Persistent Mildew on walls

I have personally experienced the ‘unexplained’ sudden growth of mildew on the walls of one room in particular.
What could possibly be causing this?, I ask myself, I followed all the right steps and left the air gap near the eaves!

Further investigation identified the problem: Lack of loft ventilation.

I have seen this problem over and over again in the houses that I Energy Audit or provide Building Energy Ratings for. The cause may seem such a small thing, but it has a massive impact.

My personal experience:

Build elements that changed, causing the problem:

• I properly insulated our loft space to 300mm of insulation
• I checked to make sure that I had left the area closest to the eaves free of insulation to allow the loft to breath (50mm is sufficient)

Contributing problem:

• The builder had placed air vents in the soffit of the eaves when he built the house, but did not leave any holes/gaps behind some of these vents to allow air ingress into loft space.

The effect the changes I made had on the house (being unaware of the contributing problem):

• Before the additional insulation, enough heat was escaping through the ceiling into the loft to keep it warm and thereby prevent condensation from occurring in the stagnant air in the loft.
• The minute I properly insulated the loft, the heat was no longer escaping into the loft space, which was great from an energy efficiency perspective, but enabled the stagnant air in the loft to cool.
• The moisture in the stagnant air was not a problem when warm, but when it got cold, condensation occurred on the underside of roof tiles and on gable walls. This condensation worked its way down the gable wall, past the ceiling and into the room below, causing damp patches on the wall and subsequent mildew growth.
• A soon as I found the problem of lack of ventilation and rectified it by making holes behind the air vents, the loft space lost the damp, stagnant air and the wall dried up.

No more mildew.