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 New or Altered. In both cases, you must describe the new assembly. In the case of altered, this will describe the assembly after alterations are complete.
In the Building Tree, navigate to the surface and click on the New Assembly which is undefined and select a valid assembly from the construction assembly library.
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