Module 2 · Buildings and How They Handle Moisture
2.2 Solid Wall and the Breathable Model
10 min read
Roughly a fifth of British homes have solid walls, and they are disproportionately represented in damp complaints — not because the construction is deficient but because it is routinely treated as a deficient version of something else. A solid wall that is getting wet and drying is working. Understanding what keeps that cycle running, and the specific things that stop it, is the difference between conserving a wall and slowly destroying it.
Learning objectives
By the end of this lesson you will be able to:
- Explain the wetting and drying cycle of a solid wall in terms of rate rather than presence.
- Identify the four things that defeat drying, and their characteristic signatures.
- Explain why mortar is specified softer than the masonry, and what happens when it is not.
- Assess a solid wall's exposure using the driving rain index approach.
Key terms
- Sacrificial mortar
- Mortar deliberately weaker and more permeable than the masonry, so evaporation, salt crystallisation and movement occur in the repointable joint rather than in the brick or stone face.
- Driving rain index
- A measure of wind-driven rain exposure, categorised in BS 8104. Governs how much water a wall must absorb and therefore how much drying capacity it needs.
- Overhang and throating
- Eaves projection, sills, string courses and drips that shed water clear of the wall face. The original rain management, and frequently lost in later alterations.
- Frost spalling
- Face loss caused by water freezing within the outer few millimetres of saturated masonry. A symptom of a wall that is not drying, not merely of cold weather.
Read the rest of Buildings and How They Handle Moisture
Module 1 is open to everyone. The remaining six modules — building science, each damp mechanism in depth, meters and thermal imaging, the full inspection method, and report writing — are free too. We just ask for an email so you can pick up where you left off, and so we can send your certificate when you finish.