Complex Fenestration Systems (CFS) can have optical and thermal behaviours dependent from the solar rays’ properties and from the air cavity type. The current most detailed calculation methods rely upon the ISO 15099 standard, which let some gaps on how to deal with such CFS. In particular, the optical and convective models might not be adequate for complex blinds and for wide air cavities. In this paper, a CFD+ RayTracing method has been setup and applied to both Standard and Complex Fenestration Systems with integrated blinds aiming at validating the approach and quantifying possible discrepancies against the ISO 15099 standard.