Our house is built on a slope so the downstairs toilet is underground and the home office has one wall underground. The house was built in 1970 so there are cavity walls above ground level but I think the bits below ground level may be solid concrete. Certainly there are some pretty big lumps of concrete at the base of the walls.
My problems are peeling paint in the toilet an a desire for better insulation in the office.
In the toilet, the paint has flaked and crumbled in a couple of local areas where something has been placed right against the wall, so restricting ventilation. Before re-decorating I am thinking of treating the wall with a 'damp seal' paint. Will this be sufficient? Would I have to strip off all the old paint first? Will this cause a greater build-up of moisture in the wall if it can't get out?
I might do the same in the office, but I was also thinking of putting Kingspan insulation on the wall first in order to improve insulation. Is that a sensible thing to do, or will I create other problems?
Both rooms seem to register humidity levels about 70-75% and there is no actual condensation forming anywhere.
What you have is almost certainly damp penetration on the earth-retaining walls. However, condensation shouldn't be ruled out either as non-cavity, earth retaining walls can also be particularly cold surfaces and so are generally more vulnerable to condensation. Further, with RH levels at around 75%, it wouldn't take much more than a moderate drop in outside temperatures for that humidity to form as condensation. The ideal RH range is widely debated, but commonly is around 40-60%, depending on the time of year.
I doubt that damp stop paint will suffice in this situation. Firstly, said paint isn't particularly effective and, in addition, is intended for much smaller jobs. If you have penetrating damp as you've described and you use the damp stop paint, this might hold it back for a short time, but, as you say, where the water is being held back, pressure builds and commonly results in a water shift (water coming through in new areas. The other (more likely) outcome is that the damp stop paint won't be effective and will break down in a short time.
Another problem that you'll likely have is salt-contaminated wall plaster. Said salts will have been carried from the earth through the masonry by the penetrating damp and will now be lodged in the plasterwork. These salts present a problem as they are hygroscopic (meaning that they attract water vapour and hold it), this can make the walls appear damp, especially with high RH levels as you have.
The solution that I would recommend is instructing a suitable CSSW surveyor to provide a quote to you for a waterproofing system to be installed. This will include the installation of a continuous waterproof membrane which will be let into the floors, joined with the DPC and sealed. Depending on the exact conditions, a sump/pump may be required. Additionally, it might be wise to cover the membrane with insulated plasterboard (and a specialist thermal plaster if you're really keen), which will help keep the internal surfaces warmer and less prone to condensation. Before that can take place, the existing wall plaster would need to be removed and the surface prepared for the membrane.
That will sound like a great deal of work, but it shouldn't take a competent company too long to complete. What's more, this is the only likely solution to achieving a cellar free of damp and condensation.
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