## Separate psi contributions with U-factors

Thanks to U-factors feature of Mold Simulator 3, it is possible to compute the contribution of every room to global psi value of a thermal bridge. In this tutorial we’ll refer to file example13.mos contained in Mold Simulator’s documentation folder.
Suppose you’ve a T type of thermal bridge:

It is very important to create two different section elements (“Top element” and “Bottom element”) to get correct results.
We want to use four different boundary conditions for the internal environments; for this reason you must pay particular attention to boundaries setup.
1- Every internal boundary must have the same temperature;
2- Grouping must be by temperature, but you must disable “Just connected boundaries” option;
3- A separate U-factor surface must be associated to each boundary;
4- U-factors of room A must be grouped under “Room A” U-factors group (same for U-factors of room B).

Now you’re ready to get separate linear thermal transmittance contributions of this thermal bridge simply passing to simulation tab.

## Internal or external linear thermal transmittance in a thermal bridge

In many thermal bridge computations, you can choose the reference point against which to calculate the linear thermal transmittance (psi). The most common example is the edge of a building:

It is possible to calculate psi compared to the internal reference point (lengths A and B) or outside (C and D). Thanks to Mold Simulator 3’s new features, you can do both with a single project following these simple steps:
1- plot the section elements to identify lengths C and D of the thermal bridge:

2- change the properties of the newly created items by enabling the “double-length” option. Some new lines will appear for each item;
3- adapt to the new lines in order to identify the lengths A and B:

Turning to “Simulation” tab, you will notice two distinct values of thermal bridge’s linear thermal transmittance (psi): one refers to the internal reference point and the other one to the outside point. For more information, please go to Mold Simulator page.