Air is a mixture of gases including water vapour. At any given temperature there is a limiting maximum vapour pressure, called the saturated vapour pressure, where the air cannot hold any more water vapour.
If air, containing water vapour, comes into contact with a cooler surface the excess moisture is deposited on the surface as condensation. The surface temperature at which condensation begins to form is called the “Dew Point”.
The ratio of the water vapour pressure at room temperature to the saturated
water vapour pressure at that temperature is known as the “Relative Humidity”, and is expressed as a
percentage. This is a measure of the water content of the air at a given temperature. Many
domestic activities (such as cooking and washing) which liberate water vapour increase the
relative humidity and thus the occurrence of condensation. In cold temperature climates, condensation
can occur on the inside of windows when the external temperature falls significantly below
temperature and cools the glass.
The four parameters for predicting the conditions under which condensation will occur are:
- Inside air temperature.
- Outside air temperature.
- Indoor relative humidity.
- U Value of the glazing.
The onset of condensation on the interior glass surface can be controlled either by reducing the humidity, thereby lowering the dew point, or by raising the inside glass surface temperature. The latter can be achieved by increasing the insulation of the window, such as by the use of an IGU as the lower the U Value the less the risk of condensation forming. Therefore an IGU reduces the likelihood of condensation forming by providing a thermal barrier between the inside and outside. The use of Argon Gas and Low E glass further enhances the U Value performance and thus the onset of condensation.
Refer Section 11.2.5 for condensation prediction chart.
Note : A maximum relative humidity of 70% is recommended for a healthy environment
and dehumidification may be required above this level.
Gauges are available from instrument supplies to
read temperature and relative humidity.
7.8 Design Limitations
Spacer Types, Sizes and Colour
GlassTech® & GlassTech® Low E+ Aluminium spacers are available in silver and black in 6, 8, 10, 12, 14, 16 and 18mm.
GlassTech® Argon+. As above but 8mm minimum.
GlassTech® Thermal+ Thermix spacers are black in 10, 12, 14 and 16mm thickness.
GlassTech® SF Silicone Foam spacers are black only and available in 6, 8, 10, 11 and 12mm thickness. These are conversions from imperial sizes.
Maximum Unit Sizes
Wind loads and deflection, unit weight, handling and
glazing implications of a large IGU can often create limitations on the size of the units
available. The maximum glass size is also restricted by manufacturing equipment
and glass size availability.
To calculate the approximate weight use the following:
Height (m) x Width (m) x 2.6 kg/m2 x
[thickness outer (mm) + thickness inner (mm)]
e.g. for a 2m x 1m unit = 2 x 1 x 2.6 x (6 + 6) = 62.4kg
Note: As a guide units should not be over 250kg.
Handling and glazing large heavy IGUs is best done with strops so both panes are supported
but in some cases suckers are required. If suckers are used on one side only
they can put shear stress on the IGU seals and damage them.
The thickness and weight of large IGUs can also put severe restrictions on the type of joinery, especially in the case of sliding and pivot doors. In some sash windows special stays, rollers and hardware are required to hold the windows open.