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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?

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The overall heat transfer coefficient U includes the air boundary layer resistances on the inside and outside of the glass, which are in series with the window glass and dominate the resistance.

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  • $\begingroup$ Even if we do the maths and include those values of air film, i am sure that it cannot compensate for that HUGE difference between 1 W/(m.k) and 0.0024 W/(m.K). $\endgroup$ – Engineer SAM Oct 12 '17 at 17:07
  • $\begingroup$ Oh yeah? According to the OP, the value of 0.0024 W/(m.K) is equivalent to an overall heat transfer coefficient of 7 W/m^2K. In Transport Phenomena by Bird, Stewart, and Lightfoot, they cite typical values of 3-20 W/m^2.K for one-sided natural convection heat transfer with air. Two-sided overall heat transfer coefficient would be half of this, neglecting the glass resistance. It all seems totally consistent. $\endgroup$ – Chet Miller Oct 12 '17 at 18:44

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