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Treating the Cause of Mould for Good: Why Wall Surface Temperature Decides

Treating the Cause of Mould: Wall Surface Temperature

Treating the Cause of Mould for Good: Why Wall Surface Temperature Decides

If mould is wiped away but the wall stays cold, it returns within a few weeks. The reason is not the fungus but the moisture available to it.

Warm room air holds more water vapour than cold air. When it meets a cold wall surface it cools there and can no longer hold that vapour, which settles as water. The process repeats daily for as long as the wall surface is colder than the dew point of the room air.

What the dew point means

The dew point is the temperature at which air can no longer hold its water vapour. At a room temperature of 20 degrees and 60 per cent relative humidity it sits at around 12 degrees.

Any wall surface colder than that becomes damp. Tackling mould therefore comes down to a single variable: the temperature of the wall surface. Keep it safely above the dew point and no condensation forms, which leaves no moisture for mould to live on.

Key point  It is not humidity alone that decides, but the relationship between humidity and wall surface temperature.

Why ventilating and heating alone are often not enough

Ventilating lowers absolute humidity and is fundamentally correct. Heating warms the room air and raises how much moisture it can hold. Neither, however, changes the surface temperature of a cold bridge by much.

At a projecting concrete slab, an uninsulated external corner or a window reveal, the surface stays noticeably colder than the rest of the wall even in a well-heated room. That is exactly where mould continues to form while the rest of the room stays dry. Behind a wardrobe against an external wall the effect is stronger still, because warm room air cannot circulate.

The four routes to a warmer wall surface

There are four methods that work in building physics terms. They differ not in principle but in how invasive they are, how much work they involve and how large an area they can cover.

Comparison of methods

Method
Principle
Intervention
Limits
Method Internal insulation
Principle Reduces heat loss so the surface stays warmer
Intervention High: build-up, lining, loss of floor space
Limits Poor execution causes condensation behind the insulation
Method Calcium silicate board
Principle Capillary-active and vapour-open, raises surface temperature
Intervention High: full-surface bonding, plastering, painting
Limits Labour-intensive, permanently alters the room, wall must be clear
Method Wall heating
Principle Warms the wall directly across a large area
Intervention Very high: installation and connection to the heating system
Limits Major building work, rarely retrofittable in existing stock
Method Infrared wall drying
Principle Raises the wall surface locally above the dew point
Intervention Low: position and plug in
Limits Effective only on the sections fitted; running electricity costs
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    FAQ

    Frequently asked questions & answers

    How warm must a wall surface be to prevent mould from developing?
    • It must remain above the dew point of the room air. At a room temperature of 20°C and 60% relative humidity, the dew point is approximately 12°C. In practice, a safety margin of several degrees is recommended.
    Is increasing the heating enough?
    • A higher room temperature can help, but it has only a limited effect on the surface temperature of a thermal bridge. The coldest area will still remain the coldest area.
    Is a calcium silicate board better than an infrared panel?
    • Both increase the surface temperature of the wall. A calcium silicate board works permanently without running costs, but requires substantial building work. An infrared panel can be retrofitted without structural alterations, but it does incur electricity costs.
    Can internal insulation cause mould?
    • Yes, if it is installed incorrectly. If the dew point shifts into the wall structure, moisture may condense behind the insulation and remain unnoticed.
    Does a dehumidifier help prevent condensation on a wall?
    • It lowers the humidity and therefore the dew point, but it does not warm the cold wall surface. This is usually insufficient where there is a significant thermal bridge.
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