When AC ducts start to "sweat," it's usually because warm, humid air is coming into contact with the cold surface of the duct. Adding the right duct insulation and sealing the vapour barrier properly will stop this moisture from forming and help keep mould, ceiling stains, and wasted energy at bay. If your ducts are old, broken, or have thin insulation, switching to better materials that are more resistant to moisture can make a big difference. If you see wet insulation, musty smells, or damp spots in your roof space, you should have your ducting checked so you can fix the problem before it gets worse.