This hopefully gives a visual demonstration using thermal imaging and graphs, and shows why mould and condensation related issues occur in certain places in buildings. I often find surveyors complicate things about mould and condensation, when really if you understand the basic issues it can be pretty simple to rectify. Simply moving furniture off cold walls and perhaps getting a balance between heating and ventilation…….. this will need another blog post, as most of the ventilation systems I see are not compliant with Approved Document F.

Micro climate

Every room in a house can have differing atmospheric conditions, these atmospheric conditions dictate certain conditions for moulds and condensation occurring.

This can also be referred to as ‘micro climates’ within a room, for example mould is often noted around window reveals on a ‘problem house’, we all know the ones. One of the reasons for this can be simply because on a cold winter night when the temperature drops, especially if there is a thick curtain, the relative humidity will rise in this ‘micro climate’ due to the drop in temperature. The temperature on the room side of the curtain can have quite a significant temperature difference (higher) and subsequent lower relative humidity.

Another example is when a wardrobe, furniture, bed is pushed up tight against a wall, particularly cold (north and east facing) external solid walls, especially if there are cold bridging issues. We have exactly the same principles as the window explanation.

Another micro climate can be in the vicinity of a radiator that is on, where the air is warmer around this area the relative humidity will be lower than behind the cold wardrobe or around a cold window.

If a room has an average relative humidity of around 70% relative humidity, the boundary layer of air can be around 5-10% and sometimes a lot more higher (see the last graph) which will then possibly put certain materials at risk of mould issues.

Mould growth needs certain conditions, in my experience there is a risk of mould if the relative humidity is in excess of 75% -80% for prolonged periods on certain materials, most particularly with lack of air flow / movement this is enough to support mould growth.

The below graph shows the temperature and relative humidity in three different places in the same room of a building, this is visually demonstrating micro climates in one room. In this graph you can see the areas that have a colder temperature, also have the higher relative humidity, where there is the risk of mould issues.


The below thermal image also gives a visual demonstration to show that behind the settee – now just removed away from the wall – is colder, and has a higher relative humidity simply due to the lower temperature as a result of a lack of heat input from air circulation. The mould was only growing behind the settee.

FLIR1131        FLIR1130

The below graph is very interesting, this shows the difference in relative humidity in the boundary layer of air and the room from the colder months through to the warmer months. The difference can be in this case up to 30% relative humidity in the colder months and then when the warmer weather occurs the relative humidity is around the same. This shows how the temperature plays a massive part regarding relative humidity.


Posted by Complete Preservation


  1. Thanks for writing this article.
    This has really helped us understand our problem.



    1. No probs Tess, glad it has helped you 🙂


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