I am getting a warning that says: "Both surfaces 'Surface 1', and 'Surface 2 reference the same construction 'Wall Interior 1' but it is not symmetric, creating a reversed copy." What does this mean?

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The problem is not with custom constructions specifically, it's with asymmetric constructions, i.e., constructions with material layer orders that are not the same when read from one side vs. another. If you assign asymmetric construction ABC to matched surfaces in zones 1 and 2, you create an impossible/ambiguous situation. The construction must look like ABC from zone 1 and CBA from zone 2 or vice versa. It cannot look like ABC from both zones. If only one surface is assigned ABC, EnergyPlus automatically creates the CBA construction and assigns it to the matched surface.