Is there a way to write these building physical property values into a Classification Library as are found in blenderbim/bim/schema/classifications? ie. can we append values to lines like in the Masterformat spec item: #864=IFCCLASSIFICATIONREFERENCE($,'03 31 00','Structural Concrete',#862,$,$); - to include thermal, mass, cost and other values and schedule or calculate from these values in a meaningful way? Would adding a property to the above look like this? (I'd need help w syntax as I have no idea)... #1533=IFCCLASSIFICATIONREFERENCE($,'07 21 13.19','Mineral Board Insulation',#1530,$, (‘ThermalConductivityTemperatureDerivative',$,IFCREAL(0.035),$; - where the U-value for Rockwool CB110 for example is 0.035W/mK
@Andyro I have no experience of Classification libraries, but in principle you should be able to create an IFC file that contains just your materials with all their physical properties, shading colours etc.. in the form of IfcMaterial and related entities. Then you can use the Project Library > Custom Library feature of Bonsai to import them into any future models.
(there are some missing features here: it would be nice to be able to import a material and have it clobber any existing materials with the same name; similarly it would be nice to be able to select several materials and Ctrl-J to merge them into a single material)
I'm not an archicad user, but per this comment, it seems you can set the 'translator' to export Material Properties.
https://forums.buildingsmart.org/t/does-these-attributes-are-defined-by-buildingsmart-or-not/1371/7
I've reached out to Stefan to see what I am missing there. His sample is from AC23, we're at AC27 now, there may be some small differences in translator settings, I will find out tx!
Finally! Got it to work. 
It was a 'Data Conversion' option in the ArchiCad translator, where 'Building Material Properties and Classifications' needs to be checked/selected. Then all of the values from our established templates import to BBim and can be viewed and presumably scheduled for wbLCA purposes.
Settings here in the IFC translator in AC: 
I have been struggling with how to represent walls with steel framing, external steel cladding and internal plasterboard lining in my geometry.
I understand Dion's comment that "nothing in IFC requires geometry" but I need the geometry for visual representation.
Do I represent the external cladding as a wall type and use this to construct the geometry? Alternatively, if I classify the external cladding as a class, what geometry do I use to include it in 2D/3D representations.
The same would apply to wooden frame structure with wood or cement/other sheet cladding.
Not being either an architect or an engineer this puzzles me.
hi @ubiquity
Do I represent the external cladding as a wall type and use this to construct the geometry? Alternatively, if I classify the external cladding as a class, what geometry do I use to include it in 2D/3D representations.
I use IfcCovering[CLADDING] for it (vertical layer)
The same would apply to wooden frame structure with wood or cement/other sheet cladding.
for the frame you can use IfcMember[STUD] or [PLATE] etc.. (extruded profile)
Not being either an architect or an engineer this puzzles me.
if that may comfort you, for me too it hasn't been an easy ride, but I got used to enjoying it :)
Thank you for your reply and sentiment!