A few years ago, my son had a friend insulate his attic. He recently discovered that a great deal of mould has since grown as it seems the attic was never properly ventilated. What can he do to get rid of the mould and prevent it coming back?
The attic is an interesting source of additional space, particularly if it has the headroom to provide for a room that complies with building regulations, namely that it has a ceiling height of 2.4m for more than half the floor area. This ceiling height requirement is to ensure adequate ventilation of the space (Part F of the Building Regulations).
But even if it isn’t suitable for habitable accommodation, it can be a useful storage space. For the purposes of my answer, I have assumed it is being used as a traditional attic storage facility for the Christmas tree, suitcases, kids’ school copy books and the like.
Due to our damp climate, mould can form for a number of reasons, namely, inadequate insulation, inadequate ventilation and inadequate heating. More often than not it’s a combination of all three.
Dealing first with inadequate insulation as a cause of dampness, consider the appropriateness of the insulation your son’s friend put in. For example, sheep’s wool will allow some moisture movement and is good for older buildings. Vapour-resistant rigid foam boards (also known as PIR boards) are good but must be fitted correctly to prevent condensation. And don’t forget the attic hatch; make sure it is well sealed and insulated to prevent warm, moist air from escaping into the attic from below.
Consider also that cold bridging may be causing a “cold spot” in the perimeter extremities of the attic space. Take care to insulate around pipe penetrations, rafters and joists effectively, using insulated board as required.
Consider removing the attic floor insulation (assuming this was used) and insulating the roof at rafter level. It will cost more to heat the house with this increased volume in the attic, but worth it if you use the space. When fitting the new insulation, you must take care that if going between the rafters, the insulation does not impede the natural ventilation between it and the slate or tile roof covering.
This brings us to the next cause of dampness on my list: inadequate ventilation. Perhaps the insulation that was installed has impeded the airflow from the soffit vents to the ridge at the peak. In most older houses, soffit ventilation is the most common form of attic insulation but is made redundant once quilts of insulation are packed in at eaves. A good flow of air must be maintained between the roof covering and the new insulation.
Venting the attic can take many forms, including the installation of traditional open vents, fans or a mechanical ventilation system. Avoid blocking airflow with any new insulation, be that a vent or a ventilation gap. Consider gable vents if cross-ventilation is required or would help.
[ Mould loves Ireland’s damp climate – what can you do to fight its spread?Opens in new window ]
Check that current vents are clear and free from blockage (and not stuffed with newspaper). Also check if a mechanical fan in the bathroom or en suite is simply ducted into the attic space above and not out through a roof tile/slate ventilator.
The third main cause, inadequate heating, may be remedied by placing a heater in the space, ideally on a timer. The mould may be caused by warm air from the house rising and condensing in the attic insulation. In this case, consider installing a vapour barrier. This should be installed on the warm side of the insulation (facing the living space) to prevent warm, moist indoor air from reaching the insulation layer. Use an airtight seal at joints to close gaps where moisture could enter.
Be sure to check for existing leaks, whether from a missing or broken slate or tile, poorly pointed or bedded ridge and hip tiles, leaking valley or parapet gutters or leaking chimney flashing. Check also that any roof timbers in proximity to these leaks are not affected by timber decay. Can these dry out properly? And, of course, check the water storage tank and associated pipework for leaks.
If your insulation and venting work in unison, and there is no way for water to leak in, you should not have a mould problem in the attic.
Pat McGovern is a chartered building surveyor and a member of the Society of Chartered Surveyors Ireland
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