This depends on whether or not the wind is blowing... or not. ASHRAE used to have tabluated values (still in some textbooks) of R = .17 Btu/ft2-hr-F for winter (15 mph wind) and .25 Btu/ft2-hr-F for summer (7 mph wind). Otherwise, for a vertical surface it would be .68 for free convection, probably less for a tilted surface, .9 if horizontal heat flow down.

Also, radiation can be significant if the surface is facing a cloudless sky. I think Kreider's solar textbook has a equation for sky temperature...which is somewhat complicated by cloud cover, etc.

I've only seenreference values of interior convection coefficients for tilted surfaces, not exterior coefficients: which are available for summer & winter conditions, due to the difference in wind velocity.

CIBSE suggest to replace g with gcos(theta) in the Grasshof number. That
means, it accounts for the roof being tilted in natural convection. When it
comes to roofs, there is another aspects that starts to be important ...
the roof finishing in tiles will have a 'detoriated' velocity underneath
the tiles. It is mainly that one directing the convective losses ...

I'm not familiar with the calculation involving the Grasshof number. The
formula given is hc= 4+4v, where hc is the convection coefficient and v
is the wind velocity. The formula is probably derived empirically (don't
know for certain). Are you definitely talking about the convection
element of heat transfer or are you talking lw radiation?

Leen,

This depends on whether or not the wind is blowing... or not. ASHRAE used to have tabluated values (still in some textbooks) of R = .17 Btu/ft2-hr-F for winter (15 mph wind) and .25 Btu/ft2-hr-F for summer (7 mph wind). Otherwise, for a vertical surface it would be .68 for free convection, probably less for a tilted surface, .9 if horizontal heat flow down.

Also, radiation can be significant if the surface is facing a cloudless sky. I think Kreider's solar textbook has a equation for sky temperature...which is somewhat complicated by cloud cover, etc.

Jeff S. Haberl, Ph.D.,P.E., FASHRAE

I've only seenreference values of interior convection coefficients for tilted surfaces, not exterior coefficients: which are available for summer & winter conditions, due to the difference in wind velocity.

Paul Hay MBA, BA(Arch)

It's true wind does dominate external convection.

Cibse guide A will give you all the numbers that you're looking for

Chris

Hi Chris,

CIBSE suggest to replace g with gcos(theta) in the Grasshof number. That

means, it accounts for the roof being tilted in natural convection. When it

comes to roofs, there is another aspects that starts to be important ...

the roof finishing in tiles will have a 'detoriated' velocity underneath

the tiles. It is mainly that one directing the convective losses ...

Am I correct on this?

Leen

Hi Leen,

I'm not familiar with the calculation involving the Grasshof number. The

formula given is hc= 4+4v, where hc is the convection coefficient and v

is the wind velocity. The formula is probably derived empirically (don't

know for certain). Are you definitely talking about the convection

element of heat transfer or are you talking lw radiation?

Regards

Chris Yates