Static And Dynamic Insulation

The thermal insulation of a building can be done in two ways: as static or dynamic insulation.

In static insulation the insulation value of still air is used.The principle requires a porous material with the greatest possible number of small air pockets. And these have to be so small that the air does not move inside them. Most insulation materials are therefore foamed or fluffy materials that are mostly air.

Dynamic insulation is a little-used approach. Here air is deliberately drawn through a porous insulation material.When cold, fresh air is led from outsidethrough the wall, rather than through ventilation ducts, it picks up heat loss lowing out of the building (Figure 14.1). Besides achieving a preheated fresh air flow into the building, the outward 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 intro-ducedinto constructionandhas beenused inonlyafewbuildings (Figures14.2 and14.3).

14.1

The principle of dynamic insulation. Source: Torgny Thoren.

14.1

The principle of dynamic insulation. Source: Torgny Thoren.

Ageing can also affect the insulation value. Certain products have shown a tendency to compress through the absorption of moisture and/or under their own weight, whilst others may shrink (mainly foamed plastics). The thickness of the layers of insulation needs to be appropriate for the local climate. Some humidity will always get into a wall, and too much insulation can cause the temperature to be too low in the outer part of the wall; thereby reducing any drying out. This can lead to fungus developing in the insulation or adjoining materials. This is primarily an issue in the new low energy buildings, such as the extremely well insulated 'passive houses'.

Insulation materials are manufactured as loose fill, solid boards or thick matting. The latter two can occasionally result in a ruptured layer of insulation since temperature or moisture content changes can cause

14.3

Barn with hay performing as dynamic insulation in the roof of the stable through the winter period. At the same time the hay is used as fodder and by the end of spring everything is eaten up. And in autumn fresh hay is refilled. Lillehammer (Norway), Gaia Arkitekter, 1995.

14.3

Barn with hay performing as dynamic insulation in the roof of the stable through the winter period. At the same time the hay is used as fodder and by the end of spring everything is eaten up. And in autumn fresh hay is refilled. Lillehammer (Norway), Gaia Arkitekter, 1995.

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 density and moisture content of the insulation. The disadvantages of hygroscopic materials become apparent here because they take up more moisture and become heavier. Settling, which is seldom more than a small amount, can be compensated for by using elastic materials which have a certain 'suspension' combined with adequate compression. Structures with hygroscopic loose fill as insulation may need topping up during the building's lifespan.

Thermal insulation materials usually occupy large volumes, but they are light and seldom take up more than about 2% of the building's total weight. Many of the most common commercial insulation products are derived from fossil oils. Plant-based materials are, however, now attracting more attention. Many of the commonly used insulation products have quite high embodied energy and associated greenhouse gas emissions. In addition, foamed plastics are often made using hydrofluorocarbons (HFCs) as foaming agents, and these are very powerful greenhouse gases. There is also a variety of environmentally problematic additives in many insulation materials, including synthetic glues, flame retardants and fungicides. For these reasons the waste materials must often be specially handled. Only in exceptional circumstances is it possible to recycle or re-use insulation materials.

Thermal stabilization

During the winter months, a building in heavy materials will store heat from warm rooms and return it when the room temperature goes down. In this way heavy materials stabilize the indoor temperature, and in summer maintain a cooling effect. This effect increases comfort and can reduce energy needs significantly; while at the same time

Table 14.2 Thermal storage capacity and thermal diffusivity of selected materials.

Material

Specific weight [kg/m3]

Thermal storage capacity [kJ/m3 K]

Thermal diffusivity [1CT4 m2/h]

Concrete

2400

2100

24

Brick

1500

1260

22

Massive wood

500

1150

4

Wood fibre board

180

415

4

Mineral wool

16

12

48

The thermal storage capacity decribes the maximum heat storing capacity of the material, not the actual speed of the uptake. Materials with both high thermal storage capacity and high thermal diffusivity will quickly respond to changes in temperature and therefore effectively buffer overheating in the interior. Materials with low thermal diffusivity will ensure that the flow of warmth from the exterior to the interior on a hot summer day is adequately dampened, see Figure 14.4.

The thermal storage capacity decribes the maximum heat storing capacity of the material, not the actual speed of the uptake. Materials with both high thermal storage capacity and high thermal diffusivity will quickly respond to changes in temperature and therefore effectively buffer overheating in the interior. Materials with low thermal diffusivity will ensure that the flow of warmth from the exterior to the interior on a hot summer day is adequately dampened, see Figure 14.4.

14.4

The variation of temperature during 24 hours on a hot summer day with a wall insulated with wood fibre boards, see Table 14.2. The time lag for the outside heat to penetrate the wall is substantial and the temperature will not reach a maximum before midnight. After that the heat can easily be exchanged with cold night air through conventional ventilation. Source: Gutex.

14.4

The variation of temperature during 24 hours on a hot summer day with a wall insulated with wood fibre boards, see Table 14.2. The time lag for the outside heat to penetrate the wall is substantial and the temperature will not reach a maximum before midnight. After that the heat can easily be exchanged with cold night air through conventional ventilation. Source: Gutex.

depending on the building's mode of heating, function, user habits and the local climatic conditions (see Figure 13.7).

The thermal stabilizing properties depend first of all on the materials. The key factors are specific weight, specific heat capacity and specific thermal conductivity. More practical terms are thermal storage capacity (specific weight x specific heat capacity) and thermal diffusivity (thermal conductivity/thermal storage capacity) (see Table 14.1 and Figure 14.4). In general, the outer layer of about 10-20 cm can be activated during a 24 hour cyclus. Location of the heat storing parts is also important. Interior walls, ceilings and the inside surface of the outer walls are most useful. The heat-storing surfaces will act less effectively if obstructed by wallpaper, hangings or large furniture.

There have been efforts to increase the heat storing capacity of wall materials by adding phase change materials (PCM) (Silva, 2007; Fossdal, 1995). These are usually fairly simple salt hydrates and react exceptionally quick to temperature changes.

Heat-reflecting materials

By mounting a material that has a low reflectivity for short-wave solar radiation on a building's facade, solar energy can be captured very efficiently, whilst a sheet of highly reflective material on the inside of the wall will reduce heat loss outwards. This is especially utilized in modern window technology. They let in a maximum of heat, but only let a minimum escape, and they can be used in the opposite way in hot climates (Table 14.3).

Also commercially available are reflecting foils, sometimes made up of several layers, which can be used in walls or roof spaces. There must be an air gap on either side, if small, and the reflecting surfaces must be kept free of dust. The reflecting foil eliminates virtually all transfer of radiant heat; however, heat loss by conduction still requires a layer of normal static insulation.

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