Comparing the Energy Efficiency of Modern and Traditional Walls

u value

Keeping the wolf from the door

The three little pigs discovered (at great cost to two of them) that bricks make better walls than straw or sticks when under attack by a voracious predator. However, does the same apply to energy efficiency, an important factor for an EPC and the cost of heating a home?

The types of materials used to build homes have changed radically over the centuries. Historic buildings generally reflect the availability of local materials such as granite, flint, clay or chalk, but today we more typically use imported bricks or system-built houses that are assembled on site from units manufactured elsewhere.

The ease with which heat passes through a wall, or its ‘thermal conductivity’, is stated as a U value. This is measured in W/m2K, representing watts of heat that pass through a square metre for each 1°C temperature difference between the inside and outside of the wall. The lower the U value, the less heat is escaping.

The current Building Regulations Part L1A requires a U value of 0.30 W/m2K or better for an external wall in a new domestic property in England, though this alone will not guarantee a positive Standard Assessment Procedure (SAP) outcome.

When undertaking an EPC, it is relatively easy to derive an accurate U value for walls in new-build and modern buildings. Each element of the wall’s construction (for example the plasterboard, bricks and cavity insulation material) has a laboratory tested R value, a measure of its inherent thermal resistance. The U value is calculated by combining the R values, with an allowance for the thickness of each.

Modern brick walls are designed to achieve the 0.30 W/m2K standard. Timber frame suppliers typically use a 140mm stud and 140mm mineral wool insulation specification to deliver a U value of 0.29 W/m2K.  For those with greater aspiration, some Passivhaus dwellings achieve U values below 0.12 W/m²K, with off-site system-built components providing more reliability than those put together from scratch on-site.

The improvement in wall U values has been one of the success stories in achieving better energy efficiency in new homes. The solid brick walls of the 1930s (around 225mm thick) have a U-value of just 2.70 W/m2K, while a typical unfilled cavity wall from the 1970s is likely to achieve 1.00 W/m2K at best.

It is more difficult to determine U values for the walls of historic and vernacular properties. The average wall width can be measured but its exact composition will be uncertain. For example, the proportions of straw and clay in a cob wall vary from building to building, or even across an individual wall. Solid stone walls differ in aspects such as the proportion of mortar, the different types and density of stone, and the presence of internal voids.

In situ measurements using thermal testing equipment rather than mathematical formulae suggest that traditional limestone can achieve a U value of 2.00 W/m2K at 200mm thickness to 1.20 W/m2K at 650mm. The corresponding figures for granite are 2.20 W/m2K to 1.25 W/m2K. Historic solid brick walls have been measured at between 2.50 W/m2K (100mm thick) and 0.75 W/m2K (600mm thick).

Many champions of traditional English cottages promote the qualities of cob walls, and their measured U values come in at around 1.00 W/m2K, irrespective of thickness (400mm to 700mm). By comparison, traditional timber-framed walls with in-fill tend to perform quite poorly. Measured results include 2.48 W/m2K for a 100mm brick infill and 2.03 W/m2K for 100mm of wattle and daub.

However, the walls of one historic timber-framed property with a 210mm hemp and lime infill yielded a respectable 0.40 W/m2K. History does not record whether hemp and lime can withstand the huffing and puffing of a wolf!

See some of our other blogs,

European Directives on Building Energy Performance

Property Size and Type and its Effect on Energy Usage and Your EPC

Property Size and Type and its Effect on Energy Usage and Your EPC

EPC

Energy usage by property size and type

It’s a simple fact that large old detached houses have a lower EPC score than modern flats.

Logic suggests that, on average, larger properties will generally use more energy for heating than comparable smaller ones. However, there are many variables involved in such a calculation, not least of which will be the type of property. Flats tend to have fewer external walls and roofs than terraced houses, which in turn have fewer than semi-detached or detached houses. The larger the external surfaces, the greater the expected loss of energy. Other variables such as the age of the property, building materials and the effectiveness of energy efficiency measures that have been installed also confuse the picture.

So although there are a lot of things you can do to influence the score of your EPC, some things you can’t.

A large sample of households would be needed to make an analysis of energy need by dwelling type statistically reliable. The Government’s Home Energy Efficiency Database (HEED) provides such a data set. While not freely available to individuals, the information is available to researchers and organisations planning and monitoring progress in home energy efficiency. It includes information collected between 1995 and 2012 on some 13 million dwellings in the UK, almost half of the country’s housing stock. The data includes property age, type, tenure and energy use, and details such as glazing type, wall type, heating systems and energy efficiency measures.  All of which are fundamental to EPC calculations too.

Researchers at the Energy Institute of University College London were given access to the database to undertake a wide range of analyses, including an evaluation of average energy usage by dwelling type and number of bedrooms.  Their research is reported in the journal Energy Policy (Energy efficiency in the British housing stock: Energy demand and the Homes Energy Efficiency Database. Hamilton I.G. et al, Energy Policy 60 (2013) pp 462-480).

The researchers analysed the data to provide an overview of the statistics for gas and electricity use in 2006 by different types and sizes of dwelling. The following tables summarise their findings, giving the median value in each case (i.e. the mid value when all data are set out in increasing order of size), as they showed this to be a more representative ‘average’ than the mean. The figures for gas and Economy 7 tariff electricity are likely to be most representative of energy demand for heating.

Median energy use for different types of property

Type

Normal tariff  Electricity (kWh/yr) median

Economy 7  tariff  Electricity (kWh/yr) median

Gas (kWh/yr) median

Flat

1,967

4,309

10,242

Bungalow

2,784

4,828

16,129

Terraced house

3,038

4,845

14,983

Semi-detached house

3,310

4,765

16,571

Detached house

4,023

5,135

20,992

Median energy use for different numbers of bedrooms

Number of bedrooms

Normal tariff  Electricity (kWh/yr) median

Economy 7  tariff  Electricity (kWh/yr) median

Gas (kWh/yr) median

1

1,934

4,685

11,137

2

2,554

4,662

13,541

3

3,357

4,637

16,590

4

4,358

5,390

21,560

5+

4,890

6,171

24,246

As expected, the results confirm that detached houses and bungalows have the highest energy usage. The figures show a clear decrease in demand as the level of detachment declines, so that flats, with the highest number of party walls and ceilings, show least energy usage.  Something which EPC scores also make clear too.

The median gas demand increases on average by 22% for every additional bedroom over one in any property type. Overall electricity use also generally increases with additional bedrooms, though not as clearly and steeply as gas.

Waste Water Heat Recovery Systems (WWHRS) and your EPC

WWHRS

Hot water down the drain

The ways you heat, store and control your hot water is an important part of your overall EPC.  This article explores how to make the water we waste work for you and how that can have an impact on your EPC score

Heating water is an energy-intensive business and it is estimated that our daily showers consume more then half of the hot water that the average household produces. After it has done its job, that water will immediately exit the house and enter the sewerage system at around 35°C, depending on how hot you like your shower. That is a considerable amount of energy and money to lose down the plug hole. It is also a significant contribution to the damaging release of greenhouse gases, with domestic hot water heating being responsible for some 6% of the UK’s total CO2 emissions.

In many circumstances it is possible to recover some of this waste heat before it is lost down the drain.  Wastewater heat recovery (WWHRS) is installed in many new-build buildings with a high demand for hot water, such as hospitals, hotels and laundries. Domestic versions that can contribute to a higher EPC rating are available.

The energy can be recovered from the wastewater through simple heat transfer. The system is entirely passive, no energy input is necessary and there are no moving parts to maintain. Few properties lend themselves to the retrofitting of WWHRS to whole-house drainage, as the plumbing tends to be too complex. However, heat recovery systems can be connected to many types of domestic shower units, provided there is sufficient space and all of the relevant pipework is metal. A typical vertical WWHRS unit is approximately 2m high so installation may not always be possible. Horizontally fitting alternatives are available, though they are generally less effective.

Heat recovery from showers has a particular advantage. As the used water runs away, there is a simultaneous on-going requirement for further hot water, so no storage is needed. A shower is supplied from hot and cold water sources and if the cold input can be pre-heated using the waste heat, then less hot water is required in the mix to achieve the desired shower temperature.

Heat exchange is more efficient at lower flow rates, with around 60% of waste heat typically being recovered from a shower operating at a flow rate of 7.5 litres/minute through a typical modern vertical WWHRS unit. Horizontal units operate at around 45%. In practice, a vertical WWHRS unit working on an average shower with wastewater at 37°C should be able to transfer approximately 17°C of heat from the wastewater to pre-heat mains water heading for the shower from 10°C to 27°C.

The system works well with thermostatically controlled showers supplied by a combination boiler, with recovery-heated cold water feeding both the shower and the boiler: however, it is important to check that the combination boiler can accept pre-heated hot water. Some WWHRS are also compatible with unvented cylinder or electric shower units.

Typical cost savings are not great in the context of an installation cost of around £1000, though routine use in new build houses with purpose-designed plumbing systems is much more effective. A family of four each taking a daily five minute shower at a flow rate of 7.5 litres per minute uses a little over 50m3 of hot water a year. 35kWh of energy is needed to heat 1m3 of incoming mains water to 40°C at the shower head, so the family uses nearly 2,000 kWh in total. A conservative estimate of 50% efficiency in real life use means that 1,000 kWh of energy would be saved each year. Depending on the fuel and tariff, this represents a saving in the region of £100 per annum, or a ten year payback period. The payback period for a smaller family using less hot water will be considerable longer.  Your EPC rating however, will show an improvement.

European Directives on Building Energy Performance

energy efficiency

The European perspective on building energy performance

Whilst looking at your EPC and considering the energy efficiency implications for your home, consider the wider perspective.

In April, the European Parliament gave its final consent to a revised Energy Performance of Buildings Directive. This is a key plank in the European Commission’s intention to lead the transition to clean renewable energy and to create ‘a resilient Energy Union and a forward-looking climate change policy’.  Whilst maintaining a fair deal for all energy consumers. It is the first of a series of measures to promote the EU’s ‘Clean Energy for All Europeans Package’, and a major contribution to its commitment to cut carbon dioxide emissions by at least 40% by 2030.

The Directive looks to up the rate of building renovations to make them more energy efficient and to foster more use of smart automation and control systems to improve energy performance in new buildings. It starts a clear path towards the creation of a low emission building stock across the EU by 2050 and underpins national roadmaps to decarbonise buildings.  This directly affects you if you require a commercial EPC.

The concept of a ‘smart readiness indicator’ is introduced. This measures the capacity of a building to use new technologies and electronic systems adapted to the needs of the occupier, optimise their operation and to successfully interact with the grid.

The EU is looking to realise the massive potential for energy efficiency improvements in the building sector, which is responsible for some 40% of final energy use.  It estimates that approximately 75% of buildings are energy inefficient and capable of improvement.  The rate of building renovation varies by Member State, but overall only 0.4% to 1.2% of the stock is renovated each year.

These data also highlight the potential economic opportunities of the programme. The construction sector generates some 9% of European GDP and directly supports 18 million jobs. Building renovation work and energy technology retrofits add almost twice as much value as the construction of new buildings, and these areas are particularly important for small and medium-sized businesses. The proposals will also help to create jobs in the smart technology sector, combat energy poverty and save consumers money through the renovation of older, less efficient buildings, and improve quality of life.

The EU recognises that significant up-front investment is required for the refurbishment of buildings and that public and private financing will need to be mobilised. The Directive is supported by financial enabling tools to help achieve the necessary investments. This include revised Eurostat guidance for energy performance contracts and a ‘Smart Finance for Smart Buildings Initiative’. The European Fund for Strategic Investments has also been expanded. This is focussed on sustainable investments that contribute to the EU’s energy and climate targets and help to deliver the transition to a resource-efficient, circular and low-carbon economy.  At least 40% of the Initiative’s infrastructure and innovation projects are intended to contribute to the European Commission’s commitments to climate action and energy transition in line with the Paris Agreement objectives.

Following the formal agreement of the introduction of the Directive by the Council of Ministers and its subsequent publication in the Official Journal of the Union, Member States will have to transpose the new elements of the Directive into national law within 20 months. This will therefore be one of the first tests of the UK’s reaction to an EU Directive once Brexit has been achieved.

How will the energy efficiency of the UK be affected?  Will it have an affect on your EPC requirements?

Minimum Energy Efficiency Standards (MEES) for Landlords

MEES

The requirements for private rental properties are being tightened under the law, as the Government brings the various provisions of The Energy Efficiency (Private Rented Property) (England and Wales) Regulations 2015 progressively into force in order to promote a new baseline minimum level of energy efficiency.

From 1 April 2018, landlords must have implemented energy efficiency improvement measures to ensure that privately rented properties in England and Wales achieve an Energy Performance Certificate (EPC) rating of E or better before they agree a new tenancy with existing or new tenants.  This requirement will be extended to all domestic rental properties in April 2020, whether or not there has been a change in tenant, and similarly to non-domestic rental properties from 2023.

This is known as the Minimum Energy Efficiency Standards, or MEES.

The required improvement measures referred to include any energy efficiency enhancement work that qualified for the Green Deal, and the installation of a gas supply provided the property is within 23m of the main.

The requirement will not apply to properties that are exempt from needing an EPC, such as some listed or temporary buildings, furnished holiday accommodation and properties that have been let continuously on a Regulated Tenancy to the same tenant since before 1 October 2008.

A landlord may be able to gain an exemption from the requirement under certain limited circumstances. These include:

  • Where all practicable energy efficiency improvements have been implemented but the property still fails to achieve EPC E banding.
  • A necessary measure cannot be implemented because it would have a negative effect on the structure or fabric of the building.
  • Third party consent is required to implement a measure, but it cannot reasonably be obtained.
  • A Chartered Surveyor’s report demonstrates that the measures would reduce the value of the property by more than 5%.

Exemptions may also be possible where landlords can demonstrate that they have been unable to access funding such as Green Deal, Energy Company Obligation or other grants to cover the full cost of installing the recommended improvements, though the details of this are subject to an on-going Government consultation exercise.

Special arrangements are also in place to provide time for landlords that have recently inherited properties to bring them up to the required standard.

Any exemption must be registered on the National PRS Exemptions Register on the BEIS website by 1 April 2018. The exemptions are for five years, during which time the landlord is expected to make further attempts to bring the property up to at least band E status, though a further exemption can then be applied for if this has not been reasonably possible. Exemptions do not pass with title, so a landlord buying a property with an existing exemption will need to apply for a new one.

The Regulations will be enforced by local authorities, who can issue compliance notices and demand copies of relevant paperwork.  They can hand out fixed penalties for non-compliance, and these can reach a maximum of £5,000 per breach. 

The Government’s guidance on the requirement is available here.