Yes, but....
A bivvy wont keep you warm, thats the job of the sleeping bag and your matress (Thermarest or whatever, I use an Exped..).
Dont buy a cheap one, you need Goretex so it can breath and you dont wake up wet from condensation.
Try and get one that the zipper is on the same side as your sleeping bag.
Something like this...
http://www.ebay.co.uk/itm/Grade-1-Genuin...4aced2391e
A bivvy wont keep you warm, thats the job of the sleeping bag and your matress (Thermarest or whatever, I use an Exped..).
Dont buy a cheap one, you need Goretex so it can breath and you dont wake up wet from condensation.
Try and get one that the zipper is on the same side as your sleeping bag.
Something like this...
http://www.ebay.co.uk/itm/Grade-1-Genuin...4aced2391e