Building regulations - energy standards and associated topics - proposed changes: consultation

Part 5 – Overheating risk in new dwellings and other new residential buildings

5.1 Introduction

5.1.1 Background

Overheating in homes occurs when conditions in excess of those acceptable for human thermal comfort or those that may adversely affect human health occur in the internal building environment. Overheating in buildings has been highlighted as a key concern for the health and welfare of people across the UK.

Whilst overheating risk may generally be considered an issue principally for southern regions of the UK, previous research carried out by the Mackintosh Environmental Architecture Research Unit, Glasgow School of Art et al assessed 26 new homes in 2013^[27] and found overheating instances in over half of the homes (based on the Passivhaus approach to determining overheating criteria).

Within the Climate Change Committees (CCC) report ‘UK Housing: Fit for the future’^[28] reference is made to there being around 40 heat related deaths per year in Scotland which is projected to potentially rise to between 70 – 280 deaths. The CCC recommend that the Scottish Government, along with the rest of the UK, introduce a new standard that requires the overheating risk to be assessed for new housing and ensure that passive cooling measures are installed at build stage where a risk of overheating is identified.

There is no direct requirement within schedule 5 of the Building (Scotland) Regulations 2004 for buildings to be designed and constructed to avoid the risks of overheating. Standard 6.6 of the energy standards of the building regulations aims to reduce the need for buildings to have active cooling/ventilation systems installed to address periods of high internal temperatures. However, this is set in the context of minimising energy use within the building as opposed to protecting the occupants from the health risks associated with overheating which occurs in buildings.

Standard 6.6 calls for dwellings with little or no cross ventilation, or a high proportion of glazing, to be assessed for high internal temperatures using the calculation methodology within Appendix P of SAP 2012. Where the results confirm that the likelihood of a high internal temperature in hot weather is slight, not significant or medium then this should avoid the occupier installing a mechanical cooling or air conditioning system at a later date. However, this provides only a simplified check of whether the home will have an overheating problem. The SAP methodology does not predict the severity of the overheating risk and the effectiveness of remedial solutions. Furthermore, the whole house / volume approach fails to identify individual rooms at higher risk from overheating. Therefore, it is considered a more robust assessment is needed.

5.1.2 Recent research to support proposals

Over-heating research commissioned in 2019 to support review of standards for new homes and residential buildings in England^[29] demonstrated that during warm years, overheating will occur in most new homes in most locations in England, particularly in London. As a consequence of the research a new mandatory requirement has been consulted upon which, if implemented, will require all new homes and residential buildings, to meet a new overheating requirement. The requirement can be met by a simplified method or by demonstrating that the overheating risk has been mitigated through dynamic thermal analysis following the CIBSE TM59 (2017) methodology. The simplified method identifies options for solutions which mitigate solar heat gains plus adequate purge ventilation to disperse the heat.

Similar research, looking at a subset of dwelling conditions, was carried out for the parallel review in Wales with prevalence of overheating found to occur in flats. As a consequence, similar provisions are being proposed for inclusion in the Welsh building regulations, that will apply to flats and houses that cannot be cross ventilated.

To understand the potential risk of overheating in Scotland the same risk assessment was carried out for 5 of the typologies that were found to be prone to a range of overheating in the English and Welsh research. This research paper is published at https://www.gov.scot/ISBN/978-1-80201-193-7.

Table 10 Dwelling typologies modelled for overheating risk.

Ref.	Dwelling Form	Size	Aspect	Ventilation Strategy	Heating System	Construction Type
1.	Apartment/ Flat	2b4p	Single	Natural ventilation	Individual	Mid-rise
1b.	Apartment/ Flat	2b4p	Single	MEV	Individual	Mid-rise
2.	Apartment/ Flat	2b4p	Dual	Natural ventilation	Individual	Mid-rise
2b.	Apartment/ Flat	2b4p	Dual	MEV	Individual	Mid-rise
3.	Semi-detached house	3b5p	Triple	Natural ventilation	Individual	Timber frame

The models were based on timber frame construction and adopted the ‘advanced’ specification (Option 2) proposed by this consultation. The floor plans, elevations, fabric specification and building geometry of the dwellings modelled are noted in the research paper.

CIBSE TM59 sets two compliance criteria for dwellings, both of which need to be met to demonstrate an acceptable risk of overheating:

• Criterion A applies to living rooms, kitchens and bedrooms. It requires that the internal temperature does not exceed a defined comfort temperature by 1°C or more for more than 3% of occupied hours over the summer period (1 May to 30 September).
• Criterion B applies to bedrooms only and requires that the internal temperature between 10 pm and 7 am shall not exceed 26°C for more than 1% of annual hours.

To assess the risk of overheating over the lifetime of new homes, the analysis of internal temperatures should be based on predicted future weather data. The CIBSE TM59 methodology requires the assessment to be carried out using DSY1 weather data (Design Summer Year) for the 2020s. DSY1 (Design Summer Year 1) is intended to represent a moderately warm summer intended to represent 2011 to 2040. CIBSE has published DSY1 weather data for two Scottish locations; Glasgow and Edinburgh. A sample simulation of a dwelling established that the Glasgow weather file resulted in a higher overheating risk. Further analysis of weather data for other locations in Scotland found that mitigation measures proposed for Glasgow should be suitable for controlling the risk of overheating for at least most other locations in Scotland.

The overheating risk has been assessed based on ‘Category II buildings’, which assumes that the dwellings have a normal level of expectation of being occupied by vulnerable and fragile persons.

Outcome

The results show the semi-detached house and the two single-aspect flats all passed both Criteria A and B of CIBSE TM59. Both dual-aspect flat typologies passed Criteria B of CIBSE TM59 (bedrooms), but both living rooms failed Criteria A albeit to a limited extent.

For the sample dwellings used in the analysis, the greater glazed area of the dual-aspect flat increased the solar gains to the extent that the net effect was to increase the risk of overheating compared to single-aspect flats. It is noted from experience that, depending on the design and location, dual-aspect flats can be at lower risk of overheating than single-aspect flats because the dual-aspect provides the potential to benefit from cross-ventilation (where wind drives greater ventilation rates through the dwelling).

Further analysis explored the impact of providing poorer performing glazing (both U-value and G-value) and being based on Category I occupancy where there is a high level of expectation the dwelling will be occupied by vulnerable and fragile persons. This identified a moderate increase in risk but no further failure.

5.2 Proposals

5.2.1 Scope of application

We propose that any new provision to assess and mitigate against the risk of summertime overheating should apply to all new dwellings, subject to the application of a set of trigger criterion (see section 5.2.2) that determine the need to consider the design and specification of the dwelling further to mitigate such risk. Detail of proposed new provisions are set out in ‘Consultation proposals - Addressing Overheating Risk in New Dwellings’.

We would also propose that the same assessment be applied to non-domestic residential buildings where the built form and occupancy is similar to dwellings (e.g. self-contained student flats accessed off a common area).

We would seek views on the broader application of such an assessment to residential buildings where such self-contained living units are not provided and the building offers a managed living environment (e.g. buildings providing residential care for vulnerable persons).

Question 41 –

Do you agree with the proposed introduction of a requirement to assess and mitigate summertime overheating risk in new homes and new non-domestic buildings offering similar accommodation?

Yes

No

Please provide a summary of the reason for your view.

If you consider that proposals should be extended to non-domestic buildings which provide other forms of residential accommodation (which are not ‘self-contained residential units’), we welcome your views on such provisions, including if the same or an alternate approach to assessment is recommended?

5.2.2 Application of criteria for assessment

The limited analysis presented in the referenced 2021 research suggests that, whilst risk may be low in Scotland compared to other parts of the UK, overheating can still occur and reasonable steps can and should be taken to assess and mitigate such risk.

Whilst analysis has identified that the greatest risk of overheating is found in flats and buildings with less scope for cross-ventilation, we propose that initial assessment should be undertaken on every new dwelling to identify relevant characteristics and also assist in embedding awareness of the issue of overheating risk. This will provide opportunities to avoid not only peak overheating but also improve summer comfort in new dwellings by understanding risk and how it can be mitigated by design choices and passive solutions.

Action to mitigate summer overheating risk must be considered against the desirability of good levels of daylight to new homes and the benefits derived from useful solar gain outwith the summer months in reducing space heating demand. Proposals are drafted with this intent in mind.

For the purpose of risk assessment, we propose that geographical location of a development in Scotland is not be considered a material factor.

Where any trigger elements are present, it is expected that either mitigating measures are put in place or that further modelling is undertaken to identify whether overheating risk, as defined in the context of TM 59 is present. Trigger elements proposed relate to:

• Extents of glazing on building elevations oriented between east, south and west (impact of solar heat gain).
• Number and adjacency of external elevations (impact of cross-ventilation on heat removal).

This enables either a simple elemental process without any additional modelling or, should the developer choose to, the flexibility of demonstrating compliance through the use of dynamic thermal modelling.

Detail of proposed new provisions are set out in ‘Consultation proposals - Addressing Overheating Risk in New Dwellings’.

Question 42 –

Do you agree with the proposal that an initial assessment of dwelling characteristics should be undertaken to help inform design choices and the delivery of new homes which provide better thermal comfort in the summer months?

Yes

No

Please provide a summary of the reason for your view.

We would also seek the views of respondents on other sources of good practice guidance which have been implemented by developers and the outcome (no reports of significant summertime overheating) evidenced through feedback from residents.

5.2.3 Options for mitigation

As noted above, two approaches to demonstrating compliance are proposed:

A. Simple elemental approach.

Where triggered by building characteristics, measures are implemented which limit unwanted solar gain in summer months and provide the means of removing heat build-up from the building. This is discussed in section 5.2.4.

B. Dynamic thermal analysis

The dynamic thermal analysis method uses CIBSE TM59 ‘Design methodology for the assessment of overheating risk in homes’ (noted in section 5.1.2) to demonstrate that any risk of overheating, assessed against the criteria defined by the methodology, are addressed. This approach uses the dwelling location, detailed information on the building form and specification and prescribed occupancy scenario to calculate the risk of overheating. It allows for greater flexibility in the selection of risk mitigation measures to meet the overheating criteria, while prioritising passive measures.

Those undertaking such modelling should be able to demonstrate competence in the application of the methodology, the underlying principles of overheating and in building construction.

The purpose of the proposed process is to address overheating risk through design choices, specification and passive measures. Active measures such as air-conditioning should only be proposed where it can be demonstrated that design changes and passive measures fail to adequately mitigate overheating risk or are impractical for the development location. This would require the applicant to demonstrate the options investigated were found, through modelling, to be insufficient.

Question 43 –

Are there circumstances where you consider specific characteristics of a dwelling should trigger a need for TM59 assessment rather than application of a simple elemental approach?

Yes

No

Please provide a summary of the reason for your view.

5.2.4 Scope of simple mitigating measures proposed

Detail of proposed new provisions for the simple assessment option are set out in ‘Consultation proposals - Addressing Overheating Risk in New Dwellings’. These are:

A. Limiting solar gain through glazed openings.

Assessment should be undertaken on all building elevations which are oriented between east (90º), through south (180º) to west (270º).

Except single aspect dwellings or those with adjacent aspect, both oriented outwith the described orientation, this will require the assessment of at least two elevations.

It is proposed assessment will be at room level rather than be based upon the total areas of glazing and the dwelling. This provides greater assurance of thermal comfort in individual apartments and other habitable spaces. Assessment should consider the area of glazed opening based upon the room or space each opening serves.

• Action: If the area of glazed opening is greater than 25% of the area of the room or space served, then provide shading or revise glazing g-value to reduce solar gain in proportion to the increase in glazing area above 25%. It is proposed that this should be based upon the assumption of equivalence with an unshaded 25% opening with a g-value of 0.6

It is not the intent of assessment to preclude larger areas of glazing, simply to recognise that increased areas of glazing which can contribute to excess solar gain in summer months. Solutions should mitigate such gain, either through a revised glazing specification (lower g-value) or by the fitting of shading such as louvres or shutters. Specification of glazing should, where practicable, seek to retain a high light transmittance value to minimise adverse impact on daylighting.

B. Ventilation to assist in cooling.

Assessment should be undertaken on all dwellings based upon their number of exposed facades which incorporate openings which provide ventilation to apartments or other habitable spaces.

• Action: Where a building has only one elevation with ventilation openings (single aspect) or two adjacent elevations with ventilation openings, the minimum area of ventilation openings in apartments should be increased from 1/20^th (5%)– as proposed in Section 4.2.2 of this document - to 1/10^th (10%) of the floor area in each room or space.
• Action: Where a building has two or more elevations, including two parallel elevations, with ventilation openings, no further action is required to support ventilation for heat removal. This will be addressed by provision of ventilation openings which provide the level of air change sought under standard 3.14 (4 air changes per hour),

Question 44 –

Recognising the level of risk identified in the published research paper, do you agree with the above proposals as a suitable means of mitigating summertime overheating in new homes through prescriptive actions?

Yes

No

Please provide a summary of the reason for your view.

5.2.5 Practicality of solutions

Where measures are implemented to mitigate overheating risk, they should be achievable in use. This will commonly require consideration of the environment within which the building sits.

This is particularly relevant to the use of ventilation to reduce levels of overheating. Issues arising from both noise from the immediate environment and air pollution should be considered. Such matters are usually material considerations in the granting of planning permission for development and any proposed mitigation measures should be compatible with issues already identified at the development (for example location of a façade adjacent to a busy road). Such issues are likely to influence other aspects of project design, including the ventilation strategy.

Similarly, where issues such as security are a concern and it is not considered practical to simply open windows to assist in cooling, further assessment of overheating risk or alternative solutions may need to be considered.

Where conflicting objectives are identified, this may make the case for dynamic thermal analysis more relevant as a means of demonstrating risk is assessed and managed.

Similarly any mitigation should not require occupants to circumvent measures intended to deliver a safe environment in respect of issues such as the risk of a fall from height through openings.

It is proposed that an assessment and statement on how these matters are considered as part of mitigating any identified overheating risk will be sought as part of the building warrant application. This should reference any relevant conditions set under the Planning regime for the development. As is the case with information on building services under the current standard 6.8, such information should be included within written information provided to the building occupant to assist in understanding how the issue of summer overheating is addressed at the building. Detail of proposed new provisions are set out in ‘Consultation proposals - Addressing Overheating Risk in New Dwellings’.

Question 45 –

Do you consider that such an approach will provide adequate assurance that ventilation measures provided to mitigate summer overheating can be used safely and conveniently in practice?

Yes

No

Please provide a summary of the reason for your view.

5.2.6 General comments

Question 46 –

We welcome any other comments you wish to make on these proposal to introduce provisions to mitigate the risk of summer overheating new homes and new residential buildings.