Best Practices

Best Practice > Best Practices

A guide to best practice in sustainable design.

Low / zero carbon design

A low or zero carbon design refers to a building that has a negligible or zero nett energy consumption in one year. A building that consumes no or little energy in one year will significantly cut down on greenhouse gas emissions and running costs.

Microgeneration

Micro Energy Generation refers to the generation of heat and power by individuals to help reduce or eliminate their dependence on the national grid and therefore contribute to the reduction or elimination of carbon emissions.

Micro Generation embraces a range of technologies such as those indicated below;

  • Small wind generators
  • Micro hydro generation
  • Small scale photovoltaic array

The use of domestic scale combined heat and power equipment such as the above can contribute significantly to self sufficient energy generation and costs.

Passive solar design principles

The use of passive solar design principles in the design of a building can reduce heating, cooling and lighting demands.

In order to get the most from passive solar design one needs to be aware of the following passive solar heating techniques;

  • Direct gain – Solar radiation directly penetrates the building and is stored there
  • Indirect gain – Collects, stores and distributes solar radiation using thermal storage. Conduction, radiation or convection transfers the energy indoors
  • Isolated gain – Conservatories or atria collect solar radiation in an area that can be selectively closed off or opened to the rest of the house

In order to take most advantage of passive solar gain it is important that the following issues are considered;

  • Siting
  • Orientation
  • Landscaping

Passive solar design and taking advantage of a site microclimate both enhances the energy and environmental performance of a building. Ideally the building should take the maximum advantage of the site’s solar radiation and daylight, as well as good shelter from the wind.

Site

  • Determine the position of the sun throughout the year
  • Establish temperature ranges – both seasonal and daily
  • Identify the direction of the prevailing wind
  • Determine seasonal characteristics e.g. cold northerly winds in winter
  • Identify topographical features that might optimise or degrade the performance of the building(s) e.g. slopes, tree belts, the shape and orientation of the site

Orientation

The main orientation of the building should be within 30° of south in an East / West direction. Houses orientated east of south will benefit from the morning sun. Those orientated west of south will catch the late afternoon sun – which can help delay the evening heating period.

A location on a south facing slope optimises solar access whilst minimising overshadowing from adjacent buildings. It also allows for higher density planning.

  • Neighbouring houses to the east and west can provide protection from low east and west sun
  • Roads should ideally run east-west to facilitate south-facing front or rear housing layouts
  • Design layouts to be self-sheltering from cold winds
  • Use tree belts around the site to promote sheltering. Arcs across the north of the site will be particularly useful against cold northerly winds

Building / room layout

  • Design to minimise the building surface to volume area
  • Orientate the house east-west to ensure a long side to face the sun. Minimising east and west facing walls and windows reduces excessive summer heat gain
  • Plan the rooms so that cooler service spaces are located with a northerly aspect and habitable rooms take advantage of the warmer southerly aspect
  • Avoid the exposed areas of the site and use any natural shelter offered

Landscaping

  • Minimise wind chill from the prevailing wind by presenting a narrow frontage in that direction
  • Think about using vegetation against walls to increase wind drag and provide an extra thermal buffer
  • Optimise solar gain in winter – ensure that south-facing windows are not overshadowed between 9am – 3pm
  • Use trees and planting to shelter from, particularly cold northerly, winds. The most effective height for trees is the height of the building and placed 1-3 heights away, or 3-4 heights where solar access is required. Use evergreen trees where solar access is not required to provide year-round shelter
  • Deciduous trees should be planted to optimise shading in the summer whilst permitting sun to penetrate at low winter angles. Ensure that the planting is not too dense that it limits daylight

Direct solar gain

Direct gain is the most basic form of solar gain. Solar energy enters through south-facing glazing and is absorbed by thermal mass incorporated into the floor and walls. Heat is stored in the thermal mass during the day and later released during the night into the living space. This re-radiation of ‘stored’ heat can maintain a comfortable temperature during cool nights and can extend through several cloudy days without ‘recharging’.

  • Three quarters of the solar energy striking the glass is converted into thermal energy
  • Solar radiation can provide a large proportion of a building’s heating requirements
  • The correct area of glazing needs to be determined in response to the duration and severity of winter temperatures; the building size; and the amount of interior thermal mass. It is important to get the correct balance between these factors in order to avoid large daily temperature fluctuations that could result in overheating, even in winter
  • Solar energy is most effectively absorbed by direct radiation – conduction and convective air currents can transmit energy to areas of mass that are not directly illuminated by the sun
  • Comfort in a living space is improved if mass is evenly distributed. Increasing the surface area reduces the incidence of localised hot and cold spots
  • The location and sizing of glazing is also dependent upon the building layout and types of spaces e.g. frequently used spaces vs. infrequently used spaces. Large glazed areas for habitable spaces and small windows to less habitable spaces, since the absorption of solar energy is most effective through direct radiation, careful planning of the building is required. Direct absorption from south facing glazing implies that walls and floor need to be close to the source. With façade glazing only, the heated room is restricted to a relatively shallow depth, typically no more than 1.5 x the height of the glazing
  • By using clerestories and roof lights, the depth of penetration of solar radiation can be extended further into the building so to allow for a deeper plan. A secondary benefit is the extra daylight provided to reduce the need for artificial lighting
  • A clerestory roof angle should be approximately the same angle as the sun at the time of the winter solstice

Indirect solar gain

Thermal mass:

  • Thermal mass acts as a ‘thermal battery’
  • Thermal mass plays an important role in the performance of a building by moderating fluctuations in space temperature. This role becomes more important as summer temperatures in the UK increase
  • The use of heavyweight construction materials with high thermal mass can reduce total heating and cooling requirements. There is no necessary correlation between thermal mass and structure. Both traditional masonry and more recent timber frame methods of construction can accommodate thermal mass
  • Materials characterised by the expression ‘thermal mass’ are those that absorb heat, store it and at a later time release it

Characteristics of effective thermal mass:

  • High heat capacity (the ability to store large amounts of heat)
  • Moderate conductance (must be able to transfer heat fairly well through conduction)
  • Moderate density (cannot be too heavy or too light)
  • High emissive (must be able to easily emit or give off heat)

Terms used with thermal mass storage:

Thermal Lag:

This term relates to the ability of a material to absorb heat and then release the heat or conduct it throughout the material. Thermal lag times are influenced by:

  • Temperature differentials between each face
  • Exposure to air movement and air speed
  • Texture and coatings of surfaces
  • Thickness of material
  • Conductivity of material

Thermal Admittance:

Thermal admittance is useful during the design process and describes the ability of a material to exchange heat with the space over a time period of 24hrs. Thermal admittance is influenced by:

  • Thermal capacity
  • Conductivity
  • Density
  • Surface resistance

Ultimately admittance has an upper limit determined by the rate of heat transfer from the material’s surface to the adjacent air – though this can be increased through ventilation providing convective heat transfer.

Materials

The most effective construction materials are those with the highest volumetric heat capacity. In general, dense materials will generally have a higher thermal mass than less dense products. For example, dense concrete blockwork, rammed earth and mud bricks have a high effective thermal mass when compared to lightweight blockwork or wood.

For thermal mass to be effective there must be minimal thermal resistance between the occupied space and the mass of the structure. The temperature fluctuations within the building fabric are greatest at the surfaces. Relatively thin layers of plaster can have a significant effect on the thermal mass by providing thermal resistance.

The seasonal effects of thermal mass:

Summer:

  • In summer, thermal mass absorbs heat that enters the building. In hot weather, thermal mass has a lower initial temperature than the surrounding air and acts as a heat sink. By absorbing heat from the atmosphere the internal air temperature is lowered during the day, with the result that comfort is improved without the need for supplementary cooling
  • At night the heat is slowly released to passing cool breezes (natural ventilation), or extracted by exhaust fans, or is released back into the room itself

Winter:

In winter, thermal mass in the floor or walls absorbs radiant heat from the sun through south, east and west-facing windows. During the night, the heat is gradually released back into the room as the air temperature drops. This maintains a comfortable temperature for some time, reducing the need for supplementary heating during the early evening.

The most difficult period in winter is the early morning. The heat released during the night has dissipated, temperatures have dropped and the sun has yet to begin the heating process. During this time it will probably be necessary to use supplementary heating to warm the thermal mass before the air temperature rises.

Locating thermal mass

  • Thermal mass is most effective where exposed to direct sun radiation
  • Where not exposed to direct radiation, thermal mass relies on efficient convection
  • Comfort is improved if the mass is distributed evenly within a room
  • Thermal mass should be insulated from external temperatures for maximum effectiveness
  • Materials that make for effective thermal mass usually perform badly as insulators
  • The most important location for thermal mass is in south-facing rooms. To heat thermal mass effectively in winter, it should be optimised for exposure to direct winter sun.
  • As the area of south-facing window increases, the more thermal mass is required to maintain a stable temperature. Thermal mass located within north-facing rooms is relatively unimportant. It is frequently argued that thermal mass should be avoided altogether in bedrooms, so reducing an associated nocturnal rise in temperature
  • Summer conditions can lead to overheating to eastern and western façades. Consideration should be paid to locating thermal mass in these locations
  • Locate additional thermal mass near the centre of the building, particularly if a heater is positioned here

Isolated gain

Sunspaces / thermal buffering

  • A ‘sunspace’ is a south-facing glazed area located outside of the main fabric envelope of the building
  • The space naturally heats and cools rapidly according to prevailing weather
  • The addition of a sunspace can realise significant gains in energy efficiency
  • Sunspaces can provide additional living space when temperature conditions make them comfortable

Sunroom location

A sunroom can be integrated into a building design in essentially two ways. Situated on the sunny side of the building they can be integrated as either a ‘lean to’ or an ‘embedded’ concept.

Both approaches have distinct advantages and disadvantages as described below:

Lean-to:

  • Simple traditional concept
  • Construction details tend to be easier
  • Can shade adjoining areas of the façade
  • Can reduce natural light into adjoining spaces

Embedded:

  • The sunspace does not restrict daylight to adjoining windows
  • Anecdotal evidence shows that they are less subject to mis-use
  • Self-shading
  • Less compact envelope to living areas

How sun spaces work

The sunspace functions as an intermediate space between the inside and outside of the building. By effectively adding another layer to the building envelope, the sunspace becomes a thermal buffer rather in the manner of air within a cavity wall.

A further effect of the sunspace is to shelter the envelope from wind chill and rain – this factor becomes increasingly important in northerly and exposed locations

Natural ventilation

Warm air can flow into adjoining spaces via openable vents located in the common wall at the top of the sunspace. Cool air is returned from the living spaces through lower vents to be heated as part of the convective loop.

Mechanical ventilation

Mechanical ventilation can extend the penetration of pre-heated ventilation into areas of the house that are not adjacent to the sunspace. Heat is collected from the upper part of the sunspace and blown via ducting to other areas of the house.

Indirect gain: Thermal mass/ storage wall

Heat is transferred to the living spaces through a masonry common wall. (see thermal walls).

Opinion is currently divided as to the effectiveness of combining storage walls with sunspaces. Debate centres around the winter period when the wall has its greatest potential, yet when equally solar radiation is at its most diffuse.

Indirect gain: Remote thermal storage

Rock stores

As distinct from the immediate use of pre-heated air, thermal storage affords the capacity to store heat for future use. The traditional form of heat storage is the ‘rock store’. Heated air from the sunspace is mechanically driven to containers of crushed rock. Heat transfer is then effected by one of two methods:

  • Air is blown through the store into the space to be heated or
  • The store is thermally coupled to the space and heat is transferred is through radiation and conduction

Though the former enables a degree of remoteness between the store and the space to be heated, the latter method has the advantage of being simple, passive and is generally more popular.

In practice, rock stores perform reasonably well. However, very large amounts of rock are required to store relatively small am heat. For example, about 60 tonnes of rock are required to satisfy the storage requirement for a solar space heating system for an area of 100 square metres.

Draught lobby

In its minimal form, the sunspace becomes a draught lobby. Heat is lost when doors and windows are opened to the outside. By providing sufficient space for the outer door to be closed before opening the inner door, the draught lobby functions something like an air lock.

Overheating

Sunspaces are easily overheated in summer. The problem can be resolved by:

  • Shading from the sun
  • External solar shading from a roof overhang or adjacent louvers
  • Deciduous trees (beware though that leafless trees will, to an extent, continue to shade.)
  • Ventilating the sunspace by placing operable vents to the exterior through the roof of the sunspace. These can be automatically opened and should be coupled with vents at low level to enhance the ‘stack effect’

Air permeability

Air tightness around windows and doors should be particularly effective to reduce heat loss to the sunspace when that space cools to temperatures lower than the adjoining living area

Controls

If automatic vent operation is not provided, the building user needs to be fully aware of the need to control the vents correctly Where automatic controls are provided, users should be able to set them for desired levels of comfort.

Building fabric – best practice

Energy labelling

What is Energy Performance Labelling?

The Energy Performance of Buildings Directive 2002/91/EC (EPBD) was passed into law by the European Parliament in December 2002 and adopted by the 25 member states, including the UK, in January 2003.

The main aim of the Directive is to promote the improvement of energy performance of buildings and it is left to each member state of the European Union to develop a framework for energy performance calculation.

Essentially, this will provide an energy rating for a building similar to a consumer item such as a fridge or freezer. The ratings vary from A (the most efficient) through to G (the least efficient).  The rating is shown on an Energy Performance Certificate (EPC)  Click here for more information.

How is an EPC calculated?

An approved assessor will gather data from the property and from this, will produce an EPC certificate. The assessor will collect data sufficient data for the assessment as indicated here.

The EPC itself is based on calculations set out in the Reduced Data Standard Assessment Procedure (RdSAP) which itself is a condensed form of the Standard Assessment Procedure (SAP)  Existing buildings that require an EPC will get one based on RdSAP calculations while new buildings will require an EPC based on the full SAP calculations.

Typical best U values

Building Control sets minimum standards for U values for various building types.

Typically, for a dwelling house the best elemental U values are as below for a dwelling with a SAP rating of 60 or less :

  • Walls – 0.45 W/MsqK
  • Floor – 0.35 W/MsqK
  • Roof – 0.2 W/MsqK

Accredited Construction Details

Accredited Details have been developed to help the construction Industry achieve the minimum construction standards so that they comply with the requirements of Part L of the Building Regulations.

It is important to understand that the above Typical Best U Values and Accredited Construction Details are minimum standards. In order to achieve minimum / ultimately zero energy useage in a building one has to look to improve these standards.

One way to improve the energy efficiency of a building is to adopt a BREEAM (Building Research Establishment Environmental Assessment Method) assessment methodology to the design of a building.

This is a government adopted agency and provides a way to measure the energy performance of a building using sustainable materials. The BREEAM assessment methodology covers all building types right through to bespoke buildings.

The following link gives an overview of how BREEAM works, what ground it covers and the scoring system associated with BREEAM, how BREEAM works and what can be achieved by employing a BREEAM approach to building design.

Air pressure tests

Reducing the amount of air leakage form a building can dramatically improve energy efficiency. The energy that we use to heat our homes is primarily created by burning fossil fuels that produces carbon dioxide. If we reduce the air leakage of a building we also reduce the amount of energy required to maintain comfort levels and in turn reduce carbon dioxide emissions.

What is air leakage?

Air leakage is the air tightness of a building through uncontrolled means such as cracks and gaps in the building envelope. Any ventilation system installed in a building is seen as a source of controlled air flow and is therefore not considered as air leakage. At a very basic level, air leakage may be seen as unwanted draughts.

What is air pressure testing?

Air pressure testing is a method of measuring and quantifying the air leakage of a building in accordance with Building Regulation requirements. Air pressure testing of a proportion of all new domestic housing is a legal requirement in accordance with the guidance given in Approved Document Part L1A – Conservation of fuel and power in new dwellings of the Building Regulations. Testing the air tightness of existing dwellings can highlight areas of problems that can be treated cost effectively to improve the energy efficiency of the dwelling as a whole.

Air tightness tests are usually carried out during the construction and commissioning process of a building when the external envelope is fully complete, with windows and external doors in place. Remedial work may need to be carried out on a building that fails an air pressure test.

Sustainable materials

A building that is sustainable must by nature be constructed using locally sustainable materials: i.e. materials that can be used without any adverse effect on the environment and which are produced locally, reducing the need to travel. There are key criteria that can be used to judge whether a material is sustainable or not:

  • It is essential that those materials are renewable, non-toxic and therefore safe for the environment. Ideally they will be recycled as well as recyclable
  • To what extent will a building material contribute to the maintenance of the environment in years to come? Alloys and metals will be more damaging to the environment over a period of years as they are not biodegradable and are not easily recyclable, unlike e.g. wood
  • To what extent is the material used locally replenishable? If the material is locally sourced and can be found locally for the foreseeable future, travelling will be kept to a minimum – reducing harmful fuel emissions

It is also important in the use of sustainable materials that an effort is made to source sustainable materials locally through suppliers and that once sourced and specified that this is communicated effectively to the contractor. If you need help with design use a ‘Green Architect.’ The Royal Institution of British Architects can advise you on finding a suitable practitioner.

Choice of fuel options – energy sources

The need to find new types of renewable energy is urgent – not only as oil reserves are running out but there is increasing competition from China and India for the limited world reserves.

There are several different types of energy being produced today. Solar and wind power are now well-known as early-stage alternative renewable energies. But there are the other potential energy sources such as those listed below:

Biomass

Biomass is a different type of energy altogether and it is still very much in its infancy, but using renewable, continually-growing plant material seems to be a sensible option. Large areas of agricultural land would have to be turned over to these crops, and using biomass fuel could make a significant contribution to reducing global warming because the fuel has a low heat output and would need to be used in greater quantity.

Hydro-electric energy

Once a dam is built, generating power from the build-up of water is inexpensive. This is quite a limited source of power as it depends upon the continued elevation of the water.

Other downsides to the use of hydro power is that dams potentially threaten human life if they collapse and they certainly affect fish stocks. They also create environmental damage for surrounding areas if they flood. Unfortunately all reservoirs eventually fill with sediment and the rate of this is unknowable.

Refuse-based fuels

This option too seems a tremendous possibility – burning our rubbish and using its byproduct. Again, this is also in its infancy. It is known to have a low sulphur dioxide emission, but the by-product created (known as fly ash) can contain metals such as cadmium and lead.

Hydrogen

Hydrogen has first to be obtained by the electrolysis of water, or by breaking down natural gas. It is an energy type which is highly explosive and must be compressed to be contained and carried. It is also very costly to produce, yet we are seeing the first generation of cars powered by this form of energy.

Energy metering

Sub-metering contributes to good energy management and the strategy for energy metering in a building should be included in the building maintenance handbook.

Most buildings have incoming meters for billing purposes. They measure the total input of the specific fuel to the site. Regular meter readings will provide some information about the overall energy consumption, but it reveals little about where the energy problems lie. Installing sub-metering throughout a building to monitor the specific uses of the fuel being metered can help identify which end-use or service (e.g. lighting, fans, pumps etc.) is performing well or badly, allowing more targeted action.