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CIBSE Technical Symposium, UK April 2022
Page 1 of 19
Design for Performance: Lessons from the NABERS
UK Independent Design Review Process
GRACE
FOO B.ENG (HONS), PRINCIPAL CONSULTANT DELTAQ PTY LTD,
CANBERRA AUSTRALIA SG.FOO@DQCS.COM.AU
DR. PAUL BANNISTER, DIRECTOR OF INNOVATION DELTAQ PTY LTD
DR. ROBERT COHEN, TECHNICAL DIRECTOR, VERCO
Abstract
The Design for Performance (DfP) Agreement is a process whereby building owners
and developers commit to the achievement of a post-occupancy NABERS UK rating
target while still in the design phase of a new building process. As part of this, an
independent detailed energy efficiency design review is undertaken to assess
whether the building design appears capable of achieving its target NABERS
rating. A review of the building simulation determines whether it makes a plausible
theoretical case for achievement of the target and a review of the design seeks to
understand the practical issues and opportunities associated with the achievement of
the target.
Based on experience from over 10 such reviews of UK commercial office
developments, this paper provides an overview of the following key findings:
• Design team and project owner motivations, comprehension and attitudes
concerning DfP Agreements
• Integration of the DfP process into the RIBA Plan of Work from stage 2 onwards,
including the timing of building simulations and independent design reviews
• Major risks inherent in the UK design, construction and building operation
processes that threaten the achievement of the NABERS targets for new
developments or major refurbishments with DfP Agreements.
The paper aims to provide an experienced-based briefing to assist project teams in
developing the capacity to understand and manage these issues and risks.
Keywords
Energy efficiency, Design for Performance, Building Energy, NABERS
UK
1 Introduction
Reflecting the NABERS Commitment Agreement framework in Australia, Design for
Performance (DfP) is ‘the process whereby a developer or owner commits to design,
build and commission a new office development or major refurbishment to achieve a
specific NABERS base building Energy rating’ (1). The associated NABERS UK
energy rating scheme launched in November 2020 and provides the mechanism for
calculating a building’s rating based on metered energy use over a year and for the
verification and public disclosure of the achievement of the original target (2).
The DfP process is an industry-led initiative developed to close the performance gap
between building design and operational performance. From its inception, it was
CIBSE Technical Symposium, UK April 2022
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supported by the Better Buildings Partnership and funded by DfP Pioneers and the
Usable Buildings Trust. To a certain extent, this performance gap is exacerbated by
the ‘design-for-compliance’ culture in the UK (2). Building Regulations and Energy
Performance Certificates (EPCs) and existing green building tools such as BREEAM
play an important role to incentivise better practice building design; however, they
lack a similar framework to ensure that buildings are operated efficiently throughout
their life in accordance with the design intent. DfP and the NABERS UK rating
scheme are administered by BRE.
In brief, the two pioneering elements of DfP are detailed dynamic energy simulation
modelling of the proposed building, notably including its building services systems
and associated controls from concept design to achievement of a target measured
rating and the Independent Design Review (IDR), which is an independent peer
review of the design and simulation with a view to identify risks and opportunities in
relation to the achievement of the proposed base building operational target.
Based on experience from over 10 such IDRs for UK office developments conducted
across two to three years (2019-21), this paper provides an experienced-based
briefing on stakeholder motivations, timing and integration of the IDR within various
RIBA stages and common issues and risks – both technical and non-technical.
Parallels are drawn to the Australian experience, alongside observations regarding
similarities or differences.
1.1 The DfP Process
DfP is suitable for buildings that are under development or are going through a deep
refurbishment. Although the principles of the process are applicable to any building
type, it is currently focused on offices because the associated mechanism of a
NABERS UK Energy rating has been developed in the UK so far only for offices.
Officially, the DfP process commences with the registration of the commitment to a
NABERS Energy target. This triggers a limited license which allows the Development
to promote the target rating using approved wording by BRE. To obtain the full
licence for target rating promotion, the process first requires the developer or owner
to conduct a compliant building energy simulation to estimate the design’s NABERS
performance. Secondly, the simulation report and building design must be critiqued
by an independent design reviewer, to assess confidence in the estimated NABERS
rating and likelihood of achieving the target rating during the operational phase. The
IDR report provides a stronger indicator of the expected performance outcome in the
form of the recommended ‘Design Reviewed Target Rating’.
2 Motivations
2.1 Stakeholder engagement
Stakeholders have varying motivations and drivers to participate in DfP. These are
mapped in Figure 2-1. This mapping is not intended to be comprehensive; instead, it
is indicative of engagement levels based on the author’s observations at the time of
writing. This mapping could change over time as the DfP program matures.
CIBSE Technical Symposium, UK April 2022
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Figure 2-1. Mapping of stakeholders' DfP advocacy levels (motivation or drive)
and knowledge.
Earlier projects reviewed were commissioned by the DfP Pioneers, market leaders
who are already invested in green building design and determined to close the
performance gap. Design and client sustainability teams were already highly
knowledgeable as to the benefits of DfP. Stakeholders were self-driven and
interested parties. That said, these traits were still observed to be championed by
client-side sustainability managers. While sympathetic to the DfP cause, the
commercial realities of the leasing market and tenant’s limited recognition of the
performance gap were still deeply ingrained within other parts of the business, such
as the leasing, marketing and facilities management teams.
Following the DfP launch and the Department for Business, Energy & Industrial
Strategy (BEIS)’s proposal to introduce mandatory energy efficiency performance-
based ratings for office buildings consulted on in 2021 (4), interest in DfP across all
stakeholders increased noticeably:
• Owner-developer interests were predicated on the marketing of DfP target
ratings to gain leasing or reputational advantage for the property. This was
both a rose and a thorn for DfP. The rose is that there is buy-in for DfP as an
objective, which empowers design teams; the thorn is the level of optimism
and pressure to register a high target rating despite being assessed to be
very risky.
• Leasing and managing agents are guided by owner objectives but are
entrenched in the business of leasing a building. Predicted ratings are
interpreted as how close the building is to a better rating, as opposed to the
more conservative Australian practice of committing to the “guaranteed”
performance level – the lower rating threshold, with internal stretch targets to
achieve the higher rating. The less mature environment in the UK drives a
CIBSE Technical Symposium, UK April 2022
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strong pressure is to reap marketing rewards in the present
1
, leaving any
building under-performance as a risk management exercise by the respective
teams. This is understandable – there are fears that by committing to a lower
rating, market perception would be that the property is inferior compared to
other available properties by the time the building is built and ready for
occupation in 2- to 3-years time.
• Motivations for modellers, M&E consultants and contractors are also led by
the client objectives and briefs, as well as budget. In general, these parties
are highly knowledgeable in design aspects but may not be accustomed to
being challenged or critiqued as part of the DfP review process. When DfP
reviews are engaged too late in the process, there is general reluctance to
make any changes as it could understandably create ‘more revision work’
which were not within the original scope. Here it is observed that the level of
knowledge of the DfP process and its principles is an important differentiator.
IDR panellists or M&E consultants with ‘advanced’ modelling expertise were
less defensive of the original design position, and more prone to accepting
DfP as a constructive and collaborative process.
• Facilities management teams are not typically engaged as part of the design
process. However, the DfP process forces early interactions with such
stakeholders through the preparation of the DfP Rating Achievement Plan.
These important stakeholders may face some of the most substantial
institutional culture change – disruption in maintenance contract structures,
tenant interactions and harder enforcement of landlord expectations. As
“recipients” of performance targets with as yet no empirical evidence of how
easily they can be achieved, these stakeholders are wary of being strong DfP
advocates with little experience of the DfP principles they will be expected to
adopt - they are largely guided by the client brief.
2.2 Understanding modelling margins
In Figure 2-2 shows the modelling margins and target ratings for nine projects
reviewed by the authors through IDRs. It can be seen that some target ratings were
assessed as risky despite seemingly high modelling margins due to non-technical
risks (management issues such as control and visibility over base building equipment
within tenancies, or, unclear accountability and responsibility for operational rating
achievement) or due to idealised simulations that do not reflect real operation when
the building is in-use. It should also be observed that target ratings reviewed as
'achievable’ have modelling margins higher than the minimum levels recommended
in the DfP guide. This is because:
1. The larger a building is, the higher the risk of deviant in-use operation and
thus excess building energy consumption.
2. As the target rating inches towards a 6-star rating, the same modelling margin
expressed in percentage terms decreases in absolute terms. A 6,000m
2
building with a 25% modelling margin can use an additional 29 kWh
e
/m
2
energy for a 4.5-star target, but only 17 kWhe/m
2
extra for a 5.5-star target.
For the same building, the 5.5-star target could easily be compromised by a
handful of large pumps operating 24/7 unbeknownst to anyone for a short
period.
1
In contrast, Australia’s experience of the commitment process over the past 20 years seems to have fostered a
mentality more accepting of reaping marketing rewards in the future when the building over-performs its target.
CIBSE Technical Symposium, UK April 2022
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The above modelling margin risks should be carefully considered by the client and
design teams before the commitment and marketing of an exceptionally high target
rating.
Figure 2-2. Target rating, modelling margin and IDR risk assessment
3 Timing and Project Integration
3.1 When should an IDR be conducted?
IDRs have been sought at all stages of the project – the authors have reviewed
design and energy simulations at stage 2 through to stage 4 of the RIBA Plan of
Work.
The assessed risks of achieving the target rating are substantially higher at earlier
RIBA stages. As such, the earlier the RIBA stage, the less suitable a single snapshot
IDR is to support a formal DfP Agreement. This is corroborated by the DfP Guide that
recommends for the IDR to be conducted during Stage 4 to ensure there is sufficient
documentation and design detail for the Simulator to produce a credible Simulation
and for the Reviewer to produce a detailed review (3).
In Australia, regardless of whether a project goes through a NABERS Commitment
Agreement (the DfP-equivalent program), many projects enlist the assistance of a
peer reviewer at the concept, 60-70% design and 90% design (typically referring to
tendered design) stages as a risk management exercise.
To harness the benefits of a peer review at all RIBA stages, multi-stage IDRs are
increasingly sought by various projects from the authors’ experience. This is the
exact opportunity noted within the DfP Guide (3), which acknowledges considerable
value-add if the reviewer can provide input to larger-scale design decisions in earlier
stages as these could be difficult to change at Stage 4.
3.2 When should the simulation be conducted?
As with the IDR, advanced building energy and thermal simulation models can be
CIBSE Technical Symposium, UK April 2022
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conducted at any stage. However, the earlier the simulation is conducted, the more
experienced the modeller needs to be as the design detail is scarce at earlier stages.
For the model to be useful as a design tool, it would need to be guided by a good
understanding of how the plant will likely be operated and reasonable model input
parameters for equipment (including where to source the information).
Some examples of how the model could be used as a design tool, for a given
building location and geometry, include:
1. Which HVAC system design is more efficient and is the difference material?
2. Is there an alternative façade design and/or amount of glazing that should be
considered to minimise operational energy use?
The model should be updated and revised in parallel with the design as it progresses
from stage 2 to stage 5. At each stage, the model should be integrated within the
feedback loop to the design team to inform evidence-based design decisions. At
stage 5, the focus of the simulation should be preparation for post-construction
monitoring and tuning - providing predicted appropriate sub-system monthly energy
targets, mapped to sub-meters, are important in this regard.
Some of these activities and modelling already occur as part of BREEAM and Part L
investigations. Essentially, the same activities could be retained albeit with a greater
focus on the simulation model being realistic and detailed using DfP principles and
co-ordination with the metering plan and specification.
3.3 Multi-stage IDRs and simulations
IDRs and simulations should be viewed along a continuum to de-risk performance
issues. Observations from reviews conducted at the various stages are discussed in
this section. The benefits and focus of the design review at each stage differs and
could be applied differently depending on the needs of the specific project.
3.3.1 Stage 2
At Stage 2, the review focuses on key design concerns that should be
considered as the design and project budget is developed in later stages. The
benefit of a design review conducted at this stage has been largely to educate the
client and design team regarding the DfP process, how the NABERS UK base
building rating works and the role of simulation in testing alternative design concepts.
Simulation models at this stage are indicative at best, with the simulations reviewed
using simplified HVAC models as well as assumed equipment and controls. Designs
tend to be conceptual: for example, the design reviewer can expect hydronic
schematics illustrating the intent to use ground-source heat pumps in conjunction
with a free-cooling chiller, without any detail regarding interlocking valves or controls.
Many contractual relationships are not in place, or even considered at this stage.
Accordingly, the review identifies design detail that should be considered in later
stages, especially if the architectural design could be reimagined to enable more
efficient services design and operation, facilitating intellectual discourse regarding
any red flags identified and identification of potential future risks that the design could
pose during the operational phase.
CIBSE Technical Symposium, UK April 2022
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Reviews conducted at this stage tend to have the highest chance of being
incorporated into the overall design due to the project being at its concept stage, and
the opportunity for the NABERS UK performance target in client briefs to be
incorporated to set the expectations of all parties within the supply chain.
3.3.2 Stage 3 or 4
Many of the earlier design reviews considered by this paper were conducted at
stages 3 or 4. This is partly due to the DfP process “landing” in the UK when
many of these projects were already at these stages. Clients and design teams
also appreciated more information would be available for critique via the IDR.
At the time of writing (January 2022), the UK’s DfP scheme’s public web site does
not differentiate a preliminary target rating from a design reviewed target rating,
despite their different license rights. Registered initial targets are not locked down
and if applicable have been revised up or down after the stage 4 design review,
rather than showing these two targets separately.
At Stage 3, MEP reports tend to have explored multiple options to optimise BREEAM
ratings and Part L compliance. Architectural detail such as layouts and coordination
schematics are generally well laid out, but the level of detail for façade construction
and thermal performance differs between projects. Hence, the review focusses on
design elements and is more critical about the veracity of the simulation report
results and modelling margin.
Arguably, a stage 3 design review is one of the most important stages as it also
brings to light non-technical risks that affect the recommended modelling margin for
the target rating. In some cases, the review assessed that the 25% modelling
margin
2
, despite being suggested as a minimum in the DfP guidebook, is extremely
risky. This could be due to simulation omissions, poor representation of actual
building operation (such as thermal inertia, lack of load diversity etc., unrealistic
controls due to lack of control or authority over tenant equipment operation), or if
design documentation does not reflect the arrangements modelled (typically control
strategies in are not codified in design detail at this stage).
In some projects reviewed, the modeller simulated equipment or controls that were
not (yet) specified to inform the design team as to what equipment or controls should
be specified in the subsequent design stage. This strategy is acceptable so long as
the simulation report states that this is the case.
In theory, the stage 4 design, being ‘for-tender’ documentation, should be more
advanced compared to the previous stage. However, like observations from stage 3
reviews, the level of detail within design documentation differed between the projects
reviewed. There is less flexibility in changes to the architectural and façade design at
this stage, as such, the focus is often on the MEP design. The overall MEP system
design is also at a more advanced stage and therefore challenging to change at this
stage. Accordingly, the focus of the design review is inclined towards equipment
2
Calculated as the ratio of predicted energy intensity minus the target energy intensity to the predicted energy
intensity.
CIBSE Technical Symposium, UK April 2022
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selection (particularly part-load performance), the intended control strategy
and the tenant fit-out brief. The efficiency of the proposed equipment selections by
the MEP consultant and the intended control strategy for the building is assessed:
• For CAT-A designs, the review suggests enhancements to HVAC system
control strategies that align with equipment selections or intended building
usage.
• For shell-and-core designs, the review identifies gaps between landlord and
tenant equipment demarcations – where risks of poor building energy
performance are driven by the lack of detailed instruction to the tenant
regarding equipment selection, maintenance and control within the tenant fit-
out brief.
The best value for the design review is derived when stage 4 documentation is
reviewed just before the issue for tender, or before the tender phase is complete.
This is because any opportunities identified as part of the design review process can
still be incorporated within the return tender pricing without the need for contract
variation or for contractor pricing to be provided for alternative designs for the value
management process.
The transition between stage 3 and stage 4 is sufficiently early for any contractual,
leasing, post-construction maintenance and property management contracts to be
structured and discussed. Before the design is locked in, the client team -
developer/owner/property managers – should consider the implications of the design
and planned tenant mix on operational energy performance. The 2018 DfP Pilot
Programme Technical Report (10) relates how the prevalence of shell-and-core
design in the premium office market leads to split incentives between landlord and
tenant, leading to major issues in energy efficiency. To overcome this, consider
introducing:
• a centralised building management system for all building equipment material
to central HVAC performance, including those within the tenant demise; or
• CAT-A services design instead of a standard shell-and-core within building
NIA.
3.3.3 Stage 5
Stage 5 designs are the most advanced as these are ready for construction and
installation. From a snapshot IDR perspective for the formal DfP agreement,
documentation in this stage is closest to actual building operation as equipment
selections would be those ordered by the contractor, any workshop drawings and
design detail show valves or dampers that are used for control as well as
commissioning. Notably, the most advanced element within stage 5, one that is
weakly represented in previous stages, is often the metering and HVAC control
strategy design. This is because most MEPs in the UK are not accustomed to a
detailed specification of HVAC control sequences
3
and Australian and American
resources (AIRAH DA28 and ASHRAE Guide 36 respectively) may not be easily
accessible by the broader market.
A Stage 5 review would focus on the metering and BMS functional description
3
While this is largely due to the lack of UK-specific published resources, the authors are aware of efforts to
revamp for this purpose the 2009 version of CIBSE Guide H: Building Control Systems.
CIBSE Technical Symposium, UK April 2022
Page 9 of 19
reverse brief by the contractor to ensure alignment with the DfP model
assumptions.
Assuming that the simulation is updated based on the stage 5 design and equipment
selections, the IDR will likely profess high confidence in the predicted rating. That
said, it may be too late for the building to make any changes should the review
outcome be that the target rating is not achievable. In this case, the only option is for
the owner/developer to revise the target rating in the DfP agreement to reflect the
design reviewed rating. This is typically the least favourable outcome as it has flow-
on consequences on leasing and marketing activities.
3.4 A shift in mindset
Comparison between design reviews conducted in 2019-20 during the DfP Pioneer
program and ahead of the NABERS UK launch versus those conducted in 2021
showed a distinct shift:
• The former revealed a tendency for Part L compliance models to be partially
‘upgraded’ and reported for DfP purposes. Shortfalls included simplified HVAC
models that were inadequate to represent real building operation and
equipment performance at partial loading, as well as idealised façade
performance that do not reflect true wall-construction layers. The simulation
was rarely used as a supporting tool to test the importance of various building
design elements and to stress test (via off-axis scenarios) the building design
against non-technical risks such as tenant behaviours. Off-axis scenarios
tended to be more ‘cookie-cutter’, mainly taken from the NABERS
commitment agreement handbook. Little thought was given to scenarios that
reflect the risk profile for the building in question, and how using a technical
tool like simulation modelling could be used to inform criticality of tenant
management, services control and monitoring, and how post-handover
contracts should be structured. This revealed another manifestation of the
mindset stemming from the need to ensure compliance with Building
Regulations. The issue of unrealistic input parameters in modelling being a
causal factor for the building energy performance gap is well documented
(6)(7), albeit published case studies for commercial offices are thin on the
ground due to an absence of commercial building performance disclosure in
the UK, to date.
• The latter revealed more sophisticated simulations, including the use of
advanced dynamic HVAC modelling programs (which are typically an add-on
to standard simulation software, such as the ApacheHVAC module by IES and
TAS Systems by EDSL, and also require more modelling inputs). The energy
modellers seem to exhibit greater awareness of simulation software
limitations, transparently reporting any compromises and the level of risk such
treatment in the model poses to estimated building performance. Off-axis
scenarios are used to stress test impacts of such compromises and deviations
from intended control such as tenant equipment operating 24/7 or inability of
landlord central plant pumps to turn down due to poor valve control by tenant
equipment. In some cases, simulation was used to inform design (via off-axis
scenarios), such as the impact of different glazing selections, significantly
oversized plant capacity that should be designed with more modular plant or
CIBSE Technical Symposium, UK April 2022
Page 10 of 19
require a thermal buffer tank to avoid plant short-cycling, or benefits of
specifying an alternative control strategy.
In essence, there has been a clear shift towards using the simulation model as a
design tool. Complementing this also revealed more realistic building simulation
models at the design stage, narrowing what Bordass et al (5) described as the
‘credibility gap’ – unrealistic assumptions that lead to modelled outcomes that bear
no resemblance to actual fuel consumption. This attests to simulators’ increased
appreciation of how real buildings and HVAC controls should be modelled instead
of idealised comparison to a reference building using Part L compliance models and
NCM profiles. Mounting confidence in the realism of simulation models also aid
acceptance of dynamic energy models as a design tool, as demonstrated in latter
reviews completed
4
.
3.5 A holistic approach for change
A change in context during 2019 was triggered by a relatively sudden mainstream
acceptance of the “Climate Emergency”, manifested by the government amending
the UK’s Climate Change Act and by the UKGBC producing operational performance
targets for commercial offices which referenced NABERS UK ratings (15). This shift
in mindset, especially for the type of developers who were spearheading DfP, largely
overcame the barriers mentioned earlier represented by the traditional commercial
realities of the leasing market and lack of tenant interest in the performance gap. At a
practical level, the change was complemented by several events, which played an
important role in the observed industry upskilling process:
• The formal launch of the NABERS UK rating scheme and DfP framework
released public documents that practitioners could reference. These
documents included guidance on modelling parameters, default operation
profiles and internal gains and introduced the concept of modelling off-axis
scenarios.
• Delivery by the BBP of the independent design reviewer training and
examination to establish a panel of experts. As part of the DfP program
launch, customised training was delivered to candidates, educating them on
the design review process, common pitfalls and using a real project as an
example, tested the candidates on the application of concepts taught.
o The training emphasised the importance of the design review process
as a conduit for educating and expanding the design team’s horizons
regarding issues and solutions to achieve operational performance. The
training further recognised the role of the design reviewer as a ‘change
agent’, challenging all parties to make this a constructive process,
encouraging teams to accept recommended solutions where
appropriate, and collaboratively brainstorm or propose better solutions
as part of the process. Setting the right culture and mindset from the
get-go has been critical in ensuring the success of the building industry
4
As opposed to a compliance check box that is ignored once ticked.
CIBSE Technical Symposium, UK April 2022
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towards achieving highly efficient building performance.
o The IDR template
5
is used by the reviewer and shared with the design
and client team – indirectly educating and upskilling the industry.
Because the same template includes all trades (architectural, MEP,
simulation) and non-technical aspects related to management such as
the rating achievement plan, it breaks down silos in the design process.
It provides all stakeholders with an appreciation of issues and risks that
affect efficient building performance including how technical and non-
technical decisions can have impacts that reverberate across the value
chain.
• Launch of the NABERS UK Assessor training by BRE. A key role of the
energy simulator is to predict the NABERS UK energy rating that the building
will likely certify at. Before the launch of the NABERS UK Assessor training,
most design teams and modellers were unfamiliar with the coverage of a
base-building NABERS UK energy rating. This lack of understanding led to
common errors such as the omission of HVAC equipment energy (such as
tertiary pumps or fan coil motors) within tenant net internal area (NIA), failure
to identify sub-metering risks or consider alternative methods for mechanical
services within NIA, lack of awareness regarding complexities of apportioning
and sub-metering requirements for shared thermal energy plant and artificially
boosted predicted ratings by fully claiming on-site renewables to discount
base building energy consumption. Launching the NABERS UK Assessor
training has significantly improved the industry’s broader understanding of the
rating coverage and a better grasp of associated risks, and thus, improved
design responses.
• Commissioning of design reviews within the industry. Many independent
design reviewers were themselves experienced energy simulators or M&E
consultants. The process of critiquing simulations and designs by others in the
role of a design reviewer, or being subject to the design review process by a
third party, also assisted in further enhancing their (or their supervised staff)
own simulations, designs and appreciation of the non-technical issues that can
impact real building energy performance.
• Delivery of the CIBSE advanced simulation modelling for DfP training.
The training course (8) was aimed at upskilling the MEP community in the
theoretical approach to simulation modelling when targeting an operational
rating. Modellers were trained on the practical application of advanced
simulation principles to reflect realistic building operation and input parameters
that need to be modelled as part of dynamic energy modelling. This course
was vital to address the issue of low building energy modelling skills in the
industry (10).
3.6 Early involvement and discourse amongst stakeholders
While the IDR process is a highly technical exercise, it would be folly to ignore the
5
The IDR template is a comprehensive spreadsheet that systematically and transparently prompts the reviewer
by asking questions about each type of specified equipment/control, enabling the reviewer to offer suggestions
for (without dictating) possible efficient design responses.
CIBSE Technical Symposium, UK April 2022
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importance of stakeholder engagement and communication across the full building
lifecycle and value chain. It is true that building operational performance is contingent
on good design and accurate simulations reflecting in-use building occupancy
profiles and facilities management; however, the success of target achievement is
highly dependent on stakeholder activities post-handover of the building (9).
This reinforces the importance of risk assessments stretching far beyond the design-
only phase – the review assesses the inherent risks across the full operational
lifecycle of the building from client brief development, design, leasing, construction,
maintenance to post-construction monitoring and tuning.
The 2018 DfP Pilot Program Technical Report (10) identifies a long list of
stakeholders ranging from investors, developers, leasing agents, managing agents,
facilities managers, occupiers, contractors, MEP consultants, modellers and many
more. Therefore, the IDR process should endeavour to engage with the different
stakeholders, as many of the non-technical issues that impact building operational
performance fall outside the sphere of control of the design team. This issue is raised
by Bannister and Cohen (14) who reported on the dilemma of designers heavily
constrained to deliver systems that conform to industry commercial norms, and
developers that feel obliged to deliver to tenant expectations.
The process adopted for the completed IDRs endeavoured to engage with
stakeholders across the lifecycle, noting that this was not always possible with all
projects reviewed due to commercial considerations.
• The briefing workshop is important as scene-setting to encourage a
collaborative instead of a combative environment. This workshop always
involved the architects, MEP design engineers and energy modeller. But in
some cases, the owner/developer and sustainability consultant, if engaged,
were also present. The briefing workshop was key to getting a basic
understanding of the development/refurbishment project brief so it was clear
which building elements were to be retained and which are new, allowing the
design engineers to describe any projects constraints that led to certain
design decisions or compromises. This workshop is key to setting up a
constructive environment instead of being combative or defensive – the
design team is encouraged to identify any challenges or elements that could
benefit from additional perusal by the reviewer to provide input. As the DfP
process is new to the UK market, it was necessary to reiterate that an
extensive list of IDR recommendations is not a fault-finding exercise.
Conversely, these should be considered as a shopping list of prioritised
suggestions, many that could or should be adopted for the project in question;
and others for integration within the next project.
• The rating achievement plan, including the tenant fit-out brief, must be
front and centre of the DfP process. In early reviews, a separate workshop
was required to educate the owner/occupier regarding the rating achievement
plan. Different approaches have been observed – (a) some projects with the
MEP being the driving force coordinating the several workshops with a large
stakeholder group including the M&E contractor, managing agent, leasing
agent, owner/developer, (b) other projects with a dedicated sustainability
consultant or ‘NABERS champion’ advising the owner with the engagement
CIBSE Technical Symposium, UK April 2022
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process with other stakeholders largely opaque to the reviewer, or (c) projects
where the owner is actively engaged in the process with the independent
design reviewer to discuss implications of the rating achievement plan and
how to set up contractual relationships such as maintenance contracts,
performance contracts for the main contractor, a structure for post-
construction monitoring and tuning contracts and interactions with facilities
management or the tenant.
4 Common Issues
The common issues and major risks identified from the IDRs are summarised in this
section.
4.1 Technical risks
4.1.1 Common technical risks for HVAC systems
The following technical risks are common for the various HVAC system types
reviewed and benefited from being modelled as an off-axis scenario in the DfP
simulation.
• Heat recovery VRF system. Zonal thermal diversity tends to favour heat
recovery VRF systems. With smaller VRF buildings delivered as a shell-and-
core base building design, stress test poor design by the tenant by removing
the diversified internal zone loads.
• Water-to-air heat pumps. Unless enforced via tenant leases or a CAT-A
HVAC design, any condenser water valves for the compressor units are likely
to be open for long durations, leading to the pumping system acting as a
constant flow instead of variable flow system. This risk is mitigated if
continuously modulating condenser water valves linked to compressor load is
specified.
• Air-to-air heat pumps. Where a single heat pump serves both interior and
perimeter zones with terminal reheats, the high efficiencies of heat pumps in
heating mode is not capitalised
6
and reheat energy will be high. Where this
result is not observed in the simulation results, revisit how the air transfer
between HVAC thermal zones is modelled by the simulation software engine
or consider adding internal partition walls to ensure this is modelled correctly
by the software. From a mechanical design perspective, this system should be
designed with separate perimeter and interior zone heat pumps.
• Centralised cooling and heating plant.
o Standard designs seem wedded to the use of plate heat exchangers at
each tenancy for hydraulic separation. This practice is advantageous to
enable tenant fit-outs without affecting the broader hydronic network
and pseudomonas risk-management. However, this is at the expense
of pressure losses
7
across the heat exchanger and more notably,
restricts execution of temperature resets
8
that increases chiller and
6
This is because the heat pumps will always operate to satisfy the warmest zones, leaving any colder zones to be
reheated using terminal reheats.
7
Typically between 10kPa to 40kPa per heat exchanger.
8
Relaxed temperatures (higher for chilled water, CHW; and lower for hot water, LTHW) decrease thermal
efficiency of heat exchangers which could be specified to be anywhere between 80 and 95% efficient.
CIBSE Technical Symposium, UK April 2022
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heating plant efficiency during building partial loading. As a result, the
system regresses to operating as a constant temperature system,
which can perpetuate the ‘low deltaT
9
’ syndrome. While uncommon,
some projects did specify a design without plate heat exchangers,
suggesting that the alternative configuration without plate heat
exchangers, the Australian norm, is possible in the UK context.
o Related to the above issue is the practice of domestic hot water (DHW)
calorifiers and space heating sharing the same LTHW plant. Space
heating demand is seasonal while DHW demand is steady-state across
the year. The common issue observed was domestic water temperature
requirements
10
restricting the ability for the LTHW temperature to be
reset downwards when space heating demand is low. Designs should
consider the ability to service the DHW load separately from the
centralised plant (e.g., via a separate hydraulic connection to a
dedicated heat pump) or at minimum, a separate hydraulic riser for
DHW so it is not on a riser shared with space heating.
4.1.2 Alternative approaches to HVAC equipment design within tenancies
Traditionally, HVAC equipment located within tenant NIA is considered ‘tenant plant’
despite being base building equipment under NABERS UK. Such tenant HVAC plant
is included within NABERS UK base building energy coverage because poor design,
control and maintenance of such equipment can substantially increase landlord
centralised HVAC plant operation (14).
A few observations associated with this issue can be made:
• UK buildings tend to be delivered as shell-and-core, with the tenant designing
and installing equipment when the space is let
11
. The predominant UK designs
reviewed under the DfP scheme often specified CAT-A lighting design but
seem to relinquish mechanical design to tenant fit-out. However, more recent
reviews, while still in the minority, show signs of market warming to the
concept of a CAT-A HVAC design combined with centralised landlord
monitoring and control. The CAT-A HVAC design approach decreases the
level of performance risk by increasing:
• Design team control over the base building systems design and
installation
• Facilities management team control of building in-use operation as well
as equipment maintenance.
• From an electrical design perspective, two options exist:
9
This is a situation where the low temperature differential between supply and return chilled water temperatures
lead to increased flow to achieve the same capacity, thereby increasing pumping energy. This is the reason why
the relationship between pump power and system flow will not follow the pump affinity laws in practice, and
one is more likely to observe a linear or at-best a quadratic relationship between the two variables (11).
10
Typically 70°C on the primary side of the heat exchanger to achieve 60 to 65°C on the secondary side
11
This is contrasted with Australian buildings which are typically delivered as ‘CAT-A’ designs across all
services, enabling a tenant to move in and occupy the space as-is if desired with some modifications to
communications to suit.
CIBSE Technical Symposium, UK April 2022
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1. to retain existing shell-and-core design but design dedicated
mechanical services switchboards for each tenancy for connection of
on-floor HVAC equipment only; or,
2. two, to design dedicated mechanical services electrical riser from the
landlord main switchboard, with sub-metered tee-off for charge-back to
each tenant. This option is the norm in Australia with the associated
energy consumption cost typically recovered from the tenant via
aggregated building outgoings.
The benefit of the second approach is decreased sub-metering requirements,
which reduce the risk of sub-metering failures. Designs reviewed indicate that
this model is possible and has been implemented in the UK context; however,
it needs to be flagged for early consideration by the owner-developer and
leasing teams as part of the client brief in Stage 2.
4.1.3 Outside air AHU design
Outside air AHUs are specified with heat recovery via thermal wheels or plate heat
exchangers, usually centralised for large buildings and on a floor-basis for medium to
smaller buildings. While conveniently aligned with tenant cost-recovery mechanisms,
this configuration is not necessarily the most efficient outcome, especially for
buildings with deep floor plates
12
.
Two design options exist:
1. Better demarcation of tempered air to the perimeter and interior zones using
separate AHUs - an ideal solution to avoid simultaneous cooling and reheat.
That way, free-cooling using air-side economy cycle to the interior zones can
be applied during temperate or cold weather, without the need for reheat on
the perimeter.
2. If the building must proceed with a tenancy-based AHU approach, then at a
minimum, VAV boxes should be designed for interior and perimeter zones
respectively. Localised heat recovery from the return/relief air stream to the
perimeter zone for reheat purposes should be designed. This option will allow
airflow to perimeter zones to be dialled back when cooling is not required, and
the cool ambient air (during economy cycle) to be reheated using recovered
heat.
4.2 Other Non-Technical Risks
4.2.1 Locked in, left behind
A less common issue but an equally noteworthy risk is the issue of locked-in obsolete
designs. This usually follows an extended hiatus in development progress due to
capital or other unforeseen issues. One project found itself locked into the use of
substantive gas heating systems supplying 70/80°C water due to the original
12
These building types accentuate the thermal difference between year-long cooling dominated interior zones
versus the perimeter zone which tracks ambient temperatures.
CIBSE Technical Symposium, UK April 2022
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BREEAM targets being set 10 years ago
13
. As the commitment had been registered,
the design team found itself under immense pressure to retain the registered design
with condensing gas boilers even though the building is anticipating a completion
date in 2024/25. This grates in the context of London’s Mayor setting a target of net-
zero carbon by 2030 for London (17) and all new buildings must be designed to meet
this target by 2025.
The NABERS UK rating tool is based on primary energy (kWh
equivalent
/m
2
), and
therefore the building is disadvantaged from the outset with gas boilers operating at a
coefficient of performance (COP) of 0.9 compared to heat pumps with COPs 2.5 or
higher. For the project in question, the compromise was to design water-to-water
heat pumps and specify controls to stage these units ahead of the gas boilers,
though it was challenging to introduce any water temperature set point reset to derive
condensing boiler and heat pump efficiency benefits due to the domestic hot water
calorifiers located downstream of the LTHW system.
This is a prime example of how a non-technical barrier led to significant technical
limitations and building operational performance risk.
4.2.2 Tenant fit-out brief
A tenant fit-out brief is important within the UK context as it sets expectations of the
tenant from the outset that may not be common at the time. The fit-out brief should
be incorporated within leasing documents, particularly shell-and-core designs.
Coverage of the brief should include:
• the base building target NABERS UK rating
• tenant equipment efficiency and design parameters,
• coverage of sub-metering, validation requirements and trend logging
requirements.
• the interface of equipment to the landlord centralised building management
system, both monitoring and control.
• landlord review and approval process of any tenant fit-outs to ensure that it
does not adversely impact the base building target NABERS UK rating
• equipment control sequences that should be programmed.
• maintenance requirements for tenant equipment.
• tenant response and participation in the building tuning program
• after-hours air conditioning request process that is aligned with the NABERS
Rules
• dispute resolution process where tenant actions or design is not approved by
the landlord
14
.
The tenant fit-out brief could be perceived as more prescriptive than usual. The brief
must be discussed with the leasing agent for negotiation purposes and for facilities
management to be empowered to enforce the tenant fit-out brief during building
13
At that time, both UK and Australia green building rating tools and building regulations had a penchant for
natural gas due to its lower emissions relative to coal-generated electricity. This has rapidly changed in recent
times as the electricity grid decarbonises with the uptick in renewable energy generation.
14
A few options explored as part of the projects reviewed such as update of the simulation model to quantify any
adverse impact of the tenant design/operation on the NABERS UK base building energy rating. Financial
responsibility for commissioning the model should ideally be disclosed in the fit out brief.
CIBSE Technical Symposium, UK April 2022
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operation.
4.2.3 Maintenance contracts during DLP
The structure of maintenance contracts and management of the defect liability period
was identified as another major risk. Designing for building performance has the best
chance for success when there is continuity between the original design and
construction team through to building operation. This is because building
performance is dependent on proactive building tuning and monitoring, which profits
from a good understanding of the original design intent.
Existing practice for maintenance contracting is not conducive for performance. M&E
contractor engagement is limited to the rectification of equipment defects during DLP
and a second M&E contractor, typically unrelated to the project M&E contractor,
responsible for service and maintenance of the building. Exacerbating this issue is
the tendency for HVAC equipment within NIA to be under direct tenant control – in a
multi-let building with 10 tenants, this could end up with 12 M&E contractors
responsible for various parts of the mechanical system that impact base building
energy consumption, all with split incentives due to engagement by different
stakeholders, varying degrees of maintenance or service quality
15
- performance
tuning which takes a system view (e.g., all fan coil units and valves in a building
impact centralised plant staging, temperature and pumping control) becomes a
logistical nightmare. Furthermore, there is no point in contractually requiring the
project M&E contractors to participate in the building tuning process during DLP if
they do not have any clout to test or tune setpoint adjustments
16
.
The rating achievement plan is the ideal forum for the discourse of these concerns
and challenging the institutional status quo, which is why designing for performance
and the IDR cannot be limited to the design and construction team. Where the IDR
process and workshops involved the owner or facilities management team –
alternative contracting methods could be debated. Approaches considered include
one that has been successful in the Australian context but requires negotiation with
the managing agents: the M&E contractor is engaged for service and maintenance
for all equipment within base building energy coverage (including those within
tenancies) during DLP, eliminating the issue of split incentives.
5 Conclusions
The Design for Performance (DfP) framework was introduced in the UK to address
the building energy performance gap, a step-change in action aimed at helping to
meet the UK’s net-zero by 2050 legislated target. DfP was modelled upon the 20-
years of success in Australia under the NABERS program, which recently reported
that an average 42% reduction in base building energy intensity upon a building’s
14
th
NABERS rating (16).
This paper has presented lessons from the NABERS UK independent design review
process across the past two- to three-years, reflecting on stakeholder motivations,
change in attitudes and skill with time and market activities, and a wide range of
15
This is not necessarily due to failure on the part of the M&E contractor as these parties could be engaged for
different service levels by their end client (various tenants, facilities management team or original developer).
16
To avoid disputes regarding liability. If anything goes wrong during that process, is the maintenance M&E
contractor responsible for rectification or the project M&E contractor?
CIBSE Technical Symposium, UK April 2022
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common issues that influence building energy performance – both technical and non-
technical. It reflects on how and when simulation and design reviews could be
conducted, and how best to extract value from these tools at each stage to de-risk
operational performance.
In 2020, Bannister and Cohen (14) reflected at the CIBSE Glasgow conference that
the UK could leapfrog the 20-years of progress in Australia learnt by slow trial and
error, by adopting these lessons in the UK ahead of the curve. The UK industry has
the right attitude and is well on the track towards delivering high-performing buildings
from the projects reviewed within the short two to three-year timeframe.
The challenge will be in changing the status quo and early engagement with
stakeholders to address many of the non-technical barriers such as maintenance and
tenant engagement during the leasing negotiation and building operational stage. A
holistic and collaborative approach to designing for performance is momentous and
will require ‘NABERS UK/DfP Champions’ within each stakeholder group to advocate
the responses to the opportunities and issues identified in this paper.
6 References
(1) BRE Design for Performance, viewed December 2021,
https://www.bregroup.com/nabers-uk/nabers-uk-products/nabers-design-for-
performance/
(2) Cohen RR, Bannister P. and Botten C “NABERS UK: Rating Performance
Outcomes”, CIBSE Technical Symposium, July 2021.
(3) NABERS UK, Guide to Design for Performance Version 1.1.
(4) BEIS consultation: Introducing a performance-based policy framework in large
commercial and industrial buildings. March 2021
https://www.gov.uk/government/consultations/introducing-a-performance-based-
policy-framework-in-large-commercial-and-industrial-buildings
(5) Bordass, B., Cohen, R. and Field, J., Energy Performance of Non-Domestic
Buildings: Closing the Credibility Gap. In Proceedings of the 2004 Improving Energy
Efficiency of Commercial Buildings Conference.
(6) Menezes, A., Cripps, A., Bouchlaghem, D. and Buswell, R. Predicted vs. Actual
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(12) BRE NABERS UK Example Simulation Report September 2021. Viewed
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(13) BCO Research: Design for Performance - Lessons from Down Under, Viewed
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(15) UKGBC: Net zero carbon: energy performance targets for offices, January 2020.
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(16) NABERS Annual Report 2020/21, Viewed December 2021,
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