Climate Change SPD

Ended on the 4 October 2023
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Chapter 4: Improving Layout and Building Design

This chapter include various layout and design principles. It is important that the design principles included should form part of a cohesive and comprehensive design approach for the wider application/proposal.

Passive Design

Passive design is the optimisation of the layout and orientation of new buildings. Passive design maximises natural environmental factors to help reduce energy needs by avoiding overshadowing, maximising passive solar gain, maximising the potential internal daylight levels and providing ventilation. There is the need to balance this with the need to mitigate overheating risk and avoiding the need for energy intensive technology for cooling.

The Essex Solar Design Guide (2022) has been produced by Etude and Levitt Bernstein on behalf of Essex County Council, to give developers, architects and homeowners an easy-to-use guide, laying out the key considerations for good solar design. The core principles of good solar design are to balance the needs of daylighting, useful solar gain and mitigating overheating. The Council recommends that applicants take into account this guidance and incorporate the 'design actions' that are included throughout the guide into development proposals.

Optimising building form can make it easier and cheaper to achieve lower levels of space heating demand (the LETI KPI for space heating demand for residential dwellings is 15-20 kWh/m2/yr – see chapter 6).

Notwithstanding the policy requirement (Policy DM15) for development proposals to be designed to a high standard which responds positively to its context, development and building form should be as simple and compact as possible. This will reduce the exposed surface area reducing the amount of heat that is lost through the walls and roof. The use of stepped roofs, roof terraces, overhangs, inset balconies, dormers and bay windows should be avoided as these features will decrease the building's energy efficiency.

Buildings with a lower form factor (form factor = exposed external surface area/gross internal floor area) are more energy efficient. This is not to say all homes should become boxes, high quality design is important, but strategic decisions should be taken on adding articulation to the building forms (such as dormers, bay windows, built in undercroft parking etc.). Joining of homes together into terraces further reduces heat loss from the building. Consideration to the number of more exposed forms such as detached and semi-detached should therefore be given.

Passive solar design should be used to harness energy from the sun for heating and for daylighting to avoid the need for artificial lighting. This reduces winter heating load, limits summertime overheating and aids natural ventilation. To maximise useful solar gains in winter, rooms where people spend most of their time should be positioned along the south side of the building to maximise natural daylight and warmth in the winter. Site layout should maximise number of dwellings with a main living room that has at least one window on a wall facing 90° due south. Bedrooms should avoid west elevations because they receive solar gain at end of the day just before they are occupied so carry risk of overheating.

Building Orientation and Massing

The massing and density of a development can influence access to sunlight, daylight, and solar gains to internal and external spaces. It is best practice to avoid placing higher elements to the south of a site, ideally these should be placed to the north to avoid excessive shading of other buildings and external amenity spaces. Consideration should also be given to surrounding buildings off site. Strategic breaks should be included in building massing to let sunlight in. When designing for sunlight, consider the sun's angle at different times of the day across different seasons. Housing layouts should be designed to maximise daylight and sunlight while taking into account other factors, such as privacy and the attractiveness of the wider streetscape whilst balancing the risk of overheating. The orientation and massing of the building should be optimized to allow useful solar gains and prevent significant overshadowing in winter. Buildings should be south facing (+/- 30°) with solar shading and dual aspect should be prioritised. Overshadowing of buildings should be avoided as it reduces the heat gain from the sun in winter.

Where urban design principles necessitate the move away from a predominantly north/south orientation, even slight twists to the building orientation can assist to reduce energy demand. Decisions on orientation and window area should also be balanced with the risk of overheating, with window shading considered to mitigate against this.

A building's form, orientation and window proportions are all aspects that do not add extra construction cost, but if optimised within the design can significantly improve the building's efficiency.

Overshadowing

Building spacing and street proportions should be assessed to reduce the extent of overshadowing. Priority should be given to the south in orientating masterplans, angling the roofs to make the most of PV opportunities to the south. It is good practice to allow a distance of 1 to 1.5 times the buildings height between buildings to avoid overshadowing and impacting the internal solar gains. High density developments should consider and demonstrate that year-round sun paths have been included in the design to allow as many dwellings as possible to receive sunlight throughout the year.

Sunlight is a welcome feature of external spaces such as communal gardens, public squares and roof terraces. The BRE guidance Site layout planning for daylight and sunlight: a guide to good practice advises that it is best practice to design for at least half of the total area of amenity space to receive direct sunlight for two hours on the 21st March (spring equinox).

While a room facing north will not receive direct sunlight, it can still be adequately daylit as it receives diffuse light (i.e. reflected or scattered light). The amount of daylight a room receives is dependent on external overshadowing from neighbouring buildings; overhangs or balconies; the size and location of windows; the depth of the room; the materials and colours used; and the visible light transmittance of the windows. These issues should all be considered as part of the design to maximise the amount of daylight entering and building.

Windows

To minimise heat loss to the north, smaller windows should be installed and to provide sufficient solar heat gain from the south larger windows should be installed. Consideration should be given to the portion of the window that is useful for daylight, solar gain, ventilation, privacy, and views. Shading should be provided to avoid overheating in summer.

Horizontal windows are more effective than vertical windows in terms of improving room lighting distribution and increasing the amount of openable area available for ventilation. Side-hung windows are favoured to top-hung windows.

Building fabric and materials

Airtightness significantly improves energy efficiency and comfort, often for a relatively modest cost. Excellent levels of insulation and airtightness, and minimal thermal bridging are required to meet the LETI KPI for space heating demand for residential dwellings of 15-20 kWh/m2/yr (see chapter 6). The list below outlines the things to consider.

  • Insulation standards, or U-values (W/m2), are a measure of how well heat passes through an element. The lower the u-value the better the insulator.
  • Thermal bridging is where a building component allows significantly more heat to travel through it than the materials surrounding it. This can create "cold" spots and sources of heat loss and mould.
  • Airtightness (m3 /h/m2) is a measure of the leakiness of a building and how much air passes between different building elements and junctions. This uncontrolled ventilation leads to heat loss.
  • Thermal mass plays a big part in thermal comfort. Thermal mass (such as brick or blockwork) inside the building helps to stabilise internal temperatures throughout the day. Lightweight buildings with little thermal mass will be subject to larger temperature swings. An allowance for appropriate wall thickness needs to be made at an early stage in the design process to ensure the number of homes expected on site will fit and can be delivered.
  • Choose materials that have certification from the Forest Stewardship Council (FSC), the Programme for Endorsement of Forest Certification (PEFC), ISO 14001 (Environmental Standard), BES 6001 Framework for Responsible Sourcing, CARES steel certification.

Ventilation and air tightness

Natural ventilation improves thermal comfort in summer. Where possible, windows should be designed to be fully openable and floors plans arranged to allow cross ventilation, which is the most effective form of natural ventilation.

Excellent levels of air-tightness and Mechanical Ventilation with Heat Recovery (MVHR) are required to meet the LETI KPI for space heating demand for residential dwellings of 15-20 kWh/m2/yr (see chapter 6).

The key to energy efficient ventilation in all buildings is being in control of where, when, and how air flows through a building. This starts with very good airtightness to limit any uncontrolled infiltration. Trickle vents should be avoided as they do not control infiltration. Practical guidance on how to achieve good levels of airtightness can be found in the Forest of Dean, Cotswold and West Oxfordshire District Councils' Net Zero Carbon Toolkit.

A key component to energy efficient, airtight homes is Mechanical Ventilation with Heat Recovery (MHVR). MVHR is suitable for all building types. Long used in non-domestic buildings, it is increasingly used in homes to ensure good indoor air quality and to remove and replace stale air in an energy efficient manner. MVHR units supply air into occupied spaces, and extract air from circulation spaces, or kitchen and bathroom spaces in the case of homes, it does this using very little energy and recovers heat energy from outgoing air. Units should be positioned close to an external wall to prevent heat loss from the ductwork that connects to the outside. These ducts should be accurately fitted with adequate insulation to prevent heat loss, and generally ductwork should avoid having sharp bends which could affect pressure loss and flow. MVHR units include filters that must be changed regularly (usually at least once per year but check the manufacturer's instructions).

Reducing overheating

Climate change is already bringing warmer summers with more extreme temperature highs. In June 2021, the Committee on Climate Change released its Independent Assessment of UK Climate Risk. It said in the last 5 years, "over 570,000 new homes have been built that are not resilient to future high temperatures". Overall in England, the summer of 2022 was the joint hottest on record. 2022 was also the hottest year on record between January-August 2022, and the driest so far since 1976. The highest ever recorded temperature of 39°C for the county of Essex was recorded in July 2022. These high temperatures led to a notable increase in wildfires, with several experienced in the borough of Colchester (in Mersea, Stanway and on Middlewick).

Overheating in buildings is becoming an increasing threat to occupants' health and wellbeing, particularly for vulnerable people. In future years, this is set to become even more of an issue.

Overheating can be reduced through good design and all developments should demonstrate how the risk of overheating has been sufficiently mitigated through good design. All developments should:

  1. Ensure glazing areas are not excessive i.e. no more than 20-25% of facade on south or west façades.
  2. Favour dual aspect homes to allow cross ventilation.
  3. Provide appropriate external solar shading. South façades should have horizontal shading over the window and the west façade should ideally have efficient movable shading e.g., shutters. Do not rely on internal blinds – these can be ineffective and removed by residents.
  4. Ensure good levels of secure natural ventilation are possible. Design window openings to take advantage of cross-ventilation (from one side to another) and/or stack ventilation (from bottom to top). Avoid fixed panes and maximise opening areas of windows. Side hung windows typically allow more ventilation than top hung.
  5. Select a g-value (the solar factor indicating how much heat is transmitted from the sun) for glass of around 0.5 where possible. Avoid reducing it too much as this would also reduce free winter solar gains.
  6. Utilise thermal mass in buildings to help dampen temperature swings throughout the day, and work with secure natural ventilation to provide passive night-time cooling
  7. Utilise green and blue infrastructure to provide natural cooling to the local environment and reduce the urban heat island effect.

The Good Homes Alliance has developed a tool and accompanying guidance which aims to help planners and design teams identify and mitigate overheating risks in new homes at an early stage.

CIBSE have a detailed methodology to assess overheating risk to occupiers over the lifetime of a development. Assessing the overheating risk and ensuring mitigation measures are incorporated into the design, will help ensure the comfort, health and wellbeing of occupiers and improve resilience of the development to a changing climate. Using the CIBSE methodology for assessing and mitigating overheating risk from not only current climate, but also projected future climate, is encouraged on major development proposals.

Working from home space

All new dwellings should be designed to accommodate the space and services necessary for comfortable home working. This will reduce the need to travel. As a guide, a suitable home office should include:

  • A high-speed internet connection.
  • A room or space with a wall length of at least 1.8m, capable of accommodating a desk and shelving.
  • Good internal daylight, reducing the need for artificial lighting.
  • Consider north facing home offices to avoid glare.

Green-blue infrastructure

The Biodiversity SPD and Active Travel SPD both refer to green-blue infrastructure. Green-blue infrastructure, such as parks, open spaces, waterways, and the connections between them, is central to Colchester's climate change adaptation and resilience. Green-blue infrastructure can improve the resilience of habitats and vulnerable species in a changing climate and help to reduce flood risk. Green-blue infrastructure reduces the environmental impact of development in terms of carbon emissions, air, soil, light, noise, and water, while also improving air, soil, and water quality. Green-blue infrastructure can also deliver a range of related benefits by improving opportunities to walk and cycle, which in turn reduces carbon emissions, and improving the health and wellbeing of local communities.

According to Natural England, good green-blue infrastructure has five key characteristics. It is:

  1. Multifunctional – Whilst traditional grey infrastructure typically has one key function, green-blue infrastructure offers a range of functions. For example, increased tree coverage may provide flood protection, reduce heat, promote biodiversity, and provide aesthetic value.
  2. Varied –Varying green and blue spaces is particularly important for wildlife in the context of a changing climate and has a positive impact on human health.
  3. Connected – Promoting connectivity addresses fragmentation, enabling the movement of people and wildlife through green networks and strengthening resilience.
  4. Accessible – For people to experience and (re-)connect with nature, green-blue infrastructure must be inclusive, safe, welcoming, well-managed and accessible for all.
  5. Responding to a local area's character - An area's natural, historical, and cultural landscape makes a place distinctive and helps people recognise and connect to their local environment. Green-blue infrastructure should preserve and maintain the existing character of an area and enhance it by strengthening existing characteristics.

Trees can provide additional shading to buildings and public realm. Deciduous trees allow for sunlight and solar gains to reach the buildings in winter when the leaves fall, while providing shading in summer. The size and age of trees can make the amount and longevity of shading difficult to predict. Therefore, shade from trees should not be relied upon as an overheating mitigation measure, they simply supplement the overall building design. When designing external spaces consider how much sunlight will be received on planted areas and select appropriate species to suit. The Council require major applications to submit a tree canopy cover assessment and have adopted guidance to explain this requirement.

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