The most significant risk when altering the thermal performance of older buildings is that of creating condensation which can be on the surface of a building component or between layers of the building fabric, which is referred to as ‘interstitial condensation’. Condensation can give rise to mould forming and damage to the building fabric through decay (‘condensation’ describes the physical process by which substances change from a gas or a vapour to a liquid phase, usually as a result of a drop in temperature).
Some of the most noticeable signs of damage to the building fabric include:
A musty, damp smell.
Black mould on the walls or ceiling
Wet walls, specifically after it’s been raining
Peeling paint or wallpaper
When the building is exposed to wind driven rain, the outer wall becomes saturated with water, the insulation then acts a bridge for this water to reach the inner wall and causes damp issues.
Examples of poor installation practices include;
Failure to perform a thorough survey to assess the property's suitability for cavity wall insulation
Ignoring the fact that a building has evidence or signs of previous signs of dampness or water penetration.
Ignoring factors which would deem cavity wall insulation unsuitable and installing it regardless
Failure to address and remedy existing building defects and obstructions in the cavity
Insufficient amounts of cavity wall insulation used
Uneven installation - leaving voids
Gaps in the insulation can lead to temperature variations and ‘cold spots' on the internal walls - when warm air touches these 'cold spots' it can result in condensation and mould. Voids in the insulation and the ensuing problems with 'cold spots' can also occur over time due to an insufficient amount of insulation material being installed in the cavity wall. As time passes, the insulation can sink to the bottom of the cavity leaving 'cold spots' at the top of the wall. It has to be said that whole wall would be at the same temperature as the ‘cold spots’ if there was no CWI present.
It is also important to remember that buildings need to "breathe" and they rely on constant air circulation. If moisture is unable to escape or evaporate this can also lead to damp problems.
When the walls are insulated, it reduces heat loss through the walls and raises the temperature of the air inside the house. This is the purpose of filling the cavities with insulation. So far, so good.
Warm air can hold more water molecules so there is more moisture held in the air. The surface temperature of the walls should also be warmer which raises the Dew Point temperature (the temperature at which the moisture in the air condenses on the walls).
Moisture vapor will naturally move from the warm side of a wall to the cooler side. If the temperature is high inside the building and lower outside the building, vapour drive will be directed outward (and vice versa). The greater the difference of this “temperature gradient,” the greater the vapor drive.
The degree of vapor drive is controlled by the porosity of the wall, together with environmental factors. Lots more could be said about Vapour permeability, Hygroscopicity and Capillarity which would allow us to more fully comprehended the effect of water on buildings. However, this may be dealt with in another Blog.
Briefly, capillarity refers to the absorption/ desorption of water as liquid, whereas hygroscopicity refers to the absorption/ desorption of water as vapour (as relative humidities change), and vapour permeability refers to the ability of a material to allow water vapour to pass through it.
Basically, the movement of moisture via diffusion is a result of differences in vapour pressure that are related to the temperature and moisture content of the air. The transport of water vapour through structures is driven partially by differential vapour pressure between both sides of the wall.
Vapour pressure arises from the amount of water gas molecules in the air. If there is a difference in the number of molecules between two areas adjacent to each other there will be a pressure differential, and the water molecules will move to equalise the pressure. In Britian, this will usually be from the inside of a building to the outside because of the higher production of moisture in a house. The rate of transmission will depend on the pressure difference, the vapour permeability of the boundary between the two areas (ie wall, roof) and the thickness of this boundary.
However, reverse vapour transmission is possible in certain circumstances. For example, with the summers getting warmer, ‘solar vapour drive’ can changes how moisture moves in the wall leading to ‘inverted’ vapour flow. The moisture now wants to move from the warmer external leaf to the cooler internal leaf. This can increase the relative humidity of the air on the surface of the cavity-facing side of the inner leaf.
Temperature is the greatest factor impacting vapor drive. Add a significant difference in humidity, and the vapor drive becomes even more vigorous. What this means is that vapor drive will act differently relative to a wall depending on the climate, or even the time of year.
Prior to insulating the cavity, any moisture getting through the single skin outer leaf of the cavity wall would run down the inner face of the outer leaf towards the bottom of the cavity below the floor level and the Damp Proof Course (DPC).
Any moisture diffusing through the inner leaf of the cavity wall from the inside would also enter the cavity (think of all that moisture produced by drying clothes, washing, cooking, showers – even breathing). Airflow in the cavity would remove excess moisture vapour while drying any residual moisture within the cavity at the same time.
The amount of moisture that can condense inside such an unprotected wall can be quite significant. What makes the issue a bit more complicated is the fact that, under certain circumstances, some degree of vapor diffusion may actually help keep wall cavities dry by allowing any trapped moisture to escape the same way it got in. For example, because brick acts as a “reservoir” material absorbing rain and condensation. In this case, moisture moves by diffusion to the surface of the brick, where it can dry by convection, helping keep moisture accumulation in check.
Now introduce cavity wall insulation (CWI) into the equation! Whenever insulation is added there is also a danger of creating thermal bridges at critical junctions where full coverage may be interrupted.
In exposed walls with CWI there is water-driven rain soaking through the outer leaf of the wall and there is also the vapour driven moisture from the house diffusing through the wall. Without a clear air space, moisture will not drain effectively. There is no air flow in the cavity. If there are any impervious coatings applied to the surface of the external wall this will prevent moisture from diffusing through the wall. The moisture gets trapped in the wall.
The insulation gets wet and the wet insulation loses its thermal resistance. This actually lowers the surface temperature of the walls and pushes the temperature gradient in the wall towards the inner leaf.
As moist, warm air moves across a falling temperature gradient, it cools. When the air reaches its dew point — the temperature at which it can no longer hold water — condensation occurs on the surface bordering the temperature gradients. Now the moisture diffusing through the wall will condensate within the wall itself when it reaches the dew point. We have interstitial condensation resulting from the cooling of the moisture diffusing through the wall.
All this leads to damp internal walls as water penetrates too far into the external surface and then gets transmitted to the internal walls. The wet insulation ensures that the cavity cannot dry out and also increases the transport of water across wall ties. In addition, the insulation loses much of its thermal resistance thus increasing the coldness of the wall and encouraging moulds on the now damp internal walls.
The movement of water vapour through parts of the construction is a key issue when considering thermal upgrading, but many other factors need to be considered to arrive at an optimum solution such as heating regimes and the orientation and exposure of the particular building.
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