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My wife would like to use our old barn that came with our house as an art studio, but the building is very damp. Construction is of thick cobbled/rubble walls with flagstone floor laid directly on earth. The flagstones clearly change colour during different weather conditions as the damp comes through from below...I'm not sure whether the walls are also damp. How would you recommend treating the walls and floor to seal against damp. This would not need to be up to normal living space standard, but used only as a studio/workshop. Hence flagstones and cobbles are fine, as long as the damp is kept out.

This building would have been designed to be damp and has been damp ever since they started to build it.
Damp in itself does no harm and when you look around there are many things that are damp.
The floor can be resolved, merely by taking it up and replacing it with a concrete slab, covered with a damp proof membrane, insulation and some sort of finish, wood is best.

I understand that for it's original purpose the barn could be damp, but it is causing damage in the form of rust to tools and other steel based items.
I was hoping someone could recommend some sort of surface treatment, liquid that could be applied and be waterproof when cured. Maybe something along the lines found on garage workshop floors?? Or would this be a bad idea?...I'm looking for advice either way.

Just google "French drain installation".

Billy is sugesting a french drain (named after a Ohio farmer) this may work in lowering the water table and therefore the hight that water will rise in the walls but, it won't get rid of all the damp.
Have you worked out if its rising damp or rain on the walls or leaking gutters or roof?
The roof and gutters are straight forward.
The exterior walls can be coated with a silicone water repelent.
The rising damp will require the whole wall system to be tanked, there are a number of systems available for this.
There is no simple cost effective way of installing a damp proof course in the walls.

Installing a new floor incorporating a damp proof membrane would stop the damp rising up through the floor. However, bear in mind that in this case the moisture may then be forced towards the edges of the new floor and cause problems at the floor wall junction which may increase problems with rising damp in the walls.

Chemical injection damp proof systems are not usually effective in rubble filled walls so an in internal tanking system may be the best option.

I have recently used the Sovereign Chemicals HEY’DI K11 “paint on” tanking system on some stone/rubble built barn conversions. The system was applied internally to a height of 1.5m and at the bottom of the wall it overlaps a new floor slab by a couple of inches. I’m not sure if you could just use this on your existing floor but it would be worth contacting them and ask the question.

I’m not sure I would use an external silicone treatment on this type of construction. Whilst it would reduce problems of rain penetration it would trap all other moisture (such as from rising damp) within the walls which could only then evaporate from the internal surfaces.

Make sure you also have ample ventilation to aid evaporation.

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