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.




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.

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.


Part L changes for 2014: here’s how they might affect you

Part L Building Regulations have been updated for 2014. Here’s our primer to keep you up to speed with the changes.

An introduction: what is Part L?

Part L is the section of the Government’s Building Regulations concerning the conservation of fuel and power. All new and rebuilt buildings, whether dwellings or non-dwellings, must comply with the specifications in Part L. This includes targets for insulation values, heating efficiency, hot water storage, air tightness and carbon emissions. It also sets out the requirements for SAP calculations, which result in an Energy Performance Certificate for the building (a mandatory requirement).

Each time Part L is updated, the targets for CO2 emissions become tighter. This means that all of the individual calculations affecting the emissions total must be carefully calibrated, to contribute to the energy efficiency of the whole building.

When was Part L updated?

Consultations for revised CO2 targets took place in 2013. The revised document comes into force on 6th April 2014, and is for use in England. If plans have been submitted for new builds before 6th April 2014, and work begins on site before that date, the 2010 Part L applies instead.

What are the changes?

The main changes from previous Part L regulations, for dwellings, are as follows:

  • Carbon Reductions – now 6% for domestic buildings and 9% for non-domestic buildings.
  • CO2 reduction target is now aggregate across building type – this means that the dwellings with the most fabric, such as detached houses, will be asked to reduce emissions by more than those with less fabric, such as mid-floor flats.
  • FEES, a new target to meet – this stands for Fabric Energy Efficiency Standard, and measures the energy demand of the dwelling.
  • The effects of heat gains in summer are now limited – calculations include heat from pipes, not just solar gains.
  • There is a new Notional Building in SAP, and a recipe approach for compliance – the parameters to follow to achieve compliance are published within Part L Section 5: Model Designs. The Notional Dwelling specifications look like this:
Element or System Values
External Walls 0.18 W/m2K
Party Walls 0.0 W/m2K
Floor 0.13 W/m2K
Roof 0.13 W/m2K
Windows, roof windows, glazed rooflights & glazed doors 1.4 W/m2K

g-value 0.63

Opaque doors 1.0 W/m2K
Semi-glazed doors 1.2 W/m2K
Air-tightness 5.0 m3/hr/m2
Linear Thermal Transmittance 1. If the thermal bridging in the actual dwelling has been specified by using the default y-value of 0.15 W/m²K, the thermal bridging is defined by y = 0.05 W/m²K.

2. Otherwise the thermal bridging allowance is calculated using the lengths of junctions in the actual dwelling and the psi values in Table R2 in of SAP 2012

Ventilation type Natural (with extract fans)
Air conditioning None
Heating system Mains gas, boiler (SEDBUK 2009 89.5% efficient) and radiators
Low energy lighting 100%
Thermal Mass Parameter Medium (TMP=250)

For existing domestic dwellings, target U-values remain unchanged.

The main changes from previous Part L regulations, for non-dwellings, are as follows:

  • 9% aggregate CO2 reduction – buildings with more fabric asked to do more than buildings with less. For example, a small warehouse must reduce CO2 by 3%, whereas the target for a hotel is 12%, and that for a shallow plan office is 13%.
  • There is a wider range of Notional Buildings – Side Lit or unlit (heating only); Side Lit or unlit (includes cooling); Top Lit.
  • Air tightness values vary by Gross Internal Area.

 How can I learn more?

The revised Part L documents can be found here, and can be downloaded. You can find out more about our assessment services on our website.

We’ll be updating our website with new guidelines soon. Until then, you can call or email us with any questions, on 01454317940, or