Avoiding Cold Bridging In Building Fabric And The Problems It Causes

According to BSRIA, cold bridging in the building fabric is a major cause of condensation and mould growth in buildings. Good design, specification and workmanship as well as thermal imaging on completion are essential in avoiding these problems.

Walls, roofs, doors and particularly window or door frames often include cold bridges that lead to cold internal surfaces. Surfaces that are below the air dewpoint, which can be as high as 18°C with air at 24°C and 70% humidity, attract condensation and if the humidity is over about 90% at the surface, mould growth can occur. More commonly in most health sector rooms in the winter, the air is only 50% saturated. With an air temperature of 22°C, moisture will condense out of this air on any surfaces below 11.5°C. Surfaces at a slightly higher temperature, 12°C in this case, are not cold enough to cause condensation but present ideal conditions for mould growth.

There are several varieties of mould that can grow on the inside surface of buildings in certain conditions and some can be toxic:

• Aspergillus refers to a group of moulds that are found worldwide occurring naturally on decaying fruit and vegetables. They are very common in the autumn and winter in the Northern hemisphere requiring temperatures over 15°C and humidity over 90%. Most healthy people are naturally immune to its effects, however weak or already ill people can be susceptible. The most problematic mould varieties are Aspergillus fumigatus, Aspergillus niger, spergillus terreus and Aspergillus flavus. The mould can cause allergic reactions in the bronchia and lungs, grow in the lungs and sinuses, and become invasive in immunosupressed people.
• Penicillium grows in damp, but not wet conditions. Some species produce toxins and may render food inedible or even dangerous. However it is rare on building surfaces and most species do not affect most humans.
• Black mould, Stachybotrys chartarum is a cellulose-decaying fungus with worldwide distribution. It grows well at room temperature and with humidity above 93%. It can cause pulmonary hemorrhage (PH). It requires water-soaked cellulose (wood, paper, and cotton products) to grow - typically from flooding, serious plumbing problems, or roof leaks - and takes about two weeks to start amplifying. While wet it looks black and slimy perhaps with white edges; when dry it looks less shiny. However, it is rarely a problem.

High ventilation rates can help to reduce humidity and therefore reduce the risk of mould growth, but this goes against energy efficiency good practice.

The safest way to avoid mould growth on building surface is to avoid cold bridges. Many commonly used building components and details are no longer appropriate for use in buildings with low leakage rates and significant moisture loads. One example of cold bridges that can be avoided is the mortar joints between lightweight concrete blocks. In the images shown of a corner of a room, the mortar joints show as 18-19°C whilst the air temperature is 23°C. In this room the location mortar joints was clear even without thermal imaging because of the mould growth along the joints in exactly the same pattern as shown in the thermal image. Thin bed jointing techniques can avoid these problems and speed up construction.

Thermal imaging after construction can be used for quality control to ensure gaps have not been left in insulation or lightweight blockwork. Research has led to the development of 'critical surface factors' (CSF) for internal wall surface temperatures. With a CSF of 0.75, appropriate for dwellings and health buildings, at least 75% of the temperature drop from inside to outside should be across the structure and insulation of the wall. This leaves slightly less than 25% between the inside air and the inside surface, 5°C in the example shown. Any internal wall surface below 18°C would indicate a defect. Therefore most of the wall shown is satisfactory, only the corner is likely to suffer from mould growth. However, if the room was a bath or shower room or swimming pool the CSF would be 0.9 and any surface below 21°C could suffer from mould growth. These factors allow thermal imaging to be used as a quantitative assessment of building quality and its use as such is rapidly increasing.