Why does my loft room keep going mouldy in winter ?

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Each spring we go up there and scrub the affected walls with mould killer but each winter it comes back

When we moved in the house hadn’t been lived in for 2 years and the loft hatch left open and there was no mould and no sign of previous mould

There is a window in the loft that we leave cracked open for extra ventilation but DH I think has occasionally shut this tight when bringing Christmas decorations down in December thinking that it’s too cold up there

Is it the shutting of the window (which he denies doing) that kicks the condensation off?

Mould = damp. Either from condensation (fingers crossed) or from a fault in your roof... I'd get a damp proofing specialist out to take a look.

The condensation will form in the coldest place in your house unless you have adequate extraction of moisture from cooking, showering and any laundry dried inside. We run a whole house dehumidifier and dry our laundry inside with no further mould issues (used to get bad mould on a bay window). We also have automatic bathroom extractors. I suspect that the window open was the previous occupiers solution.

Yes, almost certainly damp caused by condensation caused by lack of airflow.

Our conservatory has the same issue. The only way we have controlled it is by running a dehumidifier 24/7 during the winter months.

We have the same issue and apparently it’s to do with the outside temperature clashing with the inside temperature in the colder months. You can get insulation in the walls of the damp and mould don’t come in but it tends to ‘push’ the problem back into the wall where you can’t see it but it’s still there. Closing the window won’t help but it probably isn’t the root cause.

House is old Victorian and we use a dehumidifier downstairs if we are drying clothes and have a hygrometer to ensure the RH stays at around 55 throughout

The roofers have confirmed there’s no roof leaks or water ingress

Even in the wettest of summers it doesn’t go mouldy, just the cold winter time

The loft has previously be converted but back in the 80’s so insulation not up to scratch but it’d be huge job to sort out in the interim and in 2 years we are doing a proper modern loft extension that will fix it for good

So generally I think we need to leave the loft window cracked open (making sure DH doesn’t close it in December) to ensure the moisture that collects in the loft doesn’t condensate on the coolest part (the mould appears on the north facing wall)

We put a better deal around the loft hatch a few years ago but that hasn’t stopped it

Alternatively is there a way to increase ventilation in a plasterer loft room other than the window being locked open?

The other thing I was wondering is if it keeps coming back because we’ve never really treated the wall after cleaning off the surface mould

Should we be repairing with an antifungal paint after to prevent the regrow that each cold snap?

We've got condensation in our roof which has caused small leaks following really cold weather. We've had some ventilation bricks installed and are hoping that will work

If not could you get one of those vent fan things installed in the window? Or a trickle vent

Another option is a moisture absorber - they're by a brand called unibond and help reduce the amount of water in the air. There's a block that you need to change every few months. We used them in a student house year ago!

We've got a box room that seems to get significantly damper than anywhere else. It's driving me mad and I'm starting to consider a single room heat recovery ventilation system in there. Need to find how noisy they are though!