Double Glazing or Smaller Windows?
I’ve posted on Double Glazing but that not the only way to save heat loss through windows so I thought I would do a numerical comparison of the various options for glazing treatment of windows.
In a bedroom of our current house the South facing windows were approximately 4m square (We are in Australia so these windows don’t get any sun).The basis of my calculations is a difference of 15oC between internal and external temperature.
The equation used to calculate heat loss is:
Heat Loss = Area x Temperature Difference x ‘U’
‘U’ single glazing = 7*
‘U’ double glazing = 3*
‘U’ brick veneer = 0.51
* ‘U’ value includes effect of frame.
Option 1 Do Nothing
Heat loss through glass = 4 x 15 x 7 = 420watts = 0.42kw/hour
Remember this heat loss is for one room only.
Option 2 Reduce window by 40% to 2.4 m
Heat loss through glass = 2.4 x 15 x 7 = 252watts = .25kw/hour
Heat loss through brick = 1.6 x 15 x .51 = 12 watts = 0.012kw/hour
Total heat loss = 0.25kw/hr + 0.012kw/hr = 0.262kw/hour
With our builder this was a no cost option that has reduced the heat loss by 38%.
Option 3 Double Glazing
Heat loss through glass = 4 x 15 x 3 = 180watts = 0.18kW/hour
This is a heat loss reduction of 57% but at a significant cost.
Option 4 Reduce Window by 40% and Double Glazing
Heat loss through glass = 2.4 x 15 x 3 = 108watts = 0.108kW/hour
Heat loss through replacement brick wall = 1.6 x 15 x .51 = 12 watts = 0.012kW/hour
Total = 0.108kW/hour + 0.012kW/hour = 0.12kW/hour
This final option has reduced the heat loss by over 70% and will be around 30% cheaper than double glazing the original large windows.
I hope this has given you some food for thought!