Changing the temperature of the air in your home requires a lot of energy. You may be trying to make it hotter than the outside temperature during winter, or conversely cool the house down during summer. In either case, if the house is not well insulated any gains are quickly lost via heat transfer with the outside air through the walls, windows, the ceiling and the floor. This becomes a bit like trying to raise the level of water in a colander. If you don’t pour water into it fast enough, it all leaks out before the level can rise. Clearly you will use up a lot of water trying to keep the level in the colander constant and even more to increase it. The same goes for energy required to heat/cool a poorly insulated home. A well insulated home will thus save a lot of power that would otherwise have been necessary to maintain a comfortable temperature inside.
Many homes in South Africa are poorly insulated due to a combination of factors such as:
- Our very temperate climate in which most homes are only really uncomfortable for one or two months in the year.
- Building codes and builders have not emphasised insulation in domestic homes in the recent past.
- Our historically cheap electricity prices have allowed many to afford the large amounts of electricity needed for heating/cooling.
There are three forms of heat transfer:
- Radiation heat transfer is often the most effective because it is instantaneous (you feel the heat immediately). Radiant heat can travel over long distances from the heater (think about the sun’s heat which reaches us via radiation), and is unaffected by wind making it useful out of doors. Radiation however requires direct line of site between the heater and what is being heated. Insulation against radiant heat transfer is achieved by using a highly reflective surface (e.g. shiny aluminium foil). Radiant heat absorption is maximised by painting the surface matt black (i.e. make it completely unreflective).
- Convection is the transfer of heat via the circulation of a fluid or gas. Blowing hot air down a duct or piping hot water through pipes is an example of convective heat transfer. There is some delay between the start of the heater and when heat is felt in different parts of the house or room. Convection also works against you when there are air gaps where hot air can flow out of the home and/or cold air flow in. Convection heat transfer is minimised by preventing gases or fluids from circulating (many materials used for insulation do this by trapping air into many tiny bubbles which prevents any convective heat transfer between them).
- Conduction heat transfer occurs between things that physically touch each other. Insulation against conduction is important with regard to reducing heat lost through solid barriers such as the ceiling or walls. Good insulators against conduction are polystyrene foam sheeting, glass fiber blankets or polyurethane foam.
All insulation is designed to reduce one or more of these heat transfer mechanisms. Understanding how and when they are occur helps select the most appropriate solution. The building regulations specify which types of insulation are acceptable, particularly with respect to their fire hazard ratings (some older types of insulation are flammable), please consult your supplier or installer in this regard.
Ceiling and Roof Insulation
There are two main forms of heat transfer occurring in your roof. Firstly the ceiling panels allow heat to be conducted out of the house in winter when the inside is warmer than the outside and the reverse occurs in summer. Secondly the roof tiles or corrugated iron roof sheeting heat up from the sun during the day and then transfer that heat to the ceiling boards by radiation.
Some form of insulation designed to reduce conduction such as glass fibre blanket, cellulose, polystyrene sheet or foam should be laid on top of the ceiling boards to reduce conduction through the ceiling.
To reduce radiation from the roof tiles, some form of aluminiumised foil sheeting should be laid directly underneath the tiles with the shiny side facing upwards. Ideally this should be done during construction, but it can be retro fitted by attaching it to the underside of the roof brandering.
Both these types of insulation can be installed relatively easily into most homes, provided one has access to the roof space.
The older building codes used to include cavity walls as the standard way of building the external walls of a house. Cavity walls have an air gap between the inner and outer course of bricks. The air gap stops heat being transferred via conduction through the two brick courses. The air gap acts as a partial insulator because heat can only flow via convection across the gap. Convection is far less effective as a heat transfer mechanism. If necessary, one can even minimise convection by injecting polyurethane foam (as a retro fit) into the air gap, or alternatively polystyrene sheeting can be inserted into the gap during construction. Many older houses still have cavity walls.
Unfortunately, cavity walls have long since been discontinued by local builders as standard practise. No form of insulation is currently added to the walls of modern homes, unless specifically requested by the architect or owner. One should consider this if one is currently building a home in an area that experiences cold winters.
Heat flows through a single pane of glass in the following ways:
- Radiant heat is both entering and leaving the room via the window. During the day sunshine passing through the window will heat up all surfaces it lands on. During the evening warmer surfaces inside the house will radiate their heat outside if the curtains are not drawn.
- Heat will be lost via conduction through the glass pane to the air on the outside at night and the reverse will occur during the day.
To reduce heat lost via windows one should use thick curtains and keep them closed at night.
Failing that, one can apply specialised films to the glass which lowers its transparency and/or emissivity to the wave lengths of radiation that transfer heat. In colder regions such as Europe and the US, one can purchase glass that has these characteristics, but unfortunately it doesn’t seem to be readily available here in SA.
Double glazing (two panes of glass with an air gap between them) reduces heat lost via conduction quite effectively. Double glazed windows usually need to be installed during construction. Retrofitting them is usually quite costly and can require breaking into the wall to remove the previous frames. Thick curtains are much easier and cheaper!
The earth can be considered to be a giant heat sink. Heat is constantly being lost by conduction down into the earth through the floor. Anyone who has spent a night sleeping on the floor or camping will tell you that it’s more important to place insulation beneath one than above.
In colder regions of the world, it is standard practice to place a layer of insulation beneath the floor slab to reduce heat loss through the floor, but not unfortunately in South Africa. Clearly this can only be done during the construction phase.
Carpets are a practical option in South African homes of insulting the floor of your home. Laminated wooden flooring is also usually installed with a layer of insulation beneath it.