Is Double Glazing Worth the Cost for Heat Transfer?

[katehovey]

I have a question about heat tranfer across single and double glazing but have come stuck.

The question is:

A window, 3m wide by 1.5m high, originally contained a single pane of glass 6mm thick. It is proposed to install double glazing consisting of two sheets of glass (each 6mm thick) separated by a stagnant air gap 20mm thick a ta cost of £600. If heating costs 4.5p per kWh, estimate the time taken to recover the cost of the double glazing when the temperature difference between inside and outside surfaces of the glass window is 20'C.

Thermal conductivity of glass = 0.8W/mK
Thermal conductivty of air = 0.025W/mK

I worked out the heat flow over the double glazing to be:
Q = 110.43W
And over the dingle glazing to be:
Q = 12000W

Both seem far too high, so I don't think they are correct. Also, I don't know where to go from here ifthey are correct.

Thanks

 

[Mentz114]

Are those units Watts/milli-Kelvin ? If so, did you scale your temperature properly ?

 

[katehovey]

No sorry Watts per meter per Kelvin. But it's ok - I've finished the question now.