How to remediate ising damp on house exterior stone wall

[mechantbruce]

Hello I am a Kiwi living in France. I have bought a old (2 story) stone farmhouse in the Pyrenees that has been rebuilt from a complete ruin about 6 years ago.

Now that weather is turning colder and wetter, we have noticed damp rising on some of the (south facing) walls. Here is a photo from the dining room;

The walls appear to be plastered rock/rubble as per this photo taken after the trim was removed;

From digging on the outside, it seems that the walls even lack a foundation, and are sitting directly on the ground (which is rocky). It could just be the the footings are not flush with the plaster on the exterior wall, but I am reluctant to undercut to see if this is the case.

I have been googling away, but have not found too much except advetorials for companies wishing to push chemical barriers. I am more convinced however by the approach of exposing the foundation to air to allow it to dry out, as per this article;
http://www.buildingconservation.com/articles/risingdamp/risingdamp.htm

Inspired by that article, I have begun (not finished yet, plan on making it deeper) digging a small trench alongside the house, as per this snapshot;

So, my first questions, are;
-- Are there any good resources on the Interned that deal with this problem?
-- Is my approach sound?
-- Should I line the trench with landscaping fabric and gravel?
-- How necessary is it to drain this trench away? I don't *think* it will fill up with water, as it is relatively sheltered by the house, and I believe the major objective of the trench is not drainage so much as ventilation for the sub ground footings.

I have an existing storm water drain (visible in bottom of last photo, I don't know where it goes), and am wondering if there is any good way to tap into that with my trench (since the pipe will be well underground by the time my 10cm deep sloping trench reaches it, I cannot figure out any way of connecting into it without the creating erosion around the solid drain and the open(ish) trench.

Failing that, I might try extending the trench with a "french drain" across our gravel parking area.

-- Any suggestions for how to handle all this?

Thanks for the help
Bruce.

 

[RunervyRenovation]

Hello Bruce

My husband and I have had similar problems with our house in Brittany.
Firstly, you should be aware that old houses like these did not have foundations as we know them. They were built directly onto the soil/ground. Also, they were built to 'breathe', i.e. the stone walls should not be covered with non-breathable modern renders, and lime should be used for jointing/coating. This allows moisture to be wicked to the outside.

Have you checked that your wall coatings (as shown in the photo) are lime based? Has concrete been used for repairs/render etc?

We found that the floors in our house had been 'updated' in the 70s by the farmer/owner, who, to avoid higher taxes, had merely poured a 3/4" layer of concrete inside the house over an earth floor. This caused massive damp problems in the lower walls. We dug out the floors and put in hard core, sand, a damp proof membrane, insulation then a concrete floor on top. This stopped damp coming up from the ground and wicking up the walls. The farmer had also coated the interior walls with some kind of unbreathable artex which we also removed, along with any cement/concrete coatings.

On the exterior, we looked into french drains. Originally, these would have run under the floor in the interior of the house, draining downwards under the house walls to the outside, and would have been porous or perforated to allow ground moisture to be run off. With the new flooring in place, we were advised to run new french drains around the perimiter of the house. These should be about a metre away from the house walls (so as not to weaken the walls), be made of porous or perforated materials and be set about a metre deep in surrounding gravel. They should ideally be sloped downwards away from the house.
See http://www.ihbc.org.uk/guidance_notes/docs/tech_papers/French Drains.pdf.
We were lucky in that our house is built on sloping ground and so draining ground water away was fairly easy and did not involve major trench digging.
Whatever you do, do not dig your french drain ditches close against the house walls. This could undermine the strength and integrity of the walls.

I hope this helps. We took several years to figure this all out. let me know how you get on.

Jo