How do I deal with damp when adding cavity wall insulation?

The thing is, damp isn't really a result of cavity wall insulation itself - it's more that poor installation or existing moisture problems get exposed or made worse once you add it. If you're getting damp after insulation goes in, something else is usually going on. The cavity should actually help with moisture management when done properly, so the real issue is making sure the work's done right from the start.

Before you get insulation installed, have someone inspect for existing damp issues. Check for blocked gutters, cracked downpipes, or ground moisture working its way up through the walls. These need fixing first because insulation won't solve water ingress - it'll just trap moisture against the inner wall. Make sure your surveyor actually checks the cavity depth and condition. If it's too narrow or already damp, cavity fill might not be suitable at all. Some properties are better off with external or internal insulation instead, depending on what's causing moisture.

During installation, the key is that the insulation material needs to be hydrophobic (water-repellent) if your property has any moisture risk. Mineral wool and some foam products handle this better than others. You also want proper ventilation in your rooms afterward - kitchens and bathrooms especially need extraction fans vented outside, not into the cavity. Make sure all penetrations (pipes, cables) through the outer wall are sealed properly. After installation's done, keep an eye on internal walls for a few months to spot any damp patches early. If you do get issues, you'll want to know quickly so you can chase up the installer while they're still liable.