An old wall breathes.
It has been managing water for a hundred and fifty years, not by blocking it, but by taking it in and then giving it back.
A renovation that ignores how this works is not merely ineffective: it damages the building. It is the most misunderstood subject in energy renovation, and the one that produces the most expensive failures.
The rule that governs everything else
Within a wall, each layer must be more permeable to vapour than the one before it, from the inside outwards.
The vapour produced inside the home travels through the wall on its way out. If along the way it meets a layer more closed than those it has just crossed, it builds up there, cools, and condenses. The wall no longer dries. It decays from within, with nothing showing for years.
This is why a cement render on an old stone wall is a fault: it closes the outer face of a wall that was built to dry through it.
What happens within the thickness of the wall
Compare the two cases. It is the same wall, the same insulation, the same household producing the same vapour. Only the permeability of the outer layer changes.
The vocabulary, in plain terms
- Vapour openness (perspirance)
- The ability of a wall to let water vapour travel through it. A vapour-open wall is not a wall that takes in water: it lets vapour through, not rain. Confusing the two is behind a great many bad decisions.
- Capillarity
- A capillary-active material contains channels along which liquid water travels. It can therefore dry quickly after accidental wetting. Natural hydraulic lime is capillary-active, cement is not.
- Hygroscopic
- A hygroscopic material stores water vapour in its pores, then releases it again. That is what allows an old wall to absorb peaks of humidity instead of returning them to the room.
- Sd
- The equivalent air layer thickness: an Sd of 18 m means that the membrane slows vapour down as much as 18 metres of still air. The higher the Sd, the more the layer blocks.
- Vapour control layer (freine-vapeur)
- A membrane that regulates the passage of vapour without blocking it completely, unlike a vapour barrier. Humidity-variable types close in winter and open in summer, which lets the wall dry.
- Dew point
- The temperature at which the vapour held in the air turns into liquid water. Inside a wall, the place where it is reached is the place where condensation occurs. The whole question is where that place lies.
Insulating inside or outside, seen through moisture
The argument usually turns on cost and lost floor area. From the standpoint of water, the difference is sharper: it decides where the condensation will occur.
Internal wall insulation
High if badly designedThe old wall ends up on the cold side of the insulation. It receives less heat, so it dries more slowly, and the dew point moves inwards, into the masonry.
It calls for rigorous management of vapour on the inside face, a continuous vapour control layer, and treatment of the junctions with the floors, which are the most delicate points.
External wall insulation
Low if the materials are openThe old wall stays on the warm side. It keeps its temperature, and therefore its ability to dry, and the dew point moves into the insulation, out of the masonry.
It calls for external materials that are open to vapour and capillary-active. External insulation closed to vapour on an older building reproduces the fault of cement render, and worse.
The six mistakes that keep coming back
They do not come from our own experience alone: they are recorded in the two reference works cited at the foot of the page.
A vapour barrier fitted, but not continuous
A membrane pierced at electrical sockets, badly joined to window and door frames, or interrupted at the floors protects nothing. On the contrary, it concentrates all the vapour on the few remaining routes.
The ruleA vapour barrier that is not continuous merely moves the problem and concentrates it. A properly fitted vapour control layer is better than a vapour barrier full of holes.
Cement render on an old wall
Cement is vapour-tight and not capillary-active. Applied to old stone or brick, it blocks drying towards the outside. Water stays trapped in the wall and freezes in winter.
The ruleOn older buildings, lime renders are used: permeable and capillary-active. It is a design rule, not an aesthetic preference.
Insulating without dealing with rising damp
A wall that draws water up from the ground will go on drawing it up once insulated. The insulation, for its part, becomes waterlogged and loses its performance. The problem turns invisible and gets worse.
The ruleRising damp is dealt with BEFORE renovating, never after. It is the order that matters.
Sealing without ventilating
A family produces several litres of water vapour a day. In a building made airtight whose ventilation has not been overhauled, that vapour no longer leaves. It condenses on the cold spots: corners, window reveals, behind furniture.
The ruleMechanical ventilation is not a comfort option, it is what carries the moisture away. It is dealt with at the same time as the airtightness, never afterwards.
Leaving unintended air paths
Warm, humid air gets into the wall through a gap in the airtightness, meets the cold inside the insulation and condenses there. This mechanism deposits far more water than the simple diffusion of vapour through the materials.
The ruleAirtightness protects the wall from moisture as much as it saves energy. That is its least known and most important role.
Neglecting protection from driving rain
An exposed façade absorbs rain. An old porous wall left unprotected can absorb considerable amounts of it, which cancel out the whole benefit of insulation.
The ruleExposed façades are protected, and the rainwater drainage and the ground around the building are checked before insulation is discussed.