Curing damp

Hi everyone. I'm renovating a 100+year old house which has widespread damp problems caused I suspect by various problems. I'm sure a lot were caused by condensation through poor ventilation and we're doing all we can on that score but we've still got damp which I would put down either to rising or penetrating damp. We've hacked everything off the walls inside the house and I've gone round with a damp meter. The worst areas are definitely low down which suggests rising damp to me. The house had a chemical DPC injected some years ago but that hasn't cured the problem and I'm reluctant to spend more money on getting it done again. I'm having the guttering replaced but that will take time to see if it cures the problem and I'd like to get the house plastered without waiting to see if new guttering helps.

So my question is this:
Is there anything which I can apply to the walls which will contain the dampness without removing it from the wall but prevent it showing through the plaster-i.e. prevent the visible signs of damp rather than curing it? I can batten some of the walls but it isn't practicable with all of them. Even applying a coat of render is difficult in some places because of door cases. In the past someone has applied a coating of a black substance which seems to be damproof according to the dampmeter. Is anyone aware of such a product, which I could apply and then plaster over as normal.

Secondly, could I fix insulation backed plasterboard to the walls(where I have the room) without curing the damp problem. This would help with the condensation and make the house warmer but I don't know if the damp will work its way through.

Sorry the post is so long winded but I think I have a number of different problems and need a variety of solutions.

All replies will be very much appreciated.

Thanks
Keith

This dampness could have several possible causes. Just because it is showing damp low down the wall does not mean its coming up - it is just as likely to be going down and being stopped on any DPC, coming through, or it could be condensation running down.

Your damp meter is telling you nothing.

There is nothing you can paint on the walls, and you really need to identify the cause and treat that

If you cover it or even plasterboard over it, the damp blow the coating off or creep above it, will promote mould which will take hold and spread in any air gap

Damp will not go away and will permeate through any porous material (like plasterboard) or will find a joint and come through that (like between the wall and floor)

Don't think that you can just hide it and carry on with the decorating

what did you find when you looked under the floors? rubble, mud, soil, concrete? wet or dry? ventilated or stagnant?

how many airbricks are there on each side of the house, and how many of them are blocked with dirt, rubble or concrete?

how many leaking waterpipes and drains do you have?

have you looked for a 100-year-old slate DPC? is it still above ground level or has it been bridged with paths and flowerbeds?

Have leaking gutters been causing water to drip or splash onto the walls? If so they will take about a year to dry.

Thanks for the replies chaps.

To answer your specific questions John:-

1 of the downstairs floors is recent concrete with a DPM visible round the edges. The other is wooden over a cellar which is ceratinly damp but not excessively so even with all the rain we've had. The worst damp is below the windows at the front of the house which fronts directly onto the pavement, but there is some damp. according to the meter, in the internal walls which are above the walls leading down to the cellar. Both these walls had a chemical DPC injected some years ago but clearly these particular walls are unaffected by leaking guttering etc.

There are airbricks into the cellar at street level and these are not blocked. I suspect they were put in when new floor joists were fitted above the cellar, but I don't know when.

There is no slate DPC.

The guttering does leak in one area and I've already arranged to replace the whole lot.

Some damp is present in original walls where a kitchen/bathroom extension has been added at a higher level than the original floor level and I have no way of knowing what measures were taken to protect the outer original wall which was covered when the extension floor was put in.

I realise that the different areas of damp might have different causes, and so might need different solutions. I would also not like to wait for a year before replastering. The house is for my children and they are keen to move in.

1 thing I'm not sure about is how much to trust the meter readings. If the reading is high does that automatically mean damp will show if I replaster? I am not after a bodged quick fix as the kids are likely to be in the house for some years, but I don't want spend time and money fixing a "problem" which really doesn't exist except in a display on a bit of electronic equipment.

The meter readings were much worse in the mortar than the bricks-is this normal? Also do you know whether injected DPCs really work. If I was confident that they do I would spend some money on redoing it but I don't want to waste money.A builder mate has suggested apllying a sand/cement render with a waterproofing agent mixed in. Do you know if this will work?

I apologise for all these questions but I want to deal with this properly first time and at the moment don't really know which way to turn.

Thanks for your interest and replies.

Keith

If you are not trained to use a damp meter, and understand how to interpret its readings in context, then you may as well chuck it out of the window. It measures conductance not dampness and is calibrated for timber not plaster. And I presume this is not a professional one either?

Whether you treat the cause now, or rip the decorations off in a years time, is something to consider.

Your builder mates' suggestion is a solution for a particular problem. Whether you have a problem for which this is the solution is what you need to determine

Hi Woody. Are you familiar with damp meters? If so perhaps you could help me interpret the results I got. The meter I used was hired from a local tool hire firm and appeared similar to one used by a damp control firm that examined the house. To that extent it was "professional".

When you say the builder's suggestion is a solution for a particular problem, did you have a particular problem in mind? If you can help in that regard it would be useful.

thanks for your input so far.

Keith

If you poke a damp meter anywhere it will most likely give a reading. But is that normal or not? The meter may have a "normal" range from 11 to 21% but that is for timber and nothing else.

Walls are cooler at the edges so air will most likely condense on the cool surface here, and when you put the meter here it shoots up.

Damp will also come up and go down a wall and so again, put a meter near the bottom of a wall and it may shoot up the scale.

So, what damp problem does this show? Or is it a damp problem at all? Is this wall suffering dampness, or is it just damper than other parts? How damp is damp?

A meter is just a tool and the readings have to be interpreted along with other visual signs and other pertinent local conditions and construction type. A reading from a meter poked into a wall is totally meaningless on its own. In fact it is only measuring surface conductance and not how damp the wall is.
I've poked a meter into some painted timbers and it has shot up to 90% dampness, when in fact the timber was bone dry - the cause was the particular paint causing a high conductance. So don't rely on a meter on its own

So you need to know what is occurring at the wall base. Is it condensation, rising or penetrating damp, moisture from your gutter leak, seasonal moisture, hygroscopic salts etc. And to work this out, you have to either see something blindingly obvious, or narrow the possibilities down to a most likely cause by discounting the unlikely casues. You may have one or several separate causes to make one probelm.

Then when you have identified the source(s) of moisture, you work on what is causing it to get into or remain in the structure. You devise appropriate repair and preventative work, or do something to alter the local climate - such as lifestyle change or environmental change ie ventilation and heating.

Render is a good barrier to moisture. It is often done at up to 900mm off the floor to contain a damp wall. But if the wall is suffering rising dampness, then it can still rise, or get out between the render/floor junction, blow the render off or just decay the wall mortar and cause accelerated external weathering- which in turn adds to the problem.

Render can also cause a cold surface (thermal bridge) which then leads to condensation problems and internal mould growth. So it can stop one problem and start another. It is not an "end-all" solution

Your 4th prargraph raises some pertinent questions which have already occurred to me. Yes, 1 area of a wall might be damper than another but does that mean "damp" will show itself on the plaster, causing a real problem as opposed to a theoretical one. At the moment I am minded to invest in a DIY chemical DPC in one room and render/replaster and see what that achieves before moving on to the next room where there appears to be a real problem.

Do you have any view/experience of chemical DPCs? Are they all the same? How long is it before you can assess whether it's done any good? Before I inject a chemical DPC I need to be confident that it will cure rising damp, otherwise there's no point as I can't be sure 1 potential cause has been eliminated.

Keith

The damp is only worth dealing with if:

a) it is bothering you, or
b) it is causing timber decay (which could lead to structural damage)

As far as chemical DPCs are concerned there is a wide variation in performance depending on product strength. Basically go for the highest strength you can get (which is about 60% for creams and 5% for injection fluids) - it is a false economy to go for a low-strength budget material and then have to do the job again because of product failure.