Need an extractor fan that does not let cold air in!

[Showboat]

Does such a thing exist?
I need to install two extractor fans and don't want cold air blowing in when not in use. In my old house when you put you hand up to the fan you could feel the cold air coming in
Can anyone advise what I should do to prevent that or things I should look for on fans / vents that stop it?

Thanks

 

[Fr0dders]

you need one fitted with a backdraught flap

http://www.stevensonplumbing.co.uk/inline-non-return-backdraft-flap.html