Guide to Conversion of Traditional Buildings

3.28 Overheating risk

Mandatory Standard

Standard 3.28

Every building must be designed and constructed in such a way that the risk to the health of the occupants from overheating is reduced.

Limitation:

This standard only applies to –

a) a unit of residential accommodation occupied by an individual, or individuals, living together as a family or as a single household, and

b) a shared multi-occupancy residential building

3.28.1 Application of standard to conversions

In the case of conversions, as specified in regulation 4, the building as converted shall meet the requirements of this standard in so far as is reasonably practicable, and in no case be worse than before the conversion (regulation 12, schedule 6).

3.28.2 Commentary

This standard relates to the increasing risk of overheating as a public health and productivity concern in the UK. The UK climate is observed to be warming significantly, along with increased incidence of high temperature events and climate projections indicate these trends will continue to be exacerbated over the coming decades.

Overheating is defined by the standard as when indoor environments present conditions in excess of those acceptable for human thermal comfort or conditions which may adversely affect human health. These conditions are observed on an increasing frequency and thus must be addressed in buildings through passive and active mitigation measures.

Many factors influence the degree to which a building is susceptible to overheating, such as its orientation, construction methods, thermal mass, glazing, and occupant behaviour. Newer buildings may be more vulnerable to overheating generally due to higher air tightness and levels of insulation. However, buildings of traditional construction which have been converted to residential use may face high overheating risk and the opportunities for mitigation may be more limited due to the need to avoid impacting on a building’s cultural significance. As this is a ‘reasonably practicable’ standard, consideration must be given to the ways in which overheating risk can be mitigated. In all cases, the conversion to residential use must not increase the risk of overheating in the building.

The guidance in section 3.28 highlights the available methodologies for assessing overheating risk as well as the effectiveness of proposed mitigation measures and it advises that dynamic thermal modelling should generally be pursued for buildings of traditional construction due to potential limitations in interventions available.

Conversions should take a risk-based approach using the appropriate methodologies to determine mitigation actions. Conversions can initially use SAP and SBEM modelling to assess risk, and these will align with section 6 Energy compliance requirements. Practitioners should be aware, however, that these are simplified methods and, where risk is identified, Dynamic modelling (DSM) assessment against CIBSE’s TM59 criteria should be used. Interventions must be practical and communicated effectively to occupants.

3.28.3 Issues to be considered

Typical risks in the conversion of historic/traditional buildings

1. Limitation of solar gains

• Application of solar control film may impact on the appearance of windows, especially as it ages.
• Installation of external blinds is a highly effective measure, but they are uncommon in Scotland and their installation may be deemed to adversely affect the character and appearance of a building.
• Internal shading options are less effective control as the heat has already entered the building. They can form a buffer zone and can also protect fragile surfaces such as historic textiles or painted finishes from UV decay.

2. Installation of mechanical ventilation

• Ventilation ductwork, fans, and intake/exhaust terminals may impact on the historic character and features of the rooms they are installed in.
• External aspects of ventilation systems may impact on the traditional character and cultural significance of a building.

3.28.4 Recommendations to meet the standard

Design proposals for the conversion of a traditional building should aim to comply with CIBSE’s TM59 and ensure that overheating risk does not worsen. New buildings can design for overheating mitigation from the earliest stages, aided by the building’s orientation and window layout. The potential interventions for conversions are more limited but some to assess include:

a) Improvements to window g-values through solar film or full window replacement (if permitted), where appropriate consideration has also been given to the minimisation of moisture concerns.

b) Implementation of external shading devices (particularly if there is a historic precedent) oriented to interrupt summer solar gain while minimising reduction in beneficial winter solar gains. Examples include awnings or shutters.

c) Engagement with occupants to contribute to strategic natural ventilation such as off-peak purge ventilation.

d) Consider blue and green landscaping opportunities to minimise heat island effects and shield facades at risk from solar radiation.

Feasibility studies should be carefully performed to develop an overheating mitigation strategy that aligns with preserving the character of culturally-significant buildings. A risk-based approach should be taken where initial assessment with SAP or SBEM methodologies can be used with increased level of assessment and mitigation where this issue is identified. Where the risk of overheating is medium or high, dynamic modelling (DSM) should be undertaken to assess the effectiveness of any strategies deemed feasible and the most effective measures should be implemented. TM59 calculation results should confirm that overheating performance before and after the conversion to residential use is not worsened and is suitable in future weather conditions to mitigate against future overheating associated with climate change.

3.28.5 Further reading

• CIBSE (2017) TM59 Design methodology for the assessment of overheating risk in homes