09.02 Uncoated glasses

Categories: Advanced Glazings

Introduction
Solar radiation covers a broad band of wavelengths giving both light and heat.  Radiation falling on a glass surface may be reflected, absorbed or transmitted and the absorbed heat may subsequently be dissipated from both faces effectively increasing the proportion of transmitted energy.

Different glasses with different coatings respond differently to different parts of the spectrum and this can be used to control light and heat transfer through the facade.
 

Clear glass
Clear glass transmits a high proportion of both the incident light and heat, but may absorb some of the infra-red.  The temperature of the glass is dominated by ordinary heat transfer processes at the surfaces of the glass, because the thermal resistance of a single thin glass layer is insignificant.

Some typical performance values for clear float glass are:
 

T = fraction transmitted, R = fraction reflected, A = fraction absorbed
From Button and Pye (1993)

Note that these values are for the UK, where one-third of the absorbed radiation is released into the building by heat transfer.

Single clear glass has been in use for a considerable period of time.  Installation is not a problem, being possible by dry- or wet-glazing techniques.  The only significant limitation of single glass is its limited strength, particularly with regard to impact.  However, this can be improved by toughening or laminating the glass.
 

Coloured/body-tinted/stained glass
Coloured single glass absorbs a much higher proportion of the incident light but this proportion varies across the spectrum - for example green-tinted glass transmits proportionally more of the green component of light.

The absorption of solar heat radiation is much higher than for clear glass.  It is almost 50% for the thinner body-tinted glass, and can approach 80% for thicker body-tinted glasses.  Whilst body-tinted or coloured single glass is no more difficult to install than ordinary single glass care must be taken to ensure that the glass is not used with any feature on the room-side (such as additional blinds or curtains) which reduces heat loss from the glass surface and causes transmitted energy to be reflected back onto the glass.

The temperature of coloured glass is higher than for clear single glass, because of the absorbed solar radiation, and coloured glasses are usually toughened to enable them to survive the higher stresses that may be developed during use.

Some typical performance values for body tinted float glass are:
 

T = fraction transmitted, R = fraction reflected, A = fraction absorbed
From Button and Pye (1993)

The higher temperatures experienced with coloured glasses can also cause discomfort, due to heat being radiated from the glazing onto the occupants of the room.  Coloured glass is often used as the outer pane of a multiple glazing unit so that the gas-space, and any coatings on the inner panes of glass, reduce the transmission of heat into the building.
 

Low iron ‘Clear white’ glass
If viewed edge-on a pane of ordinary float glass has a distinctive greenish colour, due to the presence of iron oxides in the raw ingredients.  By using a low iron oxide mix it is possible to produce glass that is clear when viewed edge on, known as ‘clear white’ glass (the term ‘water white’ has also been used).  Clear white glass has an almost uniform transmission characteristic throughout the visible light and infra-red parts of the spectrum (Button and Pye [1993] pp63-64).  Daylight transmission is higher, but so is infra-red transmission.

This glass can be treated as an ordinary float glass.  It is often used where there is a need to give a clear view through several layers of glazing.  This glass may absorb slightly less energy than ordinary clear float glass.
 

High iron ‘Cool green’ glass
Increasing the amount of iron in the glass mix has the opposite effect - daylight transmission is reduced, but infra-red transmission is reduced even more so (Johnson [1991] pp42).  A typical such product is ‘EverGreen’ glass, from Libby Owens Ford in the USA.  A 6 mm pane of this green-coloured glass has a daylight transmission of 65% but a total solar energy transmission of only 34%, whereas a conventional 6 mm tinted glass of any other colour would have a total solar energy transmission of at least 60% for the same daylight transmission.

This glass can also be treated as an ordinary float glass, although it absorbs slightly more solar energy and will become slightly warmer.
 

Etched and patterned glass
Glass may be etched or patterned to distort or diffuse light passing through the glass.  Although usually intended for privacy such glasses may also be used to provide shading from direct light.  These glasses will absorb slightly more solar energy than clear glass, depending upon the nature of the patterning.

Etched and patterned glass is no more difficult to use than ordinary single glass, although there are issues related to the cleaning of textured surfaces, and to the edge sealing of textured surfaces if they are to be used in multiple glazing units - patterned and textured glasses are usually used with the patterned surface on the room-side of a glazing unit.