Overheating and Historic Buildings

This article explores how occupant comfort in historic buildings is being challenged under the influence of global warming trends, using Historic England offices as a living laboratory to model, predict and measure overheating.

What is 'comfort'?

Overheating can be loosely defined as occurring when internal temperatures increase to a point where occupants may experience discomfort. Whilst thermal comfort is inherently subjective and circumstantial, well-researched temperature metrics can be used to predict what comfort means for most people.

The factors driving heat-related discomfort can be both internal and external to a building. This can be broken down into internal heat gains from lighting, equipment or people and external heat gains driven by weather conditions. While it is relatively straightforward to influence our internal heat gains, environmental pressures posed through warmer weather are much more difficult to manage and contend with.

Given the United Kingdom’s long-term seasonal trends towards hotter  summers, overheating is becoming more of a concern, presenting challenges to how we maintain a healthy and comfortable internal environment all year round.

Despite all the measures being taken to control global warming, future weather predictions indicate hotter temperatures and highlights that overheating will be a growing issue. The research outlined here tries to quantify the extent of the problem and examines what can be done to make historic buildings more resilient to warmer weather whilst upholding our collective responsibility towards environmental sustainability. The aim of the research is to better understand how passive measures for summertime occupant comfort can be used to avoid the use of air conditioning and the generation of emissions, which would further accelerate climate change.

Identifying overheating risk

The Chartered Institute of Building Services Engineers (CIBSE) provide a structured methodology for assessing and reporting overheating risk in new and refurbished non-domestic buildings. CIBSE's Technical Memoranda (TM) 52 aims to quantify overheating risk through measures that are based on the relationship between the external and internal air temperature as a proxy for occupant comfort inside a building.

The CIBSE methodology is useful for understanding if and how overheating can occur within buildings and is broken up into three different criteria: 
Criterion 1: the number of hours that the temperature difference between inside and outside is greater than 1 degree. When this happens, the day is classified as ‘warm’. For a building to be defined as not subject to overheating, warm conditions must not exceed 3% of the total hours.
Criterion 2: takes a weighted average of the temperature difference between inside and outside over the course of a day. This criterion quantifies the severity of a warm day and for a building to pass the value must not exceed 6 in a single day. This is calculated as a weighted average of the temperature difference and the duration over which it occurs and it is a unitless value.
Criterion 3: the temperature inside cannot be more than 4 degrees warmer than the external temperature at any point, placing an upper limit on internal temperatures.

As occupant comfort is tied to duration as well as temperature, each of the criteria focuses on a different aspect of overheating, which includes the duration and severity of heat build-up within a space in relation to the external temperature. A space is considered to pass if it complies with either criterion 1 & 2 or criteria 1 & 3. Applying this categorisation, it is possible to assess the risk of overheating across different rooms in a building through modelling the internal and outside air temperature and testing against each of the above criteria.

Modelling Historic England offices

Using dynamic thermal modelling software (IES-VE), virtual thermal models were generated for Historic England offices in Swindon, Cambridge and York. For each location, simulations were run with current (2020) and future weather data for 2050 and 2080 to understand present and future performance. These models can simulate internal conditions under the influence of different weather data and therefore are able to predict how susceptible the buildings will be to overheating according to the TM52 criteria.

Questions that dynamic thermal modelling can answer:

• Does the building comply with TM 52?
• How do the internal conditions vary over a year?
• How does the building react under future weather scenarios?

Results vary across the different offices and are indicative of the mechanisms that are driving overheating within them, which depend on each building’s unique form and fabric. All the offices assessed currently exhibit overheating risk and the severity of this is exacerbated under the influence of future weather files.

As expected, the modelling indicates that overheating is a growing issue for historic buildings.
Making modifications to the baseline models for each office enabled testing different measures to assess how overheating could be feasibly reduced using passive measures. The baseline models were re-run under the following modifications and in the context of the future weather data for 2050:

A: Reduced occupancy to 90%
B: Solar control film applied to windows to reduce G-value (a measure of how much solar heat -infrared radiation- is allowed in through a particular part of a building) and solar gain admittance into the building
C: Night-time ventilation of 5 air changes per hour between midnight and 7am
D: Improved building airtightness from 0.7 to 0.55 air changes per hour of external infiltration.

Despite the introduction of passive measures, each office building still presents overheating risk to varying degrees, with the effectiveness of each measure varying across the modelled buildings.

Reduced occupancy reduces overheating, as expected due to a reduction in internal gains. This can be intuitively validated but it may not necessarily be considered a viable measure in the long-term for practical working spaces. Meanwhile, solar control and night-time ventilation both reduced the overheating risk, but the magnitude of this impact varies across the different offices by virtue of their unique form, fabric build-ups and climates. Solar control film is observed as being most effective in offices that suffer from high solar gains in which orientation and window geometry are important influencing factors. Conversely, night-time ventilation appears to be most effective in offices with inadequate natural ventilation as this leads to the entrapment and build-up of warm air.

Interestingly, improving the airtightness of a building leads to greater overheating risk, suggesting that infiltration of external air is advantageous for summertime occupant comfort, when assessed against the TM 52 criteria. There is, therefore, a conflict between measures to promote winter and summer occupant comfort.

Greater airtightness increases overheating risk but is at odds with imperative towards insulating and sealing buildings for improved winter thermal performance. The focus on improving thermal fabric without considering overheating risk may be driving maladaptation against well-understood climate trends.

Improving air permeability increases overheating risk but is at odds with the imperative towards greater airtightness for improved winter thermal performance.

Understanding how temperatures vary in different rooms can inform a deeper understanding of overheating patterns. A snapshot of internal temperatures in the Cambridge office highlights areas that may require urgent attention.

Taking a more detailed look at results indicates that overheating is not a homogenous phenomenon and varies across different spaces within the building thermally and temporally. This is an important observation as it may enable the introduction of specific targeted overheating mitigation measures.

Monitoring Historic England offices

To validate the findings from the modelling, ongoing monitoring is being done across the simulated offices. This will enable the establishment of a real-world baseline, which can be compared to the relative improvement predicted bymodelling different scenarios.

Key research outcomes

The initial modelling of Historic England offices highlight that overheating can be a risk in historic buildings, leading to occupant discomfort and health and safety implications. Intuitively, and through modelling future weather files, this will become more severe in the future, impacting climate change resilience of historic buildings.

A key observation from this assessment was that each building’s overheating profile is unique and influenced by a range of factors. Therefore, it is important to propose mitigation measures that are most effective towards addressing building-specific overheating drivers. As evidenced by the Historic England offices, the effectiveness of such measures varied across the different buildings. Furthermore, the extent to which passive measures can mitigate this is limited when applied in isolation. However, modelling these individually enables comparative assessment of their effectiveness as independent variables, which is beneficial for decision-making at a building management level.

The of dynamic thermal modelling tools can provide crucial insight into how different spaces within the building perform, which can inform a more targeted application of building interventions. Since overheating may be a zonal problem, this approach may minimise disruption to the existing building by supporting incremental improvements.

Dynamic thermal modelling presents advantages in predictive and remedial scenarios for overheating, under present-day and future weather conditions. It also offers detail that can inform means of comfort control on at a more zonal level within a building. 

Climate trends are well understood, but there remains a conundrum to be resolved when adapting historic buildings to meet seasonal needs for both summer and winter comfort.

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