Unusual Damp Issue

[RicardoF1RST3]

Apologies if this has been covered before but having read through the posts and searched I can't find an answer to my unique set of circumstances, which are:

We have a ground floor bedroom, with en-suite (toilet/sink/shower) which has been built into an adjourning garage with only a single breeze block thick wall. Our cold water supply is extremely cold so we have a combination of v.cold water, none insulated walls, tiled floor (probably onto concrete) and the ceiling isn't lagged either. Plaster inside is sound so no rising damp issue, hence it must be condensation related.

We can smell damp so I have used a dehumidifier in the bedroom which collected 3L of water in 24hrs so I need to do something ... ! The reason why the en-suite has single breeze block appears to be because any thicker would cause the garage door not to be able to open ... there is a hot water rad in the en-suite.

I am considering the following solution, any comments would be appreciated.

1) Lag the loft / ceiling with loft insulation (no brainer).
2) Box the toilet cistern and lag inside with polystyrene - to stop condensation from cold water / warm air reaction. The ceramic cistern gets soaking wet at the moment.
3) Attach (how?) 75mm thick polystyrene 8ft x 4ft sheets to the exterior of the en-suite (in the garage) to insulate the wall from the coldness of the garage. Use this material as I can modify thickness / channel in places to allow the garage door to open.

Initial questions:
Would the above not work?
Would the polystyrene need to be covered?
...if so with what?
Any other ideas?

Many thanks for any assistance.

[welsh brickie]

fix 50mm x 50mm batten to the wall and use 50mm drytherm cavity insulation inbetween to insulate then screw plasterboard ontop

[RicardoF1RST3]

Thanks for the tip, I was thinking polystyrene because I can cut and channel it easy - plus shave some off near the garage door - and more importantly I can get it free!...(long story)

But the DriTherm is cheap enough - so I'll have to measure the gaps involved and see if it will work.

Cheers