johnno59
Automotive
- Apr 27, 2003
- 3
I am puzzled at the magnitude of the term "downward surface convective coefficient" commonly used in calculating ceiling flux in unvented attic spaces (in hot climates).
A figure of 1.5 W/m².K seems to be the norm.
A white metal roof of low absorptivity with a radiant barrier underneath however will allow far less than than 1.5 W/m².K (radiant) even under full summer conditions.
The calculated downward conductive flow off the underside of the quite hot radiant barrier, through the air space to the ceiling is almost negligible.
Obviously there's no convective process operating (heat source at top, enclosed space, Nusselt=1).
So why do we employ such a large figure for "Hc Down" ?
A figure of 1.5 W/m².K seems to be the norm.
A white metal roof of low absorptivity with a radiant barrier underneath however will allow far less than than 1.5 W/m².K (radiant) even under full summer conditions.
The calculated downward conductive flow off the underside of the quite hot radiant barrier, through the air space to the ceiling is almost negligible.
Obviously there's no convective process operating (heat source at top, enclosed space, Nusselt=1).
So why do we employ such a large figure for "Hc Down" ?
