How Does an Injected Damp Proof Course Work?

In the majority of cases, an injected DPC will involve drilling the mortar beds in a wall, and inserting a cream, which carries materials which are considered to be a PORE LINER.

These pore lining materials are typically silane/siloxane, derived from silicone.

How do they work? If you read my previous post (link) concerning capillary action, this discussed the mechanism behind the surface tension of water, and the way in which this, and the bond between water and the sides of a container (pores/capillaries in this case), causes water to curve upwards - this being observable with water placed in a clear glass - with that upwards curve.

Concave Meniscus Curve

Concave Meniscus Curve

This upwards curve is called the meniscus curve, AND WHERE WATER CAN BOND TO A MATERIAL, that curve will be CONCAVE.

It is this combination of surface tension and bond, resulting in an upwards (concave meniscus) curve which allows moisture to rise through pores and capillaries.

So what do pore liners achieve? They are water repellents (hydrophobic) and defeat the bond between water and the pores/capillaries.

The effect of this is that the meniscus curve is not concave / curving upwards, it is convex and curving downwards, and so the surface tension cannot pull moisture upwards, hence they prevent rise of moisture via capillary action.

Concave and Conves Meniscus Curves

Concave and Conves Meniscus Curves

NB #1: Not all injected DPC's are pore liners.

NB #2: The fact that pores are not blocked maintains vapour permeability, although it may be reduced.

NB #3: Injected DPC's are not the ubiquitous solution for all damp issues and understanding of issue and structure must drive specification - my point in writing these is to provide understanding (always a good thing) and hopefully better decisions can then be made.

NB #4: A 'DPC' is typically a 'system' comprising an injected material and an associated replastering specification - the above simply covers the injection component intended to defeat capillary action.

So you may have heard that there is no such thing as 'rising damp'. The reasons why could be a lengthy post in itself, commenting on everything from the role of unscrupulous contractors who specify works not necessary, through to those that vocally denigrate and seek to position themselves as saviour against such contractors, seeking huge survey fees on the back of this.

These days, at least in professional circles, the depth of understanding around the mechanisms behind factors which contribute to all issues of moisture in buildings is greater, in part because of social media and more freely available information online.

For my part, I want to understand the technicalities, I just enjoy understanding how things work, and so briefly I will go into why moisture rises via capillary action. If you don't want to believe that this can happen within the pores and capillaries of bricks and mortar, just pretend we're speaking of blotting paper or some other porous material, dipped into water.

Water is a cohesive material, it 'sticks' to itself because of 'hydrogen bonding', which results from the opposite polarity of the oxygen (negative charge) and hydrogen (positive charge), atoms within water molecules (H2O).

Opposites attract!

Simplistically, the oxygen from one water molecule attracts the hydrogen atom from another, and they bond as a result (hydrogen bonding). Just google 'hydrogen bonding water', if you want to read up on this.

What about 'surface tension'? When many water molecules bond to each other, such as IN THE DEPTH OF LIQUID WATER, the extent of bond is distributed between ALL surrounding molecules ON ALL SIDES.

AT THE SURFACE, water molecules are still attracted to their neighbours; however there are less of them (NO WATER MOLECULES ABOVE THE SURFACE), meaning that the bond force is greater because it is distributed between FEWER molecules. (See illustration attached).

Distribution of bond strength between water molecules (hydrogen bonding).

Distribution of bond strength between water molecules (hydrogen bonding).

This greater bond creates a surface film/tension. It is this which creates the rounded shape of water droplets, lets water-boatmen insects walk on water, and allows a paper-clip to 'float' upon that surface film.

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In a pore or capillary (simplistically a thin 'tube'), water may curve upwards where there is attraction between water molecules and those of the material forming the pore/capillary.

Those water molecules bonding to the pore/capillary material then pull along neighbouring water molecules (again via hydrogen bonding) creating a curve where the liquid meets the sides of the pore/capillary. Look into a glass containing clean water and you will see this (the meniscus) curve.

In a capillary network (for example within porous mortar or tissue paper), the bonding force between water molecules, creating surface tension and causing water to curve upward (where water can also bond to the material forming the capillary), is what results in movement of moisture upwards, otherwise known as 'CAPILLARY ACTION'!

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How do we prevent this? One way is to defeat the bond between water and the materials in question, but that's for another day.

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