Why have triple-glazed windows? #Passivhaus – some maths…

Let’s look at the physics behind the Passivhaus argument for triple glazed windows vs double and in the context of different climatic regions.

WARNING – Physics & Maths – WARNING Calculations will follow…

Let’s look first at double glazing losses through a glass window of 1m2 in Dublin (Gt=60 kKh/a), typical U-value = 1.5W/m2K

nb I’m just looking at one factor (ie the glazing and I’m simplifying it a lot)

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 1.5W/m2K * 1 (exposed) * 60 kKh/a
Qt = 90 kwh/a

Let’s now look at Triple glazing for same location/window area: typical U-value = 0.8W/m2K

Qt = 1m2 x 0.8W/m2K * 1 (exposed) * 60 kKh/a
Qt = 48 kwh/a

Not surprisingly, you’ll be losing nearly twice as much heat through a double glazed window.

Let’s now look at a different location; say Lisbon (18 kKh/a)

Doubled glazed Qt = 1m2 x 1.5W/m2K * 1 (exposed) * 18 kKh/a = 27 kwh/a

Triple glazed Qt = 1m2 x 0.8W/m2K * 1 (exposed) * 18 kKh/a = 14.4 kwh/a

The losses are the same percentage wise but because of the climate difference a triple glazed window in Dublin has the same transmission loss as a double glazed window in Lisbon.

Now this isn’t the full story and there’s more to follow as we later look at solar gains through the glass, g values …

Also, I know the blog is read by a lot more knowledgeable people than me in Physics and Passivhaus; if you see anything incorrect/misleading in what I’ve written – please let me know (comment below)…