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Insulating Solid Walls – Old and Historic Buildings

Insulating Solid Walls


What Does Breathability Mean when Insulating Old and Historic Buildings?

The breathability of a building relates to the passage of water through its building structure and has nothing to do with air movement! It is concerned with how building materials can absorb and release water both as a vapour and as a liquid. Older buildings (generally built pre-1919) were constructed so that water could pass through the complete structure, so when insulating or renovating an old or historic building, it is critical to take this into account. This will ensure that this breathable capability will not be blocked which could cause the structure to decay or could adversely affect the interior air quality. Older buildings do not have a cavity and so the water vapour has nowhere to escape to except to the outside or inside of the structure where it will evaporate when favourable conditions prevail. It is estimated that about 25% of the UK’s building stock have solid walls.

3 Factors that Affect Breathability

Water Vapour Permeability in terms of buildings refers to the rate at which water vapour passes through solid materials, if it can pass through at all.

Hygroscopicity refers to the absorption and release of water as a vapour or gas, as relative humidities (RH) change. Usually there is more moisture internally from cooking, showering, breathing etc so the moisture tends to move from inside to outside, but not always. Also warmer air holds more water vapour than colder air.

Capillarity refers to the absorption and release of water as a liquid.


Older, heritage and traditional buildings are usually constructed from vapour-open materials such as brick, stone, timber frame, wattle and daub or cob with lime or earth mortar in the joints, and plaster, render or paint made from lime on the walls. All of these materials are breathable and so allow moisture to pass through them and then this moisture evaporates either externally or internally. This keeps the building dry because externally the heat from the sun and the wind will dry out the building fabric and internally the moisture will evaporate as a result of ventilation from air circulating around the building which comes in through the windows, doors, chimneys and roof eaves. Older buildings often do not have a damp proof course installed either and so this contributes to rising damp in the walls which will also evaporate when conditions are favourable.


The external surfaces of modern buildings are designed to be watertight so that rain cannot penetrate through the walls. However if these modern waterproof materials are added to the outside of an old building they will prevent the driving rain from penetrating the walls but critically they will also stop the internal moisture that is within the building fabric from evaporating to the outside. The same will happen if the internal side of the building is covered with impervious materials as the building structure may never dry out and there may be mould and damp patches on the inside of the building or interstitial condensation (see below) may occur in the middle of the structure. When breathable and naturally flexible lime mortars are replaced with hard cement based mortars the flow of water vapour is also hindered and the rigid cement-based mortar may crack as it does not move naturally with the building. If it does crack, rain will get into the fabric of the building and will become trapped causing interstitial condensation. All of these treatments affect the natural breathable balance of the building structure and may well cause dampness as well as decay especially in lime mortars, lime renders, soft bricks and soft stone work.


The Solution

In order to make older or historic buildings more energy efficient, they can be insulated with breathable materials so that the breathability balance will not be affected. If non-breathable insulation materials are used such as polystyrene or foam, a number of issues may arise such as:

  • Damp on the inside walls if non-breathable external wall insulation is used.
  • Mould growth on the inside walls if non-breathable external wall insulation is used, and there is poor ventilation internally.
  • Damage to the outside structure if non-breathable internal wall insulation is used as the wall stays too cold and wet and when the fabric of the wall contracts and expands in the frosty weather, the bricks or stone work may become damaged.

The vapour resistance of a material is a measure of its resistance to letting water vapour pass through. It is measured by its µ-value (mu-value).


The Water Vapour Resistance Factor of common insulation materials:
Mineral wool: 1

Wood fibre insulation: 5

Polyurethane foam: 60

Expanded polystyrene: 60

Extruded polystyrene: 150
All natural fibre insulation materials, e.g. Pavatex wood fibre, sheep wool and hemp, have a high degree of vapour permeability as well as having excellent hygroscopic and capillarity qualities which means they dry out quickly. Mineral wool insulation is also very vapour open (as shown above), but it has poor hygroscopic and capillarity characteristics which mean that if it does get wet through that it takes a long time to dry out. Also wet mineral wool tends to sag downwards with the weight of the water, so that it leaves gaps around the edges of the insulation where the heat will escape through and therefore cause condensation on the walls. Polystyrene and foam insulations generally do not let any moisture through their structure and so are not recommended on heritage or period properties. The best insulation materials to use are natural fibre insulations which will ensure that the whole structure will remain breathable.


External Solid Wall Insulation (EWI)

The optimal solution is to externally insulate solid walls with breathable Pavatex wood fibre thermal insulation boards because the complete building envelope will be insulated, therefore reducing cold bridging through the junctions. Also the wall will be warmer from the internal heat which means the moisture within the walls will pass through easier so that it can evaporate. This is because water vapour passes through building materials quicker and easier than water in liquid form. The wood fibre must be covered with a breathable render, preferably lime or clay, so that the water vapour can also pass through this and evaporate to the outside. Pavatex-Diffutherm and Pavatex-Diffuboard wood fibre boards are water resistant so they will resist rain from entering into the walls from the outside but they are hygroscopic and so will allow water vapour from within the wall or building to pass through both the insulation and the breathable render so that it can evaporate. The best upgrade solution is achieved when the building products become more vapour-open as they get to the outside of the structure so lime render is excellent to use in this regard. Pavatex wood fibre will also allow rising damp within the wall to be dispersed and therefore the wall will be drier and warmer and it will keep the internal air drier and healthier due to the vapour open and hygroscopic EWI board and render. The optimum internal Relative Humidity rate is between 40% and 60% because at this rate the risk to human health, as well as the health of the building, will be greatly diminished.


The wall can also be insulated externally with Isolair or Pavatherm-Plus water resistant wood fibre sarking boards and then clad with a ventilated façade. Take care to insulate around window and door reveals as these are a critical area for heat loss.


Internal Solid Wall Insulation (IWI)

Sometimes older properties have a unique external appearance or preservation order on them so that they cannot be externally insulated. In this case the walls can be internally insulated and again it is critical that a breathable insulation material is used. The recommended product is Pavadentro which should be coated in a breathable plaster – preferably lime plaster – and also breathable paints and finishes. Also take care to insulate around window and door reveals as these are a critical area for heat loss.


Pavadentro is a unique internal wall insulation product because it has an integrated mineral layer which is a vapour controlling functional layer. This helps to control the passage of water vapour through the insulation layer so as to avoid the formation of damaging moisture.


When internally insulating a solid wall, it is important not to insulate too much because the wall may always remain cold as the heat from the central heating may never get into the wall. This means that the water vapour could condense within the wall and water in the form of a liquid is much slower at escaping from the wall and may cause interstitial condensation within the structure. Therefore when internally insulating breathable walls, it may not be advisable to reach Building Regulations U-value requirements or the heritage building may become damaged. On the plus side, there have been several tests carried out over a significant period of time analysing the U-values of historic buildings using probes to test heat loss, and it has been widely agreed that these buildings are actually considerably better at retaining heat with wood fibre wall insulation, than computer U-value reports would indicate.


Understanding Interstitial Condensation

Interstitial condensation can occur in solid and cavity wall properties, and it happens when pressure and temperature differences force warm humid air through hygroscopic materials until they reach a point cold enough for it to condense upon a surface. It can occur when a variety of conditions prevail, for example it depends on:

  • The amount of humidity & evaporation and the occupancy level in the room.
  • Relative humidity, temperature and pressure both inside and outside (which constantly varies).
  • The structure and composition of the wall.
  • The surfaces (e.g. insulation, renders, plasters and paint type) both inside and outside of the wall.

During winter when warm moisture laden air is being pushed out through the wall it needs to stay warmer than the dew point, which is the temperature at which the water vapour condenses. Should the moisture condense, the liquid water may still be able to warm up during the hotter summer months and evaporate from the wall so that the wall will dry out for part of the year. However if the wall always stays cold the liquid water may never evaporate and so will cause condensation to occur in the actual wall itself which may cause the wall structure to rot. Evaporation can also be dependent on the local weather conditions and if the wall faces towards the warm sun.


Critically if the external wall insulation is not vapour open, this water cannot ever evaporate to the outside regardless of the weather conditions. It will have to rely on being absorbed back into the building which greatly increases the risk of interstitial condensation, mould growth and high internal humidity which is bad for human health. If a wall has waterproof insulation installed on either side, the water vapour can only evaporate in one direction and this will almost definitely lead to water becoming trapped in the wall and never being able to escape.


Sometimes interstitial condensation shows up as a large area of mould on the wall, rather than in the corners at the junctions, but sometimes the damage caused by interstitial condensation is not obvious until it has caused significant decay and rot. In addition a wet wall does not perform as well thermally as a dry wall, and so will allow more heat to escape.


Pavatex wood fibre is the optimum insulation material to use on a solid wall because it can hold a significant amount of water vapour without damaging its integrity or thermal performance. It will then release this water vapour as weather conditions improve. This will result in a dry building and a drier internal atmosphere which will improve the indoor air quality and is better for our health. In addition it will contribute to preserving our historic building stock for many more generations to come.