Thermal Insulation: The Ultimate Guide to A Safe Thermal Envelope

Your complete guide to building a safe thermal envelope and protecting thermal insulation.

What is a thermal envelope?

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A building's thermal envelope is a physical separator between the conditioned (indoor) and unconditioned (outdoor) environments. The thermal envelope is responsible for resistance to air, water, heat, light, and noise transfer. A well-built thermal envelope is one of the key elements of a comfortable, low energy home.​ 
The most important parts of the thermal envelope are the foundation, external walls, roofs, windows, and doors. A well-designed thermal envelope will incorporate continuous thermal insulation, the elimination of thermal bridges, an airtight layer and a windtight layer. 

Precisely defining the thermal envelope, including the continuous thermal insulation layer and the airtight and windtight layers and how these different components are to be connected, are all key steps to saving energy and protecting your building's structure. Precise installation of these components is also critical. 

The 3 main functional layers in each element of the thermal envelope

Each part of the thermal envelope of the building i.e. wall, roof, door, window has the following functional layers: ​ 

1. Wind protection (blue): this layer protects the thermal insulation from cold wind and other atmospheric influences like rain, snow and solar radiation. It also provides protection against insects.

2. Thermal insulation: this layer reduces heat transfer through the building envelope (i.e. the transfer of thermal energy between objects of differing temperature)​.

3. Airtight layer (red): this sealed layer protects the thermal insulation by preventing the passage of air and moisture through the structure.

Why is thermal insulation so important for saving energy?

Thermal insulation reduces heat transfer (i.e., the transfer of thermal energy between objects of differing temperature). Thermal insulation in buildings can be achieved with specially engineered insulation materials. The insulating capability of these materials is measured as the inverse of their thermal conductivity (k). Low thermal conductivity is equivalent to high insulating capability (resistance value).​

Every insulating material has a specific thermal conductivity, which is a fundamental property of the material independent of its thickness.

There are five main material types of thermal insulation

1. Mineral wool insulation can be either glass wool (also named fibreglass) or stone wool (also named rock wool). Both have good thermal and acoustic insulation properties as well as good fire resistance. This type of insulation is produced as a roll material, sheets, or loose fibre.

2. Cellulose insulation is one of the oldest types of insulation. It has good insulation properties as well as fire resistance. It is often made of recycled newspaper and is therefore one of the most environmentally friendly insulation materials. The amount of energy used to produce cellulose insulation is 20-40 times lower than for mineral wool. This type of insulation is mostly available as loose fibre, which can be installed either via dry or wet installation methods.

3. Wood fibre insulation is manufactured, as the name says, from wood fibres. It is produced in rigid sheets in various thicknesses and densities as well as loose fibres. It has a broader range of applications compared to cellulose insulation, as it can be used for thermal insulation, as a weather-resistant board, or as load-bearing insulation. It has a higher environmental impact compared to cellulose insulation.

4. Polystyrene insulation (EPS, XPS) is thermoplastic foam with good sound and temperature resistance. The insulation is produced as rigid sheets, and therefore it is suitable for wall/roof insulation in solid construction. The product has low fire resistance and therefore protective measures have to be considered when using this type of insulation.​

5. Polyurethane insulation (PUR, PIR) is synthetic material with excellent thermal insulation properties. It is produced as rigid sheets and is suitable for wall/roof insulation in solid construction. Another application method for this insulation type is spray insulation on site. In this case the insulation forms due to a chemical reaction on-site, so installation has to be to a very high standard. The product has low fire resistance and protective measures have to be considered when using this type of insulation material.

What is a U-value?

Also known as the heat transfer coefficient, U-values are measured in W/m²K. They are used as a measure of thermal performance. 

The U-value shows the rate of transfer of heat through a structure divided by the difference in temperature throughout the layer. The lower the U-value the better-insulated a structure is, and the lower the heating costs are likely to be. It is important to pay attention not just to the U-value but to the quality of workmanship too, as this can strongly affect thermal transmittance through the structure, and will affect heating costs. If installation quality is poor it can result in a cold bridge, a gap in the insulation where heat is lost, and this can even lead to building damage.

Why must insulation be protected from the outside?

Thermal insulation remains most effective when it is dry and there is no air movement within the material. Wet or cold insulation loses more heat.
From the outside, thermal insulation can get wet from the rain and be cooled down by wind penetration. To make sure your insulation remains effective over the lifetime of the building, it is recommended that you use wind protection materials.

Wind protection materials safeguard the thermal insulation by forming a continuous wind protection layer. These materials have one characteristic in common – they stop uncontrolled airflow into the building structure from the outside.

In many cases, the wind protection layer also functions as weather protection e.g. from rain during the construction phase. 

Wind protection materials are mostly divided into the following categories

  • Wind protection membranes — roll material which is laid over the insulation. In this category SIGA provides Majvest® 200 and Majvest® 700 SOB (fire class B, UV-resistant) for walls and Majcoat® 150, Majcoat® 250 SOB and Majcoat® 350 for roof construction. These membranes are produced using fleece, TPU, or acrylic coatings and provide relatively low Sd values (meaning they are more vapour open) to provide construction drying capacity. These membranes can also be exposed directly to the elements depending on the product for up to 12 weeks.
  • Wind protection boards — solid sheets made out of wood fibres, gypsum, or cement fibres. To ensure weather tightness with this system, all the joints must be sealed with a tape such as SIGA Wigluv®.
  • Outside plaster or render — External render is a wall covering designed to protect the bricks or blocks, but mainly the insulation, from atmospheric exposure. It also provides an exterior decorative finish.

What is the difference between wind protection for the walls and roof?

Both of these membrane types are tested for weather tightness, e.g. driving rain tests, and offer protection to the building envelope. Roofing membranes normally have a stronger top layer and higher mechanical strength. This is to withstand impacts such as walking on the roof during construction, and because they are more directly exposed to severe weather, such as driving rain or melting snow.

Why must insulation be protected from the inside?

Thermal insulation maintains its effectiveness so long as it is dry and there is no air movement within the material.

Indoor air normally contains more humidity than outside air. Because of diffusion, moist air from inside will seek to move through the thermal envelope to the outside. This is controlled by using a vapour control layer.​ 

Unsealed overlaps or weak connections in the vapour control layer allow warm and moist indoor air to penetrate the construction during winter. This warm air cools down within the building structure as it moves towards the outside. Water vapour in the air turns to liquid once it reaches the dew point, wetting the thermal insulation, reducing its thermal performance and potentially causing expensive damage to the building.

SIGA offers vapour control layers such as SIGA Majpell® 5, Majpell® 25 and Majrex® 200, plus a full system of high-performance SIGA tapes to control diffusion and convection.


Vapour control materials manage the passage of water vapour through the building structure. Their function is to protect the insulation by stopping or controlling the diffusion of water vapour from the room into the structure of the building.

Vapour control materials can be membranes with different functional properties or sheet materials like OSB, plasterboard, or plywood. Plaster and solid concrete are also considered airtight materials.

Membranes can be divided into four categories based on the diffusion capabilities of the material. The diffusion capability of the membrane is represented by its Sd-value. The Sd-value is the equivalent thickness of air, measured in metres, with the same diffusion resistance as the membrane. The higher the value the greater the resistance to the passage of water vapour.

  • Vapour stop – Most common in this category is PE-foil, which stops water vapour from moving in and out of the construction. The main drawback of the vapour stop is that it doesn't allow the construction to dry out towards the room in summertime, i.e. reverse diffusion. Therefore, it has limited usage in more complicated construction build-ups.​
  • Vapour control membrane with diffusion capabilities – These diffusion open membranes have an Sd-value between 2 and 10 m, for example SIGA Majpell® 5 (Sd-value 5 m). These membranes work well in various constructions and provide good protection to the building while allowing reverse diffusion during summer. ​
  • Variable vapour control layers – These membranes can change the rate of moisture diffusion. The rate depends on the average ambient humidity – the higher the humidity in the building structure or room, the higher the rate of water vapour diffusion. This feature is beneficial in regard to reverse diffusion. However, during the construction period when humidity levels are high in the building, the membrane's Sd-value is low (i.e. it is more vapour open), and moisture can penetrate the building structure and potentially cause damage. Humidity control is essential in such situations.
  • Variable vapour control layer with Hygrobrid® technology –  SIGA Majrex® 200  has patented Hygrobrid® technology which allows for moisture transport in just one direction – from the construction into the room. Moisture penetration from the room into the building structure is minimised due to a higher Sd-value in this direction. For even more security in-wall/roof structures on the room side of the thermal insulation, this vapour control layer with Hygrobrid® technology is ideal in cases where the building physics are challenging. It has the widest area of application compared to other types of vapour control layer. It also provides airtightness to the building if sealed properly with tapes such as SIGA Sicrall®, Rissan®, Fentrim®. These tapes have different characteristics and compositions.

What is the Sd-value?

The Sd-value is the equivalent air layer thickness in metres. As an example, SIGA Majpell® 5 has an Sd-value of 5 metres and the thickness of the material is 0.4 mm. This means that the membrane has the same resistance to water vapour passing as 5 metres of air if the pressure difference from one side to another is 1 Pa. 

A higher Sd-value means that the material is more vapour-tight. The rule of the thumb is that the inside (airtight) membrane has to be 10 times more vapour-tight than the outside (windtight) membrane for the construction to be safe. 

SIGA's Majvest® 200 windtight membrane, for example, has an Sd-value of 0.5 m, making it 100 times more vapour open than SIGA's Majpell® 5 membrane, thus providing a very safe contruction.

The same 10:1 principle should apply to the whole building envelope. 

When should variable vapour control layers be used?

Variable vapour control layers can be used in most timber frame elements e.g. walls and roofs, but they provide the most benefit in build-ups with a lower drying capacity to the outside. For example, compact unventilated roof construction where an external waterproofing material reduces drying capacity towards the outside. SIGA moisture variable vapour control layer SIGA Majrex® 200 with Hygrobrid® technology provides additional security to all construction types, because of its directional vapour diffusion properties. Read our blog about: 5 ways Majrex could save your life.

What are diffusion and convection?

In buildings, diffusion is the movement of water vapour through materials from areas of higher water vapour concentration to areas of lower concentration. 

Winter diffusion​ 
In the wintertime, the concentration of water vapour (absolute humidity g/m3) in indoor air is approximately 10 times higher than water vapour concentration in outdoor air. Therefore, water vapour will try to move from the inside to the outside.

Summer diffusion​ 
In the summertime, the concentration of water vapour (absolute humidity) in indoor air is lower than that of outdoor air. Therefore, water vapour will seek to move from the outside to the inside.

Depending on the geographical location, the length of the winter or summer diffusion period varies. In Northern European countries, the winter diffusion period is longer than the summer diffusion period. In Central Europe and Southern Europe, the summer diffusion period is either the same or the summer diffusion period is longer.​

Convection is uncontrolled air flow in the building envelope due to gaps in the airtight or windtight layer. ​ 

Convection due to temperature differences​ 
Temperature differences between the inside and outside of the building envelope are a strong driver of convection. This type of convection is a common cause of building damage, as warm air from the room penetrates the construction. This warm air is high in water vapour, which condenses to liquid water within the building structure, where the temperature decreases (dew point). 

This can lead to structural damage. Therefore, it is very important to seal the airtight layer in a quality manner and make sure that all the critical joints in the construction are sealed. The easiest way to ensure the integrity of the airtight layer is to perform a blower door test.

Convection caused by the wind​ 
Wind generates a pressurized force on a building. On the windward side the pressure is positive (over-pressurised) and on the sheltered side, it is negative (under-pressurised). If the building envelope is not sealed properly, air leaks through the windtight and airtight layer can occur due to pressure differences. Excess pressure usually cools down the insulation if the wind can penetrate the windtight layer. Negative pressure, on the other hand, can force warm indoor air out through the construction, where moisture in the air condenses to liquid.

Causes of mould

While increased heat loss due to damp insulation is uncomfortable and unnecessary, it is not usually considered a major defect. But things get messy, and even downright dangerous, once mould appears.​  ​ 

Mould indicates a failure in design, construction or maintenance of the building. The moisture could equally be rain from outside or moisture from inside.​ 

In order to grow, mould needs:

  • Air
  • Warmth (ideally between 15-25⁰C)​
  • Food (construction materials: insulation, timbers, etc.)​
  • Moisture (greater than 50% relative humidity)​

All of these are unavoidably present in buildings. However, we can control the moisture content.​

Moisture, Humidity, Steam, Damp, Condensation. Where does it come from?

  • ​Rain/groundwater (outside)​
  • Wind-driven moist air (outside)​
  • Moisture content of materials during installation (within fabric)​
  • Capillarity of material (within the fabric)​
  • Convection; the mass flow of humid air (internal)​
  • Diffusion; the movement of moisture driven by vapour differences (internal)​

So, leaks from outside can be the cause of mould damage. But equally, moisture could be coming from inside, or could have been built into the fabric from the start. 

By preventing moisture from getting into the building fabric, and by allowing any trapped moisture to escape, we can keep the humidity low & prevent the growth of mould.

How do you know if your construction is safe?

In general, it is good to know the rule of thumb, which is 10:1. Choose an internal airtight material with an Sd value that is 10 times higher than the external windtight material, to eliminate the risk of building damage due to diffusion. 

The 10:1 rule is valid only if high moisture loads into the construction are avoided. 

Uncontrolled airflow into the building structure due to convection through different joints can cause high moisture loads, leading building damage. SIGA offers a full system of high-performance tapes to stop convection.  

How do you build a safe flat roof?

SIGA Majrex® 200 provides the highest safety for compact flat roof constructions. It reduces the complicity of the build-up and helps to save space while maintaining moisture safety for the construction. See SIGA Majrex® safe construction catalogue:

How SIGA can assist with the evaluation of my build-up?

In the case of build-ups with a diffusion tight outer layer (e.g. compact wooden roofs), a hygrothermal simulation must be carried out to ensure the construction is moisture safe. Hygrothermal simulation allows a designer to predict the moisture and temperature conditions that might occur within a building envelope assembly over time. This type of analysis can improve the understanding of how the building envelope will respond to both the interior and exterior environment, and can help identify potential moisture performance problems.​​

This is a service that SIGA provides to our customers free of charge.​ Just follow the five steps below.

1. Fill in the component calculation request and send it to SIGA

As a first step fill in the component calculation request with detailed information about the building's location, orientation, shading, and materials to be used.

the component calculation request

2. A computer simulation

Based on this checklist SIGA will carry out the computer simulation. The service is free of charge if SIGA Majrex® 200 or other SIGA membranes are specified in the building.

the component calculation request

3. Assessment of the results

The moisture safety of the build-up is evaluated. Construction is only approved if the build up of moisture within the structure is avoided.

the component calculation request

4. Receive approval from SIGA

An approval letter confirming the moisture safety of the build-up is issued.

the component calculation request

5. Start building with your SIGA system

Onsite training and online training for employees can be carried out to achieve good airtightness. This training service is free of charge.

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