SASFA, the Southern African Light Steel Frame Building Association, recently commissioned the Built Environment Division of the CSIR, the Council for Scientific and Industrial Research, to conduct a comparative thermal performance analysis for light steel frame housing and masonry housing. The research findings point to the energy efficiency of the light steel frame (LSF) house in terms of a lower need for electricity to power mechanical heating and cooling – to keep the internal temperature within a prescribed comfort range.
SASFA approached the CSIR to carry out the research in order to obtain an objective prediction of the thermal performance of the two different types of houses in the different South African climate zones.
The research looked at a typical 120m² single-storey house comparing a LSF house and a masonry house, both specified to be geometrically identical, with identical orientation. The LSF house complies in all respects to the South African standard SANS 517: Light Steel Frame Building. A masonry house with double leaf external clay brick walls, without any insulation in the walls or ceilings, was used for the base case comparison. The effects of adding (i) 40mm insulation in the ceilings (Brick B), and (ii) similar ceiling insulation as used in LSF buildings (140mm) and 50mm insulation in the external walls (Brick A), were also analysed.
The Built Environment Division used the EcotectTM V 5.6 software to carry out the predictive computer analyses. In order to eliminate the effect of user input data which could influence the outcome, it was decided to use a passive analysis process, that is, without making assumptions regarding the occupancy and usage patterns of the house. The heating effect of lights and appliances was also not taken into account.
The analyses were aimed at determining the number of hours of uncomfortably high or low temperatures in each of the buildings. The houses were considered to be naturally ventilated. The thermal comfort temperature range for naturally ventilated buildings in Pretoria is 17.80C to 28.30C and the adaptive model of thermal comfort was used in calculating the levels of thermal comfort in the two houses.
The electricity needed for heating and cooling each of the houses to thermal comfort levels – ranging from 20oC to 24oC, as recommended by SANS 204 – was also determined.
The major differences between the two types of buildings are the thermal insulation and the thermal mass. The walls in a LSF building have a greater level of thermal insulation but lower thermal mass than masonry buildings. The higher thermal mass in the walls of brick buildings reduces the diurnal internal temperature swings towards the average temperature, which may be too high or too low for comfort.
The results indicate that the LSF house will be warmer than the base case masonry house in summer and in winter. If the hours of discomfort due to high and low temperatures are added together, the LSF house performs somewhat better than the masonry alternative in all locations except Durban, although the advantage is small.
For example, in Pretoria’s climate, the indoor temperature of the LSF house was within the thermal comfort range for 74% of the time compared with 71% for the base case masonry house.
However, assuming that the occupants of the houses will make use of electrically powered heating and cooling systems to maintain comfortable indoor temperatures, the analyses indicate that the electricity required to heat the base case brick house to comfortable levels would on average be double that required for the LSF house, ranging from 89% more in Pretoria to 112% more in Bloemfontein. If cooling to comfortable temperatures is required, it would take on average three times more electricity to cool the base case brick house down to thermal comfort levels compared to the LSF house.
The difference in the amount of electricity required to heat or cool the internal spaces can be ascribed to the thermal mass of the masonry building: apart from having to heat the air inside the building, the walls of the masonry building absorb some of the heat, resulting in additional energy consumption. The inverse happens when cooling, when the heavy masonry walls have to be cooled down together with the air inside the building.
When the brick house is insulated, its performance improves. However, in Pretoria’s summer climate, for example, while the LSF building will result in more hours of discomfort without heating and cooling than the masonry alternatives, the occupants will have less discomfort in winter, and less discomfort in total.
The CSIR’s comparative thermal performance analyses indicate that LSF housing offers greater energy efficiency compared to conventional masonry housing – which means significant savings (between 20% and 90%) of electricity required particularly in respect of heating of residential buildings.
The CSIR research also indicated specific areas where further gains in energy efficiency can be captured for LSF building and SASFA reports that these will be investigated and implemented in the LSF building methodology.
Reference: A predictive comparative thermal performance analysis for light steel frame and masonry residential buildings, T Kumirai and Dr D Conradie, CSIR.