Short & long wave radiation, & inputs for clad roofs

[josephbeag]

Hi Manfred,

I enjoyed the course in December where one of the practical examples we went though included a zinc roof with 'open rebate'. On the day you gave us user defined data to input in the Surface Transfer Coefficient dialogue box. Your inputs: exterior surface Sd-value 25, short wave: 0.4, long wave 0.2, rain water absorption factor: 1.0

1) Can you explain where you get the first three inputs from? Is there a table for these kind of values that I can access?

2) I'm not familiar with the term 'open or closed rebate' in relation to standing seam roofs. Am I right in saying it relates to whether the upstand is totally vapour tight or only slightly? Would it's Sd value become nearer that of a foil membrane if it were airtight (i.e. ~10000)?

3) I'm aware that using the wrong inputs for metal roofs can result in significant errors. Could you say more about the relationship of the input for long-wave radiation emissivity to the default figures for explicit radiation balance?

4) In a climate file where do you check to see that you have 'appropriate values for the atmospheric counterradiation'?

Many thanks for your help!

[Manfred Kehrer]

Dear Joseph,

1) I think this value was just a guessed. The WUFI database has sd=50m for a siding including rebate. This comes from measurements. So we just used the half for a more open siding. The radiation values are from physical handbooks. The rain water absorption factor =1.0 is typical for roofs. It means 100% of rain water hitting the surface can be uptaken.

2)I think you are right. In reality the sd value of these roofs differ a lot depending on the tightness of the rebate.

3) This is hard to say. It depends on what your final conclusion should be.

4) Most of the climate files have hourly information for the atmospheric counterradiation, either the radiation itself (ILAH) or the cloud index (CI).
If the climate files contains neither ILAH nor CI, WUFI uses a constant cloud index which can be entered at the surface transfer coefficient. This is of course a very rough approximation.
Which climate files do you use?

[Daniel]

If you use a metal roof by surface sd-values you have to swith of the rain water absorption.

Daniel