Is that foil? People will tell you it makes heat spots, I'm yet to see this to be true, but I will tell you that it will keep in a lot of heat and that's not good.
Also, your bulb has a lot more light coming off it's sides than it does it's end, you should make it so it's horizontal. Some people also report problems with heat/fire inside the CFL's ballast components when hung vertically like that.