Ventilation in the half-basement on the above-ground side

Dear forum team,

Our house is now standing, we have moved in and feel very comfortable.

My question concerns our basement, which, due to the slope, is half underground and half above ground, with the above-ground parts located to the north. It is a precast basement made of waterproof concrete with 12 cm insulation. The basement is thermally separated from the residential building.

Room 1 houses the building technology (air heat pump), room 2 is used for storage - these two rooms are not heated. Rooms 4-6 are equipped with underfloor heating (in case an apartment is installed there later), and room 5 already serves as a guest room.

With an unheated full basement, one must be very careful about humidity in the summer and should ventilate only briefly during cooler outdoor temperatures.

1. How should I handle ventilation in the partly above-ground part of the basement, especially in summer? Our terrace is right in front of the basement exterior door, and it would be quite impractical to always have to close it behind us.

2. How does heat and moisture exchange occur between the above-ground part of the basement and the rear rooms?

3. We currently do not have any doors in the basement. Should we install a special door between the "living rooms" and the "storage rooms"? (Provided the waste heat from the air heat pump allows it, we would like to keep the rear two rooms cool.)

4. The basement is currently refreshingly cool at 21 degrees, and I was considering leaving the stairwell door to the residential building open so that we can cool it from below with a fan. I know that this should absolutely not be done in a proper basement due to moisture issues, but how is it with our partly above-ground basement half?

Do we happen to have someone here who has already dealt with this topic or has experience?

Thank you very much and best regards from Alfeld Leine

2. How does heat and moisture exchange occur between the above-ground part of the cellar and the back rooms?

Basically, it’s not rocket science. The factors of indoor temperature, outdoor temperature, and relative humidity play the most important role. You can research the topic of dew point. As a basic condition, of course, everything must be properly ventilated.

Simply put, it means: Do not ventilate when the outside air is warmer and therefore carries more moisture. Of course, this can be done manually every day. But there are now ventilation systems that are dew point controlled. I would consider that. Without neglecting that air exchange in the back rooms must be ensured. But this can probably be implemented in an energy-saving way with one or two small fans that switch on when the outdoor ventilation activates.

Simply put, this means: Do not ventilate when the outside air is warmer and therefore carries more moisture. Of course, you can do this manually every day. But there are now ventilation systems that are dew point controlled. I would consider that. Without neglecting the fact that air exchange in the rear rooms must be ensured....

Ok, thank you very much. My question particularly concerns the mostly above-ground rooms. In the actual house, I open the windows in summer regardless of the temperature difference.

Can I ultimately also differentiate in the basement between areas lying in the ground and above-ground areas? So could I leave the door visible in the photo (currently still under construction) open even at higher temperatures if the doors to the underground rooms are closed?

Just as a warning, I am not a building physicist. I think you simply have to test what works and what doesn’t. It depends on many factors, including whether the basement is insulated or not, etc. But the cooler air from the basement floor usually always causes temperatures significantly below the above-ground part. Therefore, residual moisture with a 10 or 15-degree difference to the outside temperature will always first look for these cool spots in the basement to settle in comfortably. A constant draft, on the other hand, ensures that this does not happen so quickly. I would buy a proper hygrometer with a thermometer and just experiment. Unfortunately, you are not allowed to post links under threat of the death penalty. But as I said, take a look at the topic of the dew point. The ratio of humidity and temperature exactly determines when the air is able to carry moisture. Now you just have to know when the right moment is for it to happen from inside to outside and not vice versa. But even without tables and calculators, you can quite quickly get a feeling for when it’s better to keep the windows and doors closed.