I have been using a outside "Stormshield" cover for years with zero problems.With car covers you get what you pay for,so if you want one that works and is breathable you have to pay the pounds...This is the one that I have and it is still functional through all weathers after 5 years use:
http://www.prestige-carcovers.co.uk/...mart&Itemid=10.SJ.