ᐅ Is Additional Insulation of the Attic Useful in New Construction?

Hello,

we are building a house with two full stories and a pitched roof above, constructed according to the Energy Saving Ordinance standard. The construction company includes an insulated attic space (insulation between rafters with 200 mm (8 inches) thick mineral wool insulation (thermal conductivity 0.035 W/mK), with a vapor retarder foil, sealed airtight). The concrete ceiling above the upper floor (i.e., the attic floor) remains uninsulated.

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

I am somewhat concerned about the ventilation of the attic during the first year or two after construction, as the attic will not be connected to the central mechanical ventilation system (I will probably have to leave the attic windows open occasionally in weather with low dew points). Should I wait some time before insulating the attic floor for this reason? Or can I do it right away without any issues?

Or would you recommend a thicker insulation between the rafters instead of working on the attic floor? The latter would have the advantage that I could avoid the very high prices charged by the general contractor by doing it myself.

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

If not: Remove the insulation between the rafters and get a credit for it. Then roll out one layer of 120mm (5 inches) mineral wool. Lay another 120mm (5 inches) layer crosswise on top.

If yes: Will temperature-sensitive items be stored?

If not: Remove the insulation and get a credit for it. Then install walkable insulation, which is available pre-made. More insulation is better.

If yes: Have the general contractor install the insulation. Then lay the walkable boards on top.

Important: The energy-saving regulations calculation must be correct! So coordinate this with the general contractor.

Heating an attic that is not used as living space is simply pointless. That’s basically what happens if the ceiling of the top floor remains uninsulated. Even if the roof itself is insulated, you are indirectly keeping the attic, at least partially, heated.

Joedreck schrieb:

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

Ok, thanks, it will probably come down to that. For around 110-120m² (1,184-1,292 ft²) of surface area, it’s not exactly a cheap undertaking. Just simply lay some OSB insulation boards with a walkable surface next to each other without gluing them? Or is there anything to watch out for? Quote from a hardware store product: "Total thickness 100mm (4 inches), 10mm (0.4 inches) gypsum fiberboard + 90mm (3.5 inches) EPS polystyrene thermal insulation WLG 040 EPS DAD Clean, walkable finish with protective board."

Joedreck schrieb:

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

If the general contractor’s insulation between the rafters already meets the energy saving regulations, and I don’t benefit financially from KfW55 anyway, does it make any difference? Or would including it in the energy saving regulation calculation be potentially useful for a later sale of the house?

Joedreck schrieb:

Heating an attic that is not lived in is simply nonsense. That’s exactly what you do if the uppermost ceiling is left uninsulated. Even if the roof itself is insulated, you still indirectly keep the attic at least partially heated.

Yes, thanks, I was already wondering why the construction company does it that way. But they said they always do it like this, and insulating the attic floor is nonsense. Whether the investment ever pays off is another question. But apparently it’s not completely pointless. Any differing opinions? Do I need to be careful about mold or something else when adding the extra insulation?

If the general contractor doesn’t omit the insulation, you don’t need to discuss it with him further.

I consider a minimal basic ventilation system to be reasonable. Note: This is a non-expert opinion!

Whether it’s cost-effective and when can be calculated.

Install edge to edge. I don’t think there will be much slipping.

I’m not a big fan of the boards. They feel expensive, thin, and have poor thermal conductivity (WLG). But they are the easiest to install.

Joedreck schrieb:

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

Great! That makes things simpler.

Joedreck schrieb:

I’m not a big fan of the boards. They feel expensive, thin, and have poor thermal conductivity (U-value). But they are the easiest to install.

What would you suggest? Cheaper and a bit more complex to install would also be fine. Rolling out mats with stepping boards on top? Other materials? It shouldn’t be much thicker than 10–15 cm (4–6 inches), otherwise I won’t be able to move around bent over in the attic. We don’t have a basement, so the attic is quite an important storage space. Also, the temperature range in the attic should not deviate too much from the rest of the house during the coldest winter and hottest summer, so maybe the insulation on the attic floor should be somewhat lighter than the insulation between the rafters?

Joedreck schrieb:

Whether it’s cost-effective and when can be calculated.

Is there a good online tool for that? I’d basically have to add up the full thermal resistance (R-value) of the concrete ceiling, the insulation boards laid on the floor, and the 200 mm (8 inches) knauf insulation between the rafters. Then calculate the heat transmission over a year and compare that to the value without the insulation boards. How can I do this simply and pragmatically?

Using a U-value calculator. Google.

So, do you possibly want to heat the attic after all?
I think for your project, @Nordlys might be a better fit.
He turned his attic into a fully usable storage space. So far, no problems.

By (indirectly) conditioning the attic, you will consume more energy. There is more volume and therefore more exterior surface area.

My advice for you: maybe try it without. If you can accept the costs, then it’s fine.
And seriously: it won’t be a huge difference.

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