ZERO CARBON HOMES – BACK AGAIN OR JUST BACK ON THE AGENDA? Graham Suttill, Sustainable Buildings Assessor, Darren Evans Assessments


Yesterday during the Lords Report Stage of the Housing and Planning Bill, the House of Lords defeated the Government on the zero carbon homes amendment. The defeat – by 48 votes – could see the reintroduction of Zero Carbon Homes, the on-site carbon compliance standard, the Government, rather surprisingly, scrapped last July.

So all good news? Well yes and no. Whilst it is a clear sign that there is still much belief in the Zero Carbon Homes standard, the Bill will now go back to the Commons and could enter months of prolonged to-ing and fro-ing where both Houses of Parliament seek to pass their respective versions of the Bill. But the outcome could see Government ensure all new homes in England built from 1 April 2018 achieve the carbon compliance standard.

It’s fair to say, the scrapping of the policy was met with very little – if any – support.  In the eyes of many it was culled post-election so that housebuilders had one less hurdle to jump through and in doing so would help to kick start housebuilding. However this is a very naive and short term view. Killing Zero Carbon Homes simply reinforced the view that the Government has very little understand of green policies and any idea in terms of a long term strategy to create more sustainable housing.

On the back of last week’s support to the COP21 Paris, which saw over 170 countries sign the agreement, the support for Zero Carbon Homes is understandable. But if Zero Carbon Homes is going to rise phoenix like from the ashes, the question is, is it back for good or just back on the agenda?

The hope is that it is back for good. The industry has not lost faith in it and there is still momentum and desire for zero carbon homes. For example, there is still innovation in products and technology that are striving to make zero carbon homes more easily achievable, regardless of whether it is mandatory or not.

It is welcome news but this is a long way to go. However it does demonstrate there is still Parliamentary support for zero carbon homes and its reinstatement will be met with open arms from across the industry.

HOW TO ACHIEVE A GOOD U-VALUE Sophie Peters, Sustainability Assessor, Darren Evans Assessments

u value blog

U-values (measured in Watts per metre Kelvin – W/m2K) measure the rate of heat loss through building elements. As they impact on both the dwelling emission rate and fabric energy efficiency targets, they ultimately govern the result of the SAP (the Standard Assessment Procedure) test used to assess a house’s energy performance. Achieving ‘good’ (low) U-values gives you the baseline for achieving and exceeding Building Regulations. But what are the best ways to achieve a good U-value? There are so many options in terms of design and materials you can be spoilt for choice. So where should you start?

To know what may be deemed as a good U-value it is important to know how it compares to the U-values in the notional building used within SAP for Building Regulations compliance. The notional U-values within Part L Approved Document L1A 2013 are as follows: – external walls 0.18 W/m2K; party walls 0; floors 0.13 W/m2K; roofs 0.13 W/m2K; windows 1.4 W/m2K and opaque 1 W/m2K.

Achieving U-values higher than these will go against you, having a negative impact on the SAP and making it more difficult to pass.

There are a multitude of methods and construction types you can use to achieve good U-values however. Each product will differ slightly from the next, meaning that expert advice in tweaking product selections can help towards generating the best overall U-value.

There are also limitations and constraints which vary between projects. For example, lightweight aircrete blocks have improved thermal efficiency compared to a standard concrete block. However, it is not appropriate to use them for some designs due to their compressive strength.

The most important building element to focus on in achieving low U-values is the external wall element (masonry construction) as being a large exposed area it is often the cause of an SAP failure. From recent experience on projects, most contractors look to keep the overall wall thickness to 300mm, with cavities then either partially or fully filled using mineral wool or PIR insulation. A high performance full fill PIR would be sufficient to achieve a U-value of 0.18 to match the notional dwelling without requiring any internal insulation or increase in size of the cavity. (Maintaining the cavity at 100mm is desirable to avoid impacting the plot size and altering the requirement of other products, such as lintels and wall ties.) In this scenario, an aerated block would need to be used for the internal leaf; its specified thermal conductivity depending on what performance is required from the insulation in order to meet the target U-value.

Example 1: Full fill cavity wall, 0.18 W/m2K

This example of a U-value wall offering notional building U-value would internally comprise a 3mm plaster skim, 12.5mm plasterboard (lambda 0.21), 15mm minimum plaster dabs cavity with the following masonry, cavity insulation and external elements: – 100mm Aircrete block (lambda 0.15); 97mm Celotex CG5000 PIR insulation (lambda 0.021); 3mm cavity; medium density concrete block (lambda 0.57) and render.

In some areas, due to over exposure to driving rain, it may not be suitable to use full fill insulation, with a larger residual cavity being required to prevent moisture infiltration, plus foil backed PIR insulation which will offer high performance within the cavity. If a glass mineral wool product is used, offering much lower thermal performance than PIR insulation, the wall would need to be further insulated internally to meet the target U-value (this is also the case with full fill). To be able to achieve 0.18 W/m2K with a partially filled cavity wall, it is necessary to increase the cavity size or use insulated plasterboard on the internal wall.

Example 2: Partial fill cavity wall for 0.18 W/m2K (100mm cavity)

This would internally comprise 3mm plaster skim, 12.5mm plasterboard and 25mm Celotex PL4000 PIR insulation (offering 0.022 U-value) plus 15mm minimum plaster dabs cavity. Masonry, cavity insulation and external elements would include: – 100mm Aircrete block (lambda 0.15); 50mm Celotex CG5000 PIR insulation (0.021); 50mm Low-E cavity; medium density concrete block (lambda 0.57) and render.

Example 3: Partial fill cavity wall for 0.18 W/m2K (150mm cavity)

This would comprise of internally, a 3mm plaster skim plus 12.5mm plasterboard and 25mm Celotex PL4000 PIR insulation (0.022). Masonry, cavity insulation and external elements would include:- 100mm Aircrete block (lambda 0.15); 100mm Celotex CG5000 PIR insulation (0.021); 50mm Low-E cavity; medium density concrete block (lambda 0.57) and render.

If you are looking to provide U-values lower than the notional building specification for your overall construction, you need to be prepared to make improvements elsewhere further to those to external walls in order to pass the SAP calculation.



One of the two main targets that needs to be met when assessing a new domestic building against Part L 2013, together with the Target Fabric Energy Efficiency (TFEE), is the Target Emission Rate (TER) for heating. A legal requirement within Part L1A, the TER sets a minimum allowable standard for a building’s energy performance using the annual CO2 emissions of a notional building similar to the proposed building.

One of the questions we are most commonly asked regarding the TER by designers and developers is “will this dwelling pass with electric or oil heating?” Most buildings are heated using gas, and unfortunately under current Building Regulations it is very difficult to get a new dwelling to pass and exceed the TER using electricity or oil due to the notional building method by which it is calculated.

The calculation methodology has two stages: first a notional building is created to the same size and shape of the actual dwelling which is to be constructed, but using reference values from appendix R of the SAP 2012 document. These values outline the building specification which needs to be met or exceeded to improve upon the TER, and include a notional heating system which is an 89.5 per cent efficient mains gas condensing boiler.

The second stage is to apply a fuel factor to the calculations to give the final TER, and the factors for the most commonplace fuels we use in SAP calculations are 1.00 for mains gas 1.17 for oil and 1.55 for grid electricity.

This shows that different fuel types will have different effects on the TER the proposed dwelling is trying to achieve. In addition, each of the fuel types have different emissions associated with them as well as primary energy factors – which represents the amount of energy needed to delivery one unit of energy.

As the table below shows, oil and electricity have a higher CO2 per kWh of fuel when burnt than mains gas. Therefore when a notional building’s TER is based upon mains gas, and for example the proposed dwelling is using electric heaters, it is clear that associated emissions to provide the electric heating will be over twice that of mains gas.

Fuel Type Emissions kg CO2 /kWh Primary energy factor
Mains Gas 0.216 1.22
Heating Oil 0.298 1.10
Grid Electricity 0.519 3.07

In conclusion, the difficulty in getting dwellings which use oil or electricity as the main heating fuel to meet the target emission rate lies with the fact that mains gas is used in the notional calculation. There are higher CO2 emissions from oil and electricity when consumed and the energy losses from transporting electricity are significantly higher than mains gas.

This does not mean that it is not possible to meet the TER using oil or electricity as the heating fuel, however significant improvements to the building fabric or the incorporation of renewable technologies will be needed to offset the higher associated emissions.