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Having serious condensation problems on a ground floor concealed toilet cistern which is sited on an outside wall on an uninsulated concrete floor slab.The cistern is a so-called insulated one but condensation literally runs out of the entry points at the bottom (not a water leak so far as I can tell). Cistern was installed a few years ago, and the amount of leakage has caused significant problems, rotting the timber studwork to the boxing and discolouring the floor tile grout. Once renewed, I am considering filling the boxed in section with polystyrene beads up to the water level point on the cistern in order to avoid/reduce the problem. Anybody see a problem with this or should I just put ventilation grills top and bottom and leave it at that as well as lagging the cold water entry pipe, of course.

Hi Hugh

I also have the same problem with my upstairs cistern.

I have come across a product called Armaflex which is a closed cell rubber material that creates a vapour barrier once sealed stopping the formation of condensation.

Its on pipelagging.com website and comes 13mm thick 0.5m x 2m sheets, self-adhesive backed for easy application.

If you stick this directly to your cistern from the waterline down (at least) and cover the bottom face as well keeping all the joints nice and tight this will solve your condensate issue and will also help slightly with the gushing water noise after you flush the system.

I have not done this myself just yet but this is my solution/preventative measure when the bathroom replacement gets underway in the coming weeks.

My only concern with the poly bead would be if you ever need to got to the bottom of the cistern the clean up job would be a task in itself....

Hope you get this sorted soon

Regards
Craig