Given that the Epson Pro Cinema G6900 WU is an incredibly powerful projector for the home, its assumed that you chose it because you have a difficult room, but you want the benefits of large image size that only projectors can bring to the party.
It's not just the amount of ambient light in the room that matters, but where it comes from - that is, what angle from the screen.
The first thing you need to know is that if the ambient light originates from the same basic area as the projector - which is to say, straight back from the screen, then the screen selection isn't really going to help. You just don't want a window behind that projector, unless you've got good window coverings!
Note the image above. In that picture I'm using the G6900 WU projector to put a bright image on a wall! The wall, isn't even white, but a medium light tan. We won't worry about the color shift because of the wall color, but what is interesting is the lower left corner.
Yes, you are looking at sunlight hitting directly onto the wall where the lower left of the image is. Where the sunlight is, you can definitely see that the scenery is a lot more washed out than where the plane wing is, just outside the sunlit area.
Now that sun is hitting the wall from about a 35 degree angle - which is to say 55 degrees from straight back where the projector is. Unfortunately, our wall has no light rejecting properties.
But different screens do. From subtle, to dramatic. It is unfortunate that we haven't yet received the new screen for this room, as I've requested a Black Diamond screen from Screen Innovations. That screen is designed to really absorb, not reflect back to you, light hitting it from off angle, such as in this case.
How good is it? The difference should be dramatic! Unfortunately as of the publication of this review, still no answer from Screen Innovations. I've requested they place the screen here as part of our Dream Home project. (I don't really need a projector in the living room with two other rooms having screens, including my theater.) But, I figure that the media room concept is a very valid one, and this living room as tough a challenge as one will find.
If Screen Innovations doesn't come through, I'll look to some alternatives. Europe's dnp is another screen manufacturer with screen materials specifically geared for dealing with rooms like mine. Elite Screens is now offering their DarkStar surface which also should be right for the room, but they haven't managed to offer it yet in a motorized configuration, which I need.
Failing all that, I'll drop back to more traditional screens. In my last home, a Stewart Firehawk did a very impressive job of handling side ambient light, and probably can do a very good job here, although not as good as the newer screens designed for the purpose.
It should be noted, in the image above, that the image size is about 6 feet vertically, and since it's a 16:9 image, we're looking at about a 12 foot diagonal image! That's very nicely huge! BTW that dark area on the right is a mirror, it was just too heavy to remove when taking pictures.
If the G6900 projector can do this on a 144" diagonal off color, not even near white (in brightness or color) wall, a wall surface with no side ambient light handling technology, imagine what it can do on a screen this size or slightly smaller with a screen built for the job!