We recently found mould on the wall behind the wardrobe in my boyfriend’s mother’s house. We quickly cleaned and treated with bleach. The walls are covered in a polystyrene-type paper. About a week later we noticed tiny, white mites crawling all over the surface of her wardrobe. Having done some research it would appear that these were mould mites. I am at a loss to explain what has attracted them. What can we do to get rid of them? The house was built in the 1940s and has double glazing, central heating and cavity insulation.
Probably these insects are either Tyrophagus putrescentiae or T. longior, commonly referred to as mould mites. As the name suggests, they are species that feed on mould that can form on any surface, usually stored foodstuffs such as cheese or nuts but often multiplying on moulds growing due to dampness caused by condensation in buildings.
These insects have a short lifespan but can also multiply rapidly if conditions are right. In sufficiently large numbers they can cause skin or respiratory allergies. The good news is that they can be simply eradicated and if breeding conditions are removed they will not usually return.
To kill the mites a dilute solution of household bleach sprayed around the area as you suggest will usually work. However, unless the source of food, eg the mould, is removed then dormant eggs will hatch and re-infest the area quite quickly.
You say the house has had cavity insulation. There may still be cold spots which can be a location for condensation leading to mould, so you should get an infra-red thermal survey carried out to ensure the insulation is competent and there are no cold spots.
In any event, the construction of a 1940s house may only have 50mm cavities, so while it is still a good idea to insulate them, it falls far short of current demand for insulation value, and other solutions might still be needed.
You also mention that there is polystyrene thermal lining on the walls. This really has little or no energy-saving benefit, and can promote an ideal environment when condensation mould forms in the glue which the mites then feed on and multiply upon, so removal is the best policy.
One of the main causes of condensation in houses is high vapour pressure on cold surfaces. This is caused by increased temperature drawing dampness into the air from some source. In a 1940s house this typically could be the undercroft below timber floating floors. At that time there was usually no vapour barrier installed and the warmth in the house above creates an imbalance, drawing vapour upwards if the vents are compromised.
Improving air quality is a good way to reduce condensation. This can be done by installing heat recovery ventilation, but there are a number of items to get right if considering this solution.
If the problem persists ask your local building surveyor to investigate.
Fergus Merriman is a chartered building surveyor and a member of the Society of Chartered Surveyors Ireland.