ᐅ Rising damp on the basement wall after prolonged rainfall

Hello,

our inherited house was built in the mid-1980s and probably does not have any damp proofing in the basement. After the recent heavy rains a few weeks ago, we noticed quite a bit of moisture and efflorescence on the plaster.

The basement wall is made of concrete hollow blocks, above which, according to information, there is a cement plaster layer, followed by a lime plaster as the final coat.

A friend lent us a moisture meter, and we found very high moisture levels (80-100) in the first and second plaster layers. The concrete hollow blocks show less moisture (the device indicates about 30-50).

There are also "wall efflorescence" marks on the top lime plaster layer—likely some kind of salt deposits.

Is this rising damp concerning regarding the structural stability or building fabric, or is it mainly an issue of the plaster deteriorating and needing to be replaced?

Thanks in advance,

Rising moisture and efflorescence – about 20-40 cm (8-16 inches) high – here is the worst affected spot after several days of continuous rain:

zachro08 schrieb:

Is this rising damp something to be concerned about regarding structural stability/building fabric, or is it “just” about the plaster falling off and needing to be replaced?

Structural stability shouldn’t be affected quickly by this.
“Building fabric” is a broad term... and yes, with this type of damage, I would definitely say it harms the building fabric. The moisture will increase (rising up and, in the worst case, eventually reaching your bedroom or living room), and it becomes more difficult and especially more expensive to get rid of.
In addition, efflorescence often leads to mold, even on mineral-based plaster. At that point, you might have to strip everything down to the foundation walls and start all over.

So: monitor the damage. If it only occurs during heavy rainfall and then completely disappears afterward, you can live with it if you want. But if it worsens each time or the wall no longer dries out, you should bring in a professional. Possibly dig out and waterproof the exterior walls. If you do, it would be a good opportunity to insulate (or re-insulate) them at the same time.

KarstenausNRW schrieb:

The structural engineer isn’t too concerned about this.
“Building fabric” is a broad term... And yes, I would definitely say that this kind of damage harms the building fabric. Moisture tends to increase (rises upwards and, in the worst case, eventually reaches your bedroom or living room), and it becomes increasingly difficult and, above all, more expensive to get rid of it.
Additionally, efflorescence often leads to mold growth, even on mineral-based plaster. Then you might end up having to redo everything down to the foundations.

So: monitor the damage. If it only occurs during heavy rain and completely disappears afterward, you can live with it (if you want). But if it gets worse each time, or the wall does not dry out, you will need to call a professional. Possibly you’ll have to excavate and waterproof the exterior walls. If so, that would be the opportunity to install (new) insulation at the same time.

Hi Karsten,

Thank you very much for your reply.
By “building fabric” I meant the hollow concrete blocks — the load-bearing elements, so probably the structural part. I’m not an expert. 🙂

Above the 20-40 cm (8-16 inches) of visible moisture, everything was dry — the device showed 20-30 (percent). The plaster was completely hard, too.

Meanwhile, everything is dry again. We had the windows open and, to be safe, used a fan and heater. There have been a few days with less rain, and no new moisture appeared.

For now, I plan to remove the loose plaster, then apply an anti-mold spray for safety, and let it dry. I will keep measuring the moisture regularly, especially after rain.

What kind of plaster could I use to replace the damaged areas?

zachro08 schrieb:

Had they opened the windows?

Please remember that warm outdoor air contains more absolute moisture than cold basement air. This is the difference between absolute and relative humidity. Ventilating a cold basement with hot summer air can lead to condensation forming on walls and floors.
Just a note in case your basement stays nicely cool.

If you decide to replace it, I would recommend using a mineral-based cement plaster or restoration plaster again.

I would always recommend installing a fan with a differential humidity controller and then first checking whether the moisture actually comes from outside or rather from inside. I once visited an old building where a large bucket in the heating cellar was filling up with dripping water, and it wasn’t a small amount. The source was initially alarming — the main water valve. But it quickly became clear that it was simply condensation forming on the ice-cold water pipe — resulting in a huge, soaking wet cellar with real streams of water.

No one can regulate humidity as effectively as a differential humidity controller does. That’s why I think the common advice to “just ventilate!” is only one-tenth true. You need a proper fresh air inlet, a good fan with an exhaust flap, and, importantly, a well-adjusted differential humidity control system.

Of course, sometimes the moisture really does come from outside, in which case you have to dig around and seal the building. Been there, done that.