Here I have a simple Room Model, a cube with Flipped Normals, using back face culling to get HDRI Lighting into the room in the Final Render. It works fine in Eevee, but not in Cycles. In Cycles I have tried setting up a material with the shader network shown the Image. The Cycles Render is becomes a black Cube with HDRI alone. The Cycles render shown in the image is because of an area Light.
Does anyone know how to fix this in Cycles (So HDRI Lighting from Environment can get into the Room ?
First you can not compare Eevee with Cycles.
Second, Iām not sure what you are trying to achieve.
The environmental light will be blocked by the two walls.
Oh. I Am sorry my question is too complicated for you.
I will try to explain in plain English what I am trying to achieve, but I am not sure if you will try to understand it.
Firstā¦
Itās obvious you havenāt seen the technique of using Walls with back face culling to allow light from exterior to enter a room via one-sided walls, but because the walls are flipped, the light that has entered the room bounces within it. Because of backface culling the exterior walls are transparent when the camera looks through it.
So with this technique light can enter the room but gets reflected and stays in.
I canāt find the exact link but Blender Guru had a discussion in his channel somewhere in his video about this technique in interior lighting, but then that was only for Eevee.
āā
Aside: Also, Maya-ns and 3DS Max worshippers are known to use this knowledge when practicing their relgious Arts. Only Blender worshippers sound like rookie Evangelical Blender pastors avoiding questions by blocking out or denying subscribers asking questions in Q&A (or Bible Study), when they cancel out thinking the questions are ātoo complicatedā to fathom.
Got it.
A Mayan Missionary Pastor was helpful. 
This is a beginnerās course, what you are trying to do is unnatural, Light a room as though the walls are not there. Do not think I have ever heard of anyone wanting to let alone doing it.
Solution get an interior room hdri! Do not make the cube box.
I think dude was trying to make room scene with all interior setup and use HDRI to light it. So to have that you need all walls. But normally HDRI would not go trough the walls even an interior one. Question was build around that. How to light interior scene using HDRI
However impolitelyā¦ 
I think you are a very experienced user. Maybe not so in Blender!
What you are doing, is asking āintelligentā questions which can only answered by yourself.
Which leaves the impression you know it all. All your previous questions are answered by yourself, in the same tone of voice.
That is not a problem, we all are still learning here!
But to give the impression, donāt answer at forehand, because people who spending time on this forum, donāt know anything. Is the wrong attitude at this forum.
So it, would be polite to show the answer, so others can learn.
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