They are not as isolating as a set of full size headphones or not as open as Porta Pro's.
On the scale of isolation with IEM's having the best isolation with a 90% blockage, these would be in the 65% range with Porta Pro's at 40%.
I have a big head and the ear cups cover my ears. When I have them on I can't hear people talking to me. I tend to play my music halfway loud though. For the price I don't think you'll be disappointed. Just make sure you burn them in properly.