I was calculating conduction heat transfer through a glass window and I got confused conceptually. Lets assume that we have a glass window of area 1m2. Thermal conductivity (k) of glass is around 1 W/(m.k). I probed into ASHRAE's handbook to find the overall heat transfer coefficient for a 3.2mm single glazed glass window and found the U value to be around 7W/(m2.k).
We know that U = k/L where L is thickness of the body.
Here is the confusing part, if we assume the window to be 100% glass, we can calculate its thermal conductivity by multiplying the U value with thickness of the glass. By multiplying 7W/(m2.k) with 3.2mm i.e. 0.0032m we get a value of 0.0224W/(m.k).
Where am i mistaking? One one hand we have a thermal conductivity og 1 W/(m.k) and on the other hand 0.0024W/(m.k). Why such a huge difference?