Thermal Insulation Principles

Newer vs Older Buildings

• Newer buildings are built to be waterproof. Their fabric has been built with new materials and technologies designed to keep water out.
• Older buildings have been built to be water permeable (breathable). They have been built with older materials and due to the age of the building, their fabric often contain significant amounts of moisture. The presence of moisture in the fabric of older buildings is a very important element, that determines the renovation methods and materials that can be used for the renovation of these older buildings. This includes the choice of thermal insulation materials and methods.

As a result, insulating older & newer buildings should be approached differently.

How Old Walls Breathe?

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 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 must understand some of the factors that drive the movement of moisture. We have done quite a bit of research in this regard and here are the basics.

The movement of moisture is affected by ambient temperature and humidity. These changes translate to pressure changes as the movement of tiny vapour molecules in the air and building materials is primarily driven by tiny pressure differences.

In order to understand how breathability works, we need to understand some basic concepts.

• Air always carries (some) moisture. This is most commonly expressed by its relative humidity (RH).
• [Temperature] - Warm air is lighter than cold air. This is a well-known concept: heat makes air expand. Warm air becomes lighter and rises, while cold air becomes denser and sinks. 
• [Humidity] - Humid air is lighter than dry air. This is a less known concept, but it can be understood by looking at the composition and density of air and water. Clean air, composed of nitrogen and oxygen, weighs 29 g/unit of air, while water vapours (H₂O) weigh only 18 g/unit. Thus, damp air is lighter than dry air. Lighter damp air rises, while heavier drier air sinks.
• Air always flows from high pressure (cold) to low pressure (warm) areas. Cold denser air expands into warmer lighter areas, resulting in air and moisture flow.

The above principles govern the movement of air and humidity in nature.

At large scale, these interactions drive important weather phenomena. The movement of the air over large distances results in wind, while the movement of humidity results in various forms of precipitation or lack of it.

At much smaller scale, the light movement of the air is known as breathability. The moisture transported into and out of the fabric leads to various manifestations of the wetting-drying cycle, the most common ones being condensation and evaporation.

Here is an example: an old wall warmed up by the Sun. The surface of the wall becomes warmer and the surrounding air becomes lighter. The depth of the wall being somewhat colder contains denser air. The pressure difference between the denser sub-surface and the lighter surface air results in an airflow from the depth towards the surface. This outgoing flow transports humidity out of the wall fabric into the surrounding air, resulting in evaporation and drying of the fabric.

Although the scale of these events is very different (km vs mm), the guiding principles of moisture movement are the same at all levels.

Managing the Dew Point

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.

Dew Point Control in Older Buildings

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.

Dew Point Control in Newer Buildings

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 are also in use. The moisture barrier separates the external (damp) environment from the internal 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.

Why Insulating Older & Newer Buildings is a Confusing Subject?

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. 

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.   

Based on our ongoing technical research, we aimed to organize and simplify the basics of this subject on this page.