Saturday, 31 August 2013

Glazing and Thermal Performance

With a view to being able to talk intelligently with the Cretan window makers I have been researching glass and glazing and its impact upon the thermal performance of my proposed house. Whilst not certain that all options can be produced locally, for a reasonable cost!!, I thought I needed to get a clear idea of what is possible. Hence the summary below which pulls together extracts from "Your Home Technical Manual" published by the Australian Government that helped me understand the issues involved. Plus odd comments on how I could address these issues in my particular case. At the end is an appendix summarising the glazing options available.

"The impact of glazing on the thermal performance of a building is complex.

There are several aspects to consider:

- Climatic conditions at the location.

- Building design.

- Building materials - the amount of mass and insulation.

- The size and location of windows and shading.

- Thermal properties of glazing units.

Because of the complex interaction of these many variables the best way to accurately assess the impact of glazing on a building's thermal performance is to model it using using one of the sophisticated computer programmes now available".

An early design influence courtesy of "Concrete Architecture" - (1)

 However, as with all things, the application of common sense, logic and a bit of research can give one enough pointers to talk intelligently with potential suppliers and engineers.

"When considering a passive solar design there are some simple general principles that can be followed. These include:

- Locate and size windows and shading to let sunshine in when the temperature is cold and exclude it when it is hot.

- Use thermal mass to store the sun's heat and provide night-time warmth in cold conditions.

- Locate window and door openings to allow natural cooling by cross ventilation.

- Provide seals to openings to minimise unwanted draughts.

- Insulate floor, walls and roof to meet heating and/or cooling objectives.

Careful choice of glazing system provides major improvements in thermal comfort for people close to windows - especially large windows as in my case. our sense of comfort is not determined solely by air temperature. The temperature of surrounding surfaces has a great impact. The objective, therefore, should be to achieve an inside glass surface temperature as close as possible to the desired room air temperature. This means glass that is neither cold in winter or hot in summer.

Heat flows through glazed elements in several ways:




Conduction is the movement of heat energy through the glass and frame materials from the air on the warmest side to the air on the colder side. The greater the difference in temperatures the more heat flow. Different frame and glass materials have varying ability to conduct heat, specified by the U-value. The lower the U-value the less heat is transmitted.

There is a simple formula that helps quantify the impact of improved U-value:

The amount of heat conducted through a glazed unit (in Watts) = U xT x A where:

U = the U-value

T = the number of degrees difference in air temperature on each side

A = area of glazing

The U-value is important in both hot and cold climates. In my case in the hot summers the difference between indoor and outdoor temperatures may regularly be 10 to 15 degrees hotter outside than inside, so halving the U-value will halve the conducted heat gain.

Another design influence illustrating a glass wall with a shading "cowl". Courtesy  "Concrete Design" - (2)

Convection is the movement of heat energy by air that passes over the surface of the glazing unit, taking heat away from glass and frame. Higher airspeed causes greater convected heat transfer. Minimising convective heat transfer can be achieved by reducing air movement adjacent to the surfaces of glazing by walls, screens and plantings and by shielding the interior with curtains etc". In my case, apart from the shading "cowl" I am planning, it is best achieved through double glazing which creates a still gas layer between the panes since I want to maximise winter solar gain and maintain the views to the south south east.

Radiation is heat that is transmitted as electromagnetic waves. They pass through space, in the the same way visible light moves through space, until it is reflected or absorbed.

When sunlight strikes a sheet of glass, some of the solar radiation is transmitted straight through, some is reflected back and some is absorbed by the glass. The heat energy absorbed by the glass is then radiated to both the inside and outside as infrared radiation.

The total amount of solar heat that passes through the glass is the sum of the heat transmitted plus that part of the heat absorbed in the glass which is subsequently re-radiated and convected inside. This proportion of solar energy that passes through the window, both directly and indirectly is called the Solar Heat Gain Coefficient (SHGC).

The amount of infrared heat energy radiated from the surface of glass depends on its emissivity. A 'perfect' radiator has an emissivity of 1.0. Uncoated glass, whether clear or tinted, has an emissivity of 0.84. It is almost a perfect radiator.

Low-e glass has a coating on its surface that minimises the amount of heat, absorbed by the glass, being subsequently radiated into the building. It can also be designed to block some of the solar radiation transmitted through glass. Low-e glass is available with an emissivity as low as 0.03!

Reducing solar heat gain through glass can be achieved by using toned/tinted glass which absorbs a greater proportion of solar heat than clear glass. The absorbed heat is then radiated to inside and outside. Including a low-e coating on the inside facing surface reduces the proportion of absorbed heat that is radiated into the building which dramatically increases the effectiveness of the toned/tinted glass.

The solar heat gain can also be reduced by reflective glass which increases the proportion of incident solar heat that is reflected away from the glass.

Spectrally selective glazing has a low-e coating which "filters" solar radiation, allowing maximum visible light transmission while reflecting un-wanted UV and solar near-infrared wavelengths. Spectrally selective coatings have very low emissivities - as low as 0.03.

Double glazing is an effective way to reduce U-value, but its impact on solar heat gain depends upon the type of glass. One layer of clear glass has a SHGC of 0.86. Two layers have a combined SHGC of about 0.76. This may be reduced much further by using tinted, low-e or spectrally selective low-e coatings. Because low-e coatings also reduce radiative heat transfer compared with uncoated glass,the glazing system U-value may be halved again, especially if the air between the panes is replaced by argon gas.

Good example of shading with large area of glass. Courtesy of The Concrete Centre 

After the glazing, frames have the greatest impact on the thermal performance of glazing units.

Aluminium window frames are light, strong, durable and easily extruded into complex shapes, but aluminium is a good conductor of heat and can decrease the insulating value of a glazing unit by 20 to 30 per cent. Aluminium frames, especially dark coloured ones in full sun, absorb a lot of solar heat and conduct it inside.

A thermal break (typically of urethane or other low conductivity polymer) is, therefore, used to separate the exterior and interior sections of the frame.

If using toned/tinted glass it is worthwhile checking the visible transmittance if seeking to maximise natural daylighting. It is important to note that only high performance IGUs are able to simultaneously combine low U-value with low SHGC (when required) and high VT (when required).

For thermal mass to provide beneficial evening heat in cool climates it is essential that glazing is used to admit solar radiation during the day to warm the mass.

If thermal mass is used in warm or hot climates to absorb heat from the air , solar gain through glazing should be minimised and the mass should not be exposed to, or shielded from, solar heat gain.

Low mass buildings cannot store any heat to make night time warming possible so glazing with a low U-value to minimise heat loss at night and on cloudy days should be chosen.


(1) "Concrete Architecture" - Catherine Croft published by Lawrence King Publishing

(2) "Concrete Design" - daab published by ralf daab