KFW 55 EE Funding - New Construction Brick Thickness/Ventilation System?

Hello, we are building similarly in terms of dimensions but with unfilled 36.5cm Poroton T9 (0.09W/m²K) bricks, 20cm insulation between rafters and 6cm insulation above rafters, gable roof, air-to-water heat pump + ventilation with heat recovery (50/50 for comfort and reduction of energy consumption) + photovoltaic and achieve KFW 55 without problems. Ultimately, a KFW calculation is just number crunching until a certain value comes out in the end. Most windows and doors are already in the range of 0.9-1.1W/m²K by default, roof windows as well. That will work. Also, there is no "It only works this way because it has always been done that way," the envelope is always considered as a whole.

Thank you for your contribution. We will have quite large window areas. Windows are naturally always worse than walls in the energy balance – that’s clear. Would that then be enough for the 36.5cm Ytong? And assuming it would be enough, would you stick with the thinner stone or "just to be safe" (also because of soundproofing) tend toward the 42.5cm one?

Thank you for your contribution.
We will have quite large window areas. Windows are of course always worse in the energy balance than walls – that’s clear. But will that be enough for the 36.5 Ytong?

And assuming it would be enough, would you stick with the thinner stone or tend to the 42.5 one "just to be safe" (also because of sound insulation)?

That’s exactly what the energy consultant will tell you. He can then adjust..... a bit more here, less there.

Depending on how expensive or cheap the energy consultant is, it may also be worthwhile to build according to the reference value (for the individual components). This eliminates the more complex consideration of the entire building envelope, although I am not sure if this still works since 1.7, and thus the costs/efforts of the energy consultant may also decrease.

If it is done individually, meaning the entire envelope is calculated, almost all possibilities exist: thick insulating walls, seemingly open roof, insulation under the house, etc. Additionally, once the value for your envelope is calculated and additionally the baseline value in terms of 100%. Thus, it depends on the ratios of window areas, roof, walls, etc. to each other and always only on the very specific object. Then the question is how thermal bridges are considered, flat-rate approach, calculated or individually, sometimes accounting for 1/4 of the total value, so everything depends on the concrete object.

For example, we achieve KFW40 level with 42.5 07 Poroton, but that alone says nothing.

The sound insulation is around 50dB with 09 brick and the difference between 36.5 and 42.5 is 1-2dB. I would rather focus on the windows, which often only provide about 30dB instead of spending the relatively high additional cost on the stone, if there are reasons.

For example, with 42.5 Poroton 07, we reach KFW40 level, but that alone doesn't say much.

Really? Then I'm curious what the energy consultant will calculate...
If we could still reach KFW40 with little effort, that would of course be great.

As roof insulation, Integra 240mm WLG 035 is now planned as insulation between rafters.

Do you do slab or strip foundation? How is the insulation to the ground at your place?

Forget about the KFW40, it will most likely not pay off for a single residential unit with the subsidy. We have 260mm insulation between the rafters (032?) plus 140mm on the roof, cellar all around wrapped with 160mm 035 and the windows accordingly for a compact cube. The additional roof insulation alone probably consumes most of the higher subsidy, on the other hand, with 40-level, 40+ would be possible again if the photovoltaic system is planned anyway, but then with an additional battery again... a miserable rat’s tail. My main concern with the roof was thermal protection in summer, as I cannot sleep when it’s hot.

Forget the KFW40, it will most likely not be worth it with the subsidy for a single housing unit.

Yes, I suspect that too - it’s not profitable. The thing is, we definitely want to have photovoltaics, and controlled residential ventilation was actually also planned... The roof will have an attic/ second level in the children's rooms. There we also want to additionally insulate the wooden floor. We won’t be building a basement. Let’s see what comes out of the calculations...