In the Material Mapping dialog you have the choice to set the Surface Type to either "Opaque, 1-Sided" or "Opaque 2-sided".  Users familiar with AGi32 will know these as Single-Sided and Double-Sided surfaces.

In AGi32, all Rooms are single-sided by default, and all Objects are double-sided by default.  The idea is that you typically want to be able to see into Rooms in the Rendering but do not typically need to see into Objects.  With Revit there is not obvious distinction between Rooms and Objects, so everything is single-sided by default in ElumTools.

In Revit there is really no-such thing as a surface.  Pretty much everything is a 3d object, made up of multiple surfaces.  Revit does a great job of making sure each surface is "aimed" the right way, so setting everything to single-sided will almost always give you what you want, with the fastest calc time. 

There are a few things in Revit which are represented as meshes  (e.g. topography surfaces and landscape elements such as trees).  These kinds of elements should be set to 2-sided, otherwise the Rendering can look goofy.  But for interior projects, 1-sided is almost always the way to go.