Why does mould keep growing on my ceiling, and how do I stop it?

Mould grows where the ceiling has a cold spot. Warm, moist air from inside your house rises and meets that cold surface, the moisture condenses out onto it, and mould feeds on the damp. The cold spots are almost always the places where insulation is missing or thin: a gap between batts, a strip left uncovered, the line over a wall plate. That's why ceiling mould so often shows up in straight lines or patches that map the gaps in the insulation above: you're literally seeing where the cold bridges sit. The Australian Government's yourhome guide links these thermal bridges directly to condensation, and the ABCB's Condensation in Buildings Handbook warns that condensation in concealed spaces breeds mould. Fix the cold spots, and the leaks and ventilation feeding the moisture, and you take away the place mould likes to grow.

Because the lines map the gaps in the batts above. Insulation batts are laid in rows, and where two batts don't quite meet, or where one's been cut short, squashed, or left out around a downlight, there's a thin strip with little or no insulation over it. That strip stays colder than the rest of the ceiling, so when warm moist air hits it the moisture condenses there first, and the mould grows along that exact line. It's one of the clearest signs in the trade that the insulation is gapped rather than full-contact. You don't get those lines under a properly pumped, seamless cellulose ceiling, because there are no cold strips for the moisture to pick out. If you can see the mould drawing a map of your batts, the batts are the story, not a stain you just need to wipe off.

Cleaning it off treats the symptom; the mould comes back because the cold spot is still there. To stop it you have to remove the conditions it needs, and the big one on a ceiling is the cold surface where house moisture condenses. Full-contact pumped cellulose covers the whole ceiling as one seamless blanket with no gaps, so you don't get the cold strips where insulation is missing or batts leave bits uncovered. Take away the cold spot and you take away the place the moisture settles. To be straight with you, this is a thermal-gap fix, not a mould spray, cellulose isn't a mould killer, it removes the cold bridge that grows the mould. So alongside getting the insulation right you also want to fix the moisture side: roof and plumbing leaks, and ventilation in wet rooms like the bathroom, kitchen and laundry.

Plenty of moisture problems, and they often stack on top of the cold-spot issue. A slow roof or flashing leak wets the ceiling from above. Poor ventilation lets humid air from showers, cooking and the dryer build up and find the coldest surface to settle on. Blocked or absent exhaust fans, a sealed-up house in a humid SE-Queensland summer, and condensation in the roof cavity itself all feed it. The honest answer is that insulation is one part of the fix, not the whole of it. You need to deal with leaks and ventilation too, or the moisture will just keep finding somewhere to condense. The Australian Government's guidance is clear that thermal bridges and condensation go together, so a full-contact ceiling and good moisture control work as a pair. Fix the cold spots and the moisture source together and the mould has nothing left to grow on.