This came from XMFan.com, it appears windows with certain types of coatings will not allow XM to pass.
"However, there are some cases where you can get your portable to recieve reception inside the house. The XM satellites use a powerful signal that can blast through snow, rain, and even wood. However, this signal isn't the alpha and the omega, so it can't get through metal, concrete, flesh, etc. You won't be recieving direct satellite signal in a warehouse, your basement, or any building made of steel. Even if you are near a south facing window, many windows and their coatings (tinting, UV coating, etc) will completely block a satellite signal. You probably won't be getting it in your gym, office, or home UNLESS you have a terrestrial repeater nearby. However, if you are in a building with a wooden roof, you may be able to pull in a signal."
