Energy in buildings
Energy in buildings

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Energy in buildings

5 Home energy assessment

So far this course has described a wide range of different technologies. But how do you assess what makes a really energy efficient home without actually reading the fuel bills? Which retrofit measures are likely to be most cost-effective or produce the largest reduction in CO2 emissions? Fortunately home energy rating computer models can help.

The need for an energy model to assess housing energy use was identified in the UK in the 1980s. The Building Research Establishment developed a domestic energy model (BREDEM) in the 1990s. This forms the core of the Standard Assessment Procedure (SAP). Since 2006 this has been the key calculation tool to show compliance with the Building Regulations for houses and flats.

It has been through several revisions and the current (2019) version is SAP 2012 (BRE, 2014). Although the actual calculation procedure only covers about six pages, the need to deal with all possible types of houses and flats and their heating systems requires a large amount of explanatory tables.

Its use in setting the UK Building Regulations has been a little controversial. From a point of view of setting enforceable standards, it would be simplest to say that the elements of buildings should use a given thickness of insulation or achieve a given U-value. This is known as the ‘elemental’ approach. If the requirement is to set the overall fuel cost or CO2 emissions, then a more complicated calculation is required to take into account the different sizes of buildings, the heating system efficiency, etc. Finally, there is the problem of what to do about the different climates across the country. A house on a hilltop in Cumbria is likely to have a colder climate than one on the coast of Cornwall. Should it be required to have better insulation standards? Although the 2009 version of the SAP methodology did not distinguish between different climate zones, the revised 2012 version has now taken this into consideration.

Then there is the question of how much of house energy use should be ‘regulated’. The choice of a gas or electric cooker obviously influences energy use and carbon dioxide emissions, but is a matter of choice for the occupant, not the builder. As a result this is left out of the SAP calculations.

The full version of SAP is used in assessing new buildings, but for assessing existing homes a ‘reduced’ version, know as ‘RdSAP’ is used. This is used in ‘home energy audits’, particularly to produce Energy Performance Certificates (EPCs). These have been required for homes for sale in the UK since 2007 and newly rented properties since 2008. They can only be issued by qualified energy auditors.

In its original form the SAP assessment was only concerned with the cost of energy for space and water heating, ventilation and lighting. It is adjusted for floor area. It is expressed on a scale of 1 to 100, the higher the number the lower the running costs.

For example a pre-1919 house with solid walls, no loft insulation and no central heating would have a SAP rating of just over 20. This was about the average rating for the UK housing stock in 1973 (BEIS, 2018c). A newer house meeting the 2006 Building Regulations for England and Wales would have a SAP rating of about 85. Its fuel costs per square metre of floor area would be about a fifth of those for the older uninsulated house. Since the rating is cost based, electrically heated houses tend to have worse SAP ratings than gas-heated ones.

The rating also allows for houses to have their own generation technologies such as solar water heaters, PV panels or micro-CHP units.

However, the need to reduce overall CO2 emissions has now become the main driving force of the Building Regulations. An Environmental Impact Rating (EIR), based on CO2 emissions per square metre of floor area, was added. This, like SAP, is expressed on a scale of 1-100. The higher the number the better the standard.

In order to make things easier to understand for the consumer, the scales have been simplified into A-G bands (as shown in Table 14).

Table 14 Environmental Impact Rating scale

SAP or EIR ratingLabel band
≥ 92A
81 – 91B
69 – 80C
55 – 68D
39 – 54E
21 – 38F
1 –20G

The sample energy performance certificate shown below in Figure 44 gives an ‘Energy Efficiency’ (SAP) rating of 55 (a D rating) with a potential to increase it to 85 (B rating). The house was also given an Environmental Impact Rating (EIR) of only an ‘F’ (not shown in the figure). This was mainly due to its high CO2 emissions because it was totally electrically heated.

Described image
Figure 44 A sample Energy Performance Certificate

It is estimated that the average SAP rating of the UK housing stock has improved from about 18 in 1970 to about 60 in 2017.

A thorough assessment of the potential savings in a home is likely to require a careful energy audit. This video shows how one set of homeowners in Oxford made changes to the energy efficiency of their home. It also shows that home energy efficiency is only one factor in a wider appreciation of a low carbon lifestyle.

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Transcript: 1 The eco-house video

This three-bedroom terraced house on an Oxford estate was built in 1982 and doesn't look much different from its neighbours. But it's had an eco-renovation makeover, although many of the changes are hidden.
What fuel is used for heating the house?
That would be mains gas.
Click on mains gas.
Mark Luntley and Alice Brander started renovating this house soon after moving here in 2002. They wanted to reduce their carbon footprint substantially. Now they're using a carbon calculator to see whether there's more they could still do.
This is the average electricity that we used.
Mark and Alice are concerned about climate change and want to reduce their energy use and waste. They don't want to preach, but they do want to show people what can be done.
Al Gore talks about people going from denial right the way through to despondency without actually stopping off in the middle to do something. And we really wanted to say, we want to do something about it, and do something practical and influence others to do the same.
I can't think of any sacrifice in having our particular lifestyle. On the contrary, I've gained more because of the greener lifestyle. We're five minutes from the shops and central Oxford.
As you look around the house, you'll see that it looks a very ordinary house and it feels a very ordinary place. It just so happens that it uses about half the energy that it used to.
Reducing environmental impact was a high priority in the choice of this house.
The house was a trade-off. We deliberately chose somewhere that was potentially energy efficient in the centre of town. And that cut down on our transport costs. But it meant that it wasn't as large as it might've been if we'd have chosen a house in the middle of the country.
One of the advantages of this house is that it's a mid-terrace so it doesn't lose energy on either side. And it also faces south.
The house was built to pre-1982 regulations, which had poor standards for energy efficiency. So Mark and Alice commissioned an energy audit to find out how best to improve its performance.
High on the list for energy saving was replacing the old boiler with a new energy efficient one. It's in the loft to save space.
This is one of the first things that we did. It's our condensing combination boiler. This one's the smallest that we could find. It provides the hot water and the heating for the house. The old model was just about broken.
Solar water heating panels were installed on the roof at the same time as the boiler was put in. They heat water and feed it into a heat store.
This is the 100 litre heat store. This collects the heat from the solar panels and stores it. And it either provides hot water directly or it will preheat water that then goes through the boiler and then out into the taps.
Good insulation is crucial to transform a cold house into a warm and energy efficient one. Mark and Alice installed environmentally friendly sheep's wool in the loft over the existing mineral wool insulation and then tackled the heat leaking through the walls, windows and internal garage of the house.
We had the walls cavity insulated, because they were air gaps at that time. And that made almost – that was one of the least expensive things that we did. British Gas paid for part of the costs. But it made almost a noticeable difference. Within a couple of days, you suddenly realised that the house was warmer in the winter.
Adding new double glazed windows to the existing secondary glazing reduced heat loss and made the house noticeably quieter. Building a porch onto the front of the house stopped cold air coming in in winter.
But the most expensive renovation was the addition of a conservatory at the back of the house, facing south. It's a luxury not everyone can afford, but it does improve the energy performance of the house, as well as providing a light-filled, relaxing space.
We were interested in having a solar room which would take the heat from the Sun, store it within the solar room, and then release it into the house when it was needed.
There's no artificial heating in this room, only the heat from the sun absorbed by the tiles and walls in spring and autumn. But in winter, it also helps to keep the house warm.
The conservatory acts as a buffer zone in the winter. So it can be very cold outside, but the conservatory itself will never get less than 8 degrees.
These energy saving measures reduce the carbon footprint of the house itself. Gas and electricity use dropped by about 50%.
Of course, when you think about carbon footprints, the really big changes have to happen at government level, and others at community level. But as individuals, we can still make a difference.
What we found is that simply showing people some of the benefits of the house, the fact that it's quiet, the fact that it's warm, that it doesn't use very much energy. The choices that we've made mean that we don't have long commuting, that our life style is better as a result of this, and that it is something that people can do practically. People are attracted to that.
End transcript: 1 The eco-house video
1 The eco-house video
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