ᐅ KfW40+ with a basement inside or outside the thermal envelope?

You can install a photovoltaic system without any subsidies, and it usually pays off quite quickly. The problem is that high electricity consumption often occurs when solar power generation is low (heat pump in winter, cooking in the evening). A storage system can help, but usually electricity from the storage costs more than from the grid. So, you have to reframe it: your photovoltaic system is not 50% subsidized, but your storage is 100% subsidized. The jump from 40 to 40+ is therefore more or less cost-neutral.

But the problem is the jump from 55 to 40 with a basement. Get concrete quotes for both: one for a basement and house built to KfW 55 standards, and one suitable for KfW 40. You will need to invest significantly more than €6000 for that.

As for the new subsidy program, nothing is fixed yet, so don’t count on it. For example, it’s unclear whether 40+ houses will really receive more than 25% funding. New concepts are being introduced, such as "KfW 55 + renewable energy," whatever that specifically means. A photovoltaic system might be included in that. Whether this will actually be relevant remains to be seen... The new subsidy is also planned to shift the project start date from the construction start to the contract signing. So, you might have to wait until July to sign, and then face a manufacturer price increase that costs more than the subsidy is worth. This whole thing is by no means free money, contrary to what manufacturers want you to believe. I just went through this phase a few weeks ago 😉

WilderSueden schrieb:

Building a basement at the back to meet KfW 40 standards just to avoid wasting the “good wall” is not cheap and far exceeds the €12,000 grant, not to mention the hidden costs of the more expensive wall.

Provider 1: (Not Weberhaus)

Offer: House including foundation slab built to KfW 55 standard, almost finished: 345,000 euros
For KfW 40: Additional insulation under the foundation slab 5,000.00

For KfW 55, a controlled mechanical ventilation system is not mandatory, but it becomes interesting with the €10,000 BAFA subsidy. I want controlled mechanical ventilation anyway, with or without KfW 55. Therefore, the same air-to-water heat pump with controlled mechanical ventilation, whether KfW 55 or KfW 40.

A “good” (insulating) wall isn’t necessarily expensive, since thermal insulation often comes from polystyrene. Yes, Weberhaus uses ÖvoNatur Therm, which has wood fiber insulation.

Okay, the basement complicates things here. But I’m happy to spend an extra €10,000 on insulation because it’s worth it.

Have I missed something here? Maybe it’s personal, not really “de-facto” nonsense.

We are talking about basement insulation, but you are providing an offer for the slab-on-grade. These are two completely different scenarios since the basement still has four walls that need to be insulated. A total of 10,000 (10TE) won't be enough for that.

And whether or not to put Styrofoam on the outside... personally, I don't want that. I already don’t like it buried under the slab, but I definitely don’t want it on the outside of the wall.

WilderSueden schrieb:

The jump from 40 to 40+ is more or less cost-neutral.

I agree (at least not more expensive), which is why I chose the "Plus" version.

WilderSueden schrieb:

But the problem is the jump from 55 to 40 with a basement. Get some concrete quotes, once for basement and house at KfW55 and once suitable for KfW40.

I got that from provider 1.

Quote for KfW55 on slab foundation.

Extra cost 5000 euros for KfW40 (insulation of slab foundation)

Extra cost 8000 euros for KfW40 (insulation of basement ceiling, stairwell, interior door, if built with basement)

However, then the basement is not within the thermal envelope. But I want a comfortable, almost habitable basement: music studio with underfloor heating and controlled residential ventilation.

From provider 2: quote is KfW40+, no quote for KfW55. Provider 2 has a higher quality wall than provider 1!

Provider 2’s quote is cheaper than provider 1’s (KfW40 plus basement).

However, I admit I am not certain that 12cm (5 inches) + 14cm (6 inches) basement insulation (about 10,000 euros) is enough for the basement to be inside the thermal envelope. But I think I’m not far off. It might cost a bit more. Maybe not.

Therefore, I would be interested if anyone has built a KfW40 house with a basement, what insulation was used? Will 10,000 euros be far from sufficient?

WilderSueden schrieb:

We are talking about basement insulation and you bring up a quote for the slab. These are two completely different things since the basement still has four walls that need to be insulated.

I present it this way because that’s how it’s stated in the quote: Basic offer KfW55; Option KfW40 with slab, Option KfW40 on basement (outside the thermal envelope).

At least I was able to compare KfW55 to KfW40 with slab. And in that case, KfW40 is more cost-effective.

Yes, the basement complicates things, I agree.

For 12cm (5 inches) + 14cm (6 inches) insulation, there is an extra charge of 10,000 euros. If that’s not enough, what can I expect? (12m x 8m) (39 ft x 26 ft) -- 20,000 euros?

Maybe I’m being too optimistic. I admit that. But what amount and type of insulation should I expect?

Thank you

KalterKaffee schrieb:

I use this every day and it’s enjoyable.

Perhaps also an option for a gym, sauna, etc.

If you make statements like that, then the basement definitely needs to be included within the thermal envelope!