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What is SANS 10400 XA, and why would a Rational Design save you money?

Updated: May 15, 2020


SANS 10400 XA is the South African National Standard regulating energy efficiency in building design. For all new buildings and building alterations, an industry professional needs to demonstrate that the proposed building design complies with the standard.

A major factor in whether or not a building design is compliant is the windows – the size, orientation and the type of glass installed.

There are two ways to demonstrate compliance:

  • A checkbox approach, using a SANS 10400 XA calculator for the fenestration calculations, which can be done by an architect, but which often results in limitations in how large windows can be, or having to use very expensive performance glass; or

  • Whole-building energy simulation, or Rational Design, for which a specialist is needed, but it is much more accurate and in almost all cases more economical because the use of unnecessarily expensive glass is avoided.

The Rational Design process appears to still be relatively unknown to property owners, but could have major benefits in terms of reducing construction cost and increasing building comfort.

SANS 10400 XA Rational Design Overview

I recently had a conversation with a friend of mine who is in the process of building a house. I tried to explain the SANS 10400 XA Rational Design process to her, and why, in my opinion, it is the better way of demonstrating compliance to building regulations. I lost her within seconds, and realised that there is still very little understanding amongst homeowners on the national building standards and the potentially massive impact it has on the cost of a building during construction, and how comfortable the building will be once you move in.

We have been doing Rational Design modelling for about five years now, and I can easily talk through the process when speaking to architects who are familiar with building codes and construction terminology. When trying to describe it to the man on the street, I come up empty – a predicament which needed to be rectified.

So here we go – let me try my hand at offering an explanation in layman’s terms:

Anything that is built in South Africa, be it a house, office building, factory, etc., has to comply with certain regulations set by the government. There are a whole bunch of standards setting minimum requirements, called the South African National Standards (or SANS for short) to make sure buildings will function properly and safely once built. They cover everything from the structure (so it won’t collapse), the plumbing (for water to drain and connect to municipal systems properly), and so forth. One of these standards is called SANS 10400 XA, and it regulates the energy efficiency of the building.

So, what exactly is SANS 10400 XA? It regulates the energy use of a building once it is built, and tries to prevent buildings being designed in such a way that they will need massive amounts of electricity to operate. It focusses on three major energy uses:

  • Hot water generation

  • Lighting

  • Comfort - keeping the building from getting too hot or cold (e.g. air-conditioning)

Hot water generation

For hot water generation, SANS 10400 XA requires you to include solar water heating technology, or a heat pump, instead of relying on only a typical electric geyser. Electric geysers are essentially giant kettles, and they are very bad at converting electrical energy into hot water energy (1 kWh of Eskom electricity more or less equals 0.9 kWh's worth of hot water). Using solar energy to heat water is free, and heat pumps can turn 1 kWh from Eskom into about 3 kWh of hot water energy. Generating hot water usually makes up the bulk of residential energy costs, and complying to this requirement will immediately result in a lower electricity bill.

Lighting energy

With lighting, SANS 10400 XA regulates the light fittings installed in our buildings. This is a pretty simple requirement to fulfill – if you use LED lighting (which is not that expensive anymore), you’ll be set. The energy saving is massive – you can reduce your lighting energy requirements by approximately 80%, again resulting in lower electricity bills.

Comfort (heating & cooling)

Comfort is where things get a little more difficult, and where the standard can potentially cost home owners an arm and a leg. The comfort aspect addresses how much heating and cooling a building will need during a year. The required heating and cooling energy consumption is determined by the building design, building insulation (walls, floor, roof and windows), the local climate, and all of the things inside a building that will generate heat (people, lights, appliances). SANS 10400 XA has the unenviable task of setting, in less than 19 pages, requirements on how to build a reasonably comfortable building, throughout South Africa’s various different climate types, for typologies varying from residential to heavy industrial and everything in between.

SANS 10400 XA provides two routes by which compliance to the comfort aspect can be demonstrated:

1. Prescriptive Route

The first, generally known as the Prescriptive Route, is a relatively easy tick-box approach which can be completed by an architect. Minimum insulation values are set for floors, walls and the roof. For windows, you can either show that the total window area is less than 15% of the total floor area (which allows very few windows in the design), or the architect can use a fenestration calculator to calculate a ‘glazing score’. This is based on each window’s size, orientation, and the thermal properties of the frame and the glass.

The score needs to be lower than a set value for the windows to comply to SANS 10400 XA. Usually, in order to comply, performance glass (for example double glazing with a low-E coating) needs to be used, which can be expensive.

2. Rational Design

The second option, called Rational Design, is by doing whole-building energy simulation using computer software. This involves a couple of steps: A specialist consultant is required to produce the model. A computer model of the building is generated, and all the building materials (walls, roofs, floors and windows) and internal heat sources (like people and lights) are entered into the model. The software then uses hourly weather data to calculate the heating and cooling energy required during a year to keep the building between 19°C and 25°C.

SANS 10400 XA Rational Design Model Inputs

Lighting and hot water energy is then added to the cooling and heating energy. Thereafter, another model is generated for a building designed as per the Prescriptive Route above, and its energy consumption for heating, cooling, lighting and hot water is also calculated. If the original design uses less energy, and has a lower average maximum energy demand than the Prescriptive Route design, it is compliant to SANS 10400 XA.

SANS 10400 XA Rational Design Results

There are three major benefits to using Rational Design:

  • The first is that all energy uses and design elements are considered as a whole, instead of individually. This means that if, for example, energy is saved using a more efficient hot water system than the minimum requirement of the standard (easily done by using a heat pump or solar water heating for all hot water), this energy saving can be used to offset the higher heating and cooling energy required if cheaper glass is used.

SANS 10400 XA Rational Design Iterations
  • The second, and more important benefit, is that energy modelling is a lot more accurate than the standardised glazing calculation tables used in the Prescriptive Route. In many cases, higher comfort can be achieved using cheaper glass coupled with sensible passive design strategies (such as shading, window sizing and sensible use of insulation). We have also encountered cases where the Prescriptive Route forced the use of more expensive performance glass, where a straight swop with cheaper clear glass improved the energy efficiency of the design. A typical example is the use of tinted glass in houses where more heating is required in winter – the tint blocks out useful solar radiation in winter. A much better strategy would be to install single clear glass, and use external shading or internal blinds or curtains which offer the flexibility of blocking the sun in summer, but allowing it back in for natural heating in winter.

  • The third benefit is that thermal modelling is a powerful tool in determining the most efficient heating and cooling strategy, especially in larger buildings such as offices where comfort is key.

So, whether you are constructing a house and faced with the cost of installing expensive glass purely in order to comply to SANS 10400 XA, or doing a large development where comfort and energy efficiency is the top priority, spending money upfront on a Rational Design should result in financial and energy savings during construction and operation of the building.


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