Μoisture transfer mechanisms in buildings(1st part)

Moisture moves in buildings either under its gaseous form (vapor) or under its liquid form (water).

There are 4 main mechanisms of moisture transfer and they are listed here under in order of descending priority.

1) Massive water ingress through holes, honeycombs, cracks, joints, penetrations and generally discontinuities.

Governed mainly by gravity or wind forces.

2) Capillary suction or absorption.

This is a wicking phenomenon. Water can move slowly and steadily through porous building materials e.g. concrete, renderings, masonry etc.

Many times it does so against gravity.

This movement has mainly to do with the strong adhesive and cohesive forces of polarized water molecules and especially with the high surface tension of water.

 3) Moist air leakage / air convection / air movement: name it!

This mechanism might steal the second position from capillary suction.

Air convection needs a pressure differential and a path: a hole, a crack, a discontinuity in general.

 It can be induced by causes as:

 - stack effect: different in and out temperatures

- wind effect: wind action on the buildings

- mechanical ventilation systems.

 4) Vapor diffusion.

Vapor diffusion evolves at a molecular level. Vapor molecules move due to differential pressures, seeking to find passages among pore voids.

Vapor diffusion is a slow process. It’s not of the size of vapor convection. The latter is a much more powerful mechanism.

Though, vapor diffusion is always taken into account because it is persistent.

 ELABORATION 

a) Massive inflow through building envelope’s discontinuities.

This is the most powerful moisture movement mechanism and the one that could cause real catastrophes.
Rain or ground water can potentially penetrate through discontinuities as cracks, joints, penetrations, etc.

Main driving forces are gravity and wind action.

 More massive water penetrations happen – in order of descending priority – through:

- basements

- flat roofs

- pitched roofs

- facades.

Water penetration in basements can reach dramatic dimensions in case of floods and/or hydrostatic pressure build-up.

Waterproofing in such cases must be combined with effective surface and subsurface water drainage systems.

Discontinuities need to be sealed with special construction sealants or flexible cementitious mortars. Except for a good waterproofing, there may be needed some mason work.

The only hopeful fact is that these phenomena are sporadic and generally affect only limited locations of the buildings.

b) Capillary absorption or suction.

 This is a mechanism mostly – but not exclusively - encountered in basements. Liquid moisture is wicked slowly and steadily through the capillary system of porous building materials as concrete, masonry etc.

This is the dominant mechanism in basements when discontinuities are absent.

Capillary absorption can evolve laterally or upwards defying gravity.

It is a molecular phenomenon. Polarity of water molecules make them adhere very well to most of the building materials. Furthermore they cohere very well, meaning that strong cohesive forces develop among them. Cohesive forces are even more powerful at the water surface and we say that water has a high surface tension.

How does the phenomenon evolves?

In phase no1

the big adhesive molecular forces between water and material’s pores pull water molecules up the pore walls.

In phase no2

water surface tension assumes full responsibility to maintain the upward movement.

Surface tension strives to keep intact the notional elastic surface membrane by “obliging” surface molecules to follow the “leader” molecules in their upwards movement.

Imagine that you throw 2-3 links of a chain over the edge of a precipice. The momentum will pull together other links and the whole chain will move downwards.

The narrower the pores, the bigger the height of the water column.

Gravity will finally limit the height.

En passant we call rising damp the form of capillary suction in which we have mainly upwards movement.

 And finally please mark the big irony:

It’s the surface tension that prevents concrete and masonry from leaking like sieves!

Logic dictates that the minuscule water molecules – diameter of 0.28 nanometers – should pass freely through the gigantic – 2500 times bigger – material’s pores.

Surface tension creates clusters of molecules and reduces the wetting capability of water. A touch of hydrostatic pressure will act as a catalyst for the wetting process.

 Chris Strogilis