University of Pretoria : Thermal performance of walling in SA

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A thermal performance comparison between six walling construction materials frequently used in South Africa

When selecting building materials, developers and architects of green buildings take into account the embodied energy of their raw materials. While this is an excellent start, in reality energy use over the life of the building dwarfs energy used during production. Operational energy use is an expensive, long-term cost for the property owner and a drain on South Africa’s limited resources.

Annual Operational Energy is the sum of all heating, cooling and ventilation electricity costs accumulated over all four seasons in one year.

This research is of particular relevance for affordable housing, as the residents do not have the money for high electricity bills. Therefore ethical housing developers should look beyond cost-cutting during construction, to take into account the long term expense of operation and maintenance.

Methodology of the study

The size and use of a building influences its thermal performance. The design models applied in this study were used in prior research by the CSIR and the Department of Mineral and Energy Affairs. Best-practice construction methods were assumed, as required by SANS 10400 Part XA: Energy usage in Building and SANS204: Energy efficiency in building.

Three building types were analysed:

  • a large 2000m2 commercial building that is unoccupied at night and on weekends.
  • a middle-income residential home of 130 m2, and
  • a low-income home of 40m2 (with heating fuelled by coal or paraffin)

Variables such as floor, roof, windows, doors and occupancy patterns were kept constant.

Wall Types

Six wall types were analysed

  1. Double (internal and external) clay brick solid wall (nominally 220 thick, plastered)
  2. Double clay brick cavity wall with air cavity (nominally 270 mm thick with an uninsulated 50mm air cavity)
  3. Insulated double clay brick wall (nominally 280mm thick, with 30mm extruded polystyrene insulation in the 50mm cavity)
  4. 140mm hollow core concrete block (150mm thick with a single external layer of plaster, and bagged internally)
  5. Light steel frame, externally clad with 9mm fibre cement board to SANS 517 (nominally 145 mm thick with 0.2mm polymer vapour membrane, 20mm orientated strand board and 0.8mm steel studs. Internal wall of 15mm gypsum board with 75/100mm fibre sound insulation.)
  6. Timber frame to SANS 10 082 clad with external ship-lapped tiles or weatherboard (nominally 145mm thick with 20mm orientated strand board and internal cladding of 15mm gypsum plasterboard.)

A wall lifespan of 40 years was estimated, although not all the walling types have been shown to have this lifespan. US housing lifespan is taken at 32 years, influenced by the use of timber frame and lightweight construction systems in the American housing market.

Climate Zones

South Africa has six major climate zones. Energy use varies depending on the location of the building – warm climate zones use more air-conditioning and ventilation, cold climate zones need heating. High rainfall areas need to deal with humidity and condensation.  Lighting requirements have seasonal variations.

Temperatures and other parameters used in the study are acknowledged averages for that zone.

The measurements were based on the residents living in reasonable “thermal comfort” which is between 19 and 25˚C.

Analysis of the Results

The low density walling systems (timber and steel frame) show a trend towards higher annual energy use even when they have low U-values. High density walling systems (clay brick) consistently result in lower annual energy use even without additional insulation.

The results showing the variation of heating and cooling energy modelled for the three building typologies, can be summarised as follows:

Best (lowest) Energy Use

  • Residential Buildings (all climate zones, all sizes):  thermally insulated 280mm clay brick cavity walling
  • Non-residential building (climate zone 1): 140mm hollow concrete block walling
  • Non-residential building (climate zones 2-6):  220mm solid clay brick walling

Worst (highest) Energy Use

  • Residential Buildings (all climate zones, all sizes): 140mm hollow concrete block walling
  • Non-residential building (climate zone 1): timber frame walling
  • Non-residential building (climate zones 2-6):  light steel frame walling

Conclusion

“Members of the Clay Brick Association of Southern Africa are pleased to see that the study corroborates existing national and international research for clay brick products,” says CBA Executive Director Jonathan Prior.  “This research verified that any of the three clay brick walling formats provide property owners and residents with lowest energy usage and hence greatest thermal comfort.

“This also shows the value of thermal insulation for residential buildings,” comment CBA Technical Director Nico Mienie.  “It points to the need for changes in the SA building regulations to relook at the thermal resistance and heat bridging requirements of SANS 517 and SANS 10082.

“Thanks to this study, architects, and public and private section developers are able to make more informed decisions about future walling specifications,” he concludes. This will protect South Africa’s environment and give rise to high performance structures that reduce our reliance on electricity.”

No of words: 815

 

Annual Energy Usage – 40m2 Affordable House

 

GT

KZN

WC

FS

LM

NC

280mm cavity clay brick + insulation

379

296

218

496

2

1244

270mm cavity clay brick

725

454

479

1009

887

1904

220mm clay brick

1055

590

734

1464

1282

2428

Light steel frame (SABS 517)

1082

827

868

945

1135

2054

Timber Frame (SABS 10 082)

1066

786

862

1012

853

1953

140mm Hollow Concrete Block

1505

749

1079

2164

1623

3087

 

Thermal Transmittance (U-Values) of walling systems

Walling system thermal transmittance (W/m2K)

Climate Zones
1 & 6

Climate Zones
2,3,4 & 5

140mm Hollow Concrete block

3.17

3.17

220mm Solid clay brick masonry

2.22

2.22

270mm Cavity clay brick masonry

1.66

1.66

280mm Cavity clay brick masonry insulated

0.80

0.80

Timber Frame to SANS 204

0.75

0.80

Light Steel Frame to SANS 204

0.67

0.77

Caption: U-Value – long considered to be a good measurement of energy efficiency - has little correlation with energy use and electricity cost savings. Energy use does appear to correlate with material density.

Prepared by the Department of Architecture, University of Pretoria.
Authors: G Rice (Researcher), P Vosloo, H Harris, D Holm, N van Rooyen.

The authors and any participants in this study cannot be held liable for any claim of damages of any nature whatsoever, arising from this study.

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