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Modeling
About Archicad's design tools, element connections, modeling concepts, etc.
You might consider doing this with a line versus a fill. To create a insulation line is relatively simple. In you composite (other post) you would have to "split" your center part of the composite to allow a line in the middle.
That's right - hopefully this will be changing soon!
In the meantime I suggest using the InsulFib library part in the ArchiCAD Library right over the top of your sections & details. It is very flexible - you can stretch it all over the joint using editable hotspots.