By adding hydrochloric acid to a solution of potassium silicate (water-glass), silicic acid is formed in a jelly type mass, 'aerogel'. It is used as transparent thermal insulation, usually between two sheets of glass. The sun's radiant energy penetrates the gel, while it prevents the loss of heat through convection and loss of long-wave radiation (Figure 14.8).

A transparent layer of insulation on the south wall of a brick building can provide much of the heat it requires. Heavy brickwork will even out the temperature and prevent overheating or too much cooling.

Silica aerogel is at present not in general use, and has disadvantages: it does not tolerate water and has a tendency to crumble. But it has few negative consequences in relation to the environment or resource extraction.


Expanded granules of recycled glass used as thermal Insulation and capillary breaking layer In foundation. Gala Tjome, 2005.


Expanded granules of recycled glass used as thermal Insulation and capillary breaking layer In foundation. Gala Tjome, 2005.

Foamglass is usually produced by adding carbon to melted glass and heating it to 700-800 °C until it starts bubbling. A small amount (0.4%) of manganese dioxide is also added.

Foamglass is usually made in the form of slabs. These are gas and watertight with high thermal insulation properties, and they are mainly used as insulation underneath ground floors and in flat roofs, usually cemented together with a bituminous mass. The basic material is most often new glass, but up to 38% of recycled glass can be added.

A rougher loose-fill product of 95% recycled glass consists of expanded glass granules (Figure 14.9). The product absorbs little moisture and is well-suited to damp conditions. It is therefore much used as a thermal insulation and capillary break in slab-on-grade foundations. It is also used as a lightweight ingredient in mortars and concrete blocks.

Products based on new glass require a lot of energy and cause environmental emissions (see Chapter 6). Products manufactured from recycled glass are environmentally better, despite the still high temperatures needed when re-melting the glass.

These products present no problems within buildings. One possible exception is the use of bitumen as a jointing material. Glass products have no moisture regulating properties. Extensive use of them in a building can lead to an indoor climate with rapid air moisture changes and, in certain cases, the possibility of high levels of humidity in adjacent materials.

14.3.8 Mineral wool

Cleaned slabs and granules can be re-used in buildings. There is no way of recycling foamglass by re-melting. But it is practically inert and can be crushed and used as an insulating layer in road building. Components containing bitumen must be disposed of at special tips.

Mineral wool has excellent thermal insulation properties. In addition, it is a reasonably good acoustic insulator. It is normally produced in mats, but occasionally also as loose fill.

Glasswool is made from quartz sand, soda, dolomite, lime and up to 65% recycled glass. Up to 9% borax is added to reduce the energy required in production. The mass is melted and drawn out into thin fibres in a powerful oil burner. Glue is then added to the loose wool and heated to form sheets or matting in a kiln. Phenol-formaldehyde glue (PF) is commonly used in a proportion of about 5.5% of the product's weight. To give a high thermal insulation value the diameter of the fibre should be as small as possible. The usual size is about 5 mm.

Rockwool is produced in approximately the same way as glasswool, starting with a mixture of coke, diabase and limestone. Basalt and olivine can also be used. The quantity of phenol-formaldehyde glue is lower, about 2% by weight. The diameter of the fibres varies from 1 to 10 mm.

Both types of mineral wool have silicone or aliphatic mineral oils added up to 1% by weight as water repellents and dust bonding agents.

When used as insulation, both glasswool and rockwool need a tight vapour membrane of aluminium foil or plastic sheeting, partly to avoid dust entering the interior and partly because the materials cannot regulate moisture well. Research has shown that the use of rockwool and, to a certain extent, glasswool can increase mould growth and damage caused by damp on the timber framework, unlike the more hygroscopic insulating materials such as cellulose fibre (Paajanen etal., 1994).

Man made mineral fibres(MMMF) such as in rockwool and glass-wool are suspected of being carcinogenic. The risk relates both to the physical thickness and length of the fibres as well as to their chemical composition. In recent years many manufacturers have improved their products to eliminate this risk. However, the fibres are still strong irritants both to skin and respiratory organs and can lead to chronic bronchitis. Workers both in the factories and on building sites are often exposed. Acoustic panels with mineral wool are the most common source of mineral wool fibres in the indoor climate (Bakke, 1992).

It has been shown that humidity in mineral wool can lead to the emission of vapours that can later enter the building. The problem is more acute when the wall becomes warm; for example, through solar radiation. The gases released are aromatic and aliphatic hydrocarbons. Formaldehyde, which is carcinogenic, can also be emitted from glass-wool products. All of these gases irritate the ears, nose and throat (Gustafsson, 1990). Additives in mineral wool that contain nitrogen are very susceptible to mould. The amount of mould in an infected material can be 1000 to 50 000 times the amount in uninfected material (Bakke, 1992).

Raw materials are abundant for the main constituents of glasswool and rockwool. The production of glasswool occurs in relatively closed processes. The emissions from production are little and limited to formaldehyde and dust in addition to energy pollution. Phenol, ammonia, hydrogen cyanide, formaldehyde and dust are released during the production of rockwool, and large amounts of waste are produced. Phenol can leach out of rockwool waste. Unpolluted waste can be compressed and recycled for the manufacture of new mineral wool, although the industry is so centralized that this form of recycling is economically unrealistic.

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