Thermal Conductivity vs U Factor

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  • #1
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I understand that thermal conductivity is W/(m.K) and u-factor is W/m² K. Obviously U factor is energy transfer over an area but how does this compare to thermal conductivity?
 

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  • #2
Mapes
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I understand that thermal conductivity is W/(m.K) and u-factor is W/m² K. Obviously U factor is energy transfer over an area but how does this compare to thermal conductivity?
If the window is a uniform single material, then that material's thermal conductivity is essentially the U factor multiplied by its thickness (ignoring convective effects). But usually windows have internal structure, multiple panes with air gaps, etc., that make the U factor more convenient to use in practice than the thermal conductivity.
 
  • #3
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Thanks for that. So is thermal conductivity multipied by the area of the surface the U Factor (or close as damn to it)?
 
  • #4
Mapes
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So is thermal conductivity multipied by the area of the surface the U Factor (or close as damn to it)?
No, that doesn't follow. Check the units: W m-1 K-1 multiplied by m2 doesn't give W m-2 K-1.
 
  • #5
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So is it divide by a unit depth (I assume 1000mm) and multiply by area?
 
  • #6
Mapes
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So is it divide by a unit depth (I assume 1000mm) and multiply by area?
What is "it"?
 
  • #7
russ_watters
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They are just inverses of each other.
 
  • #8
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So is thermal conductivity divide by a unit depth (I assume 1000mm) and multiply by area = U factor?
 
  • #9
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Conductive heat transfer in one dimension is q=-kAdT/dt.
In chemeng, U is used as a bundling of (various) heat transfer resistances.
Q= U*A*DT
U can be composed of heat transfer resistances due to conduction of various layers of materials and convection eg representing a building wall as inside surface convection, brick/plaster conduction, insulation conduction, and external convection...etc.

U need basic ht trfr understanding - read a basic text on heat transfer.
 
  • #10
sophiecentaur
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U factor is a simpler unit because it ignores the thickness of the material. It makes it much easier to add up the heat losses for walls of different construction, windows etc.. It's purely a practical thing.; you look up the tables of U values and add up the various areas of the room and it will tell you the heat input needed to maintain a given temperature difference.
I did this once when estimating how much heating I'd need in my home but ignored to add the losses due to air exchange. I couldn't believe how small the room radiators would need to be. Then I thought again and got a much higher answer. The man who came to give me an estimate for the job just looked and came up with virtually the same figure I'd spent ages calculating.
 
  • #11
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I thought k [(W/(m*K)] was used in the case of conduction, while h [(W/(m^2 * K)] accounts for heat transfer due to force convection.
 
  • #12
russ_watters
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Whoops, sorry, I was thinking R-value was the same as thermal conductivity. Gotta read better...
 

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