John Barnard

CSIR Research Confirms the Superior Energy Efficiency of Light Steel Frame Building.

John Barnard, Director SASFA

A recent research project by the Built Environment Division of the CSIR confirmed that a light steel frame (LSF) dwelling, built to SANS 517, will result in significant savings of  electricity used for heating and cooling of the building, compared with a conventionally built heavy masonry building.

In order to obtain an objective prediction of the thermal performance of a light steel frame dwelling compared with a masonry building in the different South African climate zones, SASFA approached the CSIR to carry out the analyses.

A typical 120 m² single storey house was used for the comparison. The LSF and the masonry houses were specified to be geometrically identical, with identical orientation. The LSF house complies in all respects to SANS 517 Light Steel Frame Building. A typical masonry house with double leaf external clay brick walls, without any insulation in the walls and ceilings, was used as the base case. The effect of adding 40mm insulation in the ceilings was also evaluated.

The Built Environment Division of the CSIR decided to use the Ecotect ^TM V 5.6 software to carry out the 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, i.e. 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 firstly aimed at determining the number of hours of uncomfortably high or low temperatures in each of the buildings. The buildings were considered to be naturally ventilated and the thermal comfort temperature range for naturally ventilated buildings in Pretoria is 17.8⁰C – 28.3⁰C. The adaptive model was used in calculating the levels of thermal comfort in the two houses.

The electricity needed for heating and cooling each of the buildings to thermal comfort levels (ranging from 20 ^o C to 24 ^o C, as recommended by SANS 204) was also determined.

The major differences between the two types of building are the thermal insulation and the thermal mass. The walls in a LSF building have better 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 could be too high or too low for comfort. It should be noted that the concrete floor in both building types contribute to the thermal mass of the building.

Findings:

Results indicate that the LSF house will be warmer than a base case masonry building in winter, as well as in summer. If the hours of discomfort due to too high and too low temperatures are added together, the LSF house performs better than the masonry alternative in all locations but Durban.

As example, the indoor temperature of the LSFB was within the thermal comfort range for 74% of the time in Pretoria’s climate, compared with 71% for the masonry base case – a relatively small advantage.

However, the analyses indicate that electricity required to heat the base case brick building to comfort levels will on average be double that required for the LSF building, ranging from 89% more in Pretoria, to 112% more in Bloemfontein. If cooling to comfortable temperatures is required, it will take on average 3 times more electricity to cool the masonry building down to thermal comfort levels compared with a LSFB.

This enormous increase in the amount of electricity required to heat or cool the internal spaces of a masonry building can be ascribed to the thermal mass of the walls – 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 and a delay in the change of the internal temperature. The inverse happens when cooling, when the heavy masonry walls have to be cooled down together with the air inside the building.

When the ceiling of the brick building is insulated, its performance improves. The graph below compares the hours of thermal discomfort in a LSFB with that in two alternative masonry buildings:

• Base case: no ceiling or wall insulation
• Brick B: 40mm thick glasswool insulation in ceilings

While the LSFB will result in more hours of discomfort without heating and cooling in Pretoria’s summer climate than the masonry alternatives, occupants will have less discomfort in winter, and less discomfort in total.

As is shown below, the LSFB will require less than half the electricity to heat and cool to thermal comfort levels than the Brick Base case, and still notably less than the ceiling insulated masonry alternative.

Savings in electricity for heating only:

LSF compared with           Brick Base Case:                32.7 GJ/yr (47% saving)

Brick B                        :          13.5 GJ/yr (20% saving)

Conclusion:

The CSIR’s comparative thermal analyses indicate that LSFB offers improved energy efficiency compared with conventional masonry buildings – this means significant savings (between 20% and 47%, depending on the degree of insulation of the masonry building) of electricity required for heating of residential buildings in Pretoria, as example.

These findings are generally supported by testimonials received from occupants of LSF houses. In a recent survey carried out by SASFA, 57% of respondents reported that their LSF house was cooler in summer, while 71% said it was warmer in winter.

The CSIR research also indicated specific areas where further gains in energy efficiency can be captured for LSFB, and 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.