Calculate Assembly U-Value with THERM data

I want to calculate the U-Value of my assembly with thermal bridge. I feel that the THERM data will allow this, but I just need to figure out how to use it.
I see the U-Factor data out of HB_Read_THERM_Results, but I’m not sure how to use the tags to create useful data.
Here is an image of an assembly I’m looking at. If I can get the weighted U-Value of this assembly, I can change the overall height to approximate different girt spacings.

EDIT: This post was interesting to see the history of that HB node, and I’m realizing I think I’m looking for how to calulate the psi-value with the given HB_Read_THERM_Results outputs.

Hi @JimMarsh

If you are looking to get energy simulation results from OpenStudio which does not accept Psi-Values directly (as opposed to something like PHPP or WUFI), then yes I think that following the ASHRAE method for determining an equivalent U-Value is probably your best bet in that scenario.

Following ASHRAE HoF Chapter 27, they show the formula for calculating an Equivalent U-Value [U_e] as:

U_e = (\psi * L ) / A_t + U_o


  • U_e = The equivalent U-Value, considering the effect of the detail [W/m2k]
  • \psi = the Psi-Value for the bridging detail [W/mk]
  • L = The length of the detail across the facade [m]
  • A_t = the total surface area affected by the detail
  • U_o = The original clear field U-Value of the wall / floor /roof without the bridging [W/m2k]

So you would calculate your Psi-Value according to the ISO-10211 procedures (or get the Psi-Value from a catalog like the the TB Guide), and then use the above to figure out your equivalent U-Value for that area.

You are basically just calculating the total heat loss for the detail (\Psi * L) which gives you heat loss in W/k and just spreading that heat loss out over the affected area, which gives you W/m2k, which can then be added to your original U-Value.