Articles published in 2014
A Faltering Step Towards Zero Carbon
It is common knowledge that we need to cut carbon emissions. Achieving the necessary cuts is going to be a big challenge for industry (in developing low carbon technologies), for society (in making lifestyle adjustments), and for democratic governments (in ensuring that emission targets are achieved).
An elected government can make long term commitments to cut carbon emissions with impunity. There is too little accountability. If the same political party happens to be in power when eventually a long-term target is missed, they can blame the other lot, in power earlier, for not taking sufficient action. The taking of timely but unpopular action is very challenging for an elected government. There is too much temptation to prevaricate. An example of this can be seen in the government regulations for curtailing carbon emissions from new housing.
SAP 2012 II
As we saw last month, SAP 2012 will come into use on April 6, replacing SAP 2009. On the same date, amendments to Part L1A of the building regulations will come into force in England. Part L1A uses the Standard Assessment Procedure (SAP) to ensure that the energy requirements and carbon emissions of a new dwelling are low enough.
So far, we have looked at the first four sections of SAP 2012.
Most of the outputs are evaluated for each month of the year.
Here is an overview of the remaining sections that apply to a detached dwelling.
Thermal Stores: and multi-coil cylinders
A thermal store is an appliance for storing heat – no surprise there! The heat is stored in water, which happens to be an excellent medium for this purpose. The heat is used in a heat exchanger to instantaneously convert mains water into hot tap water. With a heat store, it is only heat that is withdrawn from the store, not the store's water – though the store's water is often circulated around a space heating circuit (UFH or radiators).
Thermal Bridging and SAP
Thermal bridges may occur at junctions in the thermal envelope of a building. If the insulation layers are interrupted by a poor insulator, a 'bridge' is formed by which excessive heat escapes. Thermal bridging commonly occurs:
- Where a wall meets the ground floor or the roof.
- Around windows and doors.
Thermal bridging must now taken into account in the Standard Assessment Procedure (SAP).
Thermal Mass: What is it, and is it useful?
Most selfbuilders are familiar with U-values. Consider a building element such as an external wall. If there is a temperature difference of ΔT between inside and outside:
Heat flow through unit area of the wall = U x ΔT.
Strictly speaking, though, this simple mathematics only applies in steady state conditions, ie, when the temperatures inside and outside remain constant. When the temperatures are varying, as they frequently are in real life, then the mathematics becomes much more complex.
In the poorly insulated houses of yesteryear, a simple estimate of heat flows based solely on U-values was good enough. But nowadays, with much reduced U values, other factors become significant. One of those factors, thermal bridging, has been the subject of my last two articles. Another factor is thermal mass.
Phase Change Materials: And the Arup thermal calculator
We've seen in the last couple of articles that high thermal mass reduces overheating in summertime and maximises solar gains in spring and autumn. For a contemporary new build, high thermal mass inside the insulation layer generally leads to lower fuel bills. (See the Footnote.)
So where does that leave a construction method like timber-frame, which essentially has low thermal mass?
SAP 2012: Coming into use soon
The Standard Assessment Procedure is the calculation method required under Part L1A of the Building Regulations. I described the procedure at some length in August and September, 2006. Those articles referred to the procedure that had been released the previous year, viz, SAP 2005. This was later superseded by SAP 2009, and that will soon be superseded by SAP 2012. Despite its name, this edition of SAP was not finalised until October 2013, and it is still not in use.
SAP Miscellany: Including targets
In the last couple of articles we've been looking at the new edition of the Standard Assessment Procedure, SAP 2012 – it comes into use in England on April 6. We saw how the procedure is used to evaluate the two ratings shown on a SAP certificate.
A SAP certificate is required on completion of a build. Of more immediate interest at the design stage are two other figures evaluated in SAP:
Dwelling Emission Rate
The DER is the amount of carbon dioxide emissions over a year per unit floor area – where the emissions result from the basic services above.
The EI rating (see above) is derived from the DER.
Fabric Energy Efficiency
The FEE is a new figure, appearing for the first time in SAP 2012.
2016 and all that: The receding zero-carbon target and the cost of reaching it
In 2007, the Labour Government published a policy statement with all-party support: Building a Greener Future. Its first sentence stated that 'climate change is a serious and urgent issue'. It went on to say that the main cause of climate change is greenhouse gas emissions, in particular carbon dioxide. More than a quarter of the UK's carbon dioxide emissions result from the energy we use in our homes.
The policy statement was based on consultations that had been held the previous year. The government proposals were to progressively improve the energy/carbon performance of new housing over the following ten years. 2016 was to become a memorable year for house builders, the year by which all new homes were to be 'zero carbon' – that means that over the course of a year, the net emissions resulting from all the energy used in the home would be zero.
How To Reduce Thermal Bridging
Last month, we looked at how the Standard Assessment Procedure assesses heat losses at thermal bridges. The worst losses are generally at the lintels, at the perimeter of the ground floor, and at gable walls (with a cold loft).
The simplest assessment of the overall heat losses due to thermal bridging is obtained by using the 'global heat loss factor', y, set by SAP to be 0.15 W/m2°C. This is likely to give a gross over-estimate of the actual heat losses. To obtain a better assessment, each bridge needs to be considered individually.
Thermal Mass and SAP
As we saw last month, thermal mass helps to smooth out fluctuations of temperature inside a home. It can be particularly helpful in smoothing out the diurnal fluctuations.
The benefits of high thermal mass are twofold:
Less overheating in summertime.
Less demand for space heating in spring and autumn.
The downside of high thermal mass is that the internal temperature responds only slowly to the central heating system.
Passive Solar Design
The aims of passive solar design are:
- To maximise the benefits to be gained from the radiant heat and light that come to a building from the sun.
- To minimise the unwanted effects – in particular, summer overheating.
Excellent insulation is a pre-requisite. Some say that natural ventilation is desirable, too.
The design is 'passive' in that no account is taken of the active gathering of solar energy, neither directly via PV and thermal panels, nor indirectly via heat pumps.
