A longitudinal assessment of indoor air quality and thermal environment in naturally-ventilated energy-efficient dwellings

Background: As buildings become more energy-efficient, increasing levels of airtightness challenge the effectiveness of natural ventilation in maintaining acceptable indoor environmental quality, specifically regarding indoor air quality and thermal environment. While natural ventilation offers passive cooling benefits and supports energy-saving goals, its performance can be compromised in airtight homes, particularly in colder months when airflow is reduced. These changes may contribute to inadequate air change rates, elevated pollutant concentrations, and increased risk of summertime overheating. The interplay between airtightness, natural ventilation, and thermal comfort is underexplored, and long-term monitoring studies in occupied dwellings are particularly sparse. This research aims to address the knowledge gaps relating to natural ventilation, airtightness, thermal comfort and occupant behaviour, and the interplay between these aspects, from the perspective of energy-efficient naturally ventilated dwellings. The research focuses on: (i) assessing long-term air exchange rate performance and its impact on indoor environmental quality; (ii) characterising seasonal variations in indoor pollutant concentrations and thermal conditions; and (iii) examining the risk of overheating and the overall implications for occupant comfort and indoor environmental quality. Methodology: A longitudinal field study was conducted in nine energy-efficient Irish dwellings over a full year. Indoor environmental conditions were monitored across the main spaces in each dwelling: bedrooms, living rooms, and kitchens, during both summer and winter and for a full calendar year, to collect data for a range of indoor air pollutants and thermal parameters. The measurements were complemented with qualitative data, collected through a survey of dwelling occupants. For investigating the ventilation performance, a longitudinal approach involving the deployment of a remotely-operated consumer grade CO2 sensors was employed over one full calendar year. The air exchange rate was calculated based on occupant-generated CO₂ decay in the main bedroom of each dwelling and annual and seasonal trends in air exchange rates were examined. For evaluating indoor air quality, research-grade equipment was deployed in the bedroom (main), living room and kitchen of each house and monitoring of key indoor air pollutants and thermal parameters took place over a week-long period during both summer and winter. Indoor air pollutants included PM₂.₅, CO₂, TVOCs, NO₂, and CO, as well as environmental parameters such as temperature and relative humidity. Air exchange rates in bedrooms were estimated using the metabolic CO₂ method. Additionally, surveys and questionnaires were deployed to gather contextual information and occupant behavioural patterns. Finally, the study evaluated the indoor thermal environment by deploying remotely-operated consumer-cost sensors to collect long-term temperature and relative humidity data in the dwellings. Analysis was performed using standardised categorisation of temperature and relative humidity data. Overheating risk was assessed following the CIBSE TM59 criteria in the bedrooms, living rooms, and kitchens. Results: The ventilation and CO2 study findings showed significant seasonal variations in indoor CO₂ concentrations, with notably higher levels during winter, primarily due to reduced ventilation. Seasonal median air exchange rates ranged from 0.15 h⁻¹ in winter to 0.27 h⁻¹ in summer, reflecting this seasonal disparity. Over half of the recorded air exchange rates values (51%) fell within the 0.1–0.3 h⁻¹ range, while 15% were below 0.1 h⁻¹, highlighting periods of insufficient ventilation. In contrast, only 0.4% of the data exceeded 1.0 h⁻¹. The overall mean air exchange rate across all dwellings was 0.28 h⁻¹, which is approximately 42–70% lower than typical values reported for naturally ventilated conventional buildings. Instances of purge ventilation (short periods of elevated airflow often associated with window opening) were infrequent and highly variable, with observed rates ranging from 0.4 to 2.7 h⁻¹. The indoor air quality study revealed that significantly higher concentrations of gaseous pollutants (p < 0.01) were recorded in bedrooms compared to living rooms, and during winter compared to summer. PM₂.₅ levels in kitchens exceeded the WHO 24-hour average threshold of 15 µg/m³ during the week-long monitoring periods in 92% of winter cases and 51% of summer cases. CO₂ concentrations in bedrooms remained above 1000 ppm for 94% of the sleeping hours in winter and 39% in summer, indicating prolonged exposure to suboptimal air quality. The weekly average TVOC concentrations across bedrooms were 463 ppb in winter and 293 ppb in summer. Indoor temperature and relative humidity generally remained within acceptable comfort and health ranges. Median air change rates in bedrooms were in the range 0.08–0.35 h⁻¹ in summer and 0.09–0.26 h⁻¹ in winter, reflecting limited ventilation across seasons. For the longitudinal thermal environment study, results showed that during the heating season, the mean temperature in bedrooms was 20.84 ±0.86 °C (Living rooms: 20.48 ±0.96 °C, Kitchens: 20.70 ±0.88 °C). In the non-heating season, it was 23.22 ±1.02 °C (Living rooms: 22.54 ±0.99 °C, Kitchens: 22.77 ±0.99 °C). Bedroom temperatures above 24 °C (The heat-related health risks guidelines by the World Health Organisation) in the non-heating season ranged from 8.5-36.6% (Living rooms: 6.6-40.8%, Kitchens: 6.9-34.2%). Regarding overheating prevalence, fourteen of twenty-seven rooms failed CIBSE TM59 Criterion A for overheating, with four of nine bedrooms failing both Criteria A and B. Overall thermal comfort was higher in winter and lower in summer, according to the survey. Conclusion: This research highlights a clear performance gap between the intended design and actual operational outcomes of natural ventilation in airtight energy-efficient dwellings, particularly under current operational patterns. Significant seasonal fluctuations were observed in both ventilation rates and indoor air pollutants concentrations, with winter conditions associated with reduced ventilation and elevated indoor air pollutants levels, and summer marked by challenges in managing overheating. While natural ventilation can, under certain conditions, provide adequate airflow delivery, its effectiveness is highly dependent on occupant behaviour and external climatic conditions. In winter, insufficient ventilation compromises indoor air quality, while in summer, passive cooling potential remains underutilised due to limited airflow and persistent overheating. These findings underscore the need for adaptive and seasonally- responsive ventilation strategies. Improvements such as hybrid ventilation systems, enhanced user guidance, and targeted mitigation measures are essential to optimise ventilation performance, safeguard indoor environmental quality, and ensure thermal comfort across the year.

Funder

Sustainable Energy Authority of Ireland (SEAI)

Publisher

University of Galway

Rights

CC BY-NC-ND

cbn

Collections

University of Galway Theses (PhD Theses)