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Using a dehumidifier to dry out damp walls - tips?
28

nomdemere · 09/09/2014 15:01

The source of the damp was inadequate flashings on 3 chimney breasts. This has (hopefully) now been fixed, but the chimney breasts are all very damp (40% moisture content according to the moisture meter we bought from Amazon - not sure exactly what that means, but apparently over 20% is bad).

Now running a dehumidifier in one of the rooms, planning to do that for a week, then switch to the second, and then the third. Does anyone know if this is sensible?

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PigletJohn · 09/09/2014 18:58

opening the windows will be cheaper. Strip off all wallpaper. If you can lay your hands on an ordinary room fan, set it to blow onto the wet walls. This will encourage water to evaporate into the air. The damp air can then escape through the open window.

If you ever use a dehumidifier, you must close all the doors and windows, or it will try to dehumidify the world.

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nomdemere · 10/09/2014 09:01

Thanks PigletJohn. DH did leave the window open the first time!

You probably know quite a bit about moisture meter readings (I read your posts avidly for information on all things plumbing and property related!) Do you know how quickly we should be able to see if the damp is receding? My concern is that the roofers have not fully fixed the problem (they did a bad job in the first place and I don't have faith in them). How long should it be before we see the moisture level falling? I know there may be a hygroscopic salts issue that keeps the reading high, but should we still see that 40% reading gradually falling over a few weeks/months?

The walls just have painted lining paper on them. I am a bit reluctant to strip it off because it will look so awful, but perhaps we should. We are going to replaster and repaper when it is fully dry - but I don't want to make the mistake of doing this before it's dry - or if the roof hasn't been fully fixed.

Thank you for any further advice!

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PausingFlatly · 10/09/2014 09:20

Opening the windows may be cheaper, but it will also be very much less effective.

After lots of damp problems, I have little humidity meters in every room and one outside. In summer, the outside air is frequently over 70% humidity.

The dehumidifier keeps a closed room at 60%, and it's taken a good six months to dry out the walls.

Moving the dehumidifier round the rooms sounds good - during the fallow period in each room, moisture will slowly come to the newly dried surface from deep in the wall, so your dehumidifier will work slightly more efficiently next time it's in there.

But however you do it, the key thing, as PJ says, is to dehumidify a closed space, so you don't try to dehumidify the world.

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PausingFlatly · 10/09/2014 09:22

BTW, when you say replaster, do you mean hack off first or just re-skim?

If the former, and you can bear it, the walls will dry faster if you hack off now.

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stealthsquiggle · 10/09/2014 09:26

We found a dehumidifier very effective in similar circumstances. I don't know the answers to your technical questions, but our dehumidifier was a great investment - we have used it for everything from dealing with general low level damp to drying rooms after spills/ floods/ carpet cleaning. It's 20 years old now and I really need to get a new one.

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PigletJohn · 10/09/2014 11:06

painted wallpaper forms a barrier which prevents water evaporating from the wet plaster. Strip it off. It will be spoiled anyway if it has got damp.

When you can see the damp wall, pencil round the outline of the damp patch and write the date on it. Do this once a month or so and you will hopefully observe that the patch gets smaller over a period of months.

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nomdemere · 10/09/2014 11:20

Okay, thanks all. Yes, I think we do need to bite the bullet and strip off the wallpaper. Will break the news to DH tonight... I will do the thing with writing the date on outlines of the damp patches too.

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PrincessOfChina · 10/09/2014 11:23

Can I jump in and ask a question here? We have a couple of damp patches. The one on the chimney breast (no actual chimney anymore) has become better since we had a vent fitted and started to use a dehumidifier but one on the bottom of a wall (internal) has remained the same over the course of nine months. It's not got worse, but not visibly shrunk either (we did the drawing thing). Anyone have any ideas?

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Middleagedmotheroftwo · 10/09/2014 11:23

We had a similar problem in a chimney breast area. You do need to strip everything off, down to bare brick, if you can bear it. It will all dry out much faster, though will still take a few months probably. We had the walls sealed before being replastered.

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nomdemere · 10/09/2014 13:08

Are there any problems with sleeping in a room with bare brick walls motheroftwo? Is it draughty?

And what does resealing provide/entail? Sounds like it might be an idea for us too.

Princess - an uneducated guess is that it might be the 'hygroscopic salts' issue which only goes away with replastering?

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Middleagedmotheroftwo · 10/09/2014 15:20

Well, it was our lounge, not the bedroom, but I can't think of any problems. If it's draughty, just chuck another blanket on!

The resealing involved painting some black stuff on the wall on top of the brick. I've no idea what it was though.

We had that done because we couldn't get to the root of the problem. We were probably just masking it, but we sold up not long after Blush.

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PausingFlatly · 10/09/2014 16:07

Yep, you were just masking it and handing a big problem to the buyer. The wall will be even more soggy because the water's now trapped.

There are additives you can put in the plaster to allow water to dry through it while keeping the salts in check. Talk to a pro damp co if in doubt.

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addictedtosugar · 10/09/2014 18:46

Only thing I'd add is to keep checking on the level of water collected, if it is like the one we loaned and is collected in a pot. The first few days (different situation - upstairs flooded their flat, and it all came through our ceiling), we got massive amounts of liquid out - and were emptying the thing several times a day.

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PrincessOfChina · 10/09/2014 20:43

Thanks for the tips. We have to get the damp people back in to check some work they did elsewhere so will get them to take a look. Sigh.

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Middleagedmotheroftwo · 11/09/2014 12:11

Pausing - in our defense, we spent a considerable amount of time and money trying to get to the bottom of our problem. Even to the extend of making our neighbours endure visits by builders, surveyors etc because their fireplace backed on to ours. They didn't have a damp problem.
No-one could get to the source of the damp, so we were advised to seal it over by the "experts".
No damp showed up in the survey when the house was sold.

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stealthsquiggle · 11/09/2014 12:45

Sometimes needs must. When I bought my first house, the survey said it had rising damp - it had a damp proof course with a guarantee which turned out, of course, not to be worth the paper it was written on. There was no way we could afford to get it all redone, and if we had it might well not have made any difference anyway. We got a dehumidifier, and when we sold it the survey showed no damp at all - which suggests it was largely condensation in the first place. The only "sneaky" thing we did was to remove the dehumidifier when buyers or surveyors came round.

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PausingFlatly · 11/09/2014 13:51

I feel your damp pain, middle.

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nomdemere · 12/09/2014 09:29

Our dehumidifier has an automatic cut-off - so it beeps and turns itself off when the water pot is full. It's filling in a few hours (though it's not that big a pot).

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Yvonne80 · 26/11/2015 16:33

Hi, sorry know this post was ages ago, however need advice please: just brought a house (showed no damp on survey) but that's ok as long as we can sort the problem, my conservatory wall is damp, so much so that if you touch it there is moisture running off, outside on the ground directly linking to the conservatory is decking, from what we can see the decking is soaking up the water and the bricks are helping, we are going to remove the decking off the wall (cutting it back) my husband will do the damp proofing, my question is do we dry out the walls with a dehumidifier before damp proofing and should we strip it back to the bricks inside?

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TipfullBoB · 06/09/2016 16:28

Open windows+door(s)increase ventilation.A Big electric FAN..fastest speed setting..on full etc,its head adjusted to move LFT to RHT-Circulates the air rapidly....weight its base down..Tilt/angle to face ceiling...pushes the excess moisture out the opened window(s) etc.superior to just only windows & doors open.new plaster will dry rapidly but naturally .

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HezzieB2B · 23/09/2017 16:48

We have had a damp proof course done downstairs, waited 6 months for it to dry out, then we decided to take off the top part of the wallpaper to redecorate. As a lot of the plaster came off with the wallpaper, we got someone in to replaster the whole room. A week later its drying out downstairs and looks great, however the bedroom directly upstairs has now got little brown spots of damp all over the room!! What can I do to make this better? Dehumidifer plus dampsealing paint to cover it? Or should I be complaining to the initial damp proof contractor? Help and advice please!

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PigletJohn · 23/09/2017 21:35

"bedroom directly upstairs has now got little brown spots of damp all over the room!! "

show us some photos and open the windows.

water vapour is lighter than air so naturally rises through a house until it either escapes or finds something to condense on.

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PigletJohn · 23/09/2017 21:36

btw you had better start a new thread. Your circumstances may be quite different from those described in this one, so will be confusing.

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HezzieB2B · 23/09/2017 22:13

Ok thanks for your advice.

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Chrissie70 · 11/01/2018 18:51

I had really black walls at the back of my sink unit and on walls. The chimney breast was really bad and my landlord got a company in to deal with it. They treated it as it was rising damp. It is gradually drying out on the wall above the sink unit, don’t know about behind the sink unit as he had a new sink unit put in. The chimney breast on the left hand side half way up, (don’t know what is below that as the fire place has been filled in with a piece of wood ) has a big wet patch so I drew round it and wrote the date on the wall, instead of it decreasing it is increasing, HELP PLEASE.... I was told not to put a dehumidifier on as it was “to late” for that...I did put one on before the wall was treated as recommended by my landlord. I was also told that it would take 6 months for it to dry out, fair enough I am a very patient person,but it seems like it is getting worse...what should I do.
Thank yo

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