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Sorry if it is one external wall I was interpreting your plan.
Even with one cold wall if you are generating steam it is unlikely that any extraction system will be able to remove the vapour fast enough to stop a proportion of of the vapour condensing on the wall and generating mould and damp. Replacement air for the air extracted will need to come from somewhere if it is outside then it will be cold and also maybe damp. All I was saying was look at your alternatives before spending £200 on core drilling. Have you looked at reducing the steam you shower makes by choosing a different shower head also could put a bigger radiator in the bathroom would cost more in fuel bills but you might still spend less than £400 over 20 years.
Zebra_Cyclist-
The right hand is the party wall with next door (terrace) and the left is shielded from the outside by the void. Also half that external wall is covered by the boiler room come shed which probably has some thermal buffering effect. I'm replacing the radiator for a tall towel rail, and getting the window replaced, the current one condensates like a motherfucker. If that doesn't sort it out then I'll have to look at insulating that wall I guess. I actually want to make the shower more steamy as it's a bit feeble at the moment (electric).
oh, and the replacement air comes from the outlet via the kitchen, with some recovered heat.
Polygon
1 external wall. How does one insulate a solid wall for less than £400?
Surely the problem is my extraction is shit, hence all the condensation and mould. I have little desire to live in a well insulated but very humid environment.