Asked by a relative for advice on this but not sure if there's really much useful to suggest.

It's a sandstone walled outhouse, once upon a time a stables. Stone floor straight on the ground. The roof seems sound, I don't think leaks are the problem.

It's perennially damp inside especially in the winter. Put a bicycle in there and it starts to go risty. Maybe some of this comes from the ground. But they report often seeing condensation on the inside of the walls. It's not heated or normally occupied.

Some of the sandstone walls have been partially rendered with cement. Obvious advice might be to get this off so that they can at least dry off towards the outside. But I'm not sure whether it's likely that would make enough difference.

Not much point suggesting extra ventilation, I don't think. The air inside is effectively 'outside' air; there's a big leaky stable door. My impression is that the condensation occurs even when there are not any moisture-generating humans in there.

So the question really is whether there's anything to suggest to improve things - or is it simply that an unheated outhouse in this location is always going to be damp.

Hack off all render inside and outside then rake out and repoint the walls with lime mortar and the damp will disappear. Stable blocks where designed to breath and remain dry even with sweaty horses in them.

It's possible that the condensation is caused by salt damp. Salts (from animal urine) are hygroscopic and can be the cause of damp problems in barns & stables.

Posted By: renewablejohnHack off all render inside and outside then rake out and repoint the walls with lime mortar and the damp will disappear. Stable blocks where designed to breath and remain dry even with sweaty horses in them.

The wall's not rendered on the inside as far as I recall.

It seems to me that if damp on the inner part of the wall can move out sufficiently fast by migrating through thick sandstone/lime mortar walls - then surely it ought also to be able to dissipate simply by evaporation from the internal surface, which is essentially connected to the outside air via the large, leaky door.

Of course if the real cause is rainwater getting into the body of the wall then I can see that removing the outside render/repointing could make a difference.

Regarding cross-ventilation; they tell me that they hear the wind whistling through the tiles/sarking boards which led me to assume that there was effectively ventilation through a non-airtight roof. But I wonder if I should suggest introducing a couple of deliberate vents on the opposite side from the door.

Posted By: WillInAberdeenYour relatives mentioned condensation on the inside of the walls. That means water in the air is being transferred to the wall, not the other way around. IE the 'ventilation' air is too damp and is part of the cause of the problem, not the solution.

Sounds like something is making the interior air damp, either a water leak in through the roof walls or floor, or rain/mist/snow blowing in through the doors.

Can you go stand inside it during heavy rain and see what's going on?

Maybe obvious but all 'ventilation' openings need to be rain-screened with a louvre or similar, a big open gap will just let in rain. If the wind can blow in between the tiles it will blow rain in with it.

Yes it's the condensation on the inside of the walls that seems significant to me. My thinking was that it's the same kind of condensation that you might get outside, ie. dew, in certain conditions, but it wasn't able to dry out quickly enough. But you're right that it does seem to suggest that something additionally is making the internal air damp.

It's not something I can look at in person for a couple of months. But might ask them a couple more questions based on the comments on this thread.

Posted By: lineweight

Posted By: renewablejohnHack off all render inside and outside then rake out and repoint the walls with lime mortar and the damp will disappear. Stable blocks where designed to breath and remain dry even with sweaty horses in them.

The wall's not rendered on the inside as far as I recall.

It seems to me that if damp on the inner part of the wall can move out sufficiently fast by migrating through thick sandstone/lime mortar walls - then surely it ought also to be able to dissipate simply by evaporation from the internal surface, which is essentially connected to the outside air via the large, leaky door.

Of course if the real cause is rainwater getting into the body of the wall then I can see that removing the outside render/repointing could make a difference.

Regarding cross-ventilation; they tell me that they hear the wind whistling through the tiles/sarking boards which led me to assume that there was effectively ventilation through a non-airtight roof. But I wonder if I should suggest introducing a couple of deliberate vents on the opposite side from the door.

Your first comment said it was cement rendered.

It's cement rendered on the outside and as far as I recall not rendered on the inside. The external render is the type you often see on gable end walls where it's somewhere between excessively generous repointing and fully rendered; maybe 2/3 of the area is behind render with the faces of some of the stones still exposed here and there.

Because it's built into the corner of the garden, two of the walls are facing into neighbouring properties so I'm not sure exactly what's on them. I will have to take a closer look at everything next time I'm there.

Posted By: Ian1961at the risk of repeating myself its highly likely that the dampness on the interior surface of the walls is caused by high levels of hygroscopic salts from horse urine that soaked into the walls when it was a stable.

This would only (I suspect) be an issue up to about 4ft, anything above that would have some other cause - but then the OP did not say how high on the walls the condensation was.

@lineweight
Unfortunately, I don't know if the salts can be neutralised chemically.
For building refurbishments in cases like yours where the walls have been contaminated the normal procedure would be to line them with a 'Newton' type profiled membrane and then plaster over the membrane
http://www.tracebasementsystems.co.uk/plastering-in-barns/
https://www.newtonwaterproofing.co.uk/11/products/cavity-drain-waterproofing-system-500