Roof insulation from KfW 55 to KfW 40 on the floor of the attic

Hello everyone,

we want to build a single-family house in BaWü with the following currently defined key data:
- approximately 12x12 m external dimensions
- basement with about 16 cm perimeter insulation and a good layer (exact height unknown) of glass foam gravel below (within the thermal envelope)
- 49 cm Poroton exterior wall filled with perlite (U-value around 0.16)
- windows/doors adapted accordingly to the exterior wall (U-value currently unknown, but roughly the same level as the exterior envelope)
- gable roof (attic outside the thermal envelope)
- domestic hot water heat pump or geothermal drilling heat pump with corresponding annual performance factor so that the BAFA subsidy applies
- photovoltaic system on the roof
- controlled residential ventilation with heat recovery

Now I have the following question about the gable roof. For a KfW house, in addition to the exterior wall, basement, and the openings in the facade (windows, doors), the roof must also be insulated accordingly.
Is it sufficient if we insulate the floor of the attic itself at the roof (about 30-40 cm of insulation materials and then clad with wood so that you can still access the attic, the technical term is unknown to me) or must the insulation be done directly under the roof (between-rafter or under-rafter insulation) for KfW 55 or ideally KfW 40?

I believe that the "attic floor insulation" would be very easy to accomplish by ourselves (DIY) and therefore cheaper than between-rafter or under-rafter insulation. Or are the costs for both variants about the same?

Which material is recommended for roof insulation for a KfW 40 house?

Are we mistaken in thinking that we can easily achieve a KfW 40 house with the materials mentioned above? Or is the effort from KfW 55 to KfW 40 not economically worthwhile?

So far, we have not yet had a conversation with an energy expert from the list.

Thanks to you all in advance,
Regards
Ann.

The most common method is the overlay insulation. This type of insulation is cost-effective and can be done yourself with some basic craftsmanship skills.

Ah exactly, I practically want an overlay insulation / ceiling insulation instead of rafter insulation because I think it is easier to manage myself and also cheaper.

Am I wrong? And is that sufficient with the corresponding U-value for KfW 55 / 40?

You will have to ask this question to your energy consultant, who will also certify the whole thing for the [KfW]. [KfW40] will be difficult, but not because of the roof, rather because of the basement. The step from [KfW55] to [KfW40] is usually not economically sensible.

I'm not very familiar with this, just a couple of notes from me on the concept.

- Are you aware of how "thick" 49cm actually is? You really have to want that.
- Hopefully with 49cm you don't just have a 0.16 U-value... you can achieve that already with 36cm. So with the castle wall thickness and perlite, you hopefully get below 0.1.
- Your overall concept in BaWü depending on what other equipment you have is more like 2500 rather 3000 /sqm, are you aware of that?
- With what you have planned, it sounds more like you're aiming for kfw40(+), KfW 55 is "easier".
- Regarding the roof, as I said I'm not very familiar... then you just have a cold roof. That means "outside" the thermal envelope. Better to have a carpenter give you an offer just in case. I wonder if it would be reasonable for me to want to build a house like that with probably a total of 250-300sqm at that standard and then do the roof insulation myself... but everyone has to decide that for themselves.

Should it be a "1.5"-story single-family house? Then there will be sloping ceilings in some rooms on the upper floor. This means that there is a transition from a heated room to the roof. At least this area of the roof must be insulated. The rest of the roof can, in my opinion, remain uninsulated if the ceiling between the upper floor and the attic is insulated.

- With 49cm you hopefully don’t just have a 0.16 U-value... you can already achieve that with 36cm. So with the castle wall thickness and perlite, you hopefully get below 0.1.
- From what you have planned, it sounds more like you want to move towards kfw40(+); KfW 55 is "easier".
- About the roof, as I said, I’m not that familiar... then you just have a cold roof. Meaning "outside" the thermal envelope. You should better get a quote from a carpenter just in case. I wonder if it would be reasonable for me to want to build such a house with a total of probably 250-300 sqm at such a standard and then do the roof insulation myself... but everyone has to decide that for themselves.

If you go to, for example, the Schlagmann site and select the T8 49er brick -> 0.16 W/(m2K) U-value (approximate price 8 EUR per brick). The newer T7 49er brick has a 0.14 U-value (price about 8.5 EUR per brick). Does the 50 cents extra per brick justify the 0.02 improvement in U-value?

Well, I actually don’t want to aim for any specific standard. I just find the non-air heat pump simply better than an air heat pump and why shouldn’t you install well-insulating materials? Of course, keeping the price-performance ratio in mind.

I don’t necessarily have to go for KfW 40, I’m just wondering if it still economically makes sense to move from KfW 55 to 40. Or if it’s rather only motivated by ecological reasons. has already rather denied that. A basement inside the thermal envelope is probably rather difficult for a KfW 40 house? But KfW 55 should still be doable, or not because of the basement?

But compared to perimeter insulation on the basement, I think the overlay insulation/floor insulation from the upper floor towards the basement would be quite easy to do by myself. That it won’t save tens of thousands of euros there is clear to me...

Is this supposed to be a "1.5" story single-family house? Then you would have sloped ceilings in some rooms on the upper floor.
That means you have a transition from heated space to roof there. At least that part of the roof must be insulated. The rest of the roof can, in my opinion, remain uninsulated if the ceiling between upper floor and attic is insulated.

Sorry, forgot to mention. No, 2-storey with about 13x11m external dimensions.