ᐅ Energy-Efficient Renovation of Older Buildings – Is a Ventilation System Necessary or Not?

Hello everyone,
we are planning to energetically upgrade a house built in 1978.

The goal is to achieve the KfW 85 standard (after consultation and calculations by the energy consultant).

Current situation:
Ground floor: 24cm (9.5 inches) expanded clay with 5cm (2 inches) internal insulation
Basement: 30cm (12 inches) concrete blocks

The exterior walls will be insulated with 10cm (4 inches) material with a thermal conductivity of 0.035 W/(m·K), and the ground floor with 10cm (4 inches) EPS insulation with a thermal conductivity of 0.032 W/(m·K).
The windows are already triple-glazed.
Additionally, a partial extension in timber frame construction will be added to meet the KfW 55 standard.

I have not yet discussed ventilation systems with my current carpenter and planner, with whom I want to carry out the project, as this topic has not come up so far. Since I am currently researching room climate and air conditioning (see: https://www.hausbau-forum.de/threads/Wärmepumpe-zur-raumkuehlung-in-kombination-mit-split-klimaanlage.42180/) and have contacted a company about this, they pointed out—independently of installing an air conditioning system—that I should definitely consider installing a ventilation system.

Does anyone have experience or recommendations on when a ventilation system is useful or even necessary, and when it is not?
What problems might I face if I decide against installing one?

I am generally inclined to prefer a decentralized ventilation system. This might even be retrofitted in the ground floor, which is not directly affected by the renovation (except for the insulation). I see a centralized ventilation system as very complex and demanding...

Best regards and thanks,
Christian

I would say it largely depends on how airtight the house ultimately is. In general, for a major renovation, I would always recommend installing a ventilation system, if only for the comfort. Mold prevention and building protection are nice additional benefits.
If you replace the heating system – which you probably will or at least should do in an energy retrofit – under certain conditions, controlled residential ventilation can also qualify for subsidies.

Regarding the question of centralized versus decentralized systems: I would only consider decentralized where it is truly unavoidable. Does the old building have an existing chimney through which ducts can be routed? Are there one or two corners in the renovation where a duct could be installed? Is there an attic that allows access to the upper floor from above? A basement for the same purpose from below?

Thank you for your response.

What exactly would be the advantage of a centralized ventilation system compared to a decentralized one?

From my understanding as a layperson, a centralized ventilation system would require supplying all rooms that need ventilation with a duct. This seems quite challenging, especially on the ground floor where no structural changes are planned.

On the upper floor, which is basically being completely renovated, it would of course be feasible. There is also a chimney with a shaft that is no longer in use.

With the decentralized units, you would only need to drill holes in the respective rooms. From what I have read, such a unit is not necessarily required in every room.

There are many websites that list the pros and cons. At a glance, for me, it looks like this:

You say it is time-consuming to install equipment in every room. But it is also time-consuming to carry out core drilling in each room. Depending on the device, this means quite a large hole. Additionally, power supply needs to be provided at each location. And—depending on how the devices communicate—they may also need to be connected to each other. Then, you end up with the exterior façade covered with many vent grilles. These are all weak points in the masonry that can become damp. Replacing vents is not very convenient (although this is probably manageable). It is also more likely that you will hear the ventilation units. There isn’t just one fan in the attic, but one or two in every ventilated room.

Furthermore, for proper decentralized units with heat recovery, once you reach a certain number of rooms, the cost quickly becomes comparable to that of a central system.

I didn’t mean "ventilator replacement" but "filter replacement."

Flitz86 schrieb:

Does anyone have practical experience or recommendations on when a ventilation system is useful or even necessary, and when it is not?

If you renovate to KfW 85 standards, you will also get new, airtight windows – I assume a blower door test will be done as well. In that case, a ventilation system is definitely advisable. It is not absolutely necessary if you can manage proper and sufficient ventilation on your own. However, for comfort reasons, it is highly recommended.