Double Glazing: Heat Loss Explained

[Gringo123]

I understand that double glazing reduces heat loss by minimising conduction of heat from the interior pane to the exterior one. Air is a very bad conductor so the heat does not travel from 1 pane the other very easily. Is that correct? If so, why doesn't all the heat just build up in the space between the 2 panes? How does double glazing help to keep the heat inside the room?

 

[mgb_phys]

The heat conduction depends on the temperature difference. So if it's 20C inside and 0C outside there is a large temperature difference across the thickness of a thin bit of glass and so a large heat loss rate. In double glazing the temperature difference is mostly across the air inside the unit. The air in the unit near to the inner pane is at almost the same temperature as the inside of the room - no temperature difference = no heat flow. Similairly the air on the other side of the gap near the outside pane is at almost the outside temperature - again no temperature difference.
All the inside-outside temperature difference is across the air trapped in the unit - air has a very poor heat conduction. It is the air in the gap between the two pains that is the point of double glazing.

In practice double glazing mostly works by replacing old draughty leaky windows with new ones - having double or triple panels of glass doesn't make as much difference.

 

[Gringo123]

Thanks a lot for your help! It's a lot clearer now.
Steve