Been working in waterproofing for years and the most common source of damp problems I get called in to fix isn't poor waterproofing on the roof or the main walls — it's the joints. The small gaps where a window meets the wall, where two different materials meet, where a facade has an expansion joint. That's where water finds its way in.
Most of the time when I investigate, there's either nothing sealing that junction properly, or someone used foam tape or silicone that's since cracked and shrunk. Both fail within a few years especially if there's any structural movement or temperature swings.
What actually works at these joints is EPDM rubber strips. It's a thin flexible rubber material that gets bonded or mechanically fixed across the joint. It stays flexible across a massive temperature range, doesn't crack with UV exposure, and stretches with movement rather than tearing. I've pulled out foam tape from window perimeters after 3-4 years that's basically disintegrated. EPDM on the same building from 10+ years ago is still doing its job.
It's also what should be used as a Damp Proof Course in masonry walls — that horizontal barrier that stops ground moisture from wicking up through brickwork. A lot of budget builds skip this or use cheap felt that doesn't last.
The fix once damp gets in is always 10x more expensive than doing the junction detail right the first time. If you're building, renovating, or getting new windows installed — just ask what's being used at the joints. If they look at you blankly, that's your answer.
Happy to answer any questions if someone's dealing with a specific damp or leakage issue.