We bought and renovated 1year ago a stone house from 1901. In the room, only room exposed North, we changed a window (double glazing, wooden window) but not the small window (single glazed wood, About 30x30cm) next to it. Molds are growing under these windows. I thought to change the window, but seeing that there is also under the new window, is it useful? Is there anything else to do? On the other hand, I think the new window is not ventilated (no holes in it), I did not pay attention to the purchase, what can I ask the carpenter + a year after the pose? Is it editable on the window in place? (I would not want to replace a new window!).
You are in the case of all people who change their windows, make the door frames airtight, without ensuring the ventilation of your interior. As a result, the moisture-laden air condenses on the cold wall or at the location of the thermal bridges (here around the window), which favors the growth of molds. It is not normal that your "window" did not offer windows with ventilation slots. Contact your carpenter to reproach him, all the professional bodies confirming the need to take into account the needs of ventilation (see the recommendations of the UFME, union of manufacturers of exterior joinery). In your place I would take advantage of the replacement of your fenestron to establish this ventilation. Thermal bridges should also be treated with good insulation.
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