jonathan18: Well, the Volere ones from Briscoes have no plug, silicon or otherwise, so essentially need to be hand-washed (and done so carefully) so as to avoid the void getting some water in it! Of course, with two scungy boys, it’s inevitable this happens and it’s a real b!tch removing the water. I keep on meaning to put a drop of glue gun glue on the hole to see if that works - does it need to remain open at all, or can it be completely sealed?
We have bought several of the Volere double wall glasses from Briscoes, and all have the silicon plug on the bottom, and all go in the dishwasher (as per website and manufacturer) without any problems. Like most glass items they do not like being dropped. I would suggest yours may have been faulty? or may have had the plug removed by someone hand washing it over-vigorously at some point?