Overheating of buildings

Another reason for the disjunction between designed and actual building performance in terms of energy use is the increased use of airconditioning and other cooling technologies. This is now becoming a major cause of energy use in the UK, and is mentioned in the new part L regulations, though only as an issue which must be considered.

A few years ago the BRE forecast that with global warming the use of air conditioning could one day be as much as 25% of the energy use in some buildings. The mass sales of cheap air conditioning units in DIY warehouses last summer are an indication of this already happening. But is it just because our climate is getting hotter that means we require air conditioning? Actually this is a minor factor. It is much more to do with how we live and work, the way we use buildings and the kind of buildings we design - for example the increasing use of rooms in roofs, of conservatories, and of glass and steel office buildings.

There are 5 main factors which affect buildings as regards over heating. These are

Location
Transparent parts of the building
Human gains (ie lights appliances cooking)
Admittance (absorption of heat by the thermal mass on the inside of the building shell)
Non transparent parts of the building shell

All of these except the human gains are related to the building shell - it's orientation, design and the choice of materials. Overheating can be avoided in much hotter climates than ours, and to resort to airconditioning is a most irresponsible sort of laziness.

Performance of Insulation Materials:

There are a number of other issues related to the nature of the building materials which will adversely affect the short and long term predicted energy performance of the building shell. These are mainly to do with the nature of the materials themselves, but also relate to building design and so called cost savings:

Loft insulation: 70% of the insulation sold in the UK is mineral or glass fibre and most of it is designed for lofts. Because of the intense price competition in this area most common insulations have very low density, as this means less material and therefore less cost. There are two problems however with this. Firstly the insulation when over 50mm thick starts to compress. With very low density insulation, 250mm of insulation will compress by up to 25% and over : ie you put in 250mm and you get less than 200mm in practice. Secondly low density insulation is very open, so if the insulation is in a vented space, the cold air passing over it will cause convected heat loss, thereby negating the thermal resistance of the top 25mm to 50mm of the insulation. So the 250mm that is bought has the effect of less than 150mm in practice. Loft insulation is also often used in timber frame walls, because of its cheapness. Because of its lack of density it has to be supported (by staples or other means). This is hardly ever done correctly and the material will sag or be gappy in most cases. It is the wrong kind of material for this application.

Insulations degrade over time. For example the fibres of glass wool are very brittle. Any one who installed 100mm of glass wool in his loft 20 years ago will wonder where most of it has gone. The fibres have collapsed under the weight of the insulation and other debris on top. Another material which degrades is any gas blown insulation (such as Polyisocyanate or Polyurethane insulation). In the past this was a more rapid process, and new gases and also a foil coating were introduced to reduce this process. However it still remains a question as to what the U value of gas blown insulations will be in 30 or 60 years time. Only air based insulations with sufficient strength and density of fibres will definitely continue to perform as designed.

Materials when wet lose thermal resistance. Our site practice and materials mean that materials often get wet in application. If mineral/glass wool in cavity construction gets wet in application, it will have almost no thermal resistance until it dries out. If it is surrounded by dense vapour resistant materials with no ventilation, then this can be several years. Further wetting is also possible through rain penetration.

Some forms of insulating blockwork also can lose their thermal resistance if wet in construction, with similarly long drying periods.

For this reason many houses fail to perform in their first few years, as well as being potentially unhealthy because of excess moisture levels in fabric.