Thanks for getting in touch with the Belfast Property Blog! The ever-present problem of mouldy walls and ceilings is one that haunts the dreams of many a landlord and property agent!
It sounds like you have has good relationships with your tenants which is great. Many people - including model tenants with the best intentions - don't realise that all windows ideally should be open every day for at least a few hours. Now that may always be feasible in Northern Ireland due to our changeable weather, but it's a good target to aim for.
Make sure you have good quality PVC windows which allow windows to be open yet locked in place - check the locks and fastenings on all windows and then advise tenants to open all windows every day, weather permitting of course.
But that's just the first step in ensuring adequate ventilation in a property. Finer controls are also required to ensure lower levels of controllable background ventilation. This is best provided with trickle ventilators - small ventilation openings, usually incorporated in window frames. Check if your existing windows have trickle ventilators (if the windows were part of a recent refurb, I would expect these to be in place). Check that ventilators are clean and free of dust and dirt, to allow for the free movement of air in and out - it's easy for ventilators to become clogged with building dust during building work and refurbishment works.
Belfast City Council's Ventilation of Dwellings literature advises that "cooking, showering, bathing and laundry can allow high levels of moisture to be released into buildings. The use of suitable mechanical extract ventilation will help to ensure that moisture is not allowed to build up to harmful levels within the dwellings... To ensure free transfer of air throughout a dwelling, guidance recommends an undercut of approximately 10mm between the floor finish and the bottom of each internal door."
If the problem persists despite extractors being in place, serviced regularly and used correctly, trickle ventilators clean and working, and good practice with windows, you could consider providing dehumidifiers or fans if you're keen to prevent this issue. Use special mould spray (available at most good hardware stores) to clean off the surface mould and maybe give problems areas a coat of mould-resistant paint (again, your local DIY shop should have plenty of options here).
I hope this has been another useful installment! Do continue to send your questions and problems to me by joining the conversation over on our social media accounts, or by emailing me - all the links you need are on the top right hand corner of this screen.