ᐅ House and garage: should the gap between the two structures be closed?

The situation: The main structure of the house is made of Ytong blocks, plastered. The garage is a precast concrete garage. Both are positioned close together, tightly adjacent.

The part of the house where the garage meets is not plastered, so the Ytong blocks are exposed. Should the gap be sealed with PU foam or silicone, or left open for ventilation? What is the professional recommendation?

I’m adding to this. The builder wants to apply a PU sealant with Sikaflex or a similar product. That will ensure it is watertight. Karsten

Completely untreated Ytong? The water that gets in there probably won’t be good for the block over time.

"Gap" and "tight"
What does it mean in centimeters (inches)?

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Ytong pure. The joints are well protected since our roof overhang is quite generous. They are about 1 cm (0.4 inches) thick, roughly the thickness of an index finger.

A PU joint would mean completely sealed. PU material is truly waterproof, even durable underwater. Leaving it open would mean air could always get in, like with brick facing in front of Ytong. Karsten

A common expansion joint is 2 cm (about 0.8 inches), which is a good size to bridge with a joint. I would use a waterproof material to seal this joint. If necessary, install small vents at the top about every half meter (around 20 inches), such as thin empty pipes, to allow air circulation. These should be placed under the roof eaves and end about a hand’s width above the garage roof. At the bottom, at foundation level, install horizontal vents as well—one on each side. This is just a suggestion to prevent the bare Ytong from being exposed to stagnant air.

I had expected a gap of around 5 cm (2 inches), which would have required more effort.

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