is additional insulation of the attic sensible in new construction?

Hello,

we are building a house with two full floors and a gable roof above according to the Energy Saving Ordinance standard, and the construction company includes an insulated attic (insulation between rafters with 200mm thick mineral wool insulation (WLG 035), sealed airtight with a vapor barrier foil). The concrete ceiling above the upper floor (i.e., the floor of the attic) remains uninsulated.

Now I am considering whether it would be sensible for summer heat protection and heating costs in winter to additionally lay insulation boards on the attic floor. For example, with EPS insulation boards. Is that sensible or just a waste of money? Can this be done well as a DIY project, and what should one pay attention to? Do the boards have to be placed/glued tightly together, or can there be a bit of a gap?

I am a bit concerned about the ventilation of the attic in the first one or two years after construction, since the attic will not be connected to the central controlled residential ventilation system (presumably, I will have to leave the attic windows open from time to time in weather with low dew points). Because of this, should I wait some time before additionally insulating the attic floor? Or can I do that right away without any concerns?

Or would you recommend going for thicker insulation between the rafters right away, instead of dealing with the floor? The latter would have the advantage that through DIY I do not have to pay the exorbitant prices of the general contractor.

Should the attic be used as storage space or remain accessible?

If no: remove insulation between the rafters and have it credited. Then unroll one layer of 120mm stone wool. Roll out another 120mm layer crosswise on top.

If yes: will something sensitive to heat or cold be stored?

If no: remove insulation and have it credited. Then lay out walkable insulation. There are ready-made products available. More is better.

If yes: have the insulation done by the general contractor. Then lay out the walkable boards.

Important: the calculation for the Energy Saving Ordinance must be correct! So clarify with the general contractor!

Heating an attic that is not inhabited is simply nonsense. That’s no different when the top floor ceiling remains uninsulated. Even if the roof itself is insulated, you indirectly keep the attic, at least partially, at temperature.

If so: Have the insulation done by the general contractor. Then lay out the walkable panels.

Ok thanks, it will probably come down to that. With about 110-120m^2 of area also not exactly a cheap undertaking. Just lay some OSB insulation panels with a walkable stepping surface next to each other, without gluing them? Or do you have to pay attention to anything in particular? Quote from a hardware store product: "Total thickness 100 mm, 10 mm gypsum fiberboard + 90 mm Styropor WLG 040 EPS DAD
Clean, walkable finish through protective panel."

Important: The calculation for the energy saving ordinance must be correct! So clarify with the general contractor!

If the general contractor already meets the energy saving ordinance with his insulation between the rafters, and I get no financial benefit from KFW55 anyway, does that make any difference? Or would considering it in the energy saving ordinance calculation possibly be useful for a later sale of the house?

Heating an attic that is not inhabited is simply nonsense. That’s all you do if the top floor ceiling remains uninsulated. Even if the roof itself is insulated, you indirectly keep the attic, at least partially, at temperature.

Yes thanks, I was already wondering why the construction company does it that way. But they said they always do it that way in principle, and insulating the attic floor would be nonsense. Whether the investment will ever pay off is another question. But it is obviously not completely pointless. Any opposing opinions? Do I have to be careful about mold or anything like that when I lay the additional insulation?

If the GU does not omit his insulation, you do not have to continue talking to him.

I consider minimal basic ventilation to be sensible. Warning: layman's opinion!

Whether it pays off or when, can be calculated.

Lay edge to edge. I don't think much will slip.

I don't think much of the boards. Feels expensive, thin, poor WLG. But the easiest.

If the general contractor doesn’t leave out his insulation, you don’t have to talk to him anymore.

Great! Makes it simpler.

I don't think much of the boards. They feel expensive, thin, poor thermal conductivity. But easiest.

What would you suggest? Cheaper and a bit more complicated to install would also be okay. Roll out mats and put walking boards on top? Other materials? It shouldn’t be much thicker than 10-15 cm because otherwise I can’t walk bent over in the attic. We don’t have a basement, so the attic serves as quite important storage space. Also, the temperature range in the attic shouldn’t differ too much from the house in the deepest winter and in the height of summer, so the insulation of the ceiling floors should possibly be somewhat weaker than the insulation between rafters?

Whether it pays off or when, can be calculated.

Is there a good online tool for that? I would have to basically add up the entire thermal resistance (R-value) of the concrete ceiling, my insulation boards laid on the floor, and the 200m compressed fleece as insulation between rafters. And then calculate the heat transmission over a year. And then compare that with the value without the insulation boards. How do I do that simply and pragmatically?

Using the U-value calculator. Google.

So you might want to heat the attic after all?
I think fits your project better.
He turned his attic into a fully functional storage room. So far no problems.

By (indirectly) tempering the attic you will consume more. You have more volume and therefore more surfaces facing outside.

My tip maybe for you: try it without. If you can live with the costs, then it’s OK.
And seriously: it won’t be worlds apart.

I don’t have it in my head. How do you want to heat?