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Thermal insulation of a passive house

Importance and rules of thermal insulation in a passive building

The thermal insulation of a passive house but also of every building is a key element to be considered. Indeed, according to ADEME, the majority of heat losses of an uninsulated building are coming through:

  • Attics and roofs (30%)
  • Walls (25%)
  • Windows and glass (between 10 and 15%)
  • Ground (between 7 and 10%).

When talking about the envelope of a building we consider all the elements that separate the interior from the exterior.

The first rule to minimize heat losses is to avoid thermal bridges as much as possible. To do so, the insulating layer has to be continuous. Moreover, it is essential to consider the heat transfer coefficient U of each building element. 

In order to meet the Passive House standard requirements, a maximum U value has been defined for each element. Obviously, the values will depend of the type of climate. The figure opposite shows the values recommended for a Central European climate, for instance Germany.

For warmer climates such as Lisbon, the value of U for exterior walls can reach 0.50 W/m²K (0.15 W/m²K for colder climates such as Germany).


Choosing an insulation material for my passive house

The choice of an insulation material is made according the following criteria:

1. Thermal performance 

Thermal conductivity

  • Insulating power of a material
  • Ability to transmit heat by conduction
  • Does not depend on the thickness of the material contrary to thermal resistance R (ability of a material to resist heat transfer)
  • For instance, a 30 cm thick brick wall has the same thermal resistance as 1 cm of glass wool.

Thermal phase shift

  • Time required for heat to penetrate the material.
  • For instance a glass wool has a low thermal phase shift, indeed heat penetrates in 3-4 hours. To the contrary cellulose wadding has a long thermal phase shift of 11 hours.

Heat capacity 

  • Ability to store heat in relation to its volume.
  • The denser a material is, the more heat conductive it is and the higher its thermal capacity. To the contrary a lightweight material is more insulating and has a lower thermal capacity.
  • Bioclimatic architecture recommends to use light materials for thermal insulation and heavy materials inside the insulation. As a result it will facilitate the regulation of temperature changes.

Thermal effusivity 

  • Ability to exchange thermal energy with its environment.
  • For example, the dense marble has an ability to store heat. If its temperature is lower than the one from our hand, it will absorb heat if touched and cool down our hand because of its high effusivity.
  • To the contrary the cork is a light material, and if touched, it feels warm because it can’t absorb our heat.

2. Humidity control

3. Non-flammable or fire-protected

4. Resistance to humidity and rodents

5. Health risks

6. Prices

7. Durability

8. Acoustic performance

Different type of insulation materials

The ACERMI (Association for the Certification of Insulation Materials) platform provides a list of certified materials. It can be used as a starting point for choosing insulation materials.


Mineral insulation materials

They are made from natural inorganic materials, they are generally fire-resistant and are in the form of panels. The most commonly used are glass wool and rock wool.


Organic insulation materials

They are produced from plant or animal materials. Among the most commonly used are cork, wood fibre, cellulose wadding, hemp, and straw.


Synthetic and new generation insulation materials 

Most of them come from fossil resources (e. g. expanded polystyrene).

Thermal conductivity of some materials – source : DIN 4108-4, EN 12524