Comparison tests of various central controlled residential ventilation systems + additional questions

Oh yes, the breathing walls. Some marketing nonsense just won't die.

Vapor pressure gradient between the inside and outside of the house

The denser and colder a building material is, the more water vapor from the indoor air condenses on it as condensate. Persistent moisture can lead to building damage. Materials that initially absorb moisture and gradually release it again regulate the indoor climate – they are said to be open to vapor diffusion.

House occupants cook, shower, sweat: in a four-person household, 10 to 12 liters of water vapor fill the indoor air per day. Its gaseous molecules distribute evenly in the room and create a certain vapor pressure, the relative humidity. Vapor pressure can also be measured outdoors. If it corresponds to the relative humidity inside the house, an equilibrium exists. If the vapor pressure inside the house is higher or lower than the relative humidity outside, this is called a vapor pressure gradient. The vapor pressure escapes to the lower-pressure side of the component; it diffuses. This creates an equalizing flow, diffusion.

Resistance to vapor pressure

Building materials offer different levels of resistance to vapor. The strength of this resistance compared to a 100 cm thick layer of air is indicated by the vapor diffusion resistance factor µ (mu) – the smaller it is, the easier water vapor can penetrate. Dense material structure with a vapor diffusion resistance factor over 100 µ is considered a vapor barrier, for example bitumen sheeting. Vapor retarders are materials that slow vapor diffusion but do not prevent it.

Moisture buffer

Building materials or components that offer little resistance to vapor pressure are called diffusion-open. Builders who desire “healthy” living appreciate the advantages of walls, ceilings, and roofs permeable to water vapor: if the indoor air is moist, they store water molecules on the surface. Conversely, they release moisture to the indoor air when it is too dry. The buffer works slowly, within hours or even days, and only about 2 percent of the indoor humidity actually passes through the component.

Diffusion-open components, airtight envelope

You cannot get rid of cooking steam and bathroom vapor by diffusion, since vapor molecules do not move through components by airflow – there are no “breathing” walls. If there is a draft, the house has open joints and loses unnecessarily much heating energy. Therefore, today great attention is paid to an airtight building envelope. The Energy Saving Ordinance (Energieeinsparverordnung) mandates it for new buildings. An advantage with risks, which makes proper ventilation all the more important.
Source: Haus.de

A little story from the neighborhood: Neighbor built a house with a ventilation system and decided against it in the basement. Now he is having the ventilation retrofitted in the basement for a lot of expensive money. Obviously, the direct comparison has convinced him that it is worth spending a lot of money on ventilation.

I always find the whole thing about diffusion-open wooden houses amusing. We are building a solid house with a 25 sqm wooden stud extension. So the area that is clearly the least diffusion-open is probably the wooden stud extension, OSB boards, sealing tapes, adhesive tapes, waterproofing, insulation between two vapor barriers, and drywall on the inside. Pure nature

In contrast, the solid house features thin-web unfilled bricks, interior lime-gypsum plaster and silicate paint, exterior plaster and silicate paint only. No adhesives - no films.

I would still accept a log house as ecological, but as soon as any insulation materials come into play, vapor barriers are also necessary.

Back to the topic: You can already tell just from the screed that it takes a long time to ventilate out the moisture in a tight house; we currently would not want to build without ventilation. Especially since the costs for us are manageable at 2.5% of the total project costs and much cheaper than retrofitting.

So yesterday I was once again with another builder. He also builds diffusion-open. Since I definitely want that, I only go to such providers. Maybe that's where the mistake lies In real life, everyone advises me against controlled residential ventilation. Here in the virtual world, everyone definitely advises me to get controlled residential ventilation. You can guess what the builder said. He installs controlled residential ventilation for only 10% of his customers. It is not necessary with his diffusion-open construction method with wood. I am very grateful for your experience reports. Maybe you always also write whether you have a vapor barrier in the walls or, as my builders always say: Do you live in a plastic bag or a Gore-Tex jacket?

We build completely diffusion-open in wood, without any plastic bag. Our building supervisor clearly advised us to get controlled residential ventilation, even though that was clear to us from the start. You can never retrofit the added quality of life! I’m so excited to finally live in well-ventilated but warm and not stuffy rooms in winter! And I’m not worried about electricity consumption at first; the photovoltaic system with storage will take care of it

I always find the thing about the diffusion-open wooden house funny. We are building a solid house with a 25sqm wooden stud extension. So the room that is clearly the least diffusion-open is probably the wooden stud extension, OSB boards, sealing tapes, adhesive tapes, sealants, insulation between 2 vapor barriers, and drywall on the inside. Pure nature

But then that is not a diffusion-open wall construction! Our wall construction in wooden studs from inside to outside: clay plaster boards - OSB - stud framework with mineral wool - wood fiber insulation - plaster..... no vapor barrier or any other plastic crap. Considered a very good wall construction by the independent building inspector of the Homeowners Protection Association.