We've got a Gründerzeit apartment in Berlin from 1910 and we're wondering whether we should insulate the exterior walls from the inside. Our tradesman says it'll help with heating costs, but I've read that it can be problematic with mold. What's your experience with it? Is the effort worth it financially?
For a Gründerzeit apartment, I'd really be skeptical about interior insulation, especially with massive external walls like yours. The problem isn't just mold, but that thick brick walls can't dry out anymore with interior insulation - the moisture basically gets trapped. You're insulating on the inside, sure, but the wall itself becomes colder and can't get rid of moisture from outside (rain) anymore. When warm indoor air hits the cold wall in winter, the moisture condenses right at the interface between the old building and the insulation. That's exactly the situation where mold really thrives.
Financially, it often doesn't add up as well as you'd think for old buildings. Heating costs might drop by 15-20% or so, but the costs for materials and labor are substantial - and you'll need a really professional vapor barrier or vapor lock, otherwise you'll create more problems than you had before. Plus you lose valuable living space because the insulation takes up several centimeters.
I'd instead look first at whether there are cheaper alternatives - proper windows (if they're still old), seal doors, maybe external insulation in accessible spots like the roof or basement. With that kind of thing you often save just as much and have less risk to the building structure. For a proper assessment, I'd get an energy consultant to come to the house, someone with specific experience with old buildings - it costs relatively little and can save you a lot of money.
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