Mmm...
In the "model world" some characteristics of the physical are split up to reflect (excuse the pun) different aspects.
1) surface roughness is used only for the convection analysis (correct me if I'm wrong)
2) material thickness, density and conductivity is used for the thermal heat transfer via conduction and the heat capacitance modelling. e+ can't handle highly conductive thin layers as the thermal diffusivity is too low. Furthermore, R-values are essentially translated internally into an equivallently thick layer of air.
3) radiative characteristics (which already incorporate surface roughness) are modelled not per every frequency in the electromagnetic spectrum, but bandwidth wise per solar, visual, infrared etc.
So...
It's not going to help to add thin layers for paint. A material only radiates from the very surface. Change the radiative properties of the outermost material's surface to that of the paint. You should change the roughness for convection purposes too.
As for emmisivity, Jer makes a good point. "normal" paint (and most other materials) have a emmisivity of .9-.96. Metals can get lower, like Aluminium .3(ish). To get low-e paints and glazings they actually spray or emulsify nano particles of metal on the surface or into the liquid to reduce the emmisivity. So, it's unlikely you'll get low-e materials with emmisivities lower than .3(ish). Low-e paint is not a post surface spray-on so I'm betting you won't get it nearly that good.
Jean
Mit freundlichen Grü�en- Sent from my iPhone (excuse the brevity)
i. A.
Jean Marais
b.i.g. bechtold
Tel. +49 30 6706662-23
On 07.07.2014, at 03:59, "Asit Mishra asitkm76@xxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:
asitRegards,Please adviseSecondly, normally what I have seen in material specifications, while values for absorptivity can be specified, there was no obvious way to specify a high emissivity (apart from indirectly doing so using surface roughness).This did not seem to produce desired resultsHello,I would like to have some idea on how you all might have approached simulating the effect of a reflective coat of paint. The way I went about it is defined a material with low absorptivity and added a thin layer (0.5 mm) to the roof/wall construct.
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