Damp in bathroom walls behind tiles - dehumidifier time?

10 Dec 2007
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United Kingdom
Hi folks,
Many thanks in advance for any advice anyone can give.

My little dilemma is that I believe I have damp in the bathroom walls behind the tiles. I thought I had dried out the walls sufficiently before tiling the bathroom.
I have concluded that using a dehumidifier would get the damp out and then I could treat the mould that has appeared in the grout. This has shown very badly in the bottom 18 inches of the shower tray in the shower end of that bathroom.The bathroom is a single story air cavity extension. Nine foot by five foot.
My questions are

a. What size dehumidifier would you recommend to get the damp out of the walls? As powerful as possible? 10 litre or much bigger?

b. What type of grout would you recommend to re grout affected parts of the shower? I have used bal on the floor tiles which seems better than the wall grout.

Many thanks,

A newbie
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I must have missed this yesterday & have replied in plumbing. Bump a post if you think it' sbeen missed but please don't double post as it causes confusion. You should ask Mods to delete the plumbing thread as this is in the more suitable forum.
What makes you think you have damp behind the tiles, is this assumption just based on mould growth you can see on the surface of the grout lines? Do you have the obligatory extract fan fitted in the bathroom? It could be the mould is the result of condensation, poor ventilation & poor quality tiling products; you need to use an anti-bacterial grout such as BAL Microban or similar, cheap DIY tiling products are generally crap. I know it sounds tedious PITA but wiping the shower enclosure down with a rubber squeegee after each use helps to reduce mould growth tremendously.

If it is indeed damp behind the tiles then you need to establish where it’s coming from, entering the wall from outside, soaking into the grout lines from the front; are there any pipes behind the tiles? Either way, damp behind will have to evaporate through the thin grout lines, it will take weeks & a de-humidifier isn’t going to help much; if you don’t cure the cause, what happens when the damp returns? You say “I thought I had dried out the walls sufficiently before tiling”, does this mean there is a history of damp or is it a new extension you have tiled too quickly? What are the walls made of block & traditional plaster or dry lined plasterboard? What preparation was done to the walls before tiling? Are the tiles still firmly attached to the wall?
take the bottom course of tiles off..and see what you have on the walls,could just be water penatration,..then come back on with more info/.pics...
also your mouls could be caused by soap,shampoo and conditioner, when getting showered if you dont rinse away properly this will lie on the tiles and grout and appear as mould, seen loads like this and even more so on OBS.

But ya gotta find the cause of the dampness and rectify, no good trying to dry it out only for it to return in a few weeks time.

personally if it was mine it would all be stripped, problem rectified then re-tiled
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Thanks guys for your input.

Well, I did assume there was still some damp behind the bottom of the shower room tiles.
I used a bal mould cleaner to remove the damp from the lower half of the shower enclosure six months ago. I then did not use the shower enclosure for six months and kept the shower room warm to see if the mould came back. The mould did come back but mainly inbetween the grout between the bottom row of tiles. Thus, I assumed there may have been some dampness behind the wall tiles in the wall.

Just a bit of background on this shower room for you, before I tiled it. This shower enclosure is in the end of an extension is 100 year old and was never used. It is constructed of brick and a cavity between the two what I think is two brick walls.
The far end of extension got damp in the shower end due to the next door neighbour knocking their adjoining extension but not sealing our shared partion wall. Thus, the walls in my extension got very damp. mainly at the bottom end. The neighbour has since built a new extension and the adjoing wall does not pass any moisture through to my side.
I dried the walls out using a heater over several weeks before tiling and stripped back the plaster to the brick.Dried the bricks out till I could get the plaster to stick to it. Then I used pva sealing stuff on top of the plaster, then tiled over the plaster on the walls.

The mould only seemed to come back on the bottom course of tlles, so I thought I may not have thoroughly dried out the other two external walls in the shower end of the extension.

Though, I do hold my hands up to making mistakes using cheap grout as I was sold this by a tiling chain that soon went bankrupt!lol..Just my luck.
Also, as you guys said, not hosing down the walls and thoroughly drying out the shower unit has caused much of the mould between the tiles in the shower enclosure.. I have fitted an extraction fan and usually open the windows to ventilate after a shower. Also, now have a dehumidifier to make sure all moisture is removed.

I will try cleaning the persistent mould off the bottom run of tiles with bleach and use a dehumidifier in the shower as it get very wet with the power shower.
If the mould persists in returning to the bottom layer of tiles, I plan to to re grout the tiles and maybe take off the bottom layer of tiles to check for any residual damp in the walls. Though, I plan to drill a small hole into this wall from outside the extension to check for dampness in the wall before taking off the tiles inside. The tiles are fixed very securely.

If any of this makes sense, What do you guys think?

Many thanks again for all your input


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