As a result, the thermal insulation of older buildings should be approached differently.
Breathability means the ability of the wall fabric to exchange moisture with the environment.
One might be tempted to assume that breathability is a one-way flow, from the wall fabric to the environment, contributing to the drying of the wall fabric. Although this mechanism exists, breathability in fact is a two-way flow, allowing moisture to move both into and away from the wall fabric.
To understand the importance of why the thermal insulation of old buildings must be breathable and how this (or lack of it) affects the long-term condition of the wall fabric, we need to understand some key factors that drive the movement of moisture. We have done quite a bit of research in this regard and here are the key principles.
The large-scale and small-scale movement of moisture is largely driven by the same principles.
At large scale, the above principles drive important weather phenomena. The movement of vapours over large distances - known as weather fronts - are driven by pressure differences across larger areas, resulting in different wind patterns between these. As air also carries humidity, based on the surrounding temperatures different forms of precipitation can also form.
At small scale inside the tiny wall capillaries we have various wetting-drying phenomena, the most well-known being condensation and evaporation. These are all driven by small pressure gradiens, which are in constant change as surrounding temperature and humidity conditions change.
Although the scale of these events is very different (km vs mm), the guiding principles of moisture movement are very similar at all levels.
Here is an example: an old wall heated by the Sun. This is what happens:
This outgoing vapour flow moves out humidity from the wall fabric, resulting in evaporation and drying of the fabric.
The thermal insulation of newer buildings is about INSULATION only.
Most newer buildings can be characterised by the following:
Applying a modern insulation material (such as a foam insulation) to the wall fabric is not a problem as
So the thermal insulation of newer buildings is about insulation only.
The thermal insulation of older buildings is about INSULATIONÂ and MOISTURE management.
Most older buildings can be characterised by the following:
Applying a modern insulation material (such as a foam insulation) onto an old fabric can cause major problems because:
As a result, the thermal insulation of old buildings must be breathable to prevent the build-up of moisture in the fabric. Additionally, because vapours also carry heat, breathability results in some heat losses - a small trade-off for old building owners to better preserve old buildings for future generations.
Thus, the thermal insulation of older buildings is a dual action of insulation and moisture management - insulating as best as best as possible while also paying attention to moisture, allowing the wall fabric to breathe. Failing with the moisture management leads to moisture accumulation and often to degradation of older buildings. Â
The dew point is the temperature at which vapours condense and transform into liquid water. Managing the dew point (or condensation point) is important to minimize interstitial condensation and to keep the wall fabric dry.
There are fundamental differences on how the dew point is managed in newer and older buildings, which must be understood to prevent insulation design mistakes.
The dew point in older buildings is controlled by natural evaporation. In a breathable wall fabric moisture moves constantly, either in or out of the fabric, depending on the dynamic equilibrium of pressures - driven by ambient temperature an humidity changes.
A key concept here is the presence of vapour flows. As long as the fabric is allowed to breathe, vapours can flow and the fabric can naturally regulate its moisture content (within limits) even under damp circumstances.
Applying some modern insulation material with a vapour control barrier - in an attempt to control the dew point in an old building - that blocks the breathability of the fabric, it will break the tiny vapour flows, leading to moisture accumulation inside the fabric. As a consequence, moisture starts piling up at the flow blockage point, usually at on one side of the membrane, leading to condensation.
Condensation is the result of a blocked vapour flow piling up at the point of blockage, leading to moisture accumulation.
This explains why can condensation form so easily on cement plaster surfaces or modern paints. When vapours flow towards the wall, the flow is stopped by the non-breathable surface and the ongoing moisture flow keeps piling up on it resulting in condensation and mould growth.
Hence, it is important that as part of the insulation solution NOT to use any non-breathable materials, membranes or moisture barriers that would block/break the native pressure-driven flow system of the wall fabric.
The dew point in newer buildings is controlled by vapour control layers (VCL). These are fundamentally moisture barriers, usually membranes or foils applied onto the back of modern insulation products, however standalone moisture barriers can also be used. The moisture barrier separates the external (damp) environment from the internal (dry) environment.
Where a membrane or moisture barrier is placed it is going to create a blocked vapour flow, leading to moisture accumulation and potential condensation. Thus, the repositioning the moisture barrier allows one to control the location of condensation, away from specific areas to other areas where the presence of condensation is acceptable.
But there is more to condensation than just temperature differences. The moisture content of the air, cavity and wall fabric is also very important. The less humidity is present in a space, the less likely to have condensation, even at higher temperature differences. After all, moisture must be present in order to condense. The higher the moisture content of the wall, the more liable it becomes to condensation under the same temperature conditions - as shown on the chart below.
Thermal insulation, particularly of older buildings, can be a confusing subject. In our opinion the confusion is introduced by the presence of moisture which also needs to be considered, figured out and handled in addition to any thermal performance and U-value requirements.Â
While the thermal insulation of newer buildings is relatively simple as it is only about one thing: thermal performance; the thermal insulation of older buildings can be much more complex, as it is always about two things: thermal performance and moisture.
The topic of moisture and of hygroscopic salts, their effects onto the building fabric and various insulation materials brings another level of complexity into an already complex subject. Lack of knowledge about the effects and importance of vapour movement, salts, breathability are often the missing ingredients and a source of confusion. Â
Based on our ongoing technical research, we aimed to organize and simplify the basics of this subject on this page.
Here are some other related pages that you might want to read to broaden your knowledge in this field.Â
Here are some practical solutions related to this topic:
Here are the some recommended materials / products that can help solving or dealing with some of the problems discussed on this page.Â
Here are some of our projects where we have dealt with some of the issues discussed on this page:
Here are some photos demonstrating these concepts. Click on any image to open the photo gallery.
The thermal insulation of this 16th century timber-frame farm house has been completed with breathable thermal insulation materials. The RasoThermo Heres superinsulating thermal lime plaster has been used for the solid parts of the masonry, resulting in high-performance low-thinkness thermal insulation, retaining the historic character of the building while saving internal space. Â
Here is an 18 metre tall 200 year-old listed wind mill, not very far from the sea. Wind driven rain created major problems, resulting in ongoing water leaks. The building has been sympathetically waterproofed and thermally insulated with lime plasters only, using breathable heritage-friendly materials only.Â
Here are some videos related to this solution. Please unmute the videos when playing them.