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 last 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’

for

‘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 sinle glazing  =  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 single glazing  =  2.4 x 15 x 7  =  252watts  =  .25kw/hour

Heat loss through new brick vineer   =  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 double glazing = 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 double glazing = 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!