This Victorian semi-detached London house has dampness up the flank wall. The wall had been render. There were cracks.
- Penetrating damp normally has a brown stain.
- You would not expect to see penetrating damp on the ground-floor chimney breast, which is why hygroscopic salts are likely.
- Visible surface salts are normally calcium sulphate a key ingredient in cement and mortar.
- Visible surface salts are a signs of water in flow and evaporating.
- By contrast hygroscopic salts are generally colourless.
- It can be hard to determine the root cause when two causes are mixed together.
Penetrating damp from render cracks mixed with hygroscopic salts from the chimney breast.
I understand that a large section of the wall was replaced by damp proofers, presumably with damp-proofing slurry which is impermeable reducing the absorption of vapour.
Staining results from water passing through bricks and over timber. These stains are a sign of a leak or penetrating damp, that is rainwater coming from the outside.
Light brown staining it’s evident through the paint
There are calcium sulphate salts on the surface. Calcium sulphate is a key ingredient in cement and other building materials. If diluted in water salts tend to move to the surface. These can be removed with sandpaper and decorated.
Well I think the primary cause of dampness is rain penetrating through external render cracks. Given that it is a Victorian chimney breast which is been damp proofed, I can’t help thinking that hygroscopic salts are a factor. See you later in the report.
Water reflects radio waves at a set frequency similar to mobile phone shields. Radio frequency meters can’t differentiate moisture from other dense matter such as metal and concrete. They help trace damp in a normal, homogeneous wall. Readings below 300 REL indicate that a wall is dry below the surface, 999 REL is the limit. These meters are for scanning, mapping and profiling, see surveyor.tips/profile.
Water is absorbed through cracks. Small cracks act like capillaries drawing water in. Water will flow down the render until it meets a brick, from there it will pass it to the building.
Generally speaking, horizontal cracks draw in more water than vertical cracks. Likewise a crack low down a wall will cause more ingress than one higher up.
There is a risk of rot. I tested the subfloor and found no signs of rot but I was unable to test the first floor ceiling void. The risk is low. Consider drilling through the floor testing for high humidity, the sign of rot, with a hygrometer probe.
Rendered walls, especially on the sunny side, are always at risk of cracking. From a damp perspective, it would be better to just have exposed bricks as the absorption and evaporation is more evenly distributed, like an overcoat. Conversely, render works like a raincoat. As a minimum rake out and fill with an external acrylic caulk.
Consider both filling and covering with external insulation.
The issue with chimney breast is there is a flue running down inside with a single brick between what is effectively the cold outside air and the inside. So when it is very cold outside a chimney breast can lose a large amount of heat, increasing condensation risk.
The kitchen extractor fan it’s not externally ducted.
Ventilation is most effective when air is extracted close to the vapour source; bathroom, kitchen, drying clothes and occupied rooms. The internal ventilation does not meet Building Regulation 2010 Part F requirements. This is best achieved with mechanical extractor fans.
See surveyor.tips/vent_regs specifically P39 and P19:
- Bathroom 15 l/s with a 30-minute overrun.
- Kitchen 30 l/s adjacent to hob; or 60 l/s elsewhere in kitchen.