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You can look for material properties online, or in the ASHRAE Handbook of Fundamentals. Since your plaster layer is pretty thin, it may be more important to focus on changes to surface properties such as Roughness, Thermal Absorptance and Solar Absorptance, instead of focusing on changes to Conductivity, Density, and Specific Heat.

If you aren't going to add 3mm to the gypsum board, then you may wan to make a copy of that material for the exterior wall, with the thickness adjusted to 18mm along with the changes to surface properties.