I think the screen porch will block some light -- how much probably depends. I've heard of people putting skylights in their screen porch and the skylights in your greatroom will help alot too. Can you put additional windows in the greatroom besides the ones next to the french doors? Is there another wall of the greatroom that could have windows?

Our plan originally called for a covered deck off our greatroom and I now want to move it to the corner of the house so that more light is let in to the greatroom where the best views are. That is just my personal preference, though. If light is important to you, you should consider another location or more windows, skylights, etc. Good luck!

Depending on the orientation of the great room and porch, you might actually appreciate some light reduction. A west-facing room can, in temperate and warm climates, get some pretty fierce sun in the afternoon. Having indirect light, rather than a direct window onto such sunlight, can be pleasant in such conditions.

--Steve

We put a sunroom on the back of our house and yes, it does block the sunlight but it was well worth it. I put 2 skylights in the diningroom and that helps a lot.

We have a screened porch off of our kitchen and it does block light. How deep is your porch? I live in Wisconsin where sunlight streaming into the house in winter is a good thing, and I personally would not want a screened porch off of my great room if the only windows in the great room looked into the screened porch. I think that putting skylights in the screened porch will help and also painting the ceiling of the screened porch white might help too. Have you considered using big windows instead of screens? If you had windows you would have option of removing the screens in the winter which would brighten it up a bit and keep it cleaner. I have been surprised at how much dirt, pollen and snow gets into our screened porch through the screens.

Our screened porch is in the SE angle formed by the breakfast room and the DR. It has a peaked roof with two skylights in the E slant of the roof. Neither adjacent room feels darkened by the porch at any time of year.