Designed to Perform

An Illustrated Guide to Providing Energy Efficient Homes

Tom Dollard

• Publisher: RIBA Publishing
• Year: 2019
• ISBN:
  9781000702316
• Edition: 1
• Topic: Architettura
• Subtopic: Architettura generale

This book is an illustrated practical design guide to delivering better energy performance in all types of new build homes. It takes the form of an annotated details book, with photos taken from live construction sites, with the content based around diagrams, drawings and photos by the author, which demonstrates valuable best practice knowledge and advice.

Chapter 1 is an introduction to the performance gap and the quality of design and construction in new build homes, explaining the typical construction sequence of homebuilding, and highlights common issues that designers need to engage with. Chapters 2-7 look at each construction fabric in turn, including a series of detailed drawings, diagrams and photos illustrating the key elements of good design. Chapter 8 contains a checklist of all performance gap issues that designers need to look for.

This book will provide valuable guidance to architects and designers on how to improve their detailing at construction stage, and therefore the overall quality of design and performance of new homes.

Chapter 1:
The performance gap and how to reduce it

What is the performance gap?

The performance gap is the common term used for the gap between predicted and as-built performance. It has been demonstrated by a wealth of evidence, including a government-funded £8 million research programme by Innovate UK into building performance. In 2012, the Zero Carbon Hub was commissioned by the government to lead a comprehensive review of the performance gap. The report, called ‘Closing the Gap Between Design and As-Built Performance’ set out the ‘2020 ambition’ for industry to be able to demonstrate that 90% of all new homes meet the designed energy performance. Determining the size and origin of the gap was a key aim for the report, and it also highlights recommendations for industry and government to close the gap. The report included input from 180 organisations and experts and reviewed over 250 new homes on 21 sites during construction to see where the gap occurred in the process of housebuilding. The conclusion from the Innovate UK and the Zero Carbon Hub study, totalling inspections and monitoring of over 350 new homes, is that this gap is endemic across the industry, complex and widespread in origin and is on average 2.6 times worse than design predictions.

Why is it important?

A significant gap in a building’s energy performance undermines its vital role in delivering the national carbon reduction plan, which in turn has major consequences for climate change and global resource depletion and major implications for national and international policy. It also presents significant reputational dangers to industry by undermining consumer confidence if energy bills are higher than anticipated.

What can we do about it?

To answer this, the Zero Carbon Hub identified three common themes across all stages of the housebuilding process. They are:

1. Lack of understanding, knowledge and skills
2. Unclear allocation of responsibility
3. Inadequate communication of information

To reduce the performance gap, we need a change in culture across the industry to place greater importance on in-use energy performance. Energy performance needs to be valued as highly as other issues such as acoustics, fire and access. The improvement in acoustic performance of new homes is a good case study in how to improve standards, and was done with cross-industry and government support through Building Regulations mechanisms such as Part E testing and Robust Details. The introduction of acoustic testing of new homes has dramatically improved acoustic performance, because the test drove associated improvement in skills, knowledge and standards. A similar improvement is evident is health and safety standards in the last 20 years. The introduction of the CDM Regulations in a clear format kick-started a whole industry and saw radical improvements in safety performance. To improve energy performance it will require similar investment and support over the next 10 years. Part L needs to be simpler and easier to use, with greater emphasis on compliance and site checks on delivered performance of the buildings. In combination with regulation change, there needs to be greater education and resources for industry professionals and operatives to understand building performance and their role in delivering quality. This lack of ‘energy literacy’ should be addressed within programmes at CITB, CIBSE, CIOB, FMB, ICE, and RIBA, with greater emphasis on practical education and skills. This book provides information on the design of better homes to reduce the performance gap but a significant improvement will need further action from policy makers, designers and contractors.

What can policy makers do?

Policy makers can reduce the performance gap with the following changes:

1. Streamline, simplify and improve compliance with Approved Document Part L, to include a simple but comprehensive as-built performance check. This will include increased building control presence on site to check energy performance and associated construction quality. Award good practice in this area with incentives such the NHBC Pride in the Job Awards.
2. Make a construction guidance web resource available to all, to share and improve levels of construction. Include helpful tips, user videos and guidance on design and installation. To include standard details and installation sequencing.
3. Improve energy literacy: Training design and site teams in energy efficiency and correct installation of fabric and services to be included in standard training and qualification routes.

These three actions would dramatically improve the quality of new homes, but will not happen in this competitive market unless there is greater enforcement of these regulations. The current energy standards in Building Regulations are overly complicated and rely on expert consultancy and detailed reports, rather than checks on site. The process and regulations need to be radically simplified to allow developers options to demonstrate as-built performance. Building regulations need to be clarified, with greater emphasis on compliance and site checks to improve as-built performance.

What can the designer do?

Designers can draw and specify a thermally efficient home that will reduce the theoretical heat demand to meet the optimum level of the Passivhaus standard, 10 W/m². However, this high standard of performance can also be more difficult and expensive to build.

To prevent this, designers must consider buildability, sequencing and supply chain with the contractor at an early stage, and agree a construction type most suited to requirements. Chapters 3 to 7 will aid these discussions and Appendix 2 is a guide for effective project management using the RIBA Plan of Work. A thermally efficient building can be more difficult to build so it is crucial that designers consider buildability and sequencing with the site team at an early stage.

Designers can improve the quality of new homes by designing better performing building envelopes and services, encompassing the following five actions:

1. Design a simple and compact building envelope, with minimal external surface area, reduced complexity of detailing, continuous insulation and airtightness.
2. Reduce thermal bridging by following design guidance and carrying out bespoke modelling.
3. Specify simple building services that perform as intended and are easy to operate and maintain.
4. Ensure drawings and specifications are clear and consider construction sequencing, buildability and as-built performance.
5. Ensure project team has the skills and experience to identify and deliver an efficient solution. Invest in training and seek expert guidance on energy analysis to improve performance.

What can the contractor do?

Whilst the designer can design out many issues like thermal bridging, the responsibility for production lies with the main contractor, who must ensure that the design is made a reality. The contractor must ensure the following:

1. Improve quality assurance on site with finishing foreman role on large sites, extra building control visits and a clerk of works to inspect quality.
2. Increase role and responsibility of designer on site, with better communication between designer and site team. Ensure design continuity by appointing the design team through to construction stage.
3. Nominate one of the project team to be ‘energy champion’ on site, to be responsible for as-built energy performance. The role encompasses airtightness, windtightness, continuous insulation, correct building services and commissioning check.
4. Install correct products according to specification and SAP.
5. Ensure the building services are thoroughly commissioned, especially ventilation, and provide information and training to residents on use and maintenance.
6. Ensure subcontractors have suitable training and experience for their job, e.g. BPEC in ventilation. Carry out toolbox talks to emphasise airtightness, thermal bridges and building services commissioning.

Contractors should refer to Appendix 1 for more detailed guidance on how to improve quality on site.

It's about quality, not just energy

Poor energy performance is symptomatic of poor design and construction quality. As-built energy performance is directly related to the wider aspects of construction quality. For example, the performance of a cavity wall will depend on a number of quality factors: the mortar joint quality, the cleanliness of the cavity, the tolerance of the insulation, the specification of the block, insulation, and cavity ties. All these items will affect the level of acoustic, thermal, damp, life expectancy, fire and structural performance. Thermal performance is a key indicator for other performance measures such as acoustics, airtightness, moisture movement, damp control, structural strength, life expectancy and fire performance. A development that achieves good thermal performance requires high levels of quality assurance, that delivers good performance in the other criteria.

Quality of construction should not be defined by the expense of finish e.g. gold standard bathroom or kitchen. It means that appropriately specified materials have been installed correctly. For example, a window installation needs the correct window, accurately positioned and fixed to the inner leaf to minimise thermal bridging and air leakage. The performance of the product goes hand in hand with the installation quality. Quality construction provides a building that meets or exceeds the brief by achieving a low energy, comfortable, enjoyable, robust and functional building that will last – a ‘sustainable’ building in its literal sense.

The quality of new housing is a product of what the customer and regulations demand. Housebuilders will typically not go further than minimum regulations unless the customer demands it, or if there is a competitive advantage for doing so. In terms of energy efficiency, this often means the minimum possible standards with very little compliance checks to ensure this is met. Consequently we produce homes that are performing considerably worse than intended.

Performance of a new home in terms of energy efficiency, comfort and indoor air quality is relatively difficult to see and evaluate. The long term impact of poorly performing homes on our health, wellbeing, the environment and our pockets are considerable. How do we check and improve performance of new homes?

Improved quality inspection on site

The simple answer to improve quality lies in increasing the frequency and quality of inspection and testing. The most effective and economical test is the 'eye-ball' test that is already used by many building surveyors and could be adapted to look for energy performance issues. A recommended ‘eye ball checklist’ is included as Appendix 1 and can be amended and used by anyone inspecting sites. Improving the regulation and inspection regime will create a level playing field of required performance for developers. Any future regulation should give flexibility in method with an easier process to achieve a higher quality as-built standard. This policy leads to improved training and communication in energy performance, and a virtuous circle of improved construction quality.

Chapter 2:
How to detail a thermally efficient building envelope

This chapter focuses on thermal performance of the fabric, but there are many synergies with these other factors. The building fabric and services design are the two key aspects of a building’s performance. A successful fabric will be thermally efficient and keep the weather, pollution, fire, noise, damp and other external factors removed from the internal environment. A thermally efficient building envelope will need to be well built, structurally sound, airtight, weatherproof, fire resistant and have good acoustic performance due to the insulation and airtightness. The attention to detail and quality assurance on site needed to deliver a thermally efficient building will also normally lead to increased performance in other areas.

Three layers

A thermally efficient building envelope will have th...