Why have triple-glazed windows? #Passivhaus – #2 – Solar Gains & more maths…

Following on from Why have tripled glazed windows?… I was intending to write this on Monday but I just couldn’t wait.

Here’s the second instalment on the same subject that focuses on Solar Gains.

As a counterbalance to the transmission losses through a window; you will also have solar gains that give (sorry) free heat to the building (if correctly sized and orientated) and these solar gains are the subject in this post:

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

Qs = r * g * Aw * G


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)

Therefore, for a window (continuing the previous blog post example) of 1m2, in Dublin and Lisbon, say r=0.5, g=0.6 G=391kWh(m2a) (South facing in Dublin)

Qs = 0.5 * 0.6 * 1m2 * 391kWh(m2a)

Qs= 117.3 kWh/a

Therefore, a 1m2 window on a south wall in Dublin gives 117.3 kWh Solar gain in a year.

So what happens when you put the same size window in a north facing wall in same location?

Qs = 0.5 * 0.6 * 1m2 * 84kWh(m2a)

Qs = 25.2 kWh/a

ie 4.65 times as much heat is gained through a south facing window as a north. Hence the Passivhaus methodology (not withstanding the calculations required to ensure overheating doesn’t occur (Passivhaus standard requires that the internal temperature does not exceed 25degC for more than 10& of days annually) are based around utilising south facing windows to benefit from solar gain.

So what happens in a hotter climate? Let’s look at somewhere hotter, Madrid – Spain:

South facing:

Qs = 0.5 * 0.6 * 1m2 * 696kWh(m2a)

Qs= 208 kWh/a

North facing:

Qs = 0.5 * 0.6 * 1m2 * 121kWh(m2a)

Qs= 36.3 kWh/a

Not surprisingly, Madrid is twice as hot as Dublin! and hence the need for smaller south facing windows in hotter climates to limit solar gain and potential overheating.

There is some doubt over the irradiation figures for Dublin which I’m going to check; watch this space for a possible blog update.

But this doesn’t help answer the question concerning why triple glazing is better than double; one of the key components in the above equations which will help us answer the question a little more is the g-value ie the total solar transmittance value of the glass; and this definitely is another blog post for Monday…

As previously, any errors/omissions/inaccuracies please let me know (comment below)…

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