The Psychrometric Calculator 

Below is an explanation of each parameter calculated by the APPs psychrometric calculator to produce figures used by the matrices.

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AIR TEMPERATURE

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Throughout the winter months an occupied property should ideally be maintained between 18 – 21 degrees in the occupied areas. By maintaining a warm internal environment, the air will have sufficient capability to retain the water produced by occupation and provided the building envelope is sufficiently insulated the surrounding floor, wall and ceiling surfaces should also be able to maintain an acceptable temperature.

 

SURFACE TEMPERATURE

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The temperature of the surfaces internally is directly related to the temperature of the atmosphere within the building and the level of insulation. The surface temperature, however, may also be influenced by the temperature outside the building. A reduction in temperature internally or externally in addition to absent or defective insulation can result in the conditions at the surface internally being conducive to mould and condensation.

 

RELATIVE HUMIDITY

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Relative Humidity (RH), is the ability of the air to retain water vapour; it is relative to its temperature. It is a measure of ‘saturation’ and expressed as a proportion (%RH). Therefore, when we say the humidity is 60% RH (Relative Humidity) we’re basically saying the air is 60% saturated in comparison to its total holding capacity (100%) at that temperature. We consider ‘dry air’ in our homes to vary between 40 - 60% Relative Humidity. Relative humidities of 75% and above increase the risk of mould growth.

 

 VAPOUR PRESSURE

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Vapour pressure is a measure of the amount of water vapour in the air which is not related to temperature. Excess production of water vapour will be represented by an increase in vapour pressure.

 

VAPOUR PRESSURE EXCESS

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This is the difference in vapour pressure between the internal and external environments. The vapour pressure internally is the sum of the external + indoor atmospheric moisture levels. By measuring and removing the external level we are left only with the measurement of indoor moisture production, this is presented as vapour pressure excess and reflects the moisture solely produced through occupation. Studies have shown that a vapour pressure excess of greater than >0.5 kPa is considered excessive. Excess Vapour Pressure is a good measure of ventilation.

 

MIXING RATIO

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The mixing ratio is the mass of water vapour in relation to the mass of dry air and is a direct measurement of water molecules in the air which is not related to temperature. Mixing ratios are usually measured in grams of water per kilogram of air or grains per pound.

Generally accepted mixing ratios vary between 5.13 gpk (18°c @ 40%RH) – 9.33gpk (21°c @ 60%RH).

 

DEW POINT TEMPERATURE

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Cool air cannot retain as much water vapour as warm air. When the air reaches its maximum holding capacity of water vapour at a specific temperature, we refer to the air as ‘saturated’. As cool air cannot retain as much water vapour as warm air, cooling causes the excess water vapour to be dropped as liquid water; this process is referred to as condensation. By evaluating the relationship between the air temperature and the surrounding surface temperatures we can establish when condensation might occur. The temperature at which condensation is determined is variable to the atmospheric conditions and is referred to as the ‘dew point’.