The missing wall is not a problem (leak) if you're modeling the lighting of a real outdoor seating area (think porch). In that case, the sky and environment will contribute far more than the missing wall would be contributing. The missing wall is a net addition when the env sphere is bright.

Re glowing couch back against wall - IDL reflection is wrong in Poser. Use D3D dialog to decrease IDL intensity to .65.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)