While working on a project, I realized that the definition of the P_Set „Thermal Transmittance“ does not comply with modeling regulations and restricts the model for flexibility. Since this value works only for all layers of an element (Outside – Inside or Up-Down), the model should have only multi layered components that work as one. The reality is off course different, where the model is built out of many single layered components that form the whole.
Therefore defining the U-Value for a single component does not make any sense. More reasonable is the conductivity of a single layer, which then combined in specific areas results in a U-Value.
May I ask why is this value anyway necessary? For permission planing (when delivered in IFC) it will still be not possible to identify which walls or slabs have a U-Value since they are modeled from at least two components. For Asset-Management in the future the conductivity of a certain material makes more sense if something has to be ordered/repaired. It is a value for/from the building physicist which considers it for energy performance simulations. When such a simulation is ready, the building physicists communicates to the planing team which materials should be adopted to fulfill regulations.
Can please anyone explain me the validity of this P_Set
Two years later and there is no discussion on this one. I wonder how this parameter is handled in all those BIM projects
This is the discrepancy I personaly see between how the designer/builder generates the model compared to a building physicist.
Maybe to follow up the discussion a bit, I will try to visualize the topic.
In this example, you see a cross-section of a building and the types of components used to model this part of the building. It could be more than this but also less, where components are modelled as groups (depending on the BIM authoring tool)
In the next graph you can clearly see the way the designer/builder thinks (modelled based) and how does the building physicist perceives these components referring to his simulation.
The green areas define the components as observed by the building physicist, respectively how he calculates the U-Value for that part. The colorful ones are how actually a model is being built, respectively the number of components.
In the example of the slab:
There is one structural slab modelled from one side to the other of the building. Above it, there is also one component for the finishing layers (either in one package or separately). Underneath the slab, there are two types of insulation, depending on whether it is outside air or a non-heated space. That is the reason why the building physicist has two different compositions, resulting in two different U-Values (U-Value 1 and U-Value 2).
These U-Values work only for the whole composition and not for single entities. That is why I raise the question if the “ThermalTransmittance” makes sense in the Pset_*Common area. It can be currently found in the IfcSlab, IfcCovering (in this case for both the FLOORING and INSULATION). In the case of the slab and flooring it is not quite clear, since we are dealing with one component (for each one of them) but with two different U-Values.
That is why I suggested (two years ago) to actually define the conductivity of the material, which is also crucial in defining the thermal transmittance. There might be (in the future) even a smart tool that could add up these conductivity values and define U-Values for parts of the building. @Moult maybe something for BlenderBIM?
This scenario would fit the modeling conventions and also fulfill the demands to calculate U-Values based on the delivered models.
I hope I was clear enough with the description…
I am open for discussions…
cheers
@agron i think you bring up one of the many factors that have emerged as part of the upcoming BEM IDM report. Trying to find consensus among the many experts has been…. challenging. I will try to add this as a topic of interest in the report that needs to be addressed.
Part of the challenge is getting (nearly) universal agreement on the methodology as one way works well for others (or is “good enough”) while others “require” the other way. Maybe we can’t come to one solution, but should work on being flexible enough to accommodate more than one solution.
@jwouellette
I understand, that fitting everything in one box is not going to be an easy task. Thank you for integrating it in the BEM IDM report. If I can be of any assistance in this case, just let me know.
My point with this topic was to address the clarity of this parameter in the BIM world. It accompanies most of the entities, either it is a wall, finishing or door. The only use case, where we could have benefits of this parameter, was for doors and windows. Everything else was simply more of a confusion than a resolution.
@kurt.battisti
the thermal resistance is simply the inverse of the thermal transmittance. When the R has a bigger value, the better, whereas the smaller value for U, the better. Aren’t we confusing here the industry of what actually means what?
From experience I know that the U-Value is better used in elements such as doors and windows. On the other hand, other components such as walls and slabs would be more suitable to use the R value. Maybe a change in the entities parameter description would be a solution?
No matter how this parameter is called or what it represents, we need to find a calculation consensus of such components, based on modeling conventions.
@agron I wrote about the R_c;op value and not the R-value. See details in my original post.
Yes, the U-value is the most common value in D-A-CH.
In other areas, the R-value is more common.
Since the property discussed is associated with an element, it should (in my opinion) only describe the property of that element.
The U-value includes too many external circumstances.
e.g. a wall: The U-value depends on the space to the right and left. If it is a wall against outside air, then it even plays a role in which building height the wall is located. The R_se changes because “more” air passes by and thus heat is dissipated more quickly.
it has been 13 years, since I was working on my master thesis about thermal performance of buildings, therefore I must admint that I am not very familiar with the R_c;op value…sorry for my ignorance.
But from what I understand from your suggestions, it could be more comprehensible, since would be documenting single layers of components. There is also no necessity to include the R_se.
Nonetheless, the building physicist could still perform an overall U-Value check based on R_c;op values. This would bring the results about the thermal performance of the building.
@kurt.battisti Would this be the added value to use the R_c;op instead of the U-Value for the properties?
