Saturday, 14 September 2013

Glass Types and Climate Considerations


Toned/tinted glass has colouring additives included during the melting process of forming glass. It is available in various colours, usually bronze, grey, blue and green. The different colours provide different SHGC and some variation in VT. This tinting does not change the U-value of the glass because glass conductivity and emissivity are unaffected by a pigment in the glass. Green and blue tones tend to have a higher ratio of visible light to solar heat transmittance.

Super toned glass has heavier pigmentation which is tuned to preferentially transmit visible wavelengths while filtering out more invisible solar near infrared wavelengths. This provides lower SHGC while preserving adequate VT.

Reflective glass has either a vacuum deposited thin film metal coating or a pyrolytic coating. Vacuum deposited coatings are soft and for protection and longevity they must be deployed inside an insulating glass cavity. Pyrolytic coatings are baked onto the surface in the factory while the glass is still hot: they are hard and durable and are normally glazed with the reflective surface to the exterior. To function to specification they must be kept clean and free of condensation. Reflective glazing also causes glare which may be rejected by planning authorities.

Low transmission low-e glass has a coating which reduces the amount of solar heat gain while still maintaining good levels of visible light transmission. Low-e coatings can be 'hard' or 'soft' and can enable a very dramatic improvement in both U-value and SHGC.

Spectrally selective glass (such as super toned and low transmission low-e glass) has a surface coating which allows maximum visible light transmission while reflecting unwanted UV and infrared wavelengths. Spectrally selective coatings generally have the lowest emissivities of any type of coated glass - as low as 0.03.

Low-e and spectrally selective coatings can be used in combination with clear, toned or reflective glass.

Glazing components. Courtesy Southwell Technologies

Heat Mirror Film

In addition to the above types of glass that are widely used to control heat flow Heat Mirror film is a window insulation material that is transparent to light but highly reflective to heat. It consists of a specially coated film that is mounted inside an insulating glass unit midway between the panes of glass. The result is a finished product which looks clear, but has a dramatically improved insulation performance. Compared with ordinary types of glass, Heat Mirror dramatically reduces heat loss and heat gain caused by conduction and radiation.

For example, Heat Mirror film can be used with clear glass when high thermal insulation and high solar heat gain is desired, such as for passive solar heating. Alternatively, Heat Mirror film can be used with solar reflective low emissivity glass when high thermal insulation and low solar heat gain is desired.

Heat Mirror can be combined with low-e coatings and two air spaces to block both radiated and conductive heat flow. This unique construction enables Heat Mirror insulating units to offer higher insulating performance than other high performance glazing options. It is claimed that the transparent Heat Mirror coating is 90% as reflective as aluminium foil, is optically clear and reflects heat back to its source.

There are products for both cooler climates where warmth from the sun is desirable for supplemental heating and warmer climates where overheating is the primary concern for homes with significant solar exposure, or glass intensive structures such as glass walls.


Cooling climates (i.e. those warmer climates where houses use more than 70 per cent of their total space-conditioning energy for cooling) present fairly simple design solutions. viz:

- Provide maximum shading of glazing.

- Use light coloured frames.

- Select glass with a low SHGC.

- Low U-value to minimise conducted heat gain.

- Choose window styles that provide maximum openable area, located on opposite sides of building to promote cross-ventilation.

Design influence - An example of "cowl" shading  in Spanish Youth Hostel - Courtesy of W@llpaper.

Heating Climates (i.e. those that typically uses more than 70 per cent of its total space-conditioning energy for heating in winter and less than 30 per cent for cooling in summer), where the objective is to maximise solar heat gain most of the year and minimise heat loss, need to consider the following:

- Locate most glazing facing south (in the northern hemisphere) where it receives maximum solar exposure in the living areas.

- Avoid shading windows except when too hot or use shading only in mid-summer.

- Choose glazing units with low U-values.

- Choose glazing units with high SHGC to maximise solar gain except if specific windows allow unwanted summer afternoon heat gains.

Mixed climates (i.e. warm and mild temperate climates where more than 30 per cent of the total space-conditioning energy is used for heating in winter and more than 30 per cent is used for cooling in summer) present more design challenges since heat gain is required in winter but needs to be avoided in summer. However:

- A low U-value will improve both summer and winter performance".

- The passive design of my building will mean the SSE facing window wall coupled with the shading "cowl" will receive more solar radiation in the winter than the summer.  It is generally recommended that these windows will perform best, year round, with a high SHGC. In my case, however, the need to take all passive measures possible to ensure a cool living environment in our off grid house throughout the long hot summers, albeit at the expense of requiring extra, wood fired, heating during the relatively mild winter months, means that I should select glass with a low SHGC.

"Mixed climates often require some compromises between summer and winter performance and here thermal modelling software is useful for determining the exact performance of the options available". I will therefore get my engineer to run these options through an appropriate modelling package.

Glass wall and shading at Old Trafford. Courtesy of Lancashire County Cricket Club