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Thermal Insulation Principles

Insulation Differences - Older vs Newer Buildings
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Newer vs Older Buildings

There are significant differences between newer and 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, the thermal insulation of older 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 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.

Basic Concepts about Moisture

  • Water always follows the path of least resistance, towards areas where it can flow the easiest. Its flow can be modified by external object or forces.  
  • The movement of liquid water is driven by GRAVITY. Liquid water flows towards the ground. However, solid objects or barriers (e.g. stone, bricks, moisture barriers, tanking etc.) can affect the movement of liquid water. 
  • The movement of water vapours is driven by PRESSURE differences. Vapours always flow from higher pressure to lower pressure areas. However, electrical surface charges (adhesion forces) - tiny electrical charges present on the surface of all solid materials - can significantly change the movement pattern and bonding of water vapours. The effect of gravity on water vapours in walls is negligible.
  • Air always carries moisture. The moisture content of air is most commonly given by its relative humidity (RH) which expresses the degree (e.g. 67%) to which air is saturated by vapours. Vapour saturation or 100% RH results in condensation or water liquification.
  • Temperature and humidity changes in the environment result in pressure changes, causing vapours to move from higher to lower pressure (closer to vacuum) areas. As temperature and humidity changes are ongoing in a building, there will be ongoing moisture movement inside the walls both horizontally and vertically. This is known as breathing. The effects of temperature and humidity onto the walls is detailed below. 
  • [Temperature effect] - 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. Thus, water flows from cold (higher pressure) areas to warm (lower pressure) areas. Cold denser air expands into warmer lighter areas, resulting in air and moisture flow. That's how heating can draw humidity out of colder walls.
  • [Humidity effect] - Humid air is lighter than dry air. This is a less known concept, but it can be understood by looking at the chemical composition of air and water. Dy air, composed of nitrogen and oxygen, weighs 29 g/unit of air, while water vapours (Hâ‚‚O) weigh only 18 g/unit - making dry air heavier than water vapours (29 vs 18 g/unit). As a result, heavier dry air (of higher pressure) sinks while lighter damp air (of lower pressure) rises. This explains while moisture evaporating from a damp soil rises, the low density of damp air being a driving force behind the physics of rising damp.
  • Vapours vs liquid water: through condensation vapours can turn into liquid water becoming a significant source of liquid water that can cause ongoing dampness problems.

Large-Scale vs Small-Scale Moisture Movement

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.

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Warm-cold or Low-high pressure interaction determines important weather phenomena

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.

An Example - The Effect of Heating

Here is an example: an old wall heated by the Sun. This is what happens:

  • The surface of the wall warms up and the surrounding air becomes lighter.
  • The depth of the wall contains colder denser air.
  • The temperature difference between the colder (denser) sub-surface and the warmer (lighter) surface results in a pressure difference, creating an air flow from the depth towards the surface.

This outgoing vapour flow moves out humidity from the wall fabric, resulting in evaporation and drying of the fabric.

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Heat or the Sun induced temperature gradient makes the wall drier

Insulating Newer Buildings

The thermal insulation of newer buildings is about INSULATION only.

Most newer buildings can be characterised by the following:

  • The fabric is made of modern, denser bricks which are relatively resistant to moisture.
  • The cavity acts as a moisture barrier against wind driven rain. A ventilated cavity allows the rainwater to evaporate from the fabric, keeping the wall dry.  
  • The building has a damp proof course (DPC), the fabric not being affected by wetting from rising damp.
  • As a result, the wall fabric is mostly dry.
  • Being dry, the fabric is less affected by humidity variations, however it can be subject to temperature variations.

Applying a modern insulation material (such as a foam insulation) to the wall fabric is not a problem as

  • The wall fabric is dry.
  • The dry fabric can tolerate a lot of temperature variations, before is affected by interstitial condensation.  
  • Modern non-breathable insulations have a vapour control layer (moisture barrier), that separates the internal environment from the external one, keeping the internal environment dry.
  • The walls being dry there is no need for the building to breathe as there is no moisture to breathe out.

So the thermal insulation of newer buildings is about insulation only.

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Modern insulation in a new building

Insulating Older Buildings

The thermal insulation of older buildings is about INSULATION and MOISTURE management.

Most older buildings can be characterised by the following:

  • The fabric is built from older, weaker porous materials - easily saturated by moisture.
  • Most old walls are solid walls with no cavity. Solid walls can be easily permeated by rain, making the fabric damp.
  • Many old building don't have a DPC (or have a failed DPC), so the fabric is also damp from rising damp.
  • As a result of the wall fabric of old buildings can contain a lot of moisture.
  • Solid damp walls subject to ongoing moisture evaporation are also colder, often perceived as radiating cold.   
  • Being damp and cold, the fabric is affected by both humidity and temperature variations.

Applying a modern insulation material (such as a foam insulation) onto an old fabric can cause major problems because:

  • The wall fabric is damp. An old fabric can contain significant amounts of moisture even if its surface looks dry.
  • Most modern insulation materials are non-breathable, thus block the evaporation of moisture. This leads to moisture accumulation which can cause persistent moisture problems in the fabric, hidden from view.
  • A damp fabric can't tolerate too much temperature variations without being affected by interstitial condensation.  
  • Moisture accumulation over time reduces the thermal performance of the walls, and can also lead to fabric-related damages.

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.  

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Modern insulation in an old building

Managing the Dew Point

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.

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 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.

insulating older& newer buildings
Condensation is caused by both temperature and humidity

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.

Related Pages

Here are some other related pages that you might want to read to broaden your knowledge in this field. 

Solutions

Here are some practical solutions related to this topic:

Recommended Products

Here are the some recommended materials / products that can help solving or dealing with some of the problems discussed on this page. 

Completed Projects

Here are some of our projects where we have dealt with some of the issues discussed on this page:

Photo Galleries

Here are some photos demonstrating these concepts. Click on any image to open the photo gallery.

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RasoThermo Heres - Insulating a Listed Farm Building

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.  

Wind Mill Project by the Sea

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. 

Videos

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