On carrying out a dampness inspection using a Flir Infrared thermal imaging camera and MR77 damp meter, and found a long-term issue caused by incorrectly installed insulation quilt in the roof void.
The property is in Scotland, and is a modern tenement building circa 2000, with a pitched roof, formed using timber trusses lined with roofing felt.
The building is oriented East West and will be subject to the prevailing Southeast winds and rain. There is continuous eve ventilation on the West and east elevation
The dampness is affecting the property below, but only occurs during wet/cold and windy weather, and the dew point is reached, in this case the cold sections, were 4-5oC above DT.
The external air flowing from the eve vents will be redirected, to the large gap between the insulation quilt, which is now a good pathway (in red) for the cold damp air from the front to the rear, the effect will be severe during stormy conditions. Also, there were sections where the insulation was completely missing. This is the type of failure we expect to see more often due to poor construction supervision and detailing by the designer.
This form of dampness is due to the incorrectly insulation installation. This is a long-known problem in Scandinavia, and we will start to see this more in the UK due to design and workmanship issues.
I relation to spray on insulation there are issues building up I suggest you read the PCA guidance document, which is available for download at https://www.property-care.org/professionals/sprayed-foam-insulation/