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Indoor Environment Quality (IEQ)

Assessing Existing Building Performance

Understanding the Building

Historic buildings were constructed using techniques and materials that often differ significantly from those used today. Their performance depends not only on their original design but also on years of repairs, alterations and changing patterns of use. Before considering any retrofit intervention, it is important to understand how the building functions as a whole, including the relationship between its structure, building fabric, services and occupants.

Gathering information through drawings, historical records, maintenance documentation and site inspections provides a valuable starting point. This understanding helps identify the building's strengths, recognise existing problems and avoid retrofit solutions that may unintentionally damage the building or reduce indoor environmental quality.

Good practice

  • Review historical drawings and available documentation. 
  • Record previous alterations and repairs. 
  • Consider the building as an integrated system rather than a collection of individual elements. 
  • Engage specialists where heritage significance requires expert advice. 

Key message: A thorough understanding of the building is the foundation of every successful retrofit.

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Construction Types

Historic buildings across Europe encompass a wide variety of construction methods, including solid masonry, timber frame, stone, brick, earth construction and mixed structural systems. Each responds differently to heat, moisture and air movement, influencing both indoor environmental quality and retrofit options.

Understanding how walls, roofs, floors and windows were originally designed to perform helps determine appropriate improvement measures. Retrofit solutions should always be compatible with the construction type to avoid unintended consequences such as trapped moisture, thermal bridging or structural deterioration.

Good practice

  • Identify the construction system before specifying retrofit measures. 
  • Recognise that different building elements may have been added during different periods. 
  • Consider how traditional construction manages moisture and ventilation. 
  • Avoid applying identical solutions to every historic building. 

Key message: Different construction types require different retrofit strategies.

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Thermal Comfort

Thermal Comfort in Historic Buildings

Thermal comfort describes how satisfied people feel with the indoor thermal environment. It is influenced by air temperature, radiant temperature, humidity, air movement, clothing and activity levels. Historic buildings often experience greater temperature variation than modern buildings due to their construction, original heating systems and natural ventilation. Occupants may therefore adapt their behaviour by changing clothing, opening windows or adjusting heating to remain comfortable.

Improving thermal comfort should focus on providing stable and appropriate indoor conditions rather than achieving a fixed temperature throughout the building. Retrofit measures should consider both winter and summer performance, recognising that interventions affecting one season may influence comfort in another. Solutions should also respect the building's original construction and avoid compromising its long-term performance.

Good practice

  • Consider thermal comfort throughout the year. 
  • Assess both winter and summer conditions before retrofit. 
  • Recognise that occupants may have different comfort preferences. 
  • Balance comfort improvements with heritage conservation objectives. 

Key message: Improving thermal comfort means creating healthier, more stable indoor conditions while respecting the way historic buildings perform.

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Heating & Cooling Strategies

Heating and cooling systems should work with the building rather than against it. Many historic buildings were designed to operate without modern mechanical cooling and relied on passive measures such as thermal mass, natural ventilation and solar shading. Retrofit provides an opportunity to improve thermal comfort by combining these traditional features with efficient modern technologies where appropriate.

Heating systems should provide consistent and controllable temperatures while avoiding rapid fluctuations that may affect occupants or historic materials. Where cooling is required, passive approaches should always be considered first, with mechanical systems introduced only where necessary and designed to minimise visual impact, energy consumption and disturbance to the historic fabric.

Good practice

  • Prioritise passive heating and cooling measures where feasible. 
  • Improve heating controls to match occupancy patterns. 
  • Avoid excessive indoor temperatures during winter. 
  • Select systems that can be installed with minimal impact on historic features. 

Key message: Well-designed heating and cooling strategies combine traditional building performance with efficient modern technologies.

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Cooling and Overheating

Although historic buildings are often associated with cold conditions, overheating is becoming an increasing concern across Europe as temperatures rise and heatwaves become more frequent. Buildings that have undergone energy efficiency improvements may also retain more heat if ventilation and solar gains are not carefully managed.

Reducing overheating should begin with passive measures such as external shading, appropriate window operation, night-time ventilation and the effective use of thermal mass. Mechanical cooling should only be considered when passive strategies cannot maintain acceptable indoor conditions or where building use requires tighter environmental control.

Good practice

  • Assess overheating risk before selecting retrofit measures. 
  • Reduce unwanted solar gains through shading where appropriate. 
  • Encourage night-time ventilation during warm periods where safe and practical. 
  • Consider future climate conditions as well as current weather. 

Key message: Managing overheating is becoming an essential part of climate-resilient retrofit.

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Moisture, Condensation and Mould

Moisture is one of the most common causes of deterioration in historic buildings and can significantly affect indoor environmental quality. Excess moisture may result from rain penetration, rising damp, plumbing leaks, condensation or everyday occupant activities such as cooking, bathing and drying clothes. Persistent damp conditions increase the risk of mould growth, material decay and poor indoor air quality.

Condensation occurs when warm, moisture-loaded air comes into contact with colder surfaces. Retrofit measures that increase airtightness or alter the building fabric should always consider how moisture will be managed. Improving insulation, reducing thermal bridges and maintaining adequate ventilation can help minimise condensation while protecting both occupants and historic materials.

Good practice

  • Investigate the source of moisture before carrying out repairs. 
  • Maintain adequate ventilation in moisture-producing spaces. 
  • Reduce thermal bridges where possible. 
  • Monitor areas with a history of dampness or mould. 

Key message: Effective moisture management protects both building fabric and occupant health.

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Building Fabric Improvements

Improving the thermal performance of the building fabric can reduce heat loss, enhance comfort and lower energy demand. Measures may include upgrading roofs, walls, floors, windows and doors, while carefully considering the original construction and moisture behaviour of the building.

Not every building requires the same level of intervention. In some cases, targeted improvements combined with better heating controls and ventilation may deliver significant benefits while reducing the need for extensive alterations. Retrofit measures should always be compatible with existing materials and avoid creating new moisture or durability problems.

Good practice

  • Improve the thermal performance of the building envelope where appropriate. 
  • Retain and repair existing elements whenever feasible. 
  • Select insulation systems compatible with traditional construction. 
  • Consider the interaction between insulation, ventilation and moisture movement. 

Key message: Building fabric improvements should enhance thermal performance without compromising the building's long-term durability.

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Managing Thermal Performance while Preserving Heritage Value

Historic buildings have cultural, architectural and social significance that should be respected throughout the retrofit process. Improving thermal performance does not necessarily require extensive alteration or the replacement of historic features. In many cases, sensitive and well-planned interventions can achieve meaningful improvements while preserving the building's character.

Retrofit decisions should consider the significance of individual building elements, the reversibility of interventions and the long-term performance of the building. Collaboration between building owners, designers, engineers and conservation professionals can help identify solutions that balance comfort, energy efficiency and heritage conservation.

Good practice

  • Understand the heritage significance of the building before intervention. 
  • Prioritise repair and maintenance before replacement. 
  • Use reversible solutions where practical. 
  • Balance energy performance with conservation objectives throughout the design process. 

Key message: Successful retrofit improves thermal performance while safeguarding the historic character and significance of the building.

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Indoor Air Quality

Key Indoor Pollutants and Sources

Indoor air quality (IAQ) plays an important role in the health, comfort and wellbeing of building occupants. Pollutants found indoors originate from a range of sources, including outdoor air, occupant activities, building materials, furnishings and heating or cooking appliances. Common indoor pollutants include carbon dioxide (CO₂), particulate matter (PM), volatile organic compounds (VOCs), nitrogen dioxide (NO₂), carbon monoxide (CO) and biological contaminants such as mould spores.

Historic buildings may face particular IAQ challenges due to ageing building services, moisture problems or changes introduced during previous refurbishments. Understanding the likely sources of pollution is the first step in selecting appropriate ventilation, maintenance and retrofit strategies that improve indoor air quality while respecting the building's heritage value.

Good practice

  • Identify both indoor and outdoor pollution sources. 
  • Consider occupant activities that affect air quality. 
  • Assess heating, cooking and combustion appliances where present. 
  • Address pollution sources before relying solely on ventilation. 

Key message: Improving indoor air quality begins with understanding where pollutants originate.

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Ventilation Principles

Ventilation supplies fresh outdoor air while removing excess heat, moisture and indoor pollutants. Effective ventilation helps maintain healthy indoor conditions, reduces the risk of condensation and supports occupant comfort. In historic buildings, ventilation often occurs naturally through windows, chimneys, vents and small air leakage paths within the building fabric.

Retrofit measures that improve airtightness can reduce uncontrolled heat loss, but they may also reduce natural air exchange if ventilation is not considered. Achieving the right balance between energy efficiency and adequate ventilation is essential to maintaining healthy indoor environments and protecting the building fabric.

Good practice

  • Consider ventilation as part of the overall retrofit strategy. 
  • Ensure adequate fresh air is provided after improving airtightness. 
  • Maintain existing ventilation paths where they contribute to building performance. 
  • Design ventilation systems that are simple for occupants to understand and operate. 

Key message: Good ventilation is essential for both healthy occupants and healthy buildings.

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Natural Ventilation in Historic Buildings

Many historic buildings were designed to rely on natural ventilation rather than mechanical systems. Windows, chimneys, roof spaces and other architectural features often work together to provide air movement, helping remove moisture and maintain acceptable indoor conditions throughout the year.

Where natural ventilation performs effectively, it should be retained and enhanced wherever possible. However, changes in building use, increased occupancy or improved airtightness may reduce its effectiveness. Evaluating how air currently moves through the building helps determine whether natural ventilation alone remains sufficient or whether additional measures are needed.

Good practice

  • Understand existing airflow patterns before making alterations. 
  • Retain traditional ventilation features where practical. 
  • Avoid blocking vents or chimneys without assessing their role. 
  • Encourage occupants to use windows appropriately where safe and practical. 

Key message: Natural ventilation remains an important asset in many historic buildings and should be considered before introducing mechanical solutions.

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Mechanical Ventilation Solutions

Where natural ventilation cannot provide sufficient fresh air, mechanical ventilation may be an appropriate solution. Modern systems can improve indoor air quality by supplying controlled amounts of outdoor air while extracting stale air, reducing pollutant concentrations and managing indoor humidity. Heat recovery systems may also improve energy efficiency by recovering heat from outgoing air.

Mechanical ventilation should be carefully designed to minimise impacts on the historic fabric and character of the building. Installation routes, equipment location and maintenance requirements should all be considered during the design process. Systems should also be easy to operate and maintained regularly to ensure long-term performance.

Good practice

  • Consider mechanical ventilation only after assessing passive options. 
  • Select systems that minimise intervention to historic fabric. 
  • Ensure ventilation systems are properly commissioned and maintained. 
  • Provide occupants with clear guidance on system operation. 

Key message: Mechanical ventilation can improve indoor air quality when carefully integrated into historic buildings.

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Moisture and Indoor Air Quality

Indoor moisture has a direct influence on air quality and occupant health. Excess humidity can encourage mould growth, increase dust mite populations and damage building materials, while very dry conditions may affect comfort and respiratory health. Everyday activities such as cooking, showering, cleaning and drying clothes contribute significant amounts of indoor moisture.

Managing moisture requires a balanced approach that combines adequate ventilation, appropriate heating and careful management of the building fabric. Retrofit measures should reduce the likelihood of condensation while preserving the ability of traditional materials to regulate moisture naturally.

Good practice

  • Control indoor moisture at its source where possible. 
  • Ventilate kitchens, bathrooms and other moisture-producing areas. 
  • Maintain consistent indoor temperatures during colder months. 
  • Inspect regularly for signs of dampness or mould growth. 

Key message: Controlling indoor moisture is essential for maintaining good air quality and protecting historic buildings.

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Monitoring Indoor Air Quality

Monitoring indoor air quality provides objective information about the conditions experienced by occupants and helps identify issues that may not be immediately visible. Measurements may include carbon dioxide, temperature, relative humidity, particulate matter and, where appropriate, volatile organic compounds or other pollutants depending on the building and its use.

Monitoring before and after retrofit allows building owners and practitioners to evaluate the effectiveness of interventions and identify areas requiring further improvement. Long-term monitoring can also support building management by detecting changes in performance and informing maintenance decisions.

Good practice

  • Measure key indoor environmental parameters over representative periods. 
  • Position sensors in occupied areas away from direct heat sources or windows. 
  • Interpret monitoring data alongside occupancy and building operation. 
  • Repeat measurements after retrofit to evaluate performance. 

Key message: Monitoring transforms indoor air quality from assumptions into measurable evidence.

 

Monitoring Indoor Air Quality

Retrofit activities can temporarily affect indoor air quality through dust generation, emissions from construction products and reduced ventilation during building works. Careful planning can minimise disruption and protect both occupants and construction workers throughout the project.

After retrofit, good indoor air quality depends on appropriate ventilation, suitable material selection, effective moisture management and regular maintenance of building systems. Low-emission products, adequate ventilation during commissioning and clear guidance for occupants can all contribute to healthier indoor environments and help ensure that retrofit delivers long-term benefits.

Good practice

  • Protect occupied spaces from dust during construction. 
  • Select low-emission materials and finishes where possible. 
  • Ventilate the building thoroughly before reoccupation. 
  • Provide occupants with guidance on operating heating and ventilation systems after retrofit. 

Key message: Achieving healthy indoor air is an ongoing process that begins during retrofit and continues throughout the building's operation.