Basic principle of thermal bridges calculation

[Anatoliy Sapunov]

Hi all!

Fairly, I started to study the topic of thermal bridges quite recently but already have a couple of questions.

I am confusing a little by some points when calculating the linear heat transfer coefficient (Psi). I am trying to understand the algorithm using this info: https://passipedia.org/basics/building_physics_-_basics/thermal_bridges/tbcalculation/basic_principle_for_calculating_thermal_bridges.  All pictures that are attached to this message are credit from this webpage.

 

The first gap is that why we use the total length of two segments that is taken for the calculation as shown in Figure 1, as for me, this leads to the double calculation of the corner of the assembly. So, the question is why we do not take the length to the corner as shown in Figure 2?

The second gap is when I calculate the Psi-coefficient, sometimes I get a negative Psi-value. I assume that this tells us about the absence of a thermal bridge. Is it a correct assumption or not? And, in this case, how we can include a negative Psi-value to overall U-value of the assembly?

Thanks in advance for your help and any ideas or information that might help me with my questions.

Regards,

Toleo!

[yal...@gmail.com]

The idea is to subtract the 1D heat loss from the total heat loss of the model. So when you do that you find a linear value that represents the loss at the corner. The length used is based on either the building reference line which can be inside of the wall or the outer face. Using the outer face you may get a negative value which would compensate what you are referring to as the double calculation of the corner. You'll need to keep the reference line consistent. 

The dimensions you've labeled would be using the inside surface so but would need to do the same for all the other conditions on the building.