Cement-based internal plaster in a cob-walled house

[Hobden]

We're looking a buying a Grade-II cob-walled house, in Devon. Sadly it's been rendered externally with a cement-based render and plastered internally with cement-based "wall finishes" (according to our surveyor, I wouldn't know how to tell lime from cement from a fried egg).

We intend to replace the external render with a lime finish, but I'm wondering whether it's worth doing anything about the internal finishes? Would an impermeable finish help to keep internal moisture (from bathrooms, kitchens and sweaty bedrooms) out of the walls and thus be beneficial to some degree? Or must it too go?

H

 

[JoceAndChris]

Hi again Hobden

As you know I don't own cob and haven't lived in one. Found this for you though, looks interesting

http://www.devonearthbuilding.com/leaflets/the_cob_buildings_of_devon_2.pdf

I don't think cement on the inside or outside of cob is great at at all but the building is still standing and I'm sure is highly unlikely to fall down. I wouldn't at this stage be making plans for the wholesale removal of internal plaster, we don't know how easily it's going to come off, how damp the building is, which are the problem walls. A building needs to be lived inat least a year before drastic decisions are made. In general if there aren't problems I'd leave internal plaster well alone. Only in the case of serious damp would I be hacking off.

Lime plaster looks very different to gypsum. Here's some 200yrold lime plaster I've been working on today, freeing it if mastic. The yellow patch in the middle is old paint. See how grey-white it is. When you drill into it, it's soft and crumbly, with brown horsehair. Gypsum is pink and brittle and delaminates in horrid ridges.

 

[Hobden]

As you know I don't own cob and haven't lived in one. Found this for you though, looks interesting

http://www.devonearthbuilding.com/leaflets/the_cob_buildings_of_devon_2.pdf

Thanks J&C - will digest that when I get time, probably tomorrow, as the sofa and my other half and the crossword are calling out to me ...

 

[tobydog]

Is the cob in direct contact with the ground inside the cement sandwich or set on a brick or stone plinth? If it's the ground then you might benefit from a structural engineer checking that the integrity of the cob hasn't been compromised with moisture. Just a thought.

 

[Hobden]

Is the cob in direct contact with the ground inside the cement sandwich or set on a brick or stone plinth?

Short answer, "don't know"! The surveyor told us that it was usual to set cob on a low stone wall, but he couldn't be certain because of the rendering.