Hi,
I just made a room with a window and put a hdr map and sun lamp outside to light it all up. It looked good but then I remembered that I hadn’t put in any glass in the window. So I added a cube, made i thin like window and just gave it the glass material and set the color to white. This is in Cycles by the way.
The scene by the window went a little bit darker but that’s ok I guess. But then I noticed that even if I bump up the power of the sun by 100 the room is not getting anymore light. If I remove the transparent window I can see that the sun is much stronger. So apparently the glass is not letting all of the sun through. Why is this?
Might have to do with the index of refraction?
A common solution to this is to forgo the glass shader and use a 70% transparent, 30% glossy-shader mix.
Aha! That did the trick. Thanks for the tip. 
I was so focused on using the glass shader.