Hello Heather, I just reread your forum question. My answer was pretty categorical, so I’ll add here that in practice, your installer could be right, but there’s no guarantee. If the proportion of glass tiles is small, the risk of cracking is probably very low on a few accent small format tiles. Even on 25mm x 25 mm toles though, the glass tile manufacturer wont guarantee the application, so if its a small area a membrane is probably a good precaution. He should have some solid or liquid membrane in stock that he can use – one of the advantages of using an installer and not having to buy all the materials yourself. John_________________