Insulating Historic Buildings with Solid Walls

Why is it necessary to use breathable insulation on historic buildings?

What Does Breathability Mean when Insulating Old and Historic Buildings?

The breathability of a building relates to the passage of water through its structure and has nothing really to do with air movement – despite the wording! It is concerned with how building materials can absorb and release water, both as a vapour (gas) and as a liquid. Older buildings (typically built pre-1919) do not have a cavity and so the water vapour has nowhere to escape to except to the outside or inside of the wall structure where it will evaporate when favourable conditions prevail. It is estimated that about 30% of the UK’s building stock have solid walls. These buildings were constructed with materials that allowed water vapour to pass through their pores, so it is critical to follow through with this capacity when insulating or renovating an old or historic building. If this breathable capability is blocked, it could lead to unintended consequences. Trapped moisture can quickly cause the structure to decay and it can adversely affect the interior air quality by creating mould and condensation.

3 Factors that Affect Breathability

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. These materials are all breathable so they allow moisture to pass through them which will then evaporate away 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 away due to good ventilation from air circulating around the building from leaky windows, doors, chimneys and roof eaves. Older buildings generally do not have a damp proof course installed and so this may contribute to some damp in the lower walls which will also evaporate away when conditions are favourable.

By contrast, modern buildings are designed to be watertight so that rain cannot penetrate through the walls, and if it does, it can escape from the cavity. However, if these modern waterproof materials are added to the outside of a solid walled building, they may 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) in the middle of the structure.

When breathable and naturally flexible lime mortars are replaced with hard and rigid cement-based mortars, the flow of water vapour is also hindered. The rigid cement-based mortar may also crack as it does not move naturally with the building and if this happens, rain will get into the fabric of the building and will become trapped causing interstitial condensation. Using vapour-closed materials will 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 stonework.  It is critical to work with the building physics of an old building, rather than against them. Wet rot and dry rot sets in when Moisture Content (MC%) reaches 20% so it is critical to keep MC below that level all year round.

The Solution to Insulate Walls on Old Historic Buildings

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, several issues may arise such as:

Source: ASHRAE HVAC Systems and Equipment Handbook, Chapter 22. 2016.

The Water 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:

Pavatex wood fibre thermal insulation and other natural fibre insulation materials, 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 means that if it does get damp 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.

Pavatex Wood Fibre Wall Insulation systems work with the physics of moisture transfer rather than attempting to prevent it with the inclusion of vapour barriers. The properties that contribute to this are:

Mould on a solid wall with tanking

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 critically, the walls will be warmer all year round as they are insulated from the cold on the external side, and the heat from the heating system will get into them from the inside. This means that moisture should stay as a gas (vapour) rather than condensing to a liquid so will pass through the building fabric much easier, allowing it to evaporate away. The wood fibre must be covered with a breathable render, preferably lime or silicate-based, so that the water vapour can also pass through this and evaporate to the outside. The Baumit range of render products have been approved for use with Pavatex wood fibre insulation. Pavatex Isolair Multi wood fibre boards are water resistant which will resist rain from penetrating into the walls from the outside, but they are hygroscopic, so will allow water vapour from within the wall or building pass through both the insulation and the breathable render so that it can evaporate. Pavatex wood fibre will also allow rising damp within the wall to be dispersed and therefore the wall will be drier and warmer.  This results in the internal air being 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.

Isolair Multi water resistant wood fibre sarking boards can also be clad with a ventilated façade, if preferred. A low U-Value can be achieved using Pavatex External Wall Insulation providing that the roof will adequately overhang the insulation. For more information see External Wall Insulation.

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 to use breathable insulation materials. The recommended products are Isolair Multi wood fibre board which is coated in breathable lime plaster or Pavadry wood fibre board which is finished with plasterboard. The wood fibre board is bonded to the wall to ensure the absence of air gaps and to ensure continuity of capillary conduction with the original wall, thus enabling two-way moisture transfer which promotes drying from both internal and external surfaces.

When internally insulating a solid wall, it is critically important NOT to over-insulate because this will lead to a very cold wall in winter, due to the heat from the central heating not getting through the insulation into the wall, and it being very cold externally. This significantly increases the risk of water vapour condensing within the cold wall, and water in the form of a liquid, is much slower at passing through the wall and may cause interstitial condensation (see below) within the structure. Therefore, when internally insulating solid breathable walls, it is not 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.

We do not recommend leaving a ventilated cavity between the wall and the insulation because this brings cold air into the space which can very easily bypass the insulation and create unwanted draughts. Secondly, the excellent thermal mass benefits of the solid wall will be totally lost as the cavity will be constantly cold in the winter due to the cold ventilated air coming into it. This also greatly increases the risk of water vapour condensing and causing decay and mould in the wall. The U-value of a wall with a ventilated cavity will be considerably higher (worse) than a wall with the Isolair Multi or Pavadry system in place. There is no requirement to put air vents into the solid wall if our installation instructions are followed correctly because the moisture will escape through the complete fabric of the wall. 

For more information see Internal Wall Insulation.

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:

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 decay, and timbers embedded in the wall may 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 can never 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 anywhere near 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. A dry wall is a warm wall. In addition, it will contribute to preserving our historic building stock for many more generations to come.