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[Sir Tigger KC]

I have a damp patch inside the back door on an internal brick/block wall which splits the kitchen and bathroom in a single story extension. 

I found out that my tenant was leaving the back door open all day and in all weathers so his dogs could come in and out from the garden, so he's had a bollocking for that and I blamed the damp on that!     To be honest at the time I thought that was what had caused it, but the other day when I was there, on a hunch I had a look under the bath and it's as damp as hell under there. 

I can't find any leaks in the waste or the supply pipes, and the walls under the bath are dry so I don't think it's leaking around the edges.  I filled the bath to the brim and left it for a while and couldn't see any leaks in the bath itself either. 

The concrete floor and the underside of the bath however are wet to the touch, but all over, not in a specific place.  I'm wondering if its a bad case of condensation.   

Any ideas chaps? TIA! 

[Doctor Gollum]

Probably water draining through the wall/floor and soaking into the concrete. Massive humidity shifts due to the weather conditions won't help.

Bath panel off and pop a dehumidifier in the bathroom for a couple of days should sort it

[Sir Tigger KC]

My tenant has a bath every day, so I'm wondering if the heat from that hitting the cold air from the concrete floor is causing condensation. 

Although when I pulled the bath panel off, I'd say it was at least 20 hours since the bath was last used. 

[ronnyd]

Had a flood when bath was emptying a couple of years ago. The strainer and the waste body under it had separated (centre screw had come loose) and the washer wasn,t sealing. Worth a check as when the plug was in it didn,t leak. Had to bale the bath out with a saucepan.

[Kevin Wood]

I doubt condensation alone would be the cause this time of year, or it would have been terrible over the winter.

[Doctor Gollum]

I only suggested it from the following experience...

Once owned a fiberglass hulled boat with a wooden top. It was in one of the sheds (steel building with concrete floor) having some work done (painting the hull being the job that day) at the yard, sat on trestles. It was this time of year, hot dry Norfolk summer, with the main shutter open. The boat was about 70 feet from the door. Anyways, crack of thunder and the heavens opened. Next thing, there was water literally pouring off all the boats in the shed where all the sudden humidity had condensed onto the hulls and was running off. Took a couple of days before anything was dry enough to work on.

[Sir Tigger KC]

So I'm resurrecting this thread as the chap who was living in this house has now moved out and the house has been empty for a couple of weeks. 

There's been no heating on or the bath used so the temperature has been stable.  Last night I popped the bath panel off and it was sopping wet under there.    Not in a specific place but everywhere and everything was wet. 

I'm convinced that there are no leaks anywhere and that it is condensation, so I have left the panel off and next week I will get a dehumidifier in there.

Once I've got it dried out, how can I stop the condensation returning?    I'll need to patch up the plaster that's blown in the ajoining kitchen, but if I don't sort out the condensation problem it will just blow again. 

Any ideas chaps?   

[Kevin Wood]

There has to be a source of moisture for the condensation to continue to form.

Given that you've eliminated the bath and any human activity inside the premises, I think damp is getting in from outside somehow, or the provisions to ventilate the wall cavities and underfloor areas are not effective.

[Viral_Jim]

What's the age and construction of the house Sir Tig?

Our old place was brick with lime mortar so needed to breathe or damp patches formed. could it be you've got the same? That said, the amount of moisture you've got sounds excessive for that....

Anything above the bath as in in the loft, header tank anything like that? Also, is there a shower over the bath? To answer your specific question, all you can do if it is condensation is create some air flow. So freestanding bath, or some sort of extractor would me my two punts.

[Doctor Gollum]

No damp proof course beneath the floor

Best bet may well be to gut the extension and tank it or rebuild 

[dave the builder]

Dry the area with a fan heater ,(RCD protected )
could just be trapped moisture
wrap bog roll round all water and waste pipes
if there is a leak ,bog roll shows it 
fill bath ,wiggle pipes ,check seal around the bath
can water get in via outside 2 walls ,roof,gutter,shared water pipe under floor leaking or hot or cold pipe in floor, sounds too wet to be sewer pipe fail.
once dried,it needs to come from somewhere to get wet again assuming you have an extractor fan 

[Sir Tigger KC]

What's the age and construction of the house Sir Tig?

It's a late Victorian end terrace, but the bathroom is in a single story extension on the back built in the early 1990's.

I've wondered whether the damp proof membrane underneath the concrete floor has failed.    The floor tiles are fine though.  I think I'll take the kick boards off the kitchen units and have a gander under there, to see if there's damp there as well. 

[Viral_Jim]

I've wondered whether the damp proof membrane underneath the concrete floor has failed.    The floor tiles are fine though.  I think I'll take the kick boards off the kitchen units and have a gander under there, to see if there's damp there as well. 

I wouldn't rule it our based on the floor being ok. Depending what its bedded on, it could push the damp to the edges of the room. Odd its onlu in the one spot though. I agree that taking the kick plates off may tell you more.

Oh, one more thought, is there concrete right up to the wall outside? Again happened on our last place and allowed moisture to sit at the base of the wall and migrate through to the inside - we had slugs living on the back of the skirting board!

[Kevin Wood]

Might be worth improving the ventilation under the bath area before doing anything more drastic. Can you put a vent in the bath panel, for example?

[shyboy]

Regardless of what is the main cause of the damp, good ventilation is essential, even if this means an air brick or similar through the outside wall.
I enjoy long hot baths in a modern well heated bathroom and condensation always occurs on all 'cold' mirrors and ceramics and has to be cleared by opening windows and operating an extractor fan to evacuate damp air.
Doing this first will give a much clearer picture of the real cause.