OK here we go, third post on ‘Why have triple glazed windows? following on from Solar Gains and Why have tripled glazed windows?…

Here’s the third instalment on the same subject that focuses on Solar Gains and that elusive g-value for glass:

Remember that:

The calculation of solar gains through a window is given as:

Qs = r * g * Aw * G

Where:

Qs=solar gains
r = Attenuating factor for glazing comprising the frame to window ratio, how shaded/dirty the glass is and factoring in any radiation that is non-perpendicular to the glass
g=Total solar transmittance value of the glass (I think this value will need a separate blog post…)
Aw=Area of the glazing
G=Global solar irradiation (this depends on location and orientation of window)

Let’s look now at the g value or as it’s known in the US as the Solar Heat Gain Coefficient (SHGC); typically a value in the region 0.55-0.6 ie 55-60% of the energy is getting through the glass (ie the amount of solar gain delivered)

So let’s look at a couple of examples, using 2 windows from a local supplier (who incidentally do excellent windows) Grady Joinery:

Window 1: double glazed with Planilux Clear Float + Low-E glass u value=1.187 W/(m2K) g value = 0.66 – Dublin

Qs = r * g * Aw * G

Qs = 0.5 * 0.66 * 1m2 * 391kWh(m2a) (using same values as previous blog post example)
Qs = 129.03 kWh/a

Window 2: Triple glazed with Diamant Low Iron + Low-E glass u value = 0.6 W/(m2K) g value = 0.6 – Dublin

Qs = r * g * Aw * G

Qs = 0.5 * 0.6 * 1m2 * 391kWh(m2a) (using same values as previous blog post example)
Qs = 117.3 kWh/a

ie 9.1% increase in solar gain using double glazed window.

Let’s look again at the transmission losses for these same 2 windows:

Window 1: double glazed with Planilux Clear Float + Low-E glass u value=1.187 W/(m2K) g value = 0.66 – Dublin

Qt = A * U * ft * Gt

Transmission losses through window = area of window * U value of window * Temperature correction factor * Heating degree hours

Qt = 1m2 x 1.187W/m2K * 1 (exposed) * 60 kKh/a
Qt = 71.22 kwh/a

Window 2: Triple glazed with Diamant Low Iron + Low-E glass u value = 0.6 W/(m2K) g value = 0.6 – Dublin

Qt = 1m2 x 0.6W/m2K * 1 (exposed) * 60 kKh/a
Qt = 36 kwh/a

ie The double glazed windows have a transmission loss of nearly twice that of the triple glazed window.

Conclusion

Although the double glazed units with a greater g value give better solar gain than triple by just under 10%; the transmission loss of the double glazed window is nearly twice that of the triple glazed window.

Therefore on balance (which is an important word here as it is a heat balance calculation) in this location (Dublin) the optimum window is triple glazed. In hotter climes the percentages aren’t so important as the transmission loss depends on the temperature difference between inside and outside air on the specific location which in a hotter climate gives a lower transmission loss (due to a lower temperature difference). And, which is why some double glazed windows (Argon filled for example) will meet the Passivhaus Certification criteria in a warmer climate. The full Passivhaus document that details all of the glazing criteria together with the classification of the different temperate regions is found HERE.