Fundamentals of Design – Insulation
The use of insulation in South African buildings was not very high until five years ago, when new building regulations forced the building industry to take a closer look at it. Although producing materials that serve a sustainable purpose, insulation manufacturers could do more to incorporate sustainability into their overall operations.
Under the SANS 10400 XA building regulations from 2011, new buildings are required to have roof insulation, pipe insulation for hot water systems and geysers, and insulation for fenestration and non-mason walls. Wall insulation for brick walls is not mandatory, but could well be part of the next round of improvements expected in early 2017 for the 10400 XA standard, says Howard Harris, MD at Structatherm Projects, who has been involved in the realisation of the regulations.
Heating and cooling systems are some of the largest energy users in a building. Together with passive design principles and the use of thermal mass, insulation could offer solutions to ensure thermal comfort in a building without using electricity.
Francois Joubert, MD at Greenplan Consultants, describes buildings as living entities in an ever-changing environment as they naturally respond to the dynamic diurnal changes in air temperature, wind, rain and solar radiation. “Some of these effects can be felt almost immediately, like solar radiation or ventilation through windows, while others, like thermal mass effects, may take many hours or even days to manifest. When selecting building material types and, in particular, insulation materials, we need to consider the dynamic performance of buildings carefully,” says Joubert.
As such, he says, the pros and cons of building insulation and material types have been discussed extensively. “The general consensus is that the use and degree of insulation applied to different areas of a building should be thoroughly considered, preferably through detailed performance modelling. In warmer countries, like South Africa, one should also be careful not to over insulate buildings as this will certainly impact on cooling energy costs,” he says.
When insulating a building, the first focus should be on the roof as this is where most energy savings will be gained, especially in low rise buildings. Considering the average South African home, the roof usually takes up a significant percentage of the building surface area and so a poorly insulated roof will have a substantial influence on annual energy use and thermal comfort. Paul Marais , local architect and academic associate at Cardiff Metropolitan University, says there are three categories of insulation: one is to stop the heat from passing through, like a blanket, or bulk insulation; the other is to reflect the heat by using a reflective foil; or a combination of the two can be used.
Joubert says heat transfer to and from a roof consists of conduction, convection (air movement) and also radiation. Roof colour can play a significant role in terms of ‘insulating’ the building against radiation. “As a general rule-of-thumb one can assume that lighter colours, especially white, will perform better, absorbing less heat during the day and radiating less heat at night. Darker colours, like charcoal, which seems to be so popular in many upmarket estates, usually represent almost ideal solar absorbers and easily reach temperatures of up to 70ºC. It is obvious that starting with such extreme surface temperatures places additional and unnecessary demands on the insulation system under the roof surface,” Joubert says.
In terms of material types and installation, there are many different options available. As long as the correct thickness is used and it is installed correctly, most products should do the trick. “Different circumstances call for different solutions,” says Harris, “and roof insulation should be an integral part of the overall design. The product has to be installed so that no heat bridges are formed.” He says typically 40% of insulation performance is lost through discontinuities, like timber joists and other elements that form heat bridges in the roof. “The way to address this is to compensate by putting in greater thickness of the insulation material, or by putting in products that don’t have discontinuities, for example insulation board, where you use the board product as your ceiling and insulation in one.”
Flexible and loose fill insulation types, installed directly on ceilings, are very common in South Africa.
With these types of insulation, it is important to ensure the material covers the entire ceiling area. Joubert says fibre blanket or batt type insulators can also offer appreciable noise reduction performance. He says: “The acoustic performance of the heavier rock wool and ceramic fibre types are usually superior and these are often used in lightweight wall constructions to reduce noise transfer. When considering flexible insulation on ceilings, note that not all materials have the same aging properties.” A major drawback of the compressible, low density products, such as the fibre products, is that over time they will lose their thickness, and with that their efficiency. Harris says the cheaper products will need to be topped up or refurbished after about ten years, while the more expensive products have a much longer life. Something else to consider is fire safety, as, for instance, rock wool has far better fire resistance properties than polyurethane foam.
Building facade insulation
Insulation for the building facade, represented by walls and glazing, should preferably be considered in combination, says Joubert, as over-insulating of walls without consideration of gains and losses through windows doesn’t make sense. Brick walls do not require insulation by law in South Africa, although Marais says he perceives a massive re-encouragement in the industry for the use of cavity walls.
“In general, solid concrete and brick walls have relatively good thermal conduction values and can often benefit from a layer of rigid thermal insulation, like expanded polystyrene. Lightweight wall construction types can be designed to have good thermal resistance (R-) values. Unlike heavyweight walls, these wall types typically have low thermal mass. Thermal mass of heavy weight wall constructions can be used effectively to control thermal energy movement through these wall types but detailed analyses are required to ensure the desired outcome. If not designed correctly, heavy walls may continue to radiate heat late into hot summer nights and consume large amounts of energy to heat up on cold winter mornings,” says Joubert.
The main glazing concern in South Africa is usually to prevent excessive solar heat from entering buildings, says Joubert. In particular double glazing, when exposed to direct sunlight, can easily lead to overheating of the building interior. Therefore, single glazing on north facing windows often works better here, in terms of both thermal comfort and cooling energy requirements, as the heat can escape from the building more readily. South facing rooms usually benefit from double glazing as this side will be exposed to direct sunlight for very limited periods of the year and double glazing will provide good insulation in colder weather. Implementing passive design principles, like louvres, and installing low-emissivity glazing could reduce solar heat on east, west and northern windows. This type of treated glass limits the amount of solar energy entering a building.
Traditional, natural, and new innovations
Although mainstream insulation products, derived from plastics or mineral wool, may not be sustainable in themselves, they serve a purpose in terms of sustainable building. However, there are other suitable insulation solutions available. According to Marais, a living roof, for instance, is the most effective form of insulation. Soil and plants form a very good layer of insulation, as long as your roof is strong enough to carry the load. “The additional cooling effect of the transpiration of the plants provides a secondary cooling function, and plants have evolved to reflect heat,” he says.
Other natural solutions include straw, cork, cotton, cellulose fibre, wool, hemp and kenaf fibres. “Some hope that these natural materials will come to the fore in future, but we have good solutions in place at the moment, especially the recycled ones. [These include options to use cellulose fibre from paper sources and also recycled PET and other recycled plastics] They may well be more environmentally friendly than growing crops for the purpose,” explains Harris. “I think the trend is that the insulation manufacturers will need to address complete overall sustainability, more than they do at the moment, and that insulation products that incorporate recycling are going to have an advantage.”
By Femke van Zandvoort