# Transmitance translation from wm²/K to 0-1 values

Hi, I’m working on a definition that takes the ammount of radiation a façade recibes and traduces it into transmittance values for the façade.
(If Rad>500 W/m², minimum transmittance, if not, stablish a value between max and min transmittance)

The values for the components of the real façade are:
5.1 wm²/K, 0.024 wm²/K, 2.8 wm²/K (…) How do I translate those values to the equivalent between 0-1 that HB Radiantion demands?

Thank you very much

NobleSix110

@NobleSix110

(If Rad>500 W/m², minimum transmittance, if not, stablish a value between max and min transmittance)

This part seems clear, you want to convert radiation values R_{ij} (where i \in simulation times; j \in simulation areas) between 0 - 1 to represent transmittance \tau_{ij}; based on some minimum and maximum threshold:

\tau_{ij} = \frac{R_{ij} - min}{max - min}

The values for the components of the real façade are:
5.1 wm²/K, 0.024 wm²/K, 2.8 wm²/K (…) How do I translate those values to the equivalent between 0-1 that HB Radiantion demands?

Can you clarify this part? Are these numbers U-values for a transparent material with units in W/m²K? I don’t recognize Wm²/K as a unit…

If they are U-values, how do you want them to relate to the transmittance values? The U-value gives the amount of thermal energy transferred through material via conduction/convection, which impacts radiation transmission, but isn’t solely responsible for it.

I dont know if I’m understandig correctly. Probably I’ve not explained myself correctly.

The Incident Radiation values shown at the left pannel are generated by the component LB_Incident Radiation (results in Kwh/m²).
I’m analyzing a facade made out of ETFE cushions. These cushions contains 3 different layers of ETFE each one, and are capable of regulating the ammount of light that goes through them.
Because of me being a newbee with HB_RAD, I simplified the ETFE-cushion-façade to a mesh wich faces correspond to each cushion. The idea was to assign a transparent material wich propperties changes proportionally to the amount of radiation recibed.

The component asks me for “transmittance” values bettween 0-1. Where can I get those?
Maybe I’m having a translation problem. I’m spanish, and in other articles, i’ve read that the U values are refered to “transmitancia térmica”. What is “transmittance” refered to in “HB_Glass modifier”?

Here you have the U values for each material in the façade.

I’m sorry, the document is written in Spanish.

Once more, thank you very much for your help.
Best regards

NobleSix110

The component asks me for “transmittance” values bettween 0-1. Where can I get those?
Maybe I’m having a translation problem. I’m spanish, and in other articles, i’ve read that the U values are refered to “transmitancia térmica”. What is “transmittance” refered to in “HB_Glass modifier”?

I think there are two issues here, first a partial translation issue, and second confusion of purpose of Radiance.

1. Translation Correction
"Transmitancia térmica” translates to “thermal transmittance”, which you are correct, is what the U-value represents. This is not what the “transmittance” refers to in the HB Glass Modifier component. That “transmittance” refers to the fraction of light transmitted through the material (with the remaining fraction of light absorbed and reflected from the material). You can’t convert “thermal transmittance” to light transmittance.

The HB Glass Modifier component specifies materials for a Radiance simulation with HB-Radiance. Radiance simulations are lighting simulations only, they do not model thermal energy transfer. Therefore, there is no U-value or "transmitancia térmica” input for Radiance. You can however use the U-values in an EnergyPlus simulation with HB-Energy, which does model thermal energy transfer.

Hopefully that all makes sense. From this point you have two choices: if you are only interested in an accurate model of the physical/optical transmission of solar radiation, you can use HB-Radiance. I think the simplest way to do this is simply search for lighting or solar-specific material specifications. The manafacturer should have this information somewhere. On the other hand, if you want to capture thermal energy effects (like the way solar radiation is driven partially by the evolving temperature differences in your assembly and interior), and are okay with using a simplified model of physical/optical radiation transmission, you can use HB-Energy, and simply use the U-values, and solar g values specified in your chart.

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Thank you very much, @SaeranVasanthakumar

You are right, I’ve missunderstood the “transmittance” input of the component. I think the definition you are giving me matches the one for “factor solar”. (A value between 0-1 that describes the relation between solar energy that goes through a glass surface, and the incident on that surface.)

Perfect! I got it now. This will help me keep my bachelor thesis focused.
For the purpose i’m searching, I’ll continue working with HB-Rad. I think I know where to find these values.

Thank you very much. Best regards.
NobleSix110

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