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The Causes of CondensatIon

Air contains moisture in the form of water vapour. The higher the air temperature the greater the amount of water vapour that the air can contain. Conversely at lower temperatures, the air is able hold less water vapour.

The relative humidity (RH) is a way of expressing how much water vapour is in the air for a given temperature. It is expressed as a percentage of the amount of moisture that would saturate the air at that temperature. A relative humidity of 100% means that the air is saturated and can hold no more moisture or water vapour at that temperature. If the temperature of the air is increased, its ability to hold moisture increases, and so the RH decreases even though the amount of moisture in the air has not changed. Conversely, if the temperature is reduced, the RH will increase for the same moisture content. With decreasing temperatures the saturation point (100% RH) will be reached. any 'excess' water vapour in the air above the saturation level will condense or turn from a vapour to a liquid. The temperature at which saturation occurs and any excess moisture in the form of water vapour condenses is called the dew point.

Thus, the main causes of condensation are changes in the temperature and/or moisture content of the air. These may occur naturally through changes in atmospheric conditions or as a result of indoor activities such as cooking or industrial processes.


Types of CondensatIon

Condensation in buildings may cause problems in two main ways:

Surface condensation is where water vapour condenses on a cold building element such as a window pane or wall that has a surface temperature below the dew point. The cold surface temperature of a building element locally depresses the adjacent air temperature to below the dew point and any excess moisture that the colder air can no longer hold condenses onto the cold surface. alternatively, a brief hot shower for example can cause condensation in a cool bath-room by increasing the moisture content beyond the saturation point or dew point for that temperature. Note that the room may feel warm but still be subject to a condensation risk. In buildings of high-thermal-capacity construction such as cavity brick masonry and concrete panel, heated intermittently, the depression of temperature at the internal surface will often be greater, as the element does not have sufficient time to warm up and remains cooler relative to the indoor temperature. also, with single-leaf walls, depending on the temperature difference, it may be difficult to heat the wall. In these situations the design of the element with respect to insulation, air gaps and vapour barriers needs to be considered more carefully.

Internal or interstitial condensation is condensation occurring inside the building element. Depending on the water vapour pressures, water vapour can be moving inward or outward through any building element. If the temperature gradient through the element is such that at any point the temperature falls below the dew point then the excess moisture that can not be held as vapour will condense within the element. When conditions change, the water evaporates into vapour and continues to move through the element.



The consequences of condensation are mostly negative for the building owner and user. If persistent, condensation on the surface of walls or roofs can create a visual problem through the damage it can cause to furnishings and fittings and unsightly mould growth. The effects of interstitial condensation are more long term but potentially more damaging as it can cause the deterioration of the fabric of the building itself. Walls and roofs become mouldy and the smell of decay and deterioration of air quality may cause health problems.


The solution

The efficient way to prevent the risk of condensation is to design it out by preventing moist air coming into contact with cold surfaces.

Ways to do this may include:

Providing improved ventilation to reduce or control the RH and internal vapour pressures and hence reduce dew point gradients.

Providing adequate heating to increase structural temperatures. This may be difficult for buildings that are not occupied all the time and heated for only short periods in the evenings as there is insufficient time to increase the structure temperature of the wall. Heating should also be throughout the building, not just in the living areas.

Providing insulation (in conjunction with heating) to assist in warming cold surfaces by controlling the loss of heat through the wall/floor.

Reducing quantity of moisture produced and/or remove it at the source to control the RH and vapour pressure.

Preventing moisture moving to colder areas of the building.

Avoiding thermal bridges.

Providing vapour barrier on internal face for cold climates. Condensation will occur only if the surface temperature falls below the internal dew point.

Providing permeable layer to outside surface to allow evaporation of water, and prevent accumulation of water within wall by selecting coatings carefully.

Whichever way best suits the project you are working on, JPA Designer Condensation Risk Analysis software will provide you with the ability to calculate the thermal transmittance of building elements in order to predict the rate of conduction heat loss through the element and analyse heat loss and condensation risk for all your design options.