Comment by flashingleds

Al-foil is ubiquitous in ultra-high vacuum labs because part of the process of reaching this vacuum level involves baking the whole chamber at 120-150dC for 24+ hours. The chambers are typically made of stainless steel which has a pretty mediocre thermal conductivity. Hence the Al-foil, to ensure even heating during the bake. Since you have to bake every time the chamber is opened, it’s easier to just leave the foil on.