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AJ Special External Envelopes G#1688 N#1592

By 15 November 2014June 26th, 2019ASWS, Issues

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AJ Special External Envelopes
Green Specifications for External Envelopes

AJ Special External Envelopes
Green Specifications for External Envelopes

Green Specifications for External Envelopes

Written by Cathy Strongman Freelance

Originally published in AJ Special External Envelopes

Brian Murphy trained as an architect but his real interest lies in materials, methods and material science, so 20 years ago he packed away his drafting tools and turned to specification writing. ‘I write recipes for buildings,’ he says. ‘I describe every ingredient of a building, down to the most miniscule detail, and match it with the most suitable construction methods. The only thing I don’t normally cover are the structure and the services. But I often influence them too. I see my role as supporting designers who want to create a specific vision, but don’t have the technical and materials knowledge to do so. I feel I can add more to an architectural solution through the specification process, than I could if I were just another person working for another architectural practice.’

Murphy’s first commission was colossal – writing the specification for the New British Library. ‘It was a scary one to start with and within the first few months I realised just how technical and detailed specification really is. A sheet of glass, for example, can have up to twenty different characteristics.’ But as well as providing him with a baptism of fire into the specification profession, the project also awakened the environmentalist in Murphy and permanently altered the direction in which his career would progress. ‘There was an incident where the designers wanted to use Afromosia wood in the exhibition room, a timber which the Friends of the Earth’s Good Wood Guide had just listed as an endangered species. Worse still they were using this rich coloured species and staining it black. I had to write the specification for this material, despite voicing my concerns, and at this point I decided that I needed to write a green specification that would guide others to make more sustainable choices.’

In 2001 Murphy launched National Green Specification NGS GreenSpec, GreenSpecDownload and now Green Building Encyclopaedia (, visitors ranging from architects, students, self-builders and construction industry players. Murphy is also working with students from Southbank, Cardiff and Sheffield Hallam Universities and runs TGR workshops and lectures on green construction, waste and specification throughout the country. These seminars are available to download from the website at a rate of £0.88 per seminar, a move that Murphy hopes will help disseminate knowledge even further.

Murphy’s mission is to promote both the specification profession and within that, a green agenda. His definition of green is primarily environmental, although much of his advice, such as encouraging the use of local materials and technologies, crosses over into the realms of social and economic sustainability. ‘Attitudes towards environmental design are definitely getting better, but there are still a lot of misinformed choices being made,’ he says. ‘Take concrete as an example. Concrete is a major problem. Cement alone is responsible for 8% of the global CO2 production, so if you have to use concrete, you need to make it greener. It’s not difficult. You just blend Portland cement with other substitutes like Ground Granulated Blastfurnace Slag Cement (GGBS) and Pulverised Fuel Ash (PFA) you can reach 65% GGBS without changing the rate of strengthening and formwork removal. You can also reach up to 30% recycled aggregate substitution in structural elements and 70% in non-structural elements. Unfortunately at the moment the Code allows such aggregates to be used, but structural engineers are not encouraging or demanding it. More often than not, they are actively discouraging it. So currently only 1% of UK concrete – a material that is fundamental in virtually every building that we construct – is green concrete. We could make an enormous difference by simply changing that specification.’

Here Murphy addresses the key challenges that will face external envelopes in the coming century and discusses ways in which we need to specify greener materials and construction methods:


Weather is an inevitable part of looking at an external envelope of a building, and as the climate changes we will be forced to rethink the specifications for our buildings. I fully expect clients to sue over buildings that don’t work in 10, 20 or 30 years time because they will be able to argue that we knew about global warming and failed to design accordingly. The planet is changing rapidly and construction has got to evolve to respond to that within the constraints of Professional Indemnity Insurance. As weather conditions become more extreme, we need to make our external envelopes airtight, watertight, stronger, more flexible and more forgiving.

Take rain as an example. Prolonged periods of heavy rain are likely to lead to greater incidences of flooding. At the moment there are two conventional means by which we keep buildings watertight. First, there is curtain walling, which repels rain and allows it to run straight off. However, high embodied aluminium framing and a 100% glass facades, which usually require air conditioning behind them, are deadly to the planet. Secondly, there is brickwork, which soaks in the water and releases it later, again possessing reasonably high embodied energy. There are also some bespoke timber curtain walling installations and proprietary off the shelf systems on the market. Most of these new products come from Germany and mainland Europe.

We need to be exploring new technologies on the market that can cope with heavy rain more effectively and are kinder to the environment. Sustainable Urban Drainage Systems (SUDS) for example, include flat green or brown roofs and permeable paving to absorb, hold and slow down rainwater and to divert it away from sewers to replenish the water table. I’m afraid, that until the Government force the industry to change by addressing Legislation Building Regulations Approved Document G, which address water efficiency, we are going to see very little change.


Air tightness is a big issue as warm air escaping through leaks in a building can squander up to 50% of the energy created to heat that building. The Building Regulations approved documents E and L deal with acoustics and airtightness and is supported by sets of robust details that address air tightness. Yet they only require the testing of a proportion of buildings, which is not enough. Furthermore, they only require a pass rate of 10, which is poor in comparison to European standards where a test rate of three, two and even below one is common. Passivhaus requires 0.6

The solution lies in informed design, effective materials and finally, good workmanship. We need to make the building fabric airtight, rather than concentrating on the final finishes, sealing leaks in finishes are often more costly and less effectual. Materials like plasterboard, might be assumed to be airtight, but even with all the joints filled they can leak air at skirting and head. Designers therefore have to make sure the abutments and laps between materials are airtight as well as the perimeters of windows, doors, the points at which timber floor joints bear into a wall, services penetrations, skirting boards, floor linings, walls and ceilings. Airtightness layers can be introduced, made from recycled paper they are vapour permeable and airtight, Pro Clima DB+ is available from Construction Resources.

Windows and doors do tend to be of a very high standard these days and GreenSpec lists ‘Ecoplus’ external doors, which are made in the UK out of FSC timber and have a very good acoustic, airtightness and thermal performance. The ‘Ecoplus’ windows have similar characteristics and both are durable and available from the Green Building Store.


There are a host of means by which you can make a building watertight, ranging from a permeable masonry wall, rainscreen backed up by a waterproof layer, to a 100% glazed wall that will attempt to allow no water in at all. Conventional methods, however, tend to use materials, such as aluminium, which have high embodied energy and are very labour intensive.

New green technologies work on the basis that the rainscreen is semi permeable, catching most of the rain, but still allowing some moisture to pass through open joints. Timber rainscreen weatherboarding is a good example, but these rainscreens could be made from a number of materials including glass, slate or metal. By accepting that there will be leaks in the external surface you avoid the material or labour intensive process of sealing the joints. A recycled glass rainscreen cladding seen at EcoBuild is Structuran by Indupart.


A different approach to external walls is to let them breathe, the key to these breathing walls is to include hygroscopic insulation. These types of insulation can tolerate moisture by absorbing moisture into the fibres themselves, they clear the airspace, further improving the insulation quality. Once weather conditions change, the water can then evaporate off. Whereas man made hydrophobic insulation materials such as glass and rock mineral fibre are unable to absorb into the fibre diminishing the performance when moisture is present. Sheep’s wool or cellulose fibres made from chopped timber fibre or coconut fibre are hygroscopic, therefore able to accommodate moisture. It is however, important to note that hygroscopic insulation needs to be matched to the construction method. Airtightness layers and breathing sheathing boards are essential in a timber stud construction. Sheep’s wool for example, is suitable for a timber construction, but should never be used in a wet masonry cavity. Thermafleece sheep’s wool is available from Second Nature UK ltd and their extensive supply chain across the UK.

Airtightness layers can be introduced, made from recycled paper they are vapour permeable and airtight, Pro Clima DB+ is available from Construction Resources.

Breathing sheathing boards include Panelvent from Excel Industries.


In the hierarchy of issues that need to be address urgently to improve the performance of our external walls, then currently U-value comes top of the list followed closely by airtightness. The immediate problems that we have to concentrate on if we really are going to save the planet are energy loss and carbon dioxide generation, and the answer lies in insulating our new and especially our existing buildings to a much greater degree. U-values are crucial in this process.

The U-value is the measure of how resistant the external envelope is in total to the passage of heat (so we need to aim for a low U-value to prevent a building from gaining or losing heat too rapidly). The U-value is calculated by adding up the k-values of each of the materials that together make up the external envelope. The k-value is in turn calculated by multiply the resistivity (R) of a material by it’s thickness.

Unfortunately, the materials with the best k-values have historically also been very damaging to the environment. Plastics turned to foam using ozone depleting gases had very good k-values, but as manufacturers have been forced to abandon the use of CFC, HCFCs, HFAs and HFC’s, employing instead pentane and CO2, the performance of the insulation has diminished. Many of these ozone depleting materials created over the past two decades lay dormant in buildings waiting to be release at the end of the buildings lives potentially leading to a second ozone crisis.

There are still some man made insulators that have reasonable k-values. Cellular glass, for example, is very strong, dimensionally stable and fire resistant but again, now made with recycled car windscreen glass but has high embodied energy from manufacture. By far the greenest option is to use natural materials such as hemp, coconut fibre and cellular fibre, all of which have a decent k-value that provide a good U-value for the external envelope. Hemp fibre and cotton insulation includes Isonat from Plant Fibre Technology Ltd.  Pavatex’s Pavetherm wood fibre boards are available from Natural Building Technologies.


The decrement delay has been known and taught since the 70s but is a new characteristic to our industry, which the Germans have been promoting for some time, they record this characteristic in technical literature on their insulation materials. It is a measure of decrement and concerns the time taken for heat to pass through materials, but also how it is held within them. In other words, there is an opportunity to exploit a combination of characteristics U-values and thermal mass from one material. So by including thermally massive, thermally insulating materials in the exterior envelope of a building you can, not only slow the movement of heat down, but store that heat as well.

Eventually the understanding of Decrement delay value will become more commonplace in Britain, but at the moment there are many insulation material on the market that exploits this characteristic. Dense cellulose fibre insulating boards.

Pavetex’s Diffutherm from Natural Building Technologies and Gutex Thermawall from Construction Resources are both render system carriers, ideal for mineral based thin render systems, replacing expanded polystyrene and synthetic/acrylic renders.

A green material that has unbelievably beneficial Decrement Delay is Hemp-lime. Three companies, Lhoist, HemCore and Lime Technologies have worked together to transform what was a cottage industry into a potential major force in the construction industry, and although it is not a load bearing material quite yet, Hemp-lime is on the edge of becoming British Standard compliant for blockwork.  In the mean time it is used as a spray applied insulating airtight monolithic building material for walls and roof with potential to find uses in many other applications. Research is underway, guides and specifications are being written now. Tradical Hemcrete from Lime Technologies is the first of many exciting products in this material that has the potential to be carbon negative, better than carbon neutral.


Acoustics and airtightness go hand in hand so if you concentrate on meeting Building Regulations Part E and L, which deal with acoustics and heat loss, you will get sound-tightness, airtightness and energy saving as a bonus.  Yet, whereas for airtightness, we spoke a lot about leakage prevention, the choice of materials used in the exterior envelope is also a crucial factor when it comes to Acoustic performance.

Weight is a valuable characteristic when it comes to acoustics and a heavy masonry wall will be able to stop sound waves from passing through it. Different arrangements of materials can also effect acoustic performance, so there are wall lining materials on the market that have very small slats on the inner surface with a cavity behind. When the sound waves enter through the slot and into the chamber they bounce around until they are absorbed.

From a green perspective, it is certain insulation materials that we are really interested. Cellulose fibre has been proved to have an excellent acoustic performance, far out stripping that of glass or rock mineral fibre. So despite not being marketed specifically as acoustic insulating products, products such as Gutex Thermawall from Construction Resources prove both kind to the environment in their manufacture, but also highly beneficial in reducing noise pollution internally and externally.


Thermal mass is a characteristic of dense materials, capturing heat in those materials for beneficial use later can save energy. So in the winter, a material with a high thermal mass can be exposed to any sunlight, the heat stored to keep the room warm into the evening; insulation beyond the mass will prevent the heat from being lost. Whereas in the summer it can prevent the heat hitting the exterior of a building from cooking the people inside, heavy masonry building in Mediterranean climates do this well. When heat is stored within interior walls, floors and roofs elements of the exterior envelope and gradually released when free heat sources are gone, exploiting thermal mass can protect us from the rapid fluctuations in external temperatures.

The important thing to realise is that thermal mass is only beneficial if you exploit it. In conventional construction where you have a cavity wall construction with a brick outer leaf that will capture heat, insulation that will stop it moving through the wall and then insulating concrete blockwork and insulating plaster on the inside, the thermal mass of the concrete is wasted. However, architects like Bill Dunster, who do understand thermal mass, are designing buildings that make the most of this characteristic, exposing concrete floor and roof surfaces in buildings like BedZED. Some have gone even further, absorbing summer heat in the building fabric and allowing it to bury itself in insulated pockets of ground behind and below the building. During the winter this heat will be released back into the building.

Thermal mass is almost always referred to in terms of concrete, but anything that has been fired and is heavy and dense will have a high thermal mass. Bricks for example, have this characteristic although it is very rarely exploited because they are mostly used on the exterior of buildings. Wood fibre reinforced dense gypsum boards that are very heavily compressed create a reasonable thermal mass without relying on concrete. Sasmox by Sasmox available from The Panel Agency and Fermacell available from Construction Resources are examples.

New technologies are also now being invented, Dupont have Energain which exploits phase-change properties, it uses wax inside the board. When heat energy is absorbs the wax melts and when it releases this heat it solidifies. A lightweight panels with heavy weight properties.


Moisture mass is a subject that most of us know nothing about and there is one architectural practice in the UK – GAIA – that is really exploiting this characteristic in its projects. Moisture mass is the ability of a material to take moisture out of the air and release it when it is appropriate. This is an important characteristic as it helps stop spores that settle on the surfaces of materials, finding moisture and growing into mould in damp humid conditions.

There is a whole range of materials that have this characteristic. Raw clay, for example, is a fantastic moisture absorber and can be utilised through clay bricks, clay blocks and clay plaster. Unfired clay bricks have been produced for centuries all over the world and they are now being made in factories, dried naturally by the passage of air through the space. One company in the UK are experimenting with clay bricks is Ibstock Ltd. and their EcoTerra Earth Brick.

Any form of earth will have a high moisture mass, as will hemp products such as Hemp Lime example Hemcrete by Lime Technology Ltd. These porous renders lend themselves to traditional methods of construction such as stone and lime based mortar walls. Yet they are equally important in modern green building. As construction methods such as Hemp or rammed earth grow in popularity so too will our understanding and application of moisture mass.

Written by Cathy Strongman Freelance Journalist
Originally published in AJ Special External Envelopes
Updated by BrianSpecMan
15th November 2014 – 7th December 2015

AJ Special External Envelopes
Green Specifications for External Envelopes
See Also:

GBE Projects

  • New British Library

GBE Materials

  • Afromosia

GBE Links

  • Friends of the Earth

GBE Library

  • Good Wood Guide
  • endangered species



  • CO2
  • Portland cement
  • Substitutes
  • Ground Granulated Blastfurnace Slag Cement (GGBS)
  • Pulverised Fuel Ash (PFA)
  • GGBS
  • recycled aggregate substitution
  • external envelopes
  • climate changes
  • global warming
  • Professional Indemnity Insurance (PII)
  • flooding.
  • watertight.
  • curtain walling,
  • high embodied
  • aluminium
  • 100% glass facades,
  • air conditioning
  • high embodied energy.
  • bespoke timber curtain walling
  • Sustainable Urban Drainage Systems (SUDS)
  • flat green or brown roofs
  • permeable paving
  • Building Regulations Approved Document G
  • Air tightness
  • Building Regulations approved documents E and L
  • robust details
  • Passivhaus requires 0.6
  • effective
  • good workmanship
  • building fabric
  • plasterboard
  • perimeters of windows, doors
  • timber floor joints
  • services penetrations,
  • skirting boards
  • floor linings
  • walls linings
  • ceiling linings.
  • Airtightness layers
  • recycled paper
  • vapour permeable and airtight
  • external doors
  • FSC timber


  • ‘Ecoplus’ doors and windows
  • Pro Clima DB+
  • Structuran
  • Thermafleece


  • Construction Resources
  • Green Building Store.


  • Indupart
  • Second Nature UK ltd.


  • watertight
  • permeable masonry wall
  • rainscreen
  • waterproof layer
  • 100% glazed wall
  • semi permeable
  • open joints.
  • H21 Timber rainscreen weatherboarding
  • recycled glass rainscreen cladding
  • EcoBuild


  • breathing wall
  • hygroscopic insulation
  • weather conditions change
  • evaporate
  • hydrophobic
  • glass and rock mineral fibre
  • Sheep’s wool
  • cellulose fibres
  • coconut fibre
  • hygroscopic
  • Airtightness layers
  • breathing sheathing boards
  • timber stud construction.
  • Sheep’s wool
  • timber construction
  • wet masonry cavity.
  • supply chain
  • recycled paper
  • vapour permeable and airtight,
  • Breathing sheathing boards
  • Panelvent
  • Excel Industries (No longer)
  • energy loss
  • carbon dioxide generation
  • U-value
  • k-values
  • resistivity (R)
  • Plastics turned to foam
  • ozone depleting gases
  • CFC, HCFCs, HFAs and HFC’s,
  • pentane
  • CO2
  • second ozone crisis
  • Cellular glass,
  • dimensionally stable
  • fire resistant
  • recycled car windscreen glass
  • high embodied energy from manufacture.
  • natural materials
  • hemp,
  • coconut fibre
  • cellular fibre,
  • Hemp fibre
  • cotton insulation
  • Isonat
  • Plant Fibre Technology Ltd.
  • Pavatex
  • Pavetherm
  • wood fibre boards
  • Natural Building Technologies.
  • decrement delay
  • characteristic
  • thermal mass
  • thermally massive
  • thermally insulating materials
  • Dense cellulose fibre insulating boards.
  • Pavetex’s Diffutherm
  • Natural Building Technologies
  • Gutex Thermawall
  • Construction Resources
  • render system carriers
  • M20 mineral based thin render systems,
  • M21 expanded polystyrene and synthetic/acrylic renders
  • Hemp-lime.
  • Lhoist,
  • HemCore
  • Lime Technologies
  • spray applied insulating airtight monolithic
  • Tradical Hemcrete
  • Lime Technologies
  • carbon negative
  • carbon neutral.
  • sound-tightness
  • Acoustic performance
  • Cellulose fibre
  • acoustic performance
  • glass or rock mineral fibre
  • Gutex Thermawall
  • Construction Resources
  • noise pollution internally and externally.
  • Thermal mass
  • dense materials
  • cavity wall construction
  • architects
  • Bill Dunster
  • BedZED
  • ZEDfactory
  • Wood fibre reinforced dense gypsum boards
  • Sasmox
  • Sasmox
  • The Panel Agency
  • Fermacell
  • Construction Resources
  • New technologies are also now being invented,
  • Dupont
  • Energain
  • phase-change properties
  • Moisture mass
  • GAIA
  • moisture vapour
  • spores
  • mould
  • damp humid conditions.
  • clay bricks
  • clay blocks
  • clay plaster.
  • Unfired clay bricks
  • Ibstock Ltd.
  • EcoTerra Earth Brick.
  • Hemcrete
  • Lime Technology Ltd.
  • traditional methods of construction
  • lime based mortar
  • rammed earth

© GBE NGS ASWS BrianMurphy
aka BrianSpecMan
15th November 2014 – 7th December 2015

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