My kitchen is an extension of the house. Most of its walls are outside walls. It's extremely cold in winter (down to 7 degrees celcius). Its unique window seems old too.
The problem is it's very humid, to the point that mold formed at several spots. Food inside cupboards has also been affected by the humidity. (for instance sugar is not powder any more but forms pieces).
Why is it so humid? Can the possibly reduced air tightness of the old window be the culprit?
It may be an addition built on a crawl space and not a real basement. Buildings on Crawl spaces tend to be more humid. Single pane windows condensate alot more than double pane which will put moisture into air.
Run a dehumidifier during colder months and A/C during warmer months to pull moisture out of air.
If you have a crawl space tell me about it. Type of vents, Flooring, insulation, how much lower than grade is it? Is it damp all time? Just after rains?
For starters, kitchens (and baths) are the humid parts of any house. That's where all the cooking happens. That's why it's important to have a hood vent in your kitchen. If you don't, that might be a big part of the problem right there...you're just dumping a lot of humidity into the air when you cook.
As you don't have any heating in the kitchen, that likely exasperates things as there's no air flow via the HVAC. So what humidity is in the kitchen stays in the kitchen.
Finally, if the walls are cold, that means there likely isn't adequate insulation in them. So what's happening is your humid air hits the cold wall in your cupboards, condenses, and then keeps them permanently wet, allowing mold to have a good ol' time.
As for how to fix this, the proper solution will be some combination of
Alas, that's not a small task.
Short term solution may to simply keep your window open in the kitchen when cooking. It will, of course, affect your heating bill. Also consider removing your cupboard doors. That will at least help with air flow so moisture doesn't get trapped in the cupboard.
