Each wall/roof/floor/slab must have a valid construction assembly selected to perform the simulation. Leaving an assembly as “undefined” is not a valid choice. In this case, the surface has been designated as either Existing or Altered. In both cases, you must describe the existing assembly. In the case of altered, this will describe the assembly before alterations are made.
In the Building Tree, navigate to the surface and click on the Existing Assembly which is undefined and select a valid assembly from the construction assembly library.
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