Reducing Building Energy Demand Through Revisions to Criteria 1 & 3 of the Building Regulations Part L2A

I proposed that:


a) A mandatory overheating risk assessment, subject to a suitable floor area threshold, be re-introduced back into Criterion 3 of the building regulations Part L2A.

Further, I propose that b) the National Calculation Method (NCM) strategy be reviewed, which currently allows buildings with intrinsically higher energy demands to be granted additional lenience through the 'Target Emissions Rate' calculated via the ‘notional building’.

These changes are proposed in order to:

  • Ensure that the quality of buildings brought to market are of an adequate standard for consumers with respect to overheating risks, for which many new buildings do not meet consumers expectations¹, and


  • To reduce the installed capacity and subsequent energy usage of air conditioning systems, for which non-domestic buildings are estimated to consume  a growing amount of energy, which is currently assessed to be approximately 10% of UK electrical energy generation ², and


  • Encourage architects and developers to adopt effective fabric first design philosophies to achieve true low-energy demand buildings in practice, rather than placing heavy reliance on heating, ventilation and air conditioning (HVAC) systems to resolve thermal comfort problems and using bolt-on renewable energy generation systems with comparatively short service lives.

A) Addressing Overheating Risks & High Cooling Energy Demand:

In order to appropriately limit air conditioning cooling demand, I proposed that new buildings are designed from the outset with an aim to limit energy demands. The current regulatory processes clearly do not bring this about.

Therefore, I suggest that as a pre-commencement building regulations condition, buildings over a certain specified floor area be tested against an appropriate industry overheating metric (eg CIBSE TM52 for commercial projects and CIBSE TM59 for domestic projects, using prescribed weather file data) These assessments should be undertaken without the inclusion of air conditioning systems. This will demonstrate that internal temperatures can be limited to an acceptable level without incurring high air conditioning cooling loads. This requirement should also be incorporated into the pre-commencement planning conditions for the project and be re-evaluated upon project completion.

I envisage that this would:

1.     Obligate architects to derive architectural designs which naturally lend themselves to minimising summertime air conditioning energy demands.

2.     By extension, reductions in summertime solar gains will naturally reduce glazed areas such that winter time heat losses through glazing are also reduced, when working in combination with the existing Part L regulatory processes.

3.     In cases where room(s) fail the assessment, justification of the cause should be made available to BCOs and planning officers, demonstrating that all other options have been exhausted prior to the application of mechanical cooling.

For clients wishing to achieve an air conditioned climate, there would be no barriers to installing air conditioning plant, so long as necessary measures have been incorporated into the design to limit energy demands, making air conditioning complimentary, rather than essential.



B) Modifications to the NCM Notional Model:


I propose that the supplementary energy demand allowances, facilitated through the National Calculation Method (NCM) process be removed for several inefficient modes of HVAC, as follows:

1) The uplift in the NCM notional building primary energy demand should not include mechanical cooling loads.

2) Further, the uplift in the NCM notional building primary energy demand should not include for natural ventilation as the only means of ventilation. Instead, the notional building should adopt mechanical ventilation with heat recovery and the availability of free-cooling.

This change shall remove the incentive to favour mechanical cooling (air conditioning) as a means to achieve thermal comfort over and above less energy intensive methods, such as demand reduction through effective building form, shading strategy and fabric specifications, and via free-cooling through natural and mechanical ventilation.

The current NCM structure allows mechanically cooled buildings to exhibit a substantial betterment of the Part L2A Target Emission Rate (TER), yet still incur substantially higher energy demands when compared to the same building designed to avoid the need for mechanical cooling. These measures actively encourage the use of mechanical cooling over other options, rather than discourage its use. The protocol does so by specifying the NCM notional building with the same HVAC system type as is specified in the actual building. This also plays out in the EPC for the building.

This is confusing for the industry and its customers, as it suggests that buildings with higher energy demands and CO2 emissions are actually more efficient than buildings which are specified with space heating only, when in actual fact, these mechanically cooled buildings are less efficient and incur higher energy demands.

Likewise, I propose that inefficient naturally ventilated spaces should also not benefit from an allowable uplift in the ‘Target Emission Rate’. Instead, naturally ventilated rooms should be benchmarked against a notional building equipped with mechanical ventilation with heat recovery. Mechanical ventilation with heat recovery has never offered a benefit in terms of Part L compliance, because where specified, the notional building also includes mechanical ventilation with a particularly low fan SFP of 0.9W/l/s. This can make naturally ventilated buildings easier to meet Part L compliance, despite incurring higher energy demands and CO2 emissions. As with above, this provides no incentive to specify this energy saving technology.








Why the contribution is important

The current revision of the building regulations Approved Document Part L2A 2013 (w/2016 amendments) (ADL2A) contains policy which aims to limit summertime heat gains in order to reduce the need for air-conditioning, or reduce the installed capacity of any air-conditioning systems which are installed. This element of the code is referred to as Criterion 3.

There is no mandatory assessment to assess the risk of overheating in buildings (see paragraph 2.52 of ADL2A), with the code stating that the developer should work with the design team to specify what constitutes an acceptable indoor environment, and also noting that this is NOT evidence that the internal environment of the building will be satisfactory, since many factors that are not covered by the compliance assessment procedure will have a bearing on the incidence of overheating. This leads to some confusion amongst building control officers (BCOs) and developers alike, concerning what Criterion 3 effectively means. Many incorrectly refer to Criterion 3 as an ‘overheating assessment’ – which it is not.

The Criterion 3 assessment itself is a crude assessment which seeks to limit solar gains to below a prescribed threshold. Criterion 3 assesses the solar gains per unit of external wall area, and does not account for room volume or floor area. When combined with National Calculation Method (NCM) zoning rules, this leads to highly misleading outcomes concerning the assessment of whether or not solar gains are excessive. There are also work-arounds available to energy assessors to omit ‘non-occupied areas’ from the BRUKL compliance document, allowing heavily glazed areas to be assigned as ‘display areas’ or ‘voids’. In reality however high solar heat gains remain to occur.

This approach pushes onus to carry out an overheating risk assessment onto developers. It is commonplace for developers not to undertake an overheating risk assessment unless expressly stated in the contract performance specification. Many developers and property buyers blindly assume that overheating risks are controlled as part of the building regulations. This allows many buildings to make their way to market which incorporate design features which place them at a high risk of overheating and in need of retrospective alterations. These retrospective alterations are expensive, wasteful and often include air conditioning as a remedial fix, rather than tackling the source of the problem.

Having personally undertaken many ADL2A assessments alongside ‘CIBSE TM52’ and ‘CIBSE TM59’ overheating risk assessments, I can testify that working to Criterion 3 compliance alone is in most cases insufficient to prevent what the chartered Institute of Building Service Engineers (CIBSE) define as ‘overheating’ in the CIBSE TM52 & TM59 codes.

This has ramifications not only to thermal comfort, but also to space cooling and space heating energy demand. The causation of these issues has its roots, and I would suggest it’s solutions, in the regulatory instruments of the planning process and the building regulations.

New construction projects typically take the following course of events:

a.      The building form, orientation and glazing quotas are defined by the architect. These concept stage design decisions are rarely based on functional performance driven objectives, but rather architectural styling.

b.     Unless expressly dictated by the client, the external aesthetic of the building will often be submitted for planning approval at this stage, with no deeper consideration regarding thermal comfort and energy efficiency taking place. Larger schemes will sometimes also be steered by advisors or ‘multi-disciplinary practices’. These may consider to a greater extend the wider repercussions of thermal comfort and energy efficiency -but not always.

c.      Glazing is usually specified with a sufficiently low g-value to satisfy the building regulations Part L2A Criterion 3.

d.     Renewable energy generation may be added as required to facilitate Part L2A Criterion 1 (CO2 emissions) compliance.


This process:

a.      Builds in potentially high overheating risks/high mechanical cooling demands into the construction fabric from the initial concept stages, unless:

1.     Architects optionally incorporate passive energy efficient attributes into their designs, or

2.     Architects are obliged to incorporate passive energy efficient attributes into their designs through external factors (eg optional additional client led performance criteria & local planning policy).

b.     Can have the effect of increasing heating loads where glazing apertures are sufficiently sized that ‘low-g’ glazing is required to limit summertime solar gains. In such cases, buildings exhibit behaviour more akin to a greenhouse with tinted glazing, incurring both moderately high summertime temperatures and/or high cooling loads and also incurring high wintertime conductive heat losses.

From experience, even in the case of projects subject to higher energy efficiency performance standards such as BREEAM and even Passivhaus, the performance of building orientation, form and glazing configuration is often the weak link in what would otherwise be an efficient system of architecture and building services working together.

In short, a heavy reliance is placed on building services and renewable energy generation to achieve desired thermal comfort and energy efficiency performance objectives; far more than is placed on architectural layouts. This can often result in absurd situations where large sums of money and resource are channelled towards various renewable energy systems, when even greater improvements in efficiency could be achieved through the informed sizing and positioning of windows at much lower monetary cost.

My estimations understand that this is caused by contractual obligations for thermal comfort and energy efficiency being largely discharged to the building services engineer and main contractor. Rarely do architects bear any contractual design responsibility for thermal comfort and energy efficiency on commercial construction schemes.

This contractual issue, combined with a limited interest and understanding of basic building physics concepts on part of many (but not all) architects, and next to no participation in post-occupancy surveys, results in architects creating the same problematic issues repetitively, with lessons not being learnt from project-to-project.

This approach is condoned through the NCM process, whereby mechanical cooling is granted an additional energy allowance, rather than acting negatively towards the assessment of energy efficiency. This can easily result in scenarios where adding additional cooling load to a building improves the Part L2A outcomes by comparison to a non-mechanically cooled alternative, whilst at the same time incurring additional energy demands.

This need not be the case. Substantial reductions in energy demand and improvements in occupancy comfort can be facilitated through no additional cost where glazing apertures are sized appropriately to suit thermal and visual comfort needs.

The knowledge and ability to instil these levels of performance is available. The industry has been allowed to rest on its laurels for eight years now, with the Part L2A 2010 regulations providing no real challenge and subsequent revisions being kicked into the long grass.  It is now time to motivate and encourage industry to provide better buildings which are fitting for the demands of today's society.

by RTibenham on July 18, 2018 at 12:47PM

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  • Posted by JSanchez July 19, 2018 at 00:21

    This is a very strong proposal that adresses an ongoing and growing issue across the country. Indeed, the architecture of our cities, villages, schools, buildings has to be better thought out. This recommendation is not only engaging, but also attainable if we commit ourselves to it. Its implementation would have both immediate and long term benefits and it would lay the first stone for the buildings of tomorrow's society. In my opinion, this is a good and legitimate starting point that deserves further interest.
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