How do I know if my cavity wall insulation has failed?

Are you seeing damp patches on your interior walls, or noticing drafts and cold spots that weren't there before? Sometimes you'll also spot discoloration or that musty smell which usually means moisture's gotten in there and the insulation's absorbed it like a sponge.

The damp patches thing is real. But honestly, the biggest tell for me after nearly two decades is when you start getting cold spots on exterior walls that make no sense - like you'll feel it on your hand before you see anything. Sometimes the insulation just settles or clumps up, especially if it's mineral wool, and leaves gaps. If you're getting condensation on windows regularly or that musty smell, it's probably failed. Best move is get a thermal imaging camera in to check - they're not expensive to rent and show you exactly what's happening in there.

I haven't dealt with this myself since I'm renting and pretty new to homeowner stuff, but from what I've picked up talking to neighbors and doing some research when we were looking at places, the big signs are basically what's mentioned above - damp patches, cold spots, and that gross musty smell. One thing I'd add is to pay attention to your heating bills too; if they suddenly spike way up for no obvious reason, your insulation might be letting air escape. If you're seeing any of this happening, it's probably worth getting someone in to have a proper look before it gets worse and costs you even more.