You experts... can you explain, please?

[Penners]

All of us on this forum are entirely familiar with one of the most fundamental principles of period-property-friendly repair and renovation. This being the mantra that lime mortars and plasters allow a building to breathe out its damp, while those impermeable Works Of The Devil, cement and gypsum, prevent this.

Now... as I sit at my computer typing this, I am facing an external wall that is rendered externally with a cement mortar, and plastered internally with gypsum (not by me, guv, honest). And yet it's as damp as a very damp thing. You can put your hand on the interior surface and feel the damp, and you can see a darkening of the paint at the damp points.

If cement and plaster are impermeable, why is this damp apparent? Shouldn't it be trapped behind them, rather than soaking through?

All explanations in very short, simple words, please. With absolutely no mention of nootkatone.

 

[DevenishEstates]

Not counting myself in the league of expert but a possible explanation....

I'm in the process of restoring an old farm, with many internal downstairs walls covered in the Devils work.

The other day whilst working in the house I noticed the air became very humid. I keep the front and back doors wide open to provide some air movement and on this day I noticed two things:

1. The blue and red clay floor tiles in the kitchen darkened considerably and clearly became very damp.
2. Those walls with cement and gypsum renders became so damp (being cold surfaces the water from the humid air was condensing on them) that water literally ran down them in rivulets.

Of course the other walls which are lime rendered merely sat there unperturbed, absorbed the damp and then quickly dried out as the humidity dropped.

Could it have been humidity in the air condensing on the cold exterior walls? or maybe you were just breathing too heavily :?

 

[biffvernon]

Cement is not impermeable. Cement may not be as 'breathable' or vapour permeable as lime mortar but liquid water can still soak through it. When you want to make a watertight tank out of concrete you have to add a waterproofing agent or it will leak like a sieve.

 

[Gervase]

...and gypsum plaster isn't wholly impermeable, either. In fact gypsum loves water. It's too fond of the stuff, and will slurp it up like there's no tomorrow, and then spend weeks sweating it out.

 

[biffvernon]

...after which it falls apart, while lime just dries out.

 

[worms]

Yes, but I wonder if it is just the coldness of concrete causing condensation to form? In my unused attic space, the gable wall is a mix of rubble construction, sandstone blocks, lime mortar and some later cement mortar. The coldest part of the wall to the touch is the cement, so I would presume that if the space was to get humid, I would get condensation forming on the cement before anywhere else?

 

[NT]

So in a word, condensation.

AFAIK gypsum is as breathable as lime, but the paints that have been put on the surface of it typically are not.

Yeskatone or nootkatone?


NT

 

[middi]

Apart from ecological concerns regarding some building materials, everything is good to build a nice cosy home ( apart those with slow release toxins). A proven fact.

A uniform rate of moisture absorbing constructional materials, as in most of our period buildings, will always be in equilibrium with the rate of evaporation, air movement and heat (as in the case of solid wattle and daub panels, soild stone wall, 9 inch victorian bricks or even solid concrete blocks).

Left in their natural state with enough provision of air movement and heat do dissipate and remove moisture as required that balance will always be kept.

The co*k up happens when we destroy this equilibrium by using materials where the permeability rate is probably at the opposite end of the scale from the existing materials, therefore creating what is in effect a one way water valve.

We unbalance the directional flow of evaporation, create stagnant mini reservoirs somewhere in these wall cavities and then winge about damp patches whilst searching for a six feet dia. industrial fan to remove the trapped slow evaporating moisture, in the mean time mushrooms are growing in the woodwork.

Sorry Penners that is the best I could come up with.

 

[biffvernon]

The coldest part of the wall to the touch is the cement, so I would presume that if the space was to get humid, I would get condensation forming on the cement before anywhere else?

That can be deceptive. Cement is a good conductor of heat so feels cold to the touch. Wool is a poor conductor so feels warm. They might actually be at the same temperature.

 

[worms]

Biff, logic (and I guess thermodynamics) would suggest that you are entirely correct!

However...

I recently filled in a section of rubble wall with field stones from the garden. Since these are in the bathroom and have yet to be covered I can see how condensation forms on different stones. One particular stone always feels colder and condensation forms on this stone before it forms on any of the others or any of the internal concrete blockwork. Maybe its a density thing? Or surface absorption characteristics?

 

[Penners]

Some interesting comments there, thanks.

However, I confess to some scepticism about the condensation theory - in the case of the wall by my computer, anyway. I can't see why condensation would form with such clear lines of delineation as those visible between the darker, damp area and the lighter, dry area ("dry" being a relative term of course).

 

[middi]

Penners, can you decifre where this " damp" is coming from.

Could it be travelling up wall from the base?.

 

[Moo]

Middi - you're surely not suggesting it might be Rising Damp, are you?

 

[Penners]

Penners, can you decifre where this " damp" is coming from.

Could it be travelling up wall from the base?.

Yes - I've looked for all the other suspects: cracked render, leaks, gutter problems, etc. and none of them exist. So every indication is that it's rising damp.

Oops! Wash my mouth out with soap.

 

[middi]

I don't really know what else to suggest Moo.

I am sure Penners come springtime will roll his sleeves up, get his very rusty pick and shovel from an hibernating state and have a dig around that section of the base wall.

 

[Gervase]

Could it be travelling up wall from the base?.

Quite possibly. I know we decry the 'rising damp' mantra that the cowboys trot out, but it does happen. Not as often as they'd have us believe, but sometimes...

 

[gsport george]

Condensation will form faster on a better conductor too... the same conductivity that draws the heat out of your hand faster (and makes it FEEL cooler) will have the same effect on moisture vapour in the air...

 

[Penners]

I don't really know what else to suggest Moo.

I am sure Penners come springtime will roll his sleeves up, get his very rusty pick and shovel from an hibernating state and have a dig around that section of the base wall.

Quite right, Middi. Although I'm not sure the job can wait until the Spring. Still, it's a good way to keep warm on a cold day.