The thermal insulation of a building can be done in two ways: as static or dynamic insulation. There are even materials that reflect thermal radiation, thereby affecting the heat loss of a building and which should be considered as representative of a particular method of insulation of their own.

Static and dynamic insulation

In static insulation the insulation value of static air is used. The principle requires the use of a porous material with the greatest possible number of air pockets. These have to be so small that no air can move within them.

In dynamic insulation air is drawn through a similar porous insulation material. When the fresh air is led from outside through the surface of the wall, rather than through small ventilation ducts, it picks up heat loss flowing out of the building. Besides achieving a prewarmed fresh-air flow into the building, the heat loss through the surfaces is reduced to a minimum.

The optimal materials for such a wall should have an open structure with pores across the whole width, plus good heat exchange properties. A high thermal capacity is also an advantage, so that sudden changes in the outside temperature are evened out. Dynamic insulation is still being introduced into construction and has been used in only a few buildings.

The technical demands of an insulating material (excluding the reflective layer) are usually as follows:

1. High thermal insulation properties

2. Stability and long life span

3. Fire resistance

4. Lack of odour

5. Low chemical activity

6. Ability to cope with moisture

7. Good thermal exchange properties (for dynamic insulation)

The thermal insulation property for static insulation is usually called lambda ( ) and can be measured with special equipment:

= W/(m°k)

Mineral wool has a lambda value of 0.04, while a woodwool slab has a value of about 0.08. This means that a double thickness piece of woodwool gives the same
insulation value as a single thickness of mineral wool.

Calculating the value of insulation

The example quoted above comparing the thermal insulation of two materials is the traditional method of calculation, making the assumption that there is a linear relationship between the lambda value and insulation/heat loss. There are limitations to the lambda values. They give no indication of the material’s structure, moisture properties or reaction to draughts (which every wall has to a certain extent). It takes no notice of the material’s thermal capacity. In buildings that are permanently heated, as in hospitals for example, there is a great energy-saving potential and improved comfort    if materials with high thermal capacity are used. The same is true for buildings where there can be wide and rapid changes in the inside temperature, for example, when opening the windows. The thermal insulation value of a material is reduced when damp. In frozen materials the ice conducts warmth three to four times better than water. This is important if using hygroscopic materials. Even if such materials seldom freeze, a lower insulation value is assumed during spring because of the higher moisture content.

Age can also affect insulation value. Certain products have shown a tendency to compress through the absorption of moisture and/or under their own weight, while others have shrunk  (mainly foam plastics). The thickness of the layers of insulation needs to be appropriate for the local climate. Too much insulation can cause low temperatures and thereby hinder drying in the outer layers, which can lead to fungus developing in the insulation or adjoining  materials. Insulation materials are sold either as loose fill, solid boards or thick matting. The latter two can result in a damaged layer of insulation, because temperature or moisture content changes can cause dimensional changes. This is especially the case with solid boards, which need to be mounted as an unbroken surface on the structure and not within it. Loose fill insulation is good for filling all the spaces around the structure, but it can settle after a time. The critical factors are the weight and moisture content of the insulation. The disadvantages of hygroscopic materials become apparent here because they take up more moisture and become heavier. Settling can be compensated for by using elastic materials which have a certain ‘suspension’ combined with adequate compression. Structures with hygroscopic loose fill as insulation need topping up during the building’s life span.
Thermal insulation materials usually occupy large volumes, but they are light and seldom take up more than about 2 per cent of the building’s total weight. Many insulation materials do, however, have a high primary energy use and use of material resources, and produce serious environmental pollution during manufacture, and use, and even as waste. The waste must often be specially treated. Only in exceptional circumstances is it possible to recycle or re-use insulation materials.

Credit to: “The Ecology of Building Materials” by Bjorn Berge

Translated from Norwegian by Filip Henley With Howard Liddell