The science of breathability

Before we can begin to discuss how ‘breathable’ or ‘non-breathable’ a material or product is, we first need to understand four very important factors.

1: The science of ‘breathability’ in building materials
The correct scientific terms for breathability / moisture movement within building materials are:

Vapour open: A vapour open material is one that has a low resistance to the passage of water vapour (the gaseous phase of water) diffusion. Diffusion is a process by which water vapor passes through the pores of a wall due to a difference in vapor pressure. Vapour open is also often expressed at low or high vapour permeability.

Capillary active: A capillary active material is one that can transport liquid water by capillary action. Capillary action occurs because of intermolecular forces between the liquid and surrounding solid surfaces. If the diameter of the micro-fractures within the material are sufficiently small, then the combination of surface tension (which is caused by cohesion within the liquid) and adhesive forces between the liquid and container wall act to propel the liquid.

Hygroscopic: A hygroscopic material is one that can absorb (physically take within) and adsorb (hold onto externally) water vapour from the air and release it again – which can help moderate internal humidity.

2: Hydrodynamics – the science of water
Water exhibits unique characteristics because of its hydrogen bonds, one of which is cohesion—the inclination of water molecules to bind together. The cohesive forces among water molecules give rise to surface tension. This phenomenon occurs because the molecules at the water’s surface lack similar molecules on all sides, causing them to adhere more robustly to those immediately surrounding them on the surface. For instance, when a small quantity of water is placed on an exceptionally smooth surface, the water molecules will adhere to each other, forming a droplet instead of dispersing across the surface.

Another crucial physical attribute of water is adhesion, which involves the attachment of a water molecule to another substance, such as the veins on the sides of a leaf. This bonding occurs because hydrogen bonds possess the unique ability to break and reform frequently. This constant rearrangement of hydrogen bonds enables a proportion of molecules in a given sample to bond with another substance. The gripping nature of water molecules gives rise to capillary action, the phenomenon where a liquid can flow against gravity in a confined space. A classic example of capillary action is observed when inserting a straw into a glass of water. The water appears to climb up the straw even before you place your mouth on it. This ascent is facilitated by hydrogen bonds formed between the water and the straw’s surface, leading to adhesion. As these hydrogen bonds continually exchange with the straw’s surface, water molecules shift positions, causing some to ascend the straw.

3: Capillary science
There’s a common misconception that masonry capillaries or pores run a singular path from start to finish, this is in fact incorrect. Capillaries within masonry and mortar are in fact a network of independent microfractures and air pockets. Further to this, capillary a action can be mechanised between two surfaces (not four as we imagine a cuboid or cylindrical tube), such as between a sheet of lead and a roof tile for instance, anywhere that surface tension can occur.

Capillaries / pores are formed within mortar in two ways, the first being the entrapment of air during the mixing of the mortar (aggregate shape and size has a role to play in this also), the second is that microfractures occur as the mortar dries. The slower the set (lime) the more it shrinks and microfractures occur. Whereas as cement quickly crystallises in its initial set, hence the greatly reduced pore structure.

4: SD Values and MVTR
What is an SD Value? The Sd value is the resistance to movement of water vapour, when compared to the resistance of a meter of air. Complicated? Yes! The paint manufacturing industry’s standard for a breathable paint is any product with an SD value below 1, which is horrendously low with regards to vapour permeability. As an example, breathable masonry paints average 0.5 SD, which equates to the same resistance of water having to travel 50 centimetres through air, where as lime wash is 0.01 and equates to travelling 1cm through air.

Microporous (branded as breathable) acrylic masonry paint – 0.5 (50cm) SD – 180g/m²/day
Stormdry masonry cream (silicone) – 0.35 (35cm) SD – *
Emperor Masonry Creme (silane) – 0.03 (3cm) SD -* **
Beeck SP Plus sealer (Silane and Siloxane) – 0.03 (3cm) SD – *
Limewash – 0.01 (1cm) SD – 1968g/m²/day

Moisture Vapour Transmission Rate is expressed as g/m2/day – grams of liquid that are able to pass through / a 1m2 surface / in 24 hours.
* Awaiting confirmation on MVTR values.
** This is a fairly new brand and further investigation of their product is required.

There are predominantly two forms of masonry sealers, topical, being those that coat the surface and non-topical (impregnating) sealers that are absorbed into the masonry and coat the walls of the capillaries.

The topical sealants (translucent paints often derived from polyurethane or acrylic) should be avoided at all costs as they have a very low SD value Not only do they greatly reduce capillary action and vapour permeability, the application of these to any form of masonry will eventually lead to a pealing and flaking of the sealant at best, and at worst, hold moisture and salts within the stone, potentially causing spalling of the masonry due to frost shattering reactions.

Impregnating sealants can be further divided into two categories, Silicates and Silicones that work by travelling inside the capillaries to coat the ‘walls’ to either reduce the width of larger pores or completely fill the smaller ones, thus reducing capillary action and vapour diffusion potential. This group includes the very popular Stormdry masonry cream and Thompson’s Water Seal.

The second group of impregnating sealants (through sealant is a less appropriate use of language in this instance) are the Silane and Siloxane based products such as Beeck SP Plus, which works by finely coating the capillary walls with a hydrophobic solution. Hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). Therefore, a rain induced wetting cycle across the masonry face is opposed by the hydrophobicity of the treated masonry, reducing capillary action by up to 95% yet only restricting vapour diffusion by 16%.

It is important to note that moisture will always move in and out of a solid wall in some form, it’s the double edged sword of capillary action. As they stand in their natural form, we can’t have ‘breathable’ walls without them being able to take in some rain water externally. In fact, the wetting cycle of masonry is often far more drastic than you might imagine, as most masonry types are extremely capillary active. To offer an example, a clay brick can reach 90% of its capacity for absorption in just 6 minutes, with the IRA (Initial Rate of Absorption) for a single clay brick averaging an incredible 223g of liquid in that time.

A Link to a study on Flexural bond strength of natural hydraulic lime mortar and clay bricks (to include IRA)

Therefore, we have to ask the question, ‘breathability’ but to what end? At what point do the benefits of the reduction in the exterior wetting cycle brought about by Silane and Siloxane based hydrophobic treatments out weigh the importance of outwards capillary action? If the wall is saturated, then the question that needs to be asked is, why is the moisture contained within a wall in liquid form and not vapour?

Might we agree, that if a wall is free from building defects and has a hydrophobic treatment within the external face, then perhaps the only necessity should be for the support of vapour diffusion?

Please note: that any form of sealant or treatment should be considered a last resort only when all other avenues, such as condition and specification of pointing and render, have been fully explored by a competent heritage specialist.