To update an *.idf file without having to start over, open the building file in TRNBuild and select „Export TRNSYS3D file…“. This will generate an *.idf file and save the modifications you have made to your *.b18. Open the newly created *.idf in SketchUp, make your modifications and save the modified *.idf, preferably under a different name, so you can keep the old one as a backup.
After returning to TRNBuild and reimporting the file, you should end up with a *.b18 that has your earlier modifications from TRNBuild as well as the modifications you just made in SketchUp.
