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EG5085 Advanced Topics 2011-2012
AN ANALYSIS OF THE FEASIBILITY AND PRACTICALITY OF
EXTERNAL WALL INSULATION IN THE UK
I .D. Midgley1 and Dr .M. Imbabi1 1. University of Aberdeen, Fraser Noble Building, Kings College, Aberdeen, AB24 3UE
Abstract:
External Wall Insulation is a rapidly growing market. As the government releases more
schemes financing domestic and non-domestic owners to enter the low carbon society, there
become more products available for more problems and scenarios than ever before. This paper
examines how feasible it is for homeowners in the domestic sector to enter the race to cut our
carbon emissions as much and as quickly as possible. External Wall Insulation is not the only
method of cutting emissions so this paper will also look into other schemes and technologies
available to homeowners. Research found that the environmental impact of various products
was significant in reducing carbon emissions for individual dwellings.
Keywords:
External Wall Insulation, carbon emissions, thermal conductivity
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Contents
1 Introduction .................................................................................................................................. 3
2 Building fabric efficiency .......................................................................................................... 4
2.1 The role of the building fabric ........................................................................................ 4
2.2 Current and future trends................................................................................................ 4
3 Background to External Wall Insulation (EWI)................................................................. 5
3.1 External versus Internal insulation .............................................................................. 5
3.2 EWI in hard to insulate properties................................................................................ 6
4 How EWI works ............................................................................................................................ 6
5 Review of EWI products currently available...................................................................... 7
5.1 Analysis of products ........................................................................................................... 9
6 Schemes to finance EWI in the transition to low carbon..............................................10
7 Economic and Environmental impact assessment.........................................................11
7.1 Economic..............................................................................................................................11
7.2 Environmental ...................................................................................................................13
8 Conclusions..................................................................................................................................13
9 Bibliography................................................................................................................................14
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1 Introduction
Energy efficiency has become a major topic over the last 10 years with governments around the world
investing billions in lowering their countries’ carbon footprint. One way this has been done is by setting
targets and incentives for developers to build more energy efficient buildings.
One quarter of the UK’s carbon emissions come from private homes. Improving the energy
efficiency of a building generally has a direct effect on lowering the carbon footprint of the building. The
burning of fossil fuels to generate energy produces vast quantities of carbon dioxide which escapes into the
atmosphere. At least 82% of the energy heating homes in the UK is generated using fossil fuels. If more
heating energy is contained within the house, the amount of energy used on heating is reduced and less fossil
fuel is consumed.
The government is committed to a long-
term plan for cutting greenhouse gas emissions
drastically, shown above in Table 1.2. As a result
of this there are several large schemes and
incentives starting up currently, such as The Green
Deal and the Renewable Heat Incentive. To the
right is a picture, taken using a thermal imaging
camera, showing a house clad in External Wall
Insulation (EWI) amongst houses with no added
insulation. It is clear to see how much of an impact
this simple technology has on the lowering the
thermal conductivity of the building fabric.
Table 1.1 UK Government targets for Carbon dioxide emissions [16, pg. 7]
Figure 1.1 [12]
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2 Building fabric efficiency
2.1 The role of the building fabric
The fabric of a building separates ‘inside’ from ‘outside’ and refers to ceilings, windows, walls, floors
and doors. The building fabric is key to the energy equation of a building and has a large role in the
building’s energy efficiency. It is important to think carefully about the building fabric when designing a
building, as this is when the most can be done to improve the energy efficiency. That said, there are also
technologies available for existing buildings. These technologies might not be as cost effective or easy to
install but can be effectively applied during a pre-planned refurbishment. This is the best opportunity to
carry out such work.
U-Value explained
The U-value is the amount of heat that passes through one square meter of structure when the
temperature differs by one degree Celsius on either side. The units are watts per square metre per degree of
temperature difference, W/m2K [1, p. 6.0.4]. This is the value most commonly used by the government or
regulatory bodies to set energy rating targets for buildings. The lower the U-value, the more energy efficient
the building is as it loses less heat through its structure.
Carbon footprint explained
A person’s carbon footprint is the amount of Carbon Dioxide that enters the atmosphere because of the
electricity and fuel that person uses. This is measured in tonnes per year.
2.2 Current and future trends
The table below shows the government regulations for required U-Values of refurbishments on
domestic properties. Scotland consistently demands a lower or equal value owing to the temperature
difference between Scotland and the rest of the UK.
Existing Buildings – Scotland (England & Wales)
Domestic altered/refurbished
Lofts 0.15 (0.16)
All other roofs 0.18 (0.18)
Walls 0.22 (0.28)
Floors 0.18 (0.22)
Table 2.1 Government Regulation U-Values (W/m2K) for refurbishments [14]
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As energy prices rise yearly, this year seeing rises of 9.4% within some of the largest suppliers, it is
expected that the government will lower the required U-Value. There are no predictions of what these values
might look like but as insulation technology progresses it could soon be possible to achieve a U-Value of 0
W/m2K. Already it is possible to achieve 0.10 W/m2K, so it is only a matter of time before the regulations
reduce required value to something similar.
3 Background to External Wall Insulation (EWI)
Solid walled properties
With new building methods and materials it is looking likely that the government’s targets for new
super high-energy efficiency ratings will be met. This doesn’t address the problem of existing houses
however. Houses built before 1920 do not have cavity walls, instead their walls are solid brick or stone.
These types of houses make up 36% of all the houses in the UK, which is currently 24.5 million homes [2].
3.1 External versus Internal insulation
Insulating the walls of existing buildings dramatically improves a building’s energy efficiency. For
buildings without cavity walls this means external or internal insulation where added insulation material is
fixed to the exterior or interior of a wall. There are advantages to both and deciding which is most suitable
depends on several aspects. Below is a table comparing External and Internal Wall Insulation:
Pros of Insulation EWI IWI
Doesn’t require good external access Renews outside appearance
Improves weather and sound proofing
Fills gaps and cracks in the wall
Increases life of the wall
Doesn’t need planning permission Doesn’t reduce internal floor space of the
room
Can be installed room by room Not disruptive to household
Doesn’t affect fittings to the internal wall Damp problems can be dealt with after
installation
Table 3.1 Comparison of pros for EWI and IWI
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Why EWI?
As shown in the table above, EWI provides more benefits than IWI for insulating homes without
cavity walls. Although IWI is cheaper, if the wall is old there is an increased likelihood of other problems
such as poor structural integrity and damp. EWI can also alleviate or help these problems as well as
dramatically reducing the building’s CO2 footprint by increasing thermal efficiency.
3.2 EWI in hard to insulate properties
There are scenarios where EWI is difficult to implement and IWI is not an option. IWI may have to be
ruled out if there are features inside the property that cannot be replaced, such as detailed architrave or wall
coverings. A property could be hard to insulate for several reasons such as: the current wall is an extremely
poor insulator; the wall cannot support any extra imposed loading; or, particularly in old stone properties, the
walls are not straight or flat. EWI offers the simplest option as with such a large market in the UK there are
wide ranges of EWI products for many scenarios. In section 5 ‘Review of EWI products currently available’,
I will look at the most competitive products available for EWI today and the specialist scenarios they are
designed for.
4 How EWI works
External Wall Insulation works like a tea cozy, by wrapping the building in a material with a low U-
value. Insulating the walls externally can lower the U-value for a solid brick wall from 2.0, to as low as 0.15
W/m2K. Insulating material is attached to the external walls of the building, sometimes in layers depending
on the original U-value, and then they are covered in a weatherproof breathable membrane and then
rendered.
Figure 4.1 [12]
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5 Review of EWI products currently available
Kingspan
Kingspan Insulation is part of Kingspan group, one of Europe’s fastest growing building materials
manufacturers. Kingspan produces one product suitable for externally insulating existing solid walled
properties, the Kooltherm K5 External Wall Board for masonry walls.
As shown in figure 5.2 below, the K5 External Wall Boards come in various thicknesses, 50-125mm,
and has a thermal conductivity of 0.02W/mK to accommodate a wide variety of needs. The consumer has the
option to lower the U-value of their property to 0.15 W/m2K using the thickest board, 125mm. The K5 is
also structurally stable enough to support a variety of cladding systems; slate, timber or a regular render.
This flexibility gives consumers a wide scope for meeting buildings regulations whilst still having a lot of
stylizing options.
PermaRock
PermaRock is one of the UK’s leading Insulation manufacturers and is based in Loughborough,
Leicestershire. Unlike Kingspan, PermaRock have five different systems specifically designed for external
wall insulation. These systems are; Mineral Fibre, Expanded Polystyrene (EPS), Phenolic Foam, Lamella
and track EPS.
Mineral Fibre is made by spinning filaments of molten rock to form a board with low thermal
conductivity of 0.035–0.036 W/mK, and is available in thicknesses 30-200mm. Mineral Fibre systems are
highly versatile and can be used where more advanced specifications are needed, such as high fire resistance,
acoustic noise reduction and water vapour permeability [9].
EPS is a low cost lightweight option for EWI with thermal conductivity as low as 0.038 W/mK for 20-
350mm thicknesses. This system is ideal for situations where imposed loads need to be kept to a minimum
Figure 5.1 [7]
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on buildings with a lower level of structural integrity. The EPS system can also be more easily profiled to fit
unusual contours and features [10].
Phenolic Foam, like Mineral Fibre, is highly versatile as it also has the highest level of fire rating (0).
The thermal conductivity is exceptionally low at 0.021–0.024 W/mK and therefore thin, only available in 20-
100mm thicknesses [11]. This is a lightweight and low maintenance system but is more expensive than the
previous two.
The Lamella system is deigned for a feature not easily
accommodated by the previous three systems, curved walls. Lamella is
made from Mineral Fibre, however the fibres are orientated
perpendicular to the wall’s surface enabling the boards to fit curved
surfaces. Lamella is also fitted using high bond strength Lamella
Adhesive so no mechanical fittings are required. These boards are
available 30-300mm thicknesses and have a thermal conductivity of
0.04W/mK.
The PermaRock Track-EPS system varies again with the insulating
EPS (0.04 W/mK) boards being fastened to a track leaving a gap between
the boards and wall. This is most beneficial for uneven surfaces where it is
necessary to create a flat even finish.
Knauf Insulation
Knauf Insulation is the UK’s leading insulation manufacturer and operates in 35 countries around the
world. Knauf Insulation has one product specifically for EWI, the EWI Slab.
This system is made from mineral wool, like the mineral fibre system produced
by PermaRock. Knauf boast their KrimpactTM technology gives them an
advantage, with a consistent density and lightweight slab compared to other
similar products on the market. The EWI Slab has Thermal Conductivity of
0.038 W/mK in thicknesses from 40-150mm. This system can be fixed
mechanically or using adhesive. The slab is water repellent preventing water
damage to the underlying wall [13].
Xella
Xella is an International German company and is one of the world’s largest aerated concrete
specialists. Aerated concrete provides structural strength whilst still being lightweight and easy to handle; the
small air bubbles in the concrete also make it a superb insulator. From their Ytong range, the Ytong Multipor
Figure 5.4 [13]
Figure 5.2 [8]
Figure 5.3 [8]
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is a mineral panel made from aerated concrete with a thermal conductivity of 0.045 W/mK. This system is
available in thicknesses of 50-200mm providing a solution for a wide range of insulating needs. The
Multipor is ideal for scenarios where structural strength is an issue as these boards will also give a wall more
strength rather than take away from it. This product has also been awarded a certificate for its ‘exemplary
ecological compatibility’ by the German Institute for Construction and the Environment [15].
5.1 Analysis of products
The External Wall Insulation market is competitive and successful in the UK. As a result of the
competitiveness of the market there are many different solutions available for many scenarios. PermaRock
have the largest number of EWI specific solutions including bespoke solutions for some very specific
scenarios such as the Lamella system that is able to curve to fit the contours of buildings. It may be an
expensive solution, yet if your house has curved walls this is the only product available for EWI that would
be able to match the contours exactly. It does have one of the highest thermal conductivities at 0.04W/mK
however is available up to 300mm thick so is able to insulate a wall to a U-Value of 0.15W/m2K.
PermaRock’s other alternative solution, Track-EPS, gives the consumer the ability to totally redesign
the look of their home. Most solutions fit directly onto the wall and therefore show up anomalous contouring
and uneven surfaces. This system uses EPS boards attached to a track that is pre-fitted to the wall, giving a
crisp even finish. Some insurers require that there is a drained cavity space between insulated render and
existing wall when used on metal or timber framed buildings. This solution has been designed with this in
mind and meets insurers’ requirements.
The remaining EWI products are fundamentally the same: a board type insulator to be attached using
adhesive or mechanical fixtures. The difference is what the boards are made of, both PermaRock and Knauf
have a Mineral Fibre product made from filaments of molten rock. Both products also have thermal
conductivity of 0.038W/mK. The PermaRock solution is very heavy, however it is available in thicknesses
up to 200mm providing a U-Value of 0.19W/m2K. Using KrimpactTM technology, Knauf Insulation’s
mineral fibre boards are lightweight and easy to handle, making them more suitable for structurally less
stable walls. These boards are only available up to 150mm thick, which can only give a U-Value of
0.25W/m2K. Although is up to the standard of today it isn’t planning for the future where energy prices are
higher and new houses are being built with a U-Value of 0.
Expanded Polystyrene is the budget option for EWI; it is lightweight and easy to fit. Due to its weight
it is available up to 350mm thick giving a U-Value of 0.10W/m2K. EPS is ideal for walls that cannot take too
much extra imposed loading or where the wall material is fragile, so fixtures can’t hold much weight. The
problem with EPS is that it is bulky and can make windows and doors look very sunken into the wall, as
large thicknesses are needed to achieve suitable U-Values.
Then are there products that reach into the ‘premium’ ranges of EWI: the Kingspan Kooltherm K5
External Wall Board, PermaRock’s Phenolic Foam Board and the Xella Ytong Multipor. Each has developed
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a more advanced and engineered material providing excellent results in the areas of fire protection, strength
and longevity. Both the K5 and Phenolic Foam have exceptionally low thermal conductivity of 0.02 W/mK.
This means thinner boards can be used making the insulation look like it has always been there. The Ytong
Multipor doesn’t have the same exceptional standards of thermal conductivity; however, being made from
aerated concrete, it does provide the best structural support for walls which otherwise wouldn’t be able to
support any EWI at all.
6 Schemes to finance EWI in the transition to low carbon
CERT (Carbon Emissions Reduction Target)
The government’s Carbon Emissions Reduction Target (CERT) requires all domestic energy suppliers
with more than 50,000 domestic customers to actively lower the quantity of CO2 emitted by households in
the UK between 2008 and 2012 [3;4]. Ofgem regulate the energy suppliers and provide quarterly reports to
the government. Suppliers meet their targets by promoting the uptake of low carbon energy solutions. The
aim of CERT is to make a contribution to the UK’s target under the Kyoto Agreement to cut greenhouse gas
emissions to 12.5% below 1990 levels by 2008-2012 [3].
CESP (Community Energy Saving Programme)
The Community Energy Saving Programme (CESP) is another government initiative aimed at helping
families improve their energy efficiency and reduce energy bills with an emphasis towards low-income
areas. 4,500 areas are eligible in the UK, selected using the Income Domain of the Indices of Multiple
Deprivation England, Scotland and Wales. In England the lowest 10% and in Scotland and Wales the lowest
15% qualify [5].
The Green Deal
The Green Deal, to be released in Autumn 2012, is the government’s flagship carbon cutting initiative
and part of the Energy Bill, introduced in Parliament on 8 December 2010. The Green Deal gives consumers
the opportunity to improve the energy efficiency of their home without having to pay anything up front. The
‘Golden Rule’ [16, pg. 5] is that the expected financial savings from the new insulation must be greater or
equal to the cost of the work done. Energy suppliers and private firms will provide the capital for the work.
Repayments will be made through the customer’s energy bills but the amount repaid won’t exceed the usual
energy bill once the new energy efficiency measures are in place. To be eligible for the Green Deal, as with
most government schemes, there are numerous hoops to jump through first.
The first step is to have an accredited assessment to determine what energy efficiency measures are
suitable for the property in question. The assessor will also give a total cost for the work, the amount saved
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in energy bills and the length of the repayment depending on the type of repayment chosen. The Green Deal
repayment scheme, meaning repayment is made through the energy bill, can also be passed on to future
occupiers.
Once finance has been approved an accredited installer must be chosen from a list accredited by the
Department of Energy and Climate Change. Accredited installers will all hold specific certification ensuring
a standard of product and material certification, and that codes of practices are adequately met; they will also
ensure warranties covering materials and work are all equal.
7 Economic and Environmental Impact Assessment
This Economic and Environmental Impact Assessment looks not only at External Wall Insulation but
other carbon cutting technologies too.
7.1 Economic
EWI cost benefit analysis
Type of solid wall
insulation
Saving per
year
Total
cost including installation
Carbon dioxide saved per
year
Internal Around £445 £5,500 to £8,500 1.8 tonnes
External Around £475 £9,400 - £13,000 1.9 tonnes
Table 7.1 [6]
Renewable Heat Incentive
The Renewable Heat Incentive (RHI) scheme
aims to make renewable heat financially attractive to
businesses and house owners. By offering financial
incentives for installing renewable heating, the
government hopes to cut down on the amount of fossil
fuel the UK uses and help work towards the
greenhouse gas reduction target. The influx of money
into the renewable energy industry will create jobs
from researchers to installation experts. This review
will focus on the RHI for domestic homes. Figure 7.1 Figure 7.1 [17, pg. 9]
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above shows the need for the RHI in the UK with only 1.5% of heating energy in 2008 coming from
renewable sources.
The RHI will be implemented in phases; Phase One has been in operation since August 2011. This
first phase has brought in the Renewable Heat Premium Payments that focuses on houses without a gas
mains supply. The government has set aside £15 million specifically for the premium payments that provide
grants for solar thermal, biomass boilers, air source heat pumps and ground or water source heat pumps. The
solar thermal is the only renewable heating measure open to all UK houses as the others are only open to
homes not connected to mains gas. Below is a table showing the grants available for each renewable heating
measure and the amount each measure costs to install.
All Houses Houses not heated by gas from the mains
Technology Grant Installation Cost Technology Grant Installation Cost
Solar
thermal £300 £4,800 [19] Biomass boiler £950 £11,500 [20]
Air source heat pump £850 £6,000-£10,000 [21]
Ground or water source heat pump £1250 £9,000- £17,000
[22]
Table 7.2 grants available and cost of installation for renewable heating measures [18]
To be eligible for these grants the house must have basic loft and cavity wall insulation. The measures
to be installed must be listed under the ‘Microgeneration Certification Scheme’ found at
www.microgenerationcertification.org. The homeowner must also complete two surveys once the system is
up and running to give the government feedback on the RHI scheme and the measures installed.
Phase Two of the RHI will come into effect sometime in 2012 to coincide with the Green Deal. Phase
Two for domestic homes will see the government bringing in tariffs to pay home owners for their renewable
heat. This will be open to anyone with renewable heating measures installed since 2009, not just those
installed through the Renewable Heat Premium Payments scheme. These tariffs provide the longterm
financial incentive needed to make domestic renewable heat technology feasible to homeowners.
Feed in Tariff Scheme (FITs)
The Feed in Tariff Scheme is similar to the RHI but for electricity production. The government has
tariffs for various renewable electricity generation methods based on the amount of electricity you generate
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per year and how much is exported to the grid. For a typical domestic solar energy scheme the homeowner
per year could earn £1,060 from the generation tariff, £40 from the export tariff and save £90 in energy bills.
7.2 Environmental
To put carbon cutting into context, the average UK home produces 10 tonnes of CO2 per year. Below
is a table showing the number of tonnes of CO2 saved by various carbon-cutting technologies. The amount
saved depends on what energy source is being replaced; for example, coal produces large quantities of CO2
whereas gas produces relatively little.
Technology CO2 saved per year
(Tonnes)
IWI & EWI 1.8 - 1.9
Solar thermal 0.25-0.5
Solar electricity 1
Biomass boiler 3 - 7.5
Air source heat pump 0.8 - 5.4
Ground or water source
heat pump 0.8 - 5.4
Wind turbines 5.2
Table 7.3 Tonnes of CO2 saved using renewable technology
8 Conclusions
External Wall Insulation is definitely feasible and practical in the UK. EWI is not the only carbon
cutting technology showing promising results, however. As shown in section 7.2 Environmental, some
measures available such as Biomass boilers can cut CO2 emissions by 75%. When one quarter of the
UK energy use comes from domestic these types of technology could have a major impact on the
national carbon footprint. EWI alone can cut 20% off the carbon emissions from a home and with
government grants this has become very feasible.
Despite the difficulties in insulating solid walled houses, due to the high demand EWI has
become a competitive market with numerous bespoke solutions for many scenarios and house types.
Even for houses with listed facades where EWI is not possible there is IWI. While it may not be as
favorable, it still has a huge impact on reducing the energy consumption.
It is beneficial financially to spend money on upgrading insulation now while the government is
funding lots of schemes. Once a house has achieved basic up to date insulation the homeowner can
apply for grants to fit renewable heat and electricity technology and actually start to earn money. At the
same time the home will be helping to contribute to the UK’s targets for carbon reduction.
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9 Bibliography
[1] The Scottish Government (SG), (15 October 2011) Technical Handbooks 2011 Domestic
Energy, http://www.scotland.gov.uk/Resource/Doc/217736/0120386.pdf
[2] National Insulation Association (NIA), (15 October 2011) Solid Wall Insulation,
http://www.nationalinsulationassociation.org.uk/householder/index.php?page=solid-wall-
insulation
[3] Department of Energy and Climate Change (DECC), (17 October 2011) Carbon Emissions
Reduction Target (CERT],
http://www.decc.gov.uk/en/content/cms/funding/funding_ops/cert/cert.aspx
[4] Ofgem, (17 October 2011) Energy Efficiency,
http://www.ofgem.gov.uk/Sustainability/Environment/EnergyEff/Pages/EnergyEff.aspx
[5] Department of Energy and Climate Change (DECC), (17 October 2011) Carbon Emission
Saving Programme (CESP), Communities: Areas of Low Income,
http://www.decc.gov.uk/assets/decc/consultations/cesp/1_20090630123736_e_@@_decccomm
unitesareasoflowincomecesp.pdf
[6] Energy Saving Trust, (17 October 2011) Solid Wall Insulation,
http://www.energysavingtrust.org.uk/In-your-home/Roofs-floors-walls-and-windows/Solid-
wall-insulation
[7] Kingspan Insulation, (March 2011) Kooltherm K5 External Wall Board,
http://www.kingspaninsulation.co.uk/getattachment/890a52c7-2425-4c29-88f2-
3cd508a8e364/Kooltherm-K5-External-Wall-Board.aspx
[8] PermaRock, (26 October 2011) Products/ External Wall Insulation,
http://www.permarock.com/products/external_wall_insulation_systems.aspx
[9] PermaRock, (29 October 2011) External Wall insulation Systems/ Mineral Fibre,
http://www.permarock.com/products/external_wall_insulation_systems/mineral_fibre.aspx
[10] PermaRock, (29 October 2011) External Wall insulation Systems/ EPS,
http://www.permarock.com/products/external_wall_insulation_systems/eps.aspx
[11] PermaRock, (29 October 2011) External Wall insulation Systems/ Phenolic,
http://www.permarock.com/products/external_wall_insulation_systems/phenolic.aspx
[12] PermaRock, (29 October 2011) PermaRock existing buildings brochure,
http://www.permarock.com/downloads/PermaRockExistingBuildingsBrochure.pdf
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[13] Knauf Insulation, (29 October 2011) EWI Slab,
http://www.knaufinsulation.co.uk/products/rock_mineral_wool_slabs/external_wall_slabs/ewi_
slab.aspx
[14] Kingspan, U-Value Health Check, 2006
[15] Xella, (31 October 2011) Ytong Multipor Brochure, http://www.mhe-
multipor.com/Ytong%20Multipor%20MHE.pdf
[16] Department of Energy and Climate Change (DECC), (1 November 2011) The Green Deal
– A summary of the Government’s proposals, 2010,
http://www.decc.gov.uk/assets/decc/legislation/energybill/1010-green-deal-summary-
proposals.pdf
[17] Department of Energy and Climate Change (DECC), (2 November 2011) Renewable Heat
Incentive, March 2011,
http://www.decc.gov.uk/assets/decc/What%20we%20do/UK%20energy%20supply/Energy%20
mix/Renewable%20energy/policy/renewableheat/1387-renewable-heat-incentive.pdf
[18] Department of Energy and Climate Change (DECC), (2 November 2011) Renewable Heat
Premium Payments- factsheet, 2011,
http://www.decc.gov.uk/en/content/cms/meeting_energy/renewable_ener/incentive/factsheet/fac
tsheet.aspx
[19] Energy Saving Trust, (2 November 2011) Solar Water Heating, 2011,
http://www.energysavingtrust.org.uk/Generate-your-own-energy/Solar-water-heating
[20] Energy Saving Trust, (2 November 2011) Wood Fuelled Heating, 2011,
http://www.energysavingtrust.org.uk/Generate-your-own-energy/Wood-fuelled-heating
[21] Energy Saving Trust, (2 November 2011) Air Source Heat Pumps, 2011,
http://www.energysavingtrust.org.uk/Generate-your-own-energy/Air-source-heat-pumps
[22] Energy Saving Trust, (2 November 2011) Ground Source Heat Pumps, 2011,
http://www.energysavingtrust.org.uk/Generate-your-own-energy/Ground-source-heat-pumps