Can somebody tell me what this is on my walls?

[Jblakes]

We live in a 300yr cottage, I think we are getting condensation on the wall as its damp. I've knocked it back to the wall (the hole in picture) it's got a gypsum skim on it, then this white stuff, guessing lime plaster. Then a sandy coloured material underneath.

What are these materials and do they breathe as read old houses need to breathe? If not what should I do to sort it?

Kind regards
James

[RBailey]

Welcome to the group.

gypsum skim on it

Looks like it.

white stuff

Probably the top coat, lime plaster is a three (of four) coat system. This may be the top coat which is normally finer, un-haired and with a higher lime content.

sandy coloured

Probably lime base coat. The base coats have coarser aggregate mix, a lower lime content and haired.
(Although it depends on your wall, if you've got cob and you go too deep you will expose the cob which is a random dirt / earth type mix (cob mix varies considerably by location).

You can test if its lime by dropping a small chunk into some vinegar, lime will fizz but Portland cement / gypsum will not.

What are these materials and do they breathe as read old houses need to breathe?

You are correct, old hoses definitely need to breathe. This is the key to understanding and treating your house properly.
Lime is breathable, Portland cement and gypsum will not.

what should I do to sort it?

Probably strip all the gypsum off and repair with lime.
Alternatively you could potentially repair as smaller area but the bigger the better.

Also remember to check for other sources of the damp, blocked / damaged rain water goods, external (cracked) cement render etc..

Cheers,
Richard B.

[Jblakes]

Thank you very much, I'll hack some of tomorrow and hope it's lime. Look like the rest is thr house is same with gypsum skim. So hopefully not as big of a job as I thought

Kind regard
James

[malcolm]

If it's soft and crumbly it'll likely be old and breathable. Hard and grey will be cement, and pink Gypsum. Lime plaster takes well on old and crumbly stuff.

When you have damp it's good to do something about it, but it's not always due to internal wall coverings. Sometimes the ground levels outside are higher than inside and that will cause damp wherever you do. Impervious coatings on outside walls will cause trouble. Leaky drainage is bad.

Most of us haven't got the ideal lime internal walls. But it's rare it is the cause of a damp problem.

[Jblakes]

Thank you for the reply. We had a slipped tile, think that was the problem and part condensation.

[RBailey]

Most of us haven't got the ideal lime internal walls.

Very true, us included
When you hear some people talk you would think that a square centimeter of anything other than traditional hot mixed lime and your house will collapse.

I take a middle ground which I feel is a sensible and practical approach. I am not putting anything none lime one my walls but I am also not panicking to hack off every centimeter of plaster.
Downstairs (room by room as work progresses) I am hacking of gypsum and replacing it with lime, especially the lower areas of the walls which got attacked by the chemical damp man. Upstairs I am only fixing broken bits.

We had a slipped tile, think that was the problem and part condensation.

Well done, very common suspect.

Cheers,
Richard B.

[MatthewC]

I would think that condensation on the walls is the classic result of gypsum plaster. I see roof tiles and high ground outside already mentioned as items to watch out for so I think for completeness one should also mention old lead flashing on the roof, blocked/broken rainwater goods, drains that don't work properly, non-breathable flooring and my least favourite - rampant ivy which can get everywhere.

I reckon I had the full set on my house and we worked down from the top - it's now over 11 years (really?) since I started and only a few months ago I reckon we did the last one by laying a limecrete floor in the hall. See my blog at http://houseintheenchantedforest.blogspot.com/