Xypex modified concrete, either by use of the coatings or byuse of the admixtures, definitely continues to breath and thus yes will allow water vapour transfer. There has been some testing done in regards to reduction of vapour and air transfer through Xypex treated concrete and these tests show that Xypex reduces vapour and air transfer by a significant amount. Will the reduction be significant enough to eliminate your issue? It is probably impossible to know. Most sheet membranes on the other hand, when properly installed, will be a complete barrier to water vapour and air.
I would probably do both Xypex and install a membrane as the work associated with exposing the wall and prepping is such that the additional steps associated with doing both is not a lot more work and you really want this to solve the problem the first time. A multiple “belt and suspenders” approach would definitely give you a higher chance of success.
I hope that this helps.
Posted on 06 Mar 2014