ᐅ Building without Mechanical Ventilation according to KfW40 or the Building Energy Act
Created on: 21 Jan 2026 15:47
S
Simon1988
Hello everyone,
I don’t want to start a debate about the pros and cons of mechanical ventilation with heat recovery; we have decided against it for various reasons.
Now we are wondering whether it would be worth building to KfW 40 standard with only a small additional cost (photovoltaics and battery storage are already included in the offer) in order to receive the subsidy—compliance with the ventilation concept is not an issue according to the general contractor—or if it might be better to build according to the Building Energy Act or KfW 55 without mechanical ventilation. Does this make a significant difference?
We are just concerned that with even tighter insulation but no mechanical ventilation, we may end up having to ventilate manually more often or face problems with mold.
Thanks and best regards,
I don’t want to start a debate about the pros and cons of mechanical ventilation with heat recovery; we have decided against it for various reasons.
Now we are wondering whether it would be worth building to KfW 40 standard with only a small additional cost (photovoltaics and battery storage are already included in the offer) in order to receive the subsidy—compliance with the ventilation concept is not an issue according to the general contractor—or if it might be better to build according to the Building Energy Act or KfW 55 without mechanical ventilation. Does this make a significant difference?
We are just concerned that with even tighter insulation but no mechanical ventilation, we may end up having to ventilate manually more often or face problems with mold.
Thanks and best regards,
M
MachsSelbst21 Jan 2026 19:36There are actually people who buy a BMW 5 Series and choose to forego the heated seats...
Skipping a ventilation system is nonsense; you won’t save any money that way. While a ventilation system recovers a significant portion of the heat energy, by airing out your home 2-3 times a day, you are letting warm air escape and then reheating it. And you’ll do this with a heating system designed for a well-insulated house, which is relatively small.
Don’t underestimate how much moisture is released in a new build over the first 2-3 years from plaster, screed, and so on. The house is far from fully dried out even after using the heating to dry the screed.
No one will actually air out the entire house 3 times a day during the first years. I found it annoying enough to have to do that 3 times a day for 3 weeks during the drying phase.
Get a ventilation system... if you don’t have the budget, it’s better to build according to the building energy law (Gebäudeenergiegesetz) with ventilation than to meet KfW40 standards without it.
Skipping a ventilation system is nonsense; you won’t save any money that way. While a ventilation system recovers a significant portion of the heat energy, by airing out your home 2-3 times a day, you are letting warm air escape and then reheating it. And you’ll do this with a heating system designed for a well-insulated house, which is relatively small.
Don’t underestimate how much moisture is released in a new build over the first 2-3 years from plaster, screed, and so on. The house is far from fully dried out even after using the heating to dry the screed.
No one will actually air out the entire house 3 times a day during the first years. I found it annoying enough to have to do that 3 times a day for 3 weeks during the drying phase.
Get a ventilation system... if you don’t have the budget, it’s better to build according to the building energy law (Gebäudeenergiegesetz) with ventilation than to meet KfW40 standards without it.
Simon1988 schrieb:
I don’t want to start a discussion pro and contra controlled residential ventilation; we decided against it for various reasons. Now we face the question whether to build to KfW 40 standard with only minor additional costs (photovoltaics and battery storage are already included in the offer) to receive the subsidy (complying with the ventilation concept is no problem according to the general contractor), or whether it might be better to build according to the Building Energy Act or KfW 55 standard without controlled residential ventilation. Does this make a big difference? We are just concerned that with even tighter insulation without controlled ventilation, we might have to ventilate more manually or risk mold problems. It seems you misunderstand the function of thermal insulation: it does not slow down gas transport, but rather the heat transfer through the wall. Anyone who “hears” air drafts will also continue to notice street noise (even from a residential street) to the same extent. The main disadvantage of controlled residential ventilation compared to permanent window tilt or trickle vents is possibly some fan noise, which can be minimized by choosing a central system with the technical equipment located away from sleeping areas. Nighttime intervals without air exchange are too long to fully eliminate moisture from perspiration.
If subsidy conditions are relevant to your ability to afford the house, it is better to build smaller — this is a more effective way than changing the efficiency class of the building.
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