diff --git a/src/Mod/BIM/bimcommands/BimArchUtils.py b/src/Mod/BIM/bimcommands/BimArchUtils.py
index 801cacc41f..9b31ef04d4 100644
--- a/src/Mod/BIM/bimcommands/BimArchUtils.py
+++ b/src/Mod/BIM/bimcommands/BimArchUtils.py
@@ -54,6 +54,10 @@ class Arch_Add:
             FreeCAD.ActiveDocument.openTransaction(translate("Arch","Add space boundary"))
             FreeCADGui.addModule("Arch")
             FreeCADGui.doCommand("Arch.addSpaceBoundaries( FreeCAD.ActiveDocument."+sel[-1].Name+", FreeCADGui.Selection.getSelectionEx() )")
+        elif Draft.getType(sel[-1]).startswith("Ifc"):
+            FreeCADGui.addModule("nativeifc.ifc_tools")
+            for s in sel[:-1]:
+                FreeCADGui.doCommand("nativeifc.ifc_tools.aggregate(FreeCAD.ActiveDocument."+s.Name+",FreeCAD.ActiveDocument."+sel[-1].Name+")")
         else:
             FreeCAD.ActiveDocument.openTransaction(translate("Arch","Grouping"))
             if not Arch.mergeCells(sel):
@@ -89,6 +93,10 @@ class Arch_Remove:
             FreeCAD.ActiveDocument.openTransaction(translate("Arch","Remove space boundary"))
             FreeCADGui.addModule("Arch")
             FreeCADGui.doCommand("Arch.removeSpaceBoundaries( FreeCAD.ActiveDocument."+sel[-1].Name+", FreeCADGui.Selection.getSelection() )")
+        elif Draft.getType(sel[-1]).startswith("Ifc"):
+            FreeCADGui.addModule("nativeifc.ifc_tools")
+            for s in sel[:-1]:
+                FreeCADGui.doCommand("nativeifc.ifc_tools.aggregate(FreeCAD.ActiveDocument."+s.Name+",FreeCAD.ActiveDocument."+sel[-1].Name+",mode='opening')")
         else:
             FreeCAD.ActiveDocument.openTransaction(translate("Arch","Ungrouping"))
             if len(sel) > 1:
diff --git a/src/Mod/BIM/importers/exportIFC.py b/src/Mod/BIM/importers/exportIFC.py
index d235cf4959..7088e923bc 100644
--- a/src/Mod/BIM/importers/exportIFC.py
+++ b/src/Mod/BIM/importers/exportIFC.py
@@ -2362,6 +2362,10 @@ def getRepresentation(
             # actual ifcmaterial)
             shapecolor = obj.ViewObject.ShapeColor[:3]
             transparency = obj.ViewObject.Transparency/100.0
+            if transparency == 1:
+                # fix buggy fully transparent materials
+                # TODO there is some problem somewhere in ShapeAppearance that needs solving.
+                transparency = 0.
             if hasattr(obj.ViewObject,"DiffuseColor"):
                 diffusecolor = obj.ViewObject.DiffuseColor
         if shapecolor and (shapetype != "clone"): # cloned objects are already colored
diff --git a/src/Mod/BIM/nativeifc/ifc_diff.py b/src/Mod/BIM/nativeifc/ifc_diff.py
index bb0b078654..be418fca63 100644
--- a/src/Mod/BIM/nativeifc/ifc_diff.py
+++ b/src/Mod/BIM/nativeifc/ifc_diff.py
@@ -26,6 +26,7 @@ import difflib
 import FreeCAD
 import FreeCADGui
 import ifcopenshell
+from nativeifc import ifc_tools
 import Arch_rc
 
 translate = FreeCAD.Qt.translate
@@ -41,7 +42,12 @@ def get_diff(proj):
         # than ifcopenshell and diff does not work
         f = ifcopenshell.open(proj.IfcFilePath)
         old = f.wrapped_data.to_string().split("\n")
-    new = proj.Proxy.ifcfile.wrapped_data.to_string().split("\n")
+    if not old:
+        return ""
+    ifcfile = ifc_tools.get_ifcfile(proj)
+    if not ifcfile:
+        return ""
+    new = ifcfile.wrapped_data.to_string().split("\n")
     # diff = difflib.HtmlDiff().make_file(old,new) # UGLY
     res = [l for l in difflib.unified_diff(old, new, lineterm="")]
     res = [l for l in res if l.startswith("+") or l.startswith("-")]
diff --git a/src/Mod/BIM/nativeifc/ifc_tools.py b/src/Mod/BIM/nativeifc/ifc_tools.py
index da35ef8616..0f1cc44bec 100644
--- a/src/Mod/BIM/nativeifc/ifc_tools.py
+++ b/src/Mod/BIM/nativeifc/ifc_tools.py
@@ -414,6 +414,8 @@ def get_ifcfile(obj):
             if getattr(project, "Proxy", None):
                 project.Proxy.ifcfile = ifcfile
             return ifcfile
+        else:
+            FreeCAD.Console.PrintError("Error: No IFC file attached to this project")
     return None
 
 
@@ -977,14 +979,17 @@ def save(obj, filepath=None):
     obj.Modified = False
 
 
-def aggregate(obj, parent):
-    """Takes any FreeCAD object and aggregates it to an existing IFC object"""
+def aggregate(obj, parent, mode=None):
+    """Takes any FreeCAD object and aggregates it to an existing IFC object.
+    Mode can be 'opening' to force-create a subtraction"""
 
     proj = get_project(parent)
     if not proj:
         FreeCAD.Console.PrintError("The parent object is not part of an IFC project\n")
         return
     ifcfile = get_ifcfile(proj)
+    if not ifcfile:
+        return
     product = None
     stepid = getattr(obj, "StepId", None)
     if stepid:
@@ -1001,11 +1006,14 @@ def aggregate(obj, parent):
         newobj = obj
         new = False
     else:
-        product = create_product(obj, parent, ifcfile)
+        ifcclass = None
+        if mode == "opening":
+            ifcclass = "IfcOpeningElement"
+        product = create_product(obj, parent, ifcfile, ifcclass)
         shapemode = getattr(parent, "ShapeMode", DEFAULT_SHAPEMODE)
         newobj = create_object(product, obj.Document, ifcfile, shapemode)
         new = True
-    create_relationship(obj, newobj, parent, product, ifcfile)
+    create_relationship(obj, newobj, parent, product, ifcfile, mode)
     base = getattr(obj, "Base", None)
     if base:
         # make sure the base is used only by this object before deleting
@@ -1179,7 +1187,7 @@ def get_subvolume(obj):
     return tempface, tempobj
 
 
-def create_relationship(old_obj, obj, parent, element, ifcfile):
+def create_relationship(old_obj, obj, parent, element, ifcfile, mode=None):
     """Creates a relationship between an IFC object and a parent IFC object"""
 
     if isinstance(parent, FreeCAD.DocumentObject):
@@ -1280,7 +1288,8 @@ def create_relationship(old_obj, obj, parent, element, ifcfile):
                     relating_object=container,
                 )
     # case 4: void element
-    elif parent_element.is_a("IfcElement") and element.is_a("IfcOpeningElement"):
+    elif (parent_element.is_a("IfcElement") and element.is_a("IfcOpeningElement"))\
+    or (mode == "opening"):
         uprel = api_run(
             "void.add_opening", ifcfile, opening=element, element=parent_element
         )