What is porous?
A material is porous if it contains voids — pores. Virtually every building material is, to some degree: brick, calcium silicate, concrete, mortar, plaster, timber and natural stone all contain a network of voids between the solid particles.
The measure of this is porosity: the proportion of void in the total volume, usually as a percentage. But that figure alone does not say enough. What matters more is what kind of pores they are.
Open, closed and capillary
- Open pores connect with each other and with the surface. They can pass water and vapour.
- Closed pores are sealed bubbles. They lower the weight and improve insulation, but take up no water. That is exactly the difference between XPS (closed cell) and an open-cell foam.
- Capillary pores are so fine that they actively draw water in against gravity. That is the mechanism behind rising damp in old masonry.
| Open pores | Connect with each other and the surface; pass water and vapour |
|---|---|
| Closed pores | Sealed bubbles; lower weight and improve insulation, take up no water |
| Capillary pores | So fine they draw water in against gravity; behind rising damp in old masonry |
A material can therefore be highly porous and only slightly permeable — aerated concrete is the textbook example.
What this means in practice
- Insulation. Air in small, closed voids conducts heat poorly. The insulating action of aerated concrete, lightweight block, cork and foam board rests on this.
- Water absorption and frost resistance. Water in pores expands on freezing. Whether that causes damage depends on the distribution of pore sizes and on how much space is left free. Facing masonry therefore has frost resistance categories; for external work always choose a unit tested for it.
- Adhesion. A porous background pulls the mixing water out of adhesive, mortar and plaster. If it sucks too hard, the mortar burns and fails to bond. Hence wetting highly absorbent bricks and priming absorbent walls.
- Vapour diffusion. An open, porous build-up lets vapour through and can dry out. Seal such a wall vapour-tight on both faces and you trap moisture inside it.
- Sound. Open, porous material absorbs sound; closed, heavy material blocks it. Those are two different things that are often confused.
- Staining. Porous natural stone and concrete take up stains easily. On worktops, sills and patios, impregnation is worthwhile.
Rule of thumb
Ask of every material: should this be able to take up moisture and give it back, or should it keep moisture out? Do not mix the two in one build-up without thinking about where the moisture can go.
Related terms
- Porosity
- Capillary action
- Frost resistance
- Water absorption
- Vapour diffusion
- Aerated concrete
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