I'm pretty sure we have talked about this before, but I can't find the thread about preventing sunlight through a window from causing light fluctuations on set.
After some experimentation, I found the best and cheapest sulution is to make a diffusion curtain out of heavy black garbage bags. Some windows will be higher or wider than the bag will cover, so you might have to tape two or more of them together. But you will end up with a large sheet that does a very nice job of blocking out any light you don't want, even you can still see some light coming through with your eyes. At the end of the day, if the camera doesn't see it, it doesn't exist. I like to take test shots of the set with no lighting to make sure I'm getting a completely black image. Then I light from there. What this does is keep you from bringing the exposure up too high to where ambient light will be seen on the camera and cause fluctuations as the sun goes behind a cloud or comes out from behind one. More often than not, you will find that if you light and expose from complete darkness, you will be wanting to stop down the camera to close up the iris more than you will want to be opening it up. In some cases, the bags might need to be more than a layer thick, as in where you need a stop of F/16. In the case of using a higher stop like F/5.6, you won't need to completely block the window, but just dim it several stops.
I've tried blankets, cotton sheets, cardboard, and aluminum foil - and so far, the black bags attached with masking tape have worked the best. They even allow access to the window on a hot day when you need some air. The cardboard tended to not want to stay up, the aluminum foil bowed in and bounced light everywhere, and the cotton sheets weren't thick enough, so the black bags ended up being perfect for my setup. Hope this helps someone who is wondering how they're going to animate in a room that wasn't designed to be animated in. You can even use this setup in hotel or motel rooms when you're on vacation.