Envelope reflectance impact on thermal perfomance - Sefaira

We know that envelope reflectance properties have an important impact on thermal gains, but apparently assigning different colors to walls and roofs on Sketchup has no impact on internal temperature or energy use. Is that so, or am I missing something? Isn´t there a way to assign different reflectance values to envelope, so that it impacts simulation?

No one can help??

It’s strange that @niraj.poudel didn’t reply. Let’s see if he does now.

Thanks @colin.

Hi @donadesapa thanks for sending in the question. Not sure how it fell through the cracks but glad to be able to help now.

So yeah, Sefaira creates a separate Energyplus file and a separate Radiance file from your SketchUp model to run the Energy Analysis and daylight analysis on. And in the process, regardless of your wall colors it will assign a default reflectance value for the walls, floor, roof, windows etc. These numbers are “baked in” on the basis of the IES LM 83 standard.

As for energy side of things, because Sefaira is running early stage analysis on your project, the assumption is that once the sunlight penetrates a space that energy will be converted into heat indoors. In other words once sunlight penetrates a space it is essentially “trapped” indoors, ignoring the negligible amount of sunlight that bounces off of the surfaces and escapes through the windows again. That heat gain is distributed across the floor area regardless of the number of bounces (i.e., reflectance off of the walls) because the surfaces will end up absorbing the heat sooner or later. And this is where the specific heat capacity of the walls come in. The ability of those surfaces to retain and dissipate heat. Sefaira also has a heat capacity number “baked in” for each surface. If you want to change those numbers then you are able to do so by downloading the Energyplus input file after each Sefaira run.

Finally, if you are looking to change the Solar Reflective Index (Albedo) of the roof then that too can be done by downloading the EnergyPlus file .

All in all, the surface reflectivity numbers are “baked in” per the IES LM 83 standard and that any other thermal properties that you would like to change (that is not already visible through the Sefaira interface) can be changed by downloading the Energyplus file. I hope this helps clarify things a bit?

Thank you a lot for the reply! Yes, my concern is about external surfaces Albedo, since it´s a crucial property that can increase external surface temperatures significantly, when exposed to sunlight, and thus, increase heat gains as well. Although we can change it diretcly on IDF file, it´s a pity there´s no way to change it through Sefaira interface, so that we could take advantage of its optimization capabilities, like clones and response curves. Hope you guys implement such feature someday!

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