Damp in the corner

[Guitar_man]

Hi I have a corner of a NE facing stone cob wall. Heavily covered in mould, it has had foam wallpaper added on it to cover up, but mould/damp coming though from the bottom. There were two large crack in the render which looks like its been patched in the past, I have cut the cracks and filled with silicon temporarily until I decide what to do. The render is not blown. There is also an outside drain near the corner in front of the house.
Does anyone have any ideas as to what is causing it? Rising damp? Or condensation or penetrating damp?
What would a more permanent outside repair be, repair the render with new for 1inch with new render?
If the render is leaking still how long should the mould take to clear once the repair is good? Or should I rip off the affected area and tank it?
Thanks in advance.
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[jeds]

Condensation. Increase ventilation and particularly air movement around the corner. Obviously do any external repairs as well but the mould is condensation.

 

[Flyboytim]

NE is the coldest corner of a building in the UK. No sun, and the raw Arctic and Scandinavian winds.

What is the composition of the render and the textured finish? Cob walls should have a breathable render that is basically lime, and should not have a latex paint coat. If the render cannot breath, it will eventually build up what is to all intents and purposes, a 'water table' within the cob which can only come out on the inside. On a lesser scale of problem the base of the wall is cold due to having water evaporating and then condensing on it when the air is moisture laden. The wallpaper is also preventing the wall from breathing, a simple emulsion, not a vinyl will allow the wall to continue to dry.

If an oscillating, unheated, fan is pointed at the (stripped) wall, it will move air and eventually the wall will warm to the same temperature as the ambient air. At the same temperature there will be no condensation possible. If you heat intermittently, and allow the room to cycle between hot and cold, this will encourage condensation and higher humidities indoors. Try to keep a constant lower temperature throughout the colder months.

If you have mould, you may want to remove it before you start blowing air on it. I use neat thick bleach applied with a paintbrush and then turn the fan on to dry the wall off.

The mould is not coming through the wallcovering, although you might find it underneath as well - it is feeding on the adhesive and minerals from the wall and actually fixing nitrogen and carbon dioxide from the air in association with algae and bacteria.

 

[Guitar_man]

Tim, thanks for a good description of the what is happening. Given that the walls have been rerendered in the past, then this would effectively mean that the whole house could be rendered in cement/sand. Is there a way to tell what the render is?
Given that reredering is not going to be possible(cost/time) are there any other solutions to what appears high levels of condensation throughout? Fans in every room?
Yesterday I had a dry day, so I repaired the render cracks. On further investigation there was a fence post which had not been removed when the re render was done, consequently there were holes behind it in the render. So it's likely there was considerable water ingress and was probably not helping. The stone wall was damp to the touch behind the render. There were was a large 'blown' area above the top of the fence post.
Does heating enable the mould to grow faster? Currently the house is unheated, but once a few problems are fixed it will be heated and lived in.
Is the polystyrene wallpaper likely to be breathable and helping or hindering the breathing of the wall from inside?
The property is on the coast, and from what I can tell, the internal humidity appears very much consistent with the outside temperature/weather patterns.

Cheers.

 

[Flyboytim]

What is the render? If it is quite hard to break then it is cement, I would guess. It has the grey component of cement between the grains and the colour of the sand - e.g. yellow or red, depending on source.

Lime is softer and usually whitish, creamy or pinkish, - usually burnt from local limestone or chalk, and the source of sand was probably from a local beach, or sandpit.

I would have to say that one of the the best sources of advice would be a local well-established builder who has experience of renovating traditional cob buildings with traditional materials. There may be adult education courses you can go on that will enable you to tackle the job yourself over time. http://lmgtfy.com/?q=cob+stitch+repair# may be a good place for you to start. Public libraries, Council building control and Conservation officers may also offer helpful contacts and sources of advice.

It does seem that render is not absolutely necessary - the clay component of the cob is proofing enough, as long as exposed to the elements.

Cob is slightly different geographically and temporally, so where in the UK, and how old is the building? You have cracks which indicate movement.

Once the house is occupied, heated and ventilated, and problem areas attended to, it will improve. However, expectations of modern living and this type of building do not really get on that well. Don't expect your chipboard fitted kitchen to last or your 56" TV to hang on the wall very easily!

When the cob walls were built they were wet, and dried out gradually over years, sometimes getting damper during a season, drying out over the next. No condensation occurred since the walls absorbed the moisture, and there was always a fire burning, airflow, and very little furniture against the walls. Decoration was a limewash inside or out. Air trapped in the straw in the cob acted as insulation. The stone bases of the wall would probably remain damp, since they act as a drain for any water shed through the cob. If not for the plastic wallcovering inside and the impermeable render outside, this would not be a problem.

 

[Guitar_man]

Tim, sorry I don't think I have described the construction correctly, I said cob, but you mentioned straw which it isn't.. Doh. My apologies. It is what they call in Cornwall as mud and stone. So it's stone with some sort of mortar in between. Yes, the render was hard to remove and grey. So I'm guessing the whole house has been re rendered probably in cement. I'm not going to even consider rerendering!
Given that my only solution is to allow the walls to breathe internally, would a PIV system work to remove the moisture that is coming up through the walls but unable to escape though the walls. I'll also look into adding vents through the walls. The construction is aporox 1880.
Thanks.

 

[Guitar_man]

Further investigation reveals there is an unused gas fire flue, at about 2ft above floor level. Would opening this hole up adding adding a fan/vent be a better way to ventilate? What height above floor level is the best? As low as possible as water sinks? I could open up holes in the walls, but it would be a job as they are 24inches thick!

 

[Flyboytim]

The impervious cement render and paint is still the problem, whether the cob is reinforced with stone or not. Wind, sun, and atmospheric temperature and humidity changes cannot act on the surface of the wall, and it will become increasingly damp, the clay in the cob will become plastic, and eventually the weight of the wall will cause movement and collapse.

With a rigid cement mortar holding back an unstable wall, the collapse can be sudden and catastrophic - see the picture of the barn in the link:

http://www.cornishlime.co.uk/articles/DecoratingOldBuildings.php

I doubt if it will dry from the inside especially if the walls are 24" thick at the base, where the water enclosed in the wall will be doing the most damage.

I reiterate that you should seek advice from someone experienced with traditional building maintenance in that locale.

 

[Guitar_man]

Interesting article. So what is the correct render for outside waterproofing? 3 sand, 1 lime?
It's quite surprising, I doubt anyone down here is even aware there cement renders are causing these problems, including local builders!

 

[Flyboytim]

http://www.devonearthbuilding.com/leaflets/leaflet.pdf

Page 3 has an excellent diagram that describes your situation.

Later there are recipes for render and limewash, but there may be other mixtures more appropriate to your property.

 

[munchingB]

my house has clunch walls that havnt been rendered and we have a little mould on all four corners of the house. our walls are about 35cm thick and it seems to us that it is because each corner has no direct surface internally and 2 external faces and takes much longer to warm up..if ever! hope that make sense..?

we think that our internal walls have been lined with modern materials and this might also be contributing to the little mould we have.

 

[Flyboytim]

I think your analysis is correct - modern materials and modern living - higher air temperatures on demand, but without draughts and constant air changes due to open fires in the winter and better insulation causes condensation and mould.

It may be countered by use of unheated fans to move air around (artificial draughts!) the coldest walls and corners, and behind furniture will help to prevent condensation as will maintaining a more constant temperature - not allowing heating to go off altogether during the night and during the daytime when nobody is at home, but instead thermostatting to a level of 13-15 deg C, if acceptable.

Mould also needs to be cleaned off before it takes deep hold - if it has been found in one area before, clean on a regular basis using a disinfectant/antibacterial surface cleaner, and if the mould cannot be removed, paint on neat thick bleach and allow to dry, using a fan to dry the area, and do not rinse off.