Glass is different, in contrast with a number of other common materials such as metal, plastics or wood, glass exhibits brittle fracture, i.e. glass can break suddenly and without any apparent warning. This often unpredictable nature of glass breakage can be both spectacular and frightening when it happens, so we must and do design glass to be as safe as possible as economically as possible.

The first step toward designing architectural glass is to understand what glass actually is, so let us firstly take a look at glass as a material.

Glass as a Material

Glass is a liquid that has cooled to a solid state without any crystallisation occurring in the material. Glass has often been described by less well informed people as super cooled liquid, which it is not. A super cooled liquid is a liquid at temperature below which it would normally solidify, while glass is a solid with an amorphous random non crystalline structure. The use of the term “super cooled liquid” in relation to glass is seriously misleading as it implies that glass can flow. In fact glass is far too rigid to flow at normal temperatures.

Typical soda lime silica float glass (window glass) is comprised of the following primary chemicals;

• Silica (SiO₂) – 73%
• Soda (Na₂O) – 14%
• Calcium Oxide (CaO) – 7%
• Magnesium Oxide (MgO) – 4%
• Alumina (Al₂O₃) – 1%

Traces of secondary materials such as Potassium Oxide (K₂O), Iron Oxide (Fe₂O₂) and Sulphur Tri-oxide (SO₃) are also used to improve the glass surface quality and durability etcetera.

On an atomic scale window glass is basically a network of silicon-oxygen-silicon bonds, this network being randomly modified by the presence of calcium ions and sodium ions. The arrangement of these atoms is completely random, just as it would be in a liquid and is not orderly or regular like the molecules in a crystal of sugar, salt or ice. See Figure 1.

Because of this random network arrangement, glass is completely non ductile and it is this which sets glass apart from most other materials.

When stress is applied to any material it deforms through stretching of the interatomic and intermolecular bonds. The amount of deformation, or strain, depends on the arrangement of the atoms and molecules. At higher levels of stress most materials deform plastically, that is, the atoms or molecules which are organised in a geometric or linear formation can slide past each other. They can thus accommodate large strains without failure although they may be permanently deformed.

Solar Radiation

Not to be confused with insulation, insolation refers to the incident solar radiation that reaches the Earth from the Sun. At an average distance of 150 million kilometres from the Sun, the outer atmosphere of the Earth receives approximately 1353 W/m² of insolation (NASA 1971).

This varies by around +/- 2% due to fluctuations in emissions from the Sun itself as well as by +/- 3.5% due to seasonal variations in distance and solar altitude.

Atmospheric Effects

As solar radiation passes through the Earths atmosphere, some of it (25%) is absorbed and scattered by air molecules, small airborne particles, water vapour, aerosols and clouds. Some of the radiation is reflected straight back out into space (20%) but much more with increased cloud cover. The rest arrives at the surface of the Earth, where some of it is reflected immediately back into the sky. This amount depends on the nature and type of the actual surface – fresh snow can reflect up to 95% while desert sands reflect 35% – 45%, grasslands 15%-25% and dense forest vegetation 5%-10%.

Sometimes less than 50% of the solar radiation arriving at the outer atmosphere actually reaches the surface of the Earth.

It is the scattered component that makes the sky look bright and provides the ambient diffuse daylighting used in buildings. Without it, the sky would look as black as night with the Sun appearing a very large bright star occasionally passing through it. Most of the particles in the atmosphere that are responsible for scattering solar radiation are around 0.5 microns in size. As radiation with longer wavelengths is not affected by these particles, shorter wavelength radiation tends to be scattered more. This is what makes the sky appear blue.

The Solar Spectrum

Electromagnetic radiation from the Sun is spread over a wide frequency range. Insolation contains electromagnetic wavelengths as short as 200nm (Ultraviolet) with maximum energy centred at around 400nm (Blue light).

In addition to the spectrum of solar radiation there is also a spectrum of terrestrial radiation with a range spanning from 3000nm to 7500nm. This is basically the heart radiating from the surfaces of materials that have been heated by the sun.

As previously discussed the solar spectrum includes energy at a wide range of wavelengths. When considering or calculating the spectrophotometric properties of any glazing product we are primarily interested in the energy wavelengths between 280nm and 2500nm, which covers ultra violet through visible light and on into the infra red. Figure 1 indicates the distribution of solar energy across this range of wavelengths.

Visible Light

The visible range of solar radiation is the range of wavelengths of the spectrum between 380nm and 780nm. These wavelengths include the colours of the spectrum and taken together constitute what the human eye perceives as visible daylight.

Ultraviolet

Ultraviolet (UV) radiation makes up a small part of the total energy content of insolation amounting to approximately 8% – 9% between 280nm and 380nm.

(UV-B range from 280nm to 315nm and UV-A range from 315 to 380nm)

Whilst overexposure to ultraviolet light has been linked to skin cancer some exposure is required for the bodily production of certain vitamins and the healthy growth of plants. Ultraviolet radiation is the primary cause of the fading of fabrics and the degradation of many building materials

Infrared

From wavelengths 780nm through to 2500nm lays the infrared range of the solar spectrum. Solar radiation in this range of wavelengths gives us the physical sensation of heat, as this range of infra red radiation lies close to the visible part of the spectrum it is generally referred to as the “near infra red”.It is this infra red radiation that we feel on our skin when exposed to the Sun or a naked flame.

In Addition to the spectrum of solar radiation there is also a spectrum of terrestrial radiation from the near infra red through to the far infra red at 7500nm. This is basically the heat radiating from the surfaces of materials that have been heated by the sun.

Glass & Solar Control

When we consider glass for solar control there are a wide range of elements to take into account. Wherever glass is specified to reduce cooling loads in buildings it is important to consider the ability of the glass to control the transmission of heat not only from the direct influence of the Sun, but also the re-radiated heat from adjacent buildings after the Sun has gone down.

ArcGlass coated solar control products have been developed to offer protection not only from direct solar radiation during daylight hours but also protection extending well into the infra red and therefore offer protection against long wave heat energy throughout the hours of darkness.

Glass and Solar Energy

Glass transmits solar radiation from the Sun by three mechanisms, reflection, transmission and absorption, which for solar control purposes are defined in terms of the following parameters:

Direct Solar Energy Transmittance (Te) is the proportion of solar radiation at near normal incidence that is transmitted directly through the glass.

Solar Energy Reflectance (Re) is the proportion of solar radiation at near normal incidence that is reflected by the glass back into the atmosphere.

Solar Energy Absorptance (Ae) is the proportion of solar radiation at near normal incidence that is absorbed by the glass.

Total Solar Energy Transmittance (TET) also referred to as the Solar Factor (SF) or g Value in Europe, or as the Solar Heat Gain Coefficient (SHGC) in the USA, is the proportion of solar radiation at near normal incidence that is transferred through the glazing by all means. It is composed of the direct transmittance, also known as the short wave component and the part of the absorptance that is dissipated inwards by long wave radiation and convection, known as the long wave component.

Condensation will form on any surface as soon as the surface temperature falls below the dew point of the air. External condensation will only occur on cloud free nights when there is little or no wind and usually when a warm front follows a cold dry spell.

As gardeners know, the air temperature in their garden can vary on any day or night from one part of the garden to another. A hedge, a shrub, an open flowerbed or a projecting wall or garage can all affect the air temperature in their close proximity.

It is a combination of weather conditions and local microclimate, which can contribute to the formation of external condensation. On occasions and as a result of the local conditions it is possible to see clear and condensed windows in the same home.

However, as would be expected the formation of external condensation on glazing is also affected by the thermal insulation performance of the glass.

The thermal insulation provided by single glazing is very poor and heat from the home passes through the glass and escapes to the outside world.

As a result of this the external surface of a single glazed window is warmer than the dew point of the outside air and this prohibits the formation of condensation on the glass.

With ordinary double-glazing the level of insulation is improved. However, sufficient heat still escapes through the glass to warm the external pane sufficiently to prohibit the formation of condensation.

Low emissivity glass works differently to ordinary glass. The low emissivity coating reflects heat back into the room and as a result the amount of heat passing through the glazing is greatly reduced.

The external pane of a double glazing unit containing low emissivity glass is not warmed up by escaping heat and therefore presents a colder surface to the outside environment.

When the glass surface temperature is lower than the dew point of air and conditions are comparable to those mentioned earlier, condensation could form on the external glass surface.

Unfortunately, it is not possible to quantify the number of occasions when external condensation will occur because nobody can predict the coincidence of still air and clear night skies. However, it will be a relatively rare and transient occurrence.