What does it all mean...

[Jemster]

I've had too much time to poke around the house at the moment and raise questions in my mind.

I posted a thread over on the building forum about some issues with a chimney that has had 2 of 4 flus removed.

viewtopic.php?f=4&t=100234

And we've now had this done. But the wall (internal) still is showing weird damp readings. Now, I know damp meters are not the be-all-and-end-all and that they are measuring salts effectively in the wall. But the paint finish is blowing out so I need to deal with this one...

Here's the wall:

IMG_4730.jpg (154.84 KiB) Viewed 1644 times

And a bit closer detail on the state of the paint (painted 6 months ago, 2 coats of Zinser BIN, 2 coats of dulux matt white emulsion)

IMG_4731.jpg (256.58 KiB) Viewed 1644 times

So this wall is only a single-brick thickness behind a pair of flus - one in use, open top and bottom, nearest to camera, with chimney cap to prevent rain entering, the other one unused but vented top and bottom with terracotta vented pot.

If I take damp readings around the paint it is alternating between approx. 5% and 22% ... I mean... as close together as a couple of inches in any direction from a high reading, I'll get a low reading. I am thinking this may mean there is damp in the mortar but not in the bricks? You can see one of the salt areas is a short vertical line, so no doubt a mortar joint. Either that, or the meter is just always going to read higher over the mortar joins than over the brickwork and this is not a problem to be solved but something that damp meters 'do' inherently.

Part of my reasoning is that if I walk around this house and take damp readings at random places, pressing hard through the wallpaper, I will get many places where readings are of a similar pattern - low readings, then something high approaching 20% then back to low <5%. It's not necessarily low down on the ground floor (so most likely not a rising damp - the house is solid-brick, no cavity, has the original slate damp proof course intact and also has had an injection damp-proofing performed some point in the past).

So my thoughts are that we aren't leaking water, either penetrating or rising. The walls aren't cold enough to be hitting dew-point, ventilation is good (we have a PIV fan fitted upstairs as well just to be sure) and these readings are confusing as hell. I want to get the place decorated, but how to deal with this sort of thing so I don't end up having to re-decorate every 6 months? Is dot and dab foil-backed plasterboard on a wall like this a feasible solution? I used the Zinser BIN in the first place as the wall had a soot smell to it, doubtless because it was originally inside a chimney flu and also backing on to an active flu. This smell has gone totally and no sign of any staining.

All-in-all feeling a little out-at-sea as I don't have a 'damp-guy' I can trust and the builders I've chatted to so far have no particular interest (or knowledge beyond 'yeah mate, it'll be condensation' which it obviously is not). I can take temperature readings (IR) and damp readings if anybody thinks it will help in diagnosing...

[wine~o]

Pronged damp meters are great for measuring damp in wood. Not so good for measuring damp in walls. See Peter Wards channel on you tube

https://www.youtube.com/channel/UC55-Nf ... 3wgwUFtruw

[Jemster]

Pronged damp meters are great for measuring damp in wood. Not so good for measuring damp in walls. See Peter Wards channel on you tube

Hi Wine-o, thanks for the reply!

I've read a lot on Peter Ward, both his online heritage house writings and the Haynes Period Property manual. Also aware he's a little controversial and I'm not sure I agree with absolutely everything - on a practical level there are plenty of houses like mine that are solid-wall red-brick rendered in gypsum that have been that way for years and aren't damp everywhere, but he has 100% put the fear of so-called damp-experts in me :). I do believe each damp problem is an individual problem to be solved, a house isn't just "damp" all over. I've fixed many of the problems here - window seals, faulty window fitting, lack of underfloor airflow, broken slates... we've come a long way in 18 months but this has me stumped.

I know pronged damp meters aren't great... But as a rough-and-ready reckoner they do give a bit of guidance if you read them in conjunction with other factors. This is why I'm wondering where to go from here. I mean, if the damp meter says damp, but the paint stays on the walls, then I don't have a problem and I don't worry about it. However in this case the damp meter says damp and, despite a couple of coats of sealer and paint, the salts are pushing their way through.... and I need to deal with it somehow.

It's not condensation - room relative humidity is around 45-48% at 19 degrees. Wall surface temperature is around 17 degrees (IR reader). Dew point for that is going to be around 8 degrees. the wall is simply never anywhere near that cold. Air flow is good. If it wasn't for the paint lifting, I'd be thinking it was just a duff reading caused by typical chimney contamination. The other side of the wall makes only limited contact with the environment (i.e. ventilation in the flu).

As I see it, there's a couple of options... either I make a real mess hacking off what appears to be perfectly good, sound plaster to get back to the bricks, get a plasterer in, have him apply some kind of sealer to the brick work, a new bonding layer and skim and hope it doesn't happen again. Or... I dot and dab foil backed plasterboard to the wall and get somebody in to skim it (how I wish I could plaster!!).

I like the second option... I don't like it if it's going to trap damp in the wall that leads to problems down the line. I like to fix things once and have them stay fixed :)

That's why I'm wondering what opinions people have - both of this individual issue, but also of this strange behaviour in random places around the house where I'll get a high reading followed by low readings either side - in a 3" space it'll go from 3% up to 20% and down to 3% again. At the moment there's wallpapers everywhere, some sticking better than others, but I'm wondering if it is going to cause real headaches when I get to re-decorating if I don't understand how to deal with it.

[Jemster]

So during these furloughed days I’ve done a bit of wall excavation work and chiselled out a small area around one of the salt ridden areas. Turns out the wall is plastered about 10-15mm deep directly on to the brickwork. It looks like a couple of thick coats of skim rather than a bonding layer and a couple of thin skim coats.

Once I got down to the brick, it is black (sooty) in colour and smells, unsurprisingly, of chimney flu. So I’m guessing, but I’d say it wasn’t properly sealed up, bonded and plastered. Which brings me back to does it need chiselled off (given that the plaster is sound and certainly not boast) or should I simply stick plasterboard over it and re-skim? Seems a lot of work to remove sound plaster over a large section of the wall if not needed.