Landlord concerned that there was penetrating damp coming through gaps in render.
The root cause was vapour through insufficient ventilation, causing condensation and mould.
- No signs of penetrating damp, would see external cracks and internal tea bag stain like discolouration.
- Hygroscopic salts in top floor chimney breast.
- Fascinating example of old chainsaw retrofitted DPC – the work of Peter McDonald I believe.
There is no tea stain like discolouration, which is the tell-tale sign of penetrating damp, therefore penetrating damp is unlikely. Semi-circular dampness at a wall base is consistent with condensation. Corners lose heat rapidly, as there is greater external surface for heat loss.
A small section of paint has flaked off, I don’t believe the water is penetrating, but it should be repainted, because the paint will retain some dampness resulting additional heat loss. And in theory there could be a render crack behind the flanking paint.
However, when the property is being lived in, there is likely to be a greater relative heat loss, especially behind furniture.
Metal courses rapid heat loss and could account for the water damage to the plaster.
I tested walls with a Protimeter in radio frequency mode. Water reflects radio waves at a set frequency similar to mobile phone shields. 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.
This tells me that the wall is dry enough to replaster.
Although the timber was slightly humid it was less than the 28 WME necessary for rot to start growing.
The data doesn’t tell us a great deal because the property has been empty with limited heating. I will email an Excel spreadsheet which you can add data to visualise changes over time, once the property is tenanted, see section later on monitoring humidity.
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.
I recommend to landlords that they install a continuous flow fan see recommendations, as I have done myself.
Consider adding anti-condensation paint or insulation into this difficult to heat corner.
Flaking masonry paint should be removed and repainted. Any cracked render should be raked out and filled. I tend to fill small cracks with external acrylic caulk, and larger cracks with a render similar to your current render, which I believe is cement-based.
These cracks should be filled, but I don’t believe that they are the causing any internal dampness.
Hygroscopic salts come from the historic burning of coal in a fireplace below. They cause condensation at normal levels of humidity, see later in the report.
The dark paint obscures the presence of hygroscopic salts. These salts are benign, and not a source of moisture. So there’s no need to do anything. If the patches concern you, a solution is to cover with a solvent-based, primer-sealer such as Zinsser Cover Stain.
The benefit is that you can set up multiple temperature settings, without allowing the temperature to drop below the dew point. In addition, as a landlord you can monitor the temperature making sure the temperature never drops to low. Nest (Amazon) allows you to monitor humidity. Other Wi-Fi thermostats include Hive – British Gas and Honeywell. They allow you to site the meter near to the cold wall. However, you need Wi-Fi for it to work.
Kitchen extractor fans on adjacent walls should have her 60 l/s speed, and ideally be humidity activated.
Damp proofing against rising damp
I suspect this retrofitted DPC was installed by cutting out a course of bricks with a chainsaw. Possibly the work of Peter McDonald (now retired, formerly CEO of SafeGuard, PCA trainer – when he worked for or ran South London Damp Proofing – as was, as I recall).