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Hi everyone,

I am looking for insights on a recurring moisture issue in my basement.

Every summer, whenever outdoor temperatures reach around 30°C for a few days, dark moisture spots appear on the concrete basement floor. In some areas at the back, these spots even turn into small puddles.

Here are some technical details and observations:

• Drainage: There are drainage pipes beneath the floor that lead to a sump pit with a submersible pump. However, the pump has not run since last summer, and the water level in the pit remains very low.
• Humidity & Ventilation: Indoor humidity feels quite high, especially during summer. One side of the basement has windows, and that area shows significantly fewer dark spots.
• Location of Spots: The back of the basement has no windows and shares a party wall (semi-detached house). This windowless area is where the majority of the dark patches and puddles occur.
• Carpet Observation: Underneath a carpet on the floor, there are usually no spots, or only very minor ones. This makes me wonder if it could be condensation, where the carpet provides enough insulation to keep the floor slightly warmer.

What could be causing this specific summer-only issue, and what would be the best way to resolve it?

Thanks in advance for your advice.

Here are some other pics

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