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Does thermal emissivity and solar absorptance matter for all layers?

asked 2021-03-16 15:44:41 -0600

Wolfgang Kaiser gravatar image

updated 2021-03-28 09:40:41 -0600

Hi all,

I have a wall and roof section. I would like to know should I just include the thermal emissivity and solar absorptance for the innermost and outer most layers or should I define it for all of them? Furthermore, is it enough to for instance, use the solar absorptivity of white paint for a gypsum wall? Does it make sense?

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It is easy enough to empirically test, right? Try changing the values for materials in the middle of a construction and see if the EnergyPlus results change.

shorowit gravatar image shorowit  ( 2021-03-18 09:37:11 -0600 )edit

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answered 2021-03-17 15:59:20 -0600

updated 2021-03-18 04:53:01 -0600

Thermal and solar absorptances are only considered in EnergyPlus when appropriate. For instance when a material surface sees the outside environment, other interior surfaces, etc. Otherwise, those attributes are ignored at run time, as in the case of conductive heat flow between 2 materials. See: Input Output Reference. If uncertain, stick to the default values e.g., 0.9 (thermal) or 0.7 (solar).

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@Denic Bourgeois yes, I am seeing from ASHRAE that values are different for some paints and other stuff. So, I was wondering does the solar absorptance only affects the outer most layers or not. I guess the thermal absorptance is important for all layers

Wolfgang Kaiser gravatar image Wolfgang Kaiser  ( 2021-03-18 02:49:46 -0600 )edit

I edited my initial answer for clarity (hopefully). Those attributes are indeed important for materials intended to be exposed. For unexposed materials, you need not spend any time on this. For instance, spray foam insulation (polyurethane) as a raw material shouldn't be exposed to indoor environments (fire hazard) or to the outdoors (UV exposure) - I wouldn't pay attention to spray foam thermal/solar absorptances.

Denis Bourgeois gravatar image Denis Bourgeois  ( 2021-03-18 04:59:51 -0600 )edit

@Denis Bourgeois thank you, so, when we have paint on the wall, the layer after paint does not matter as well.

Wolfgang Kaiser gravatar image Wolfgang Kaiser  ( 2021-03-19 00:45:08 -0600 )edit

Correct, with the caveat that one shouldn't attempt to insert paint as a distinct layer within an EnergyPlus multilayered construction! One simply adjusts the solar/thermal absorptance attributes of the underlaying material (e.g. 12.7mm gypsum board) to reflect the contribution of its coating (e.g. opaque low-e paint). So if your gypsum board has a default 0.9 for its thermal absorptance, yet your coating offers 0.6 ... then your 12.7mm gypsum board layer should instead have 0.6.

Denis Bourgeois gravatar image Denis Bourgeois  ( 2021-03-19 07:35:50 -0600 )edit

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Asked: 2021-03-16 15:44:41 -0600

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Last updated: Mar 24 '21