Some Theory

Double glazing refers to using two sheets of glass, separated by a gap of approximately 12 mm. They are sealed so moisture cannot enter the gap and cause condensation. The gap can be filled with argon rather than air to improve the insulating performance and the glass can be coated on the inside (low-E or tinted) to improve radiant heat performance. Normally the gap contains crystals to absorb any moisture.

Secondary glazing is the addition of a second layer of glass, again separated by an air gap. It has the advantage of being able to be applied to an existing window. However it is not sealed and so moisture and fogging can be a problem.

While wall and roof insulation is measured using R ratings, glass manufacturers tend to use U-Factor and Solar Heat Gain Coefficient (SHGC). An R rating is a measure of the resistance to heat loss. A standard insulation batt has an R rating of 3.5. Two layers of batts will half the heat flow and so have an R rating of 7. The U-Factor is the inverse of an R rating and is the measure of the energy loss per square metre per degree centigrade. The U-Factor for an insulation Batt is around .3. This means that one square metre will transmit .3 watts if there is a 1 degree difference across the batt.

The U-Factor for 3 mm glass is 5.9. This means that one square metre will transmit 5.9 watts if there is a 1 degree difference ocross the glass. If there is a 10 degree difference from inside to outside the house, each square metre loses the same heat as a 60 watt light globe. This is nearly 20 times the heat loss through an insulated ceiling. Double glazing reduces the U-factor to 2.7 and 2.5 if argon is used.


This is only half the story for glass. As well as losing/gaining heat by conduction, it also loses/gains heat via radiation. To understand how glass reacts to radiation. It is first useful to look at the radiation from the sun that hits the top of the atmosphere and the radiation that reaches the ground.

You can see that 50% of the energy from the sun arrives in the visible range. The remainder is in the UV range out to 2500 nm.

When designing windows, you must decide how you handle the direct sunlight. Ideally you have north facing windows that receive the winter sun, but are protected by the eaves in summer. East and west windows are best handled by external blinds, so that you can allow the winter sun in but keep the summer sun out. While tinting reduces summer heat, it also reduces winter heat and so shading is the preferred option


As well as solar radiation entering the house, there is long wavelength heat leaving the house. This graph extends out much further to longer wavelengths. We see that a room will radiate heat at around 10 micrometers. In winter we want the windows to transmit the solar radiation (.4 to 3) microns but reflect radiation beyond 3 microns. It is also useful to stop UV below 3 microns since it causes carpets etc to fade.


This graph shows how different types of glass respond to each wavelength. Clear glass transmits 80-90% of all frequencies. Tinted glass transmits 40-60% of all frequencies and so helps control summer heat, but also reduces winter heat. The Low-E windows transmit around 80% of solar light but blocks 80% of the heat radiating from inside the house.

The windows with a Low-E coating therefore provide a good compromise for cold climates. They let most of the solar radiation through while blocking most of the radiated heat. This makes the room “feel” warmer in winter. On a cold winter night, if you stand 50 cms from the window where the room temperature is 20 degrees and the outside temperature is 5 degrees it feels cold. When you close the curtains it instantly feels warmer, even though the room temperature is still 20 degrees. This is because the curtains reflect the radiant heat rather than transmit it. The low-E windows have a similar effect. Combining Low-E glass and an Argon gap gives the best performance, reducing the heat loss by nearly 75% compared to a single pane of glass.


This photo was taken with a thermal imaging camera by a customer, Denny from Hopper's Crossing. It clearly shows the improved thermal performance. Even though it was not very cold outside, the inside temperature of the double glazed unit on the left is much warmer than the single glazed unit on the centre right.


This graph shows the performance figures for all options. While Argon and Low-E glass will add 25% to the cost of the double glazing, its performance improvement is such that if cost is a concern, it is better to reduce the number of windows done by 25% and use the Argon and Low-E. This means that you can come back later and replace the windows not done previously when finance is available.


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