Rising damp - really?

[plasticpigeon]

Well looking at the problem from a different perspective, what pore size is necessary to wick water 1m or more up a wall? If the pore size comes out at something ridiculous for a house brick, than it probably isn't happening. If I can find the time, I shall see if I can find a calculation method and come up with a number even though I am content that my once slightly damp house is now no longer damp enough to bother me in the slightest.

 

[Penners]

For tonight's homework:-

1. Weigh a dry housebrick.
2. Stand the brick on one end, immersing the bottom inch or so in water overnight.
3. The next day, remove the brick from the water and dry its immersed surface with a towel.
4. Immediately weigh the brick again.
5. Note the colour of the brick - has it darkened along the bottom (immersed) inch only, or does the colour change extend further than this?
5. Write up the results of your experiment, using one side of the paper only.

 

[Keith Bowman]

Time to look at the problem in a slightly different way:

This analysis could indeed shed light on the pre-flood condition. The practicality perhaps founders on the quantum of the disputed claim relative to the cost of Lindy having the analysis carried out. Alas, the insurers are unlikely to volunteer their services.

However, if Lindy were to say that she will if necessary have this investigation done but the cost will be added to the claim, that would be a negotiating point in her favour. The mere fact that she knows how to analyse the origin of the damage, will be impressive.

Sadly the insurance industry has rarely been good at paying claims. Deny, dispute, delay, dissimulate, diminish... the name of the game.

Ivor

I declare an interest but feel honour bound to point out that the NFU Mutual has an excellent reputation for paying out promptly and fairly. It's premiums also tend to be at the upper end of the range - but what's the good of cheap insurance which isn't there when you need it?

 

[kreyszig]

Well looking at the problem from a different perspective, what pore size is necessary to wick water 1m or more up a wall? If the pore size comes out at something ridiculous for a house brick, than it probably isn't happening. If I can find the time, I shall see if I can find a calculation method and come up with a number even though I am content that my once slightly damp house is now no longer damp enough to bother me in the slightest.

One expression I've seen banded about for capillary action (idealised to a cylinder) is h = 2T/r*g*rho where in SI units

h is the wick height (capillary potential)
T is the surface tension of water, around 72.75e-3
r is the pore radius
g is gravitational acceleration- 9.81
rho the density of water 1000

rearranging for a 1m height you get a pore radius of 1.478e-5 m. I.e. ~15 microns, or diameter of 30 microns.
This link seems to be a reasonable source of information on pore sizes in clay brick (first one I googled, but it looks reasonable). 15 microns is on the high side, and as you can see in the above formula a lower pore size will generate a higher potential.
If you take some of the more common pore sizes, say 0.4 microns, the potential works out at 37m!
Of course a brick isn't a single cylinder with a radius of 0.4 microns but I'd say given the porosity of some of the bricks in the article above is around 30%, it doesn't seem unreasonable at all that water could wick up over a meter high.

I've seen a few rate dependent equations as well so remember that this could take some time to achieve.

 

[Penners]

One expression I've seen banded about for capillary action (idealised to a cylinder) is h = 2T/r*g*rho

Yes, that expression is frequently bandied about at garden parties, here in rural Suffolk.

 

[kreyszig]

One expression I've seen banded about for capillary action (idealised to a cylinder) is h = 2T/r*g*rho

Yes, that expression is frequently bandied about at garden parties, here in rural Suffolk.

It's amazing how often it comes up in conversation.. .
- just making the point that I haven't studied this properly and given that the formula showed up in quite a few of the quick skims I did it seemed worth having a go with.

 

[Keithj]

Then there is the evaporation calculation, as the water works its way up the wall and there's less of it.

Can the water wick through the lime (or concrete if you must) between the bricks? We need to be told!

 

[kreyszig]

Yep water can wick through concrete. A study here looks at cement/concrete and notes the importance of evaporation - some of their samples exposed to air actually decreased in weight during the experiment!
They point out that although their equations don't predict an end to the capillary rise, they are probably valid for use over a period of years.