NEUTRAL IS POSITIVE – CARBON NEUTRAL, BACK ON THE AGENDA By Darren Evans, Managing Director, Darren Evans Assessments


Earlier this year the government scrapped its zero carbon homes plan – again. Were the plans too ambitious or were they just simply not required or valued by the industry? Many believe it was the wrong decision. But the question remains, are zero carbon or carbon neutral buildings financially achievable in the current climate and does the desire still exist to strive to create them?

In May of this year the government overruled the House of Lords and scrapped the zero-carbon homes policy – a policy it had scrapped in July 2015. The House of Lords had attempted to reinstate the standard for all new homes through an amendment to the Housing and Planning Bill but the proposals were thrown out. Instead, the government committed to a review of energy standards in current Building Regulations. To many, including the UK Green Building Council (UKGBC), this is seen as a very ‘weak clause’.

The abandoned zero-carbon rules, which were due to come into force this year, would have required new housing developments to generate energy through renewable sources such as solar panels or ground-source heat pumps. So why the u-turn and where does that leave our drive for lower energy homes?

One reason given for the scrapping of the regulations was to boost housebuilding. This seems a contraction given the fact that in July, two months after scrapping zero carbon homes, Housing minister, Gavin Barwell, said that the Government remains committed to building 1 million new homes.

So whilst we are getting mixed messages from Government and a lack of legislation to drive the carbon neutral agenda, how does the market view our position? Is there a place for zero carbon homes?

Sustainability is now considered a norm. However all too often motives for sustainability, especially in the commercial sector, are short term and driven by motives such as quick financial gain. For example, carbon reduction with the driving force being reducing immediate energy costs rather than long term resource efficiency. In the housing sector, whilst there is no doubt that housebuilders are ‘making hay whilst the sun shines’, there are more discerning clients, especially housing associations, looking at the future and realising that the great gains are made by playing the long game.

You only have to look at the growth of BREEAM the internationally recognised measure of sustainability for buildings and communities. More than 530,000 certificates have been issued under BREEAM on more than 24,000 projects in over 70 countries and over 2.2 million buildings and communities are registered for certification. This has to be the largest, global, indicator that developers, tenants and clients see the value in sustainability.

Whilst achieving zero carbon isn’t easy and it can come at a cost, many are now understanding the long term gains. It is estimated that a mixed-use development built to BREEAM Outstanding will add around 4.8% to the overall capital costs. However the payback in terms of lower running costs can be less than 10 years. Long game? 10 years isn’t that long!

The growth in the application of passive techniques in the UK and the reduction in cost of renewables are now making zero carbon a commercial viability. Yes there is still work to do to educate homeowners what living in a zero carbon homes means and the lifestyle changes required. However at a time when consumers are looking at how they can save themselves money, present someone with an opportunity for low or even zero utilities bills and they will bite your arm off.

I am pleased to see more clients looking forwards and talking about how they can achieve carbon neutral developments. We need to make sure that zero carbon is seen as a long term positive and like sustainability, becomes a norm rather than a aspiration.

AIRCRETE AIDS THERMAL DESIGN BUT BEWARE OF A SHORTAGE Marcus Eves, Sustainability Consultant, Darren Evans Assessments


Marcus Eves 3The demand for Aircrete blocks has been steadily growing over the past few years. This can be thanked by a combination of the change in building regulations (Part L 2013) and a housing crisis that has led to strong growth in the residential sector.

With Part L 2013 putting more emphasis on the requirement of an even more thermally efficient fabric, through compliance with the Fabric Energy Efficiency Target (FEE) Aircrete blocks have become the preferred choice to achieve lower U-Values and better PSI Values (Thermal Bridging/heat loss at junctions) and an overall easier route to compliance.

The pace of the industry growth and a lack of raw materials have started to cause a shortage of Aircrete blocks, unwelcome bad news which is slowly rippling through the construction industry.

Pulverised Fuel Ash (PFA) is the main waste product from coal fired power stations and this raw material is integral in the production of most Aircrete blocks in the UK. This lack of material could similarly impact on other concrete blocks, cement and ready mix concrete as these products also use PFA.

A handful of factors have combined to see levels of PFA production drop. A mild winter has reduced the overall level of electricity generation in the UK, lower gas prices has seen electricity generators burn less coal and more gas and as we drive ourselves forward to a greener energy infrastructure reliance on coal power has diminished and will continue to do so.

There is potential to import PFA from coal fired power stations across Europe, but this has never been needed before so the transportation network and infrastructure just isn’t there yet. For now, the most important thing to do is be aware and plan ahead.

A change from Aircrete blocks to a denser block will ultimately see a rise in emissions through high heat losses through the walls and the junctions. This will cause some buildings to fail to meet the Emission and FEE targets, where previously the design assessment was compliant. Compliance with Part L can still be achieved with a change in blockwork as long as the assessor is informed early enough to propose ways to offset the additional CO2 and heat losses.

So the advice is to anticipate the shortage and to begin to design denser blocks into your SAP and SBEM calculations as early as possible.


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.



The Energy-related Products (ErP) Directive sets out a legal framework across the EU to help drive specification of more efficient products which reduce energy and resource consumption. The idea is that this will make a big contribution to meeting the EU’s ‘20-20-20’ target, namely a 20% reduction in EU greenhouse gas emissions, a 20% increase in energy efficiency, and 20% energy from renewables, which is targeted to happen by the year 2020.

The ErP Directive is not new by any means, having been implemented back in October 2009, and we are all familiar with seeing the G to A+++ rating on appliances such as refrigerators, washing machines and dishwashers. As of September 26th 2015 however the range of the Directive has been extended to encompass all residential and commercial heating products.

The ErP Directive actually comprises two separate directives: Ecodesign and Energy Labelling. The Ecodesign Directive is chiefly for manufacturers and sets out rules and mandatory requirements for the energy efficiency of products which they have to comply with in order to sell those products within the EU. The Energy Labelling Directive compliments the Ecodesign aspect with mandatory labelling requirements intended to make life easier for consumers.

The Energy Labelling Directive is intended to help communicate complicated information in a simplified format to consumers in order to allow them to make informed decisions without having any specialist knowledge of the product. All new heating products should now have an ErP energy label ranged from G to A+++ however if one or more heating products are used in combination to make a system (such as a boiler with heating controls or solar device) then an installer will be required to produce a document called a ‘fiche.’ This essentially calculates the efficiency of the combination of heating products to give an overall system efficiency/rating.

Installing more than one heating product in this way is very commonly seen currently as boilers now have separate heating controls. Most large boiler manufacturers offer the production of a fiche as a free service, however there are templates available online if a specifier wished to produce one themselves. The fiche should be left with the property owner when completed so they know how efficient their system is.

What does this mean for specifiers? Best practice would normally dictate that a specifier would always choose from latest range of products produced by a boiler manufacturer to achieve optimum energy efficiency, depending on the boiler type required. In doing this, technically they will receive an ErP compliant boiler as all new heating products available to the UK market should be ErP compliant, however the specifier should always double-check.

This blog also features as a Guest Blog on Buildingtalk, you can read it here:


CLEARING UP CONFUSION AROUND THE CRITERION 3 CHECK ON SUMMER HEAT GAIN Marcus Eves, Sustainability Consultant, Darren Evans Assessments

DEA blog - sunSummer overheating has been a hot topic in recent months, however the Building Regulations do make provision for limiting the effects of solar gain. Unfortunately despite being a requirement for compliance with Part L2A of the Building Regulations, the importance of the Criterion 3 check on how much buildings are being heated via windows is often overlooked and misunderstood.

Whilst we are all familiar with the requirements of Criterion 1 (Compliance with the Building Emission Rate), Criterion 3 (Limiting the Effects of Solar Gains in Summer) always seems to get forgotten about. And when it comes to finding a route to achieving compliance it can leave some confused and frustrated.

In commercial buildings compliance with the emissions targets is straightforward, proceeding through the reliable time-honoured menu; lower the U-values, improve the building services, enhance the controls and finally consider renewables. However many are unfamiliar with how the Criterion 3 check is performed in SBEM (the BRE’s Simplified Building Energy Model for non-domestic buildings). When it comes to compliance the implications of this can be drastic.

For the purposes of Part L, a ‘limiting solar gain check’ is performed for Criterion 3 compliance where the aggregated solar gains between April and September must be below a certain benchmark. It is crucial to understand that this does not mean a straightforward overheating check is carried out and is related to solar gain via windows only. Provisions must be taken to limit internal temperature rise due to solar gain in summer; this will reduce or eliminate the need for air conditioning or mechanical ventilation which will increase the building’s energy demand.

In Part L 2006 the Criterion 3 check carried out in the SBEM was based on overheating assessment and therefore any zones which were mechanically cooled were exempt. This gave a quick fix to developers: if a zone failed Criterion 3, a cooling system was added and a pass could be demonstrated. This is not the case anymore. Mechanical plant cannot simply be introduced to offset an issue as a result of poor façade design.

Consideration needs to be given to the design of the building very early on to limit solar gains entering the spaces. There are three main factors to consider:

  • window size and orientation;
  • solar protection through internal or external shading or improved glass specification;
  • high thermal capacity.

When the Criterion 3 limit is exceeded by a small amount – up to 30% – consider the glazing specification and blinds. Windows with lower G-values will restrict the amount of solar radiation entering the space. Blinds will also have the same effect, but these are not appropriate for all spaces.

When the Criterion 3 limit is exceeded by a larger amount (up to 50%) consideration must be given to the glazed area proposed. A reduction in the number of openings or their size may be required. If curtain walling is proposed, spandrel panels can be introduced to reduce the area of glass but keep the overall feel of the building.

Finally external shading must be considered; such as the introduction of overhangs to windows, vertical or horizontal fins, or brise soleil. This will have significant impact on the building form and so will require resubmissions to planning, therefore it’s vital the impact of Criterion 3 compliance is identified at the earliest stage.

It must be noted that if external shading is required, SBEM does not take this fully into account. A full overheating study using a bespoke Dynamic Simulation Model will be required in order to demonstrate to building control that the internal comfort conditions will be met.

In summary, recognising that compliance Criterion 3 is restricted to solar gain from windows, it is important to also acknowledge there is a trade-off between maximising the benefit of daylight and limiting solar gains. Also it is essential to recognise the impact of Criterion 3 compliance prior to planning submission and that the introduction of building services such as air conditioning or mechanical ventilation are not a fix. Lastly, achieving compliance can be expensive and in some instances affect a building’s form.