Why does an old solid wall need to breathe?

Almost every argument about damp in an old house is really an argument about which of two completely different building philosophies applies. Modern construction keeps water out with a physical barrier — a cavity, a membrane, a sealed skin. Traditional construction lets water in and lets it out again. Both work. Applying the rules of one to a building of the other is the origin of an enormous amount of unnecessary damp and unnecessary expenditure.

Step by step

1. Establish the age and the construction.Broadly, UK houses built before about 1919 are solid walled and lime built; between the wars is a transition; after about 1930 most are cavity walled. Look at the brick bond: a wall showing header bricks in a regular pattern is solid, a wall of nothing but stretchers is almost certainly a cavity.
2. Measure the wall at a window reveal.A single-skin solid wall is roughly 225mm thick with plaster, a solid one-and-a-half brick wall is thicker still, and a cavity wall is typically 250 to 300mm with a hollow middle. The reveal is where you can measure it.
3. Look at the mortar.Lime mortar is soft enough to scratch with a coin, is usually a warm buff or grey, and often has visible lime lumps in it. Cement mortar is hard, uniform and grey, and if it is on an old wall it is almost always a later repointing.
4. Understand what breathability actually means.Not draughts, and not air movement through the wall. It means vapour permeability: the ability of the wall's materials to let water molecules pass through and evaporate away, so the wall does not accumulate moisture over successive winters.
5. Identify what has been done to the wall already.Cement render, cement pointing, gypsum plaster, plastic emulsion, masonry paint, foam-filled cavity in a wall that never had one, internal tanking. Each is a layer that slows evaporation, and their effects stack.
6. Work out where the moisture is being pushed.A wall sealed at the bottom pushes moisture higher. A wall sealed outside pushes it inwards. A wall sealed on both faces holds it in the masonry, where it freezes and spalls the brick faces in winter.
7. Reverse the sealing rather than adding to it.The usual sequence in an old house is: repoint in lime, remove cement render where it is cracked and holding water, strip plastic paint, replace gypsum with lime plaster on the affected walls, and use a mineral paint. It is slow and it is done in that order.
8. Keep the water off the wall in the first place.Breathability manages the rain a wall absorbs. It does not excuse an overflowing gutter, a broken downpipe or a ground level over the barrier — those put far more water into a wall than weather ever does.

Tips

• Internal wall insulation on a solid wall is the highest-risk fabric intervention in an old house, because it makes the masonry colder and moves the dew point into the construction. It can be done well, and it needs designing rather than buying off a shelf.
• Insulating a solid wall externally with a breathable system is usually the safer option where planning and appearance allow it, because it keeps the masonry warm and on the dry side of the insulation.
• For a listed building or one in a conservation area, materials on the external face are often controlled. Check before removing render or repointing.

Common mistakes

• Repointing an old wall in cement — Cement is harder and less permeable than the brick around it, so the wall evaporates through the brick face instead of the joint. Salts and frost then blow the brick faces off, and the wall loses material that cannot be replaced.
• Applying masonry paint or a water repellent to an old solid wall — It reduces the rate at which the wall can give up the water it has already absorbed. Any water that gets in behind — around a window, through a crack, from a leaking gutter — is then trapped.

Questions people ask