Well, occasionally something really cool comes along, and you just
gotta share it. 
This is semi-operational, so....
http://news.com.com/Sheet+could+shelter+Wi-Fi+from+eavesdroppers/2100-1029_3-5685431.html
..there.
- ferg
Assuming your walls, roofs and floors have the same level of protection, and you need windows then this product is a good fit.
Certain British institutions I have been involved with in the past don't bother with windows and the walls are faraday cages (internal ones as well!).