Beyond BER: Rethinking Retrofit Policy for Indoor Environmental Quality in Social Housing

Abstract

Energy efficiency retrofits are central to climate policy, yet their implications for indoor environmental quality (IEQ) and occupant health remain underexplored. This study investigates IEQ outcomes following staged retrofits in Irish social housing, where achieving Building Energy Rating (BER) targets is the primary performance metric. Four dwellings, three retrofitted and one control, were monitored over six weeks during the heating season. Built in the 1980s, these homes represent the typical social and private housing stock of that era. Continuous measurements of carbon dioxide, temperature, relative humidity, and thermal performance were complemented by analyses of vapour pressure excess and ventilation rates. While all retrofitted homes achieved BER improvement targets, persistent IEQ challenges were identified. Elevated pollutant concentrations and increased condensation/mould risk occurred in the presence of inadequate ventilation. Thermal anomalies and cold bridging were associated with cavity wall insulation, whereas external wall insulation provided more stable surface temperatures and reduced moisture-related risks. These results underscore the complex interplay between retrofit measures, occupancy patterns, and ventilation performance. The study highlights the need for retrofit strategies that integrate energy efficiency with occupant health objectives. At scale, retrofit programmes risk embedding systemic vulnerabilities unless ventilation and moisture control are prioritised, with implications that extend to health, wellbeing, and long-term building resilience.

1. Introduction

Ireland’s Climate Action Plan (CAP) 2025 [1] is positioned as a pivotal policy instrument in the national drive to enhance energy efficiency. It reflects a broader European agenda aimed at reducing energy consumption in the building sector, which currently accounts for approximately 40% of total energy use. Meeting these targets demands an immediate reduction in fossil fuel consumption and a rapid improvement in building energy performance across the entire built environment stock. From a social housing perspective, the Republic of Ireland’s local authorities (municipalities) collectively own approximately 143,000 units [2], with plans to retrofit 36,500 of these dwellings over the current decade [3]. Supporting this effort, the current Energy Efficiency Retrofit Programme (EERP) (commenced in 2013 but revised in 2021), is designed as a ten-year initiative to align with our CAP goals. The current phase of the programme incorporates previously applied singular retrofit measures, reflecting both the incremental nature of energy upgrades historically undertaken by local authorities and the strategic leveraging of previous investments to align with the current funding framework.

This approach is similar to other jurisdictions [4,5,6,7], aligning with the Energy Efficiency Directive [8] principle of ‘energy efficiency first’ as a central pillar of retrofit policy. However, it is imperative that retrofit measures aimed at reducing regulated energy use do not compromise the integrity or quality of the indoor environment. Ensuring that interventions improve conditions within dwellings is essential to safeguarding occupant health and wellbeing. This exploratory study investigates the potential impact of BER/cost-optimal focussed retrofit measures currently applied to local authority housing. As the chosen retrofit measures have historically yielded variable outcomes, this study evaluates ventilation performance, moisture load and vapour pressure excess alongside the thermal performance of external walls. The research focuses on the operational outcomes of these strategies and contributes to a broader research agenda that draws on post-retrofit data to explore whether the uniform application of retrofit strategies across a heterogeneous housing stock may result in unintended consequences for both buildings and their occupants. This exploratory work may highlight the necessity for larger-scale investigations to validate observed patterns and strengthen the evidence base for future policy and practice.

Literature Review

The European Performance of Buildings Directive (EPBD) requires EU member states to establish minimum energy performance standards for buildings and building elements, based on cost-optimal methodologies [9]. In Ireland, this process is implemented using the Dwelling Energy Assessment Procedure (DEAP), the official calculation method and software for assessing building energy performance in accordance with EPBD guidelines [10]. The cost-optimal analysis evaluates a range of energy-efficient measures and renewable technologies, factoring in capital, operational, maintenance, and carbon costs. Regulation 244/2012 [11] further supports the EPBD by stipulating that national minimum energy performance requirements must not fall more than 15% below the cost-optimal benchmark. In an Irish context, a BER of B2 is considered the benchmark for excellent energy performance and home comfort [12] relating to a primary energy performance of less than 125 kWh/m²/yr. This is the minimum energy performance requirement that should be achieved in so far as this is technically, functionally and economically feasible. For the majority of existing dwellings, the energy performance requirement is within 15% of the cost optimal primary energy level as required by the cost optimal guidance [13]. The BER is calculated according to the energy performance of the house and its associated carbon dioxide (CO₂) emissions, accounting for regulated energy use under specific standard conditions, considering dwelling typology, year of construction, orientation, overall and room specific dimensions, insulation levels, ventilation characteristics, lighting and space/water heating systems. Energy performance is expressed via (a) primary energy use per unit floor area per year (kWh/m²/yr) represented on an A to G scale and (b) associated CO₂ emissions (kgCO₂/m²/yr) [14].

From a social housing perspective, the EERP was introduced in 2013 to support the upgrade of local authority owned social homes in need of insulation and energy improvements. Phase 1 focused on cavity wall and attic insulation, while Phase 2 expanded to include broader fabric and heating upgrades. These phases concluded in 2020 and have since been replaced by a more comprehensive approach. The current scheme includes eligible measures such as attic and cavity wall insulation, external wall insulation, window and door replacements, heat pump installations, and related ancillary works. It also permits the inclusion of mechanical ventilation once a post-works air permeability of less than 5 m³/hr/m² is achieved and independent ventilation validation is demonstrated [15]. Annual funding is allocated based on performance against previous targets and local authority forecasts, and may be reallocated between local authorities to ensure national retrofit targets are met [2,15,16,17]. Grants are conditioned by a funding ceiling, with maximum available allocations ranging from €42,350 for mid-terrace homes and apartment units to €48,850 for end-of-terrace, semi-detached, and detached homes [15]. Local authorities are encouraged to select a balanced mix of properties across BERs to ensure that high-cost retrofits are offset by lower-cost interventions, enabling targets to be met within the allocated budget. This approach facilitates the achievement of retrofit targets within the constraints of allocated budgets as sub-optimal property selection may result in cost overruns, necessitating additional financial contributions from local authority resources. Only properties that achieve a post-retrofit BER of B2 or its cost-optimal equivalent are eligible for inclusion in the annual programme. Works that are ineligible, overdesigned, or exceed the necessary performance threshold will not receive financial support [2,15,16,17].

The revised EERP (2021–to date) marks a notable shift from shallow intervention to a staged approach aimed at achieving deeper retrofit status over time. The EERP retrofit model is structured to accommodate evolving BER requirements through a phased, incremental upgrade strategy. Viability is underpinned by several factors including the affordability of retrofit measures, the historical tendency of local authorities to implement energy improvements in stages and the strategic alignment of previous investments within the structure of annual funding frameworks. The relevance of this approach has been further reinforced by the recast EPBD 2018/844/EU, which introduces the concept of a building renovation passport under Article 19a [8].

While the current EERP aligns with prevailing regulatory and fiscal frameworks, the prioritisation of BER targets as the principal metric of success may overlook critical dimensions of IEQ. Poor IEQ is strongly associated with adverse health outcomes, including respiratory diseases, allergies, asthma, and severe illnesses such as lung cancer. However, efforts to improve energy efficiency and thermal comfort through increased airtightness and insulation have been shown to inadvertently elevate indoor pollutant and moisture concentrations [18]. Previous research has highlighted how a narrow focus on the reduction in regulated energy use and associated CO₂ emissions can result in unintended consequences associated with retrofit interventions, including increased condensation and mould risk due to enhanced insulation and airtightness [19,20,21], elevated levels of some indoor pollutants [22,23,24,25] and the occurrence of overheating [18,26,27]. These issues often result from and are exacerbated by inadequate ventilation and reduced air exchange rates post-retrofit, highlighting the need for integrated strategies that balance the requirement for energy efficiency with the proper design, installation and ongoing maintenance of effective ventilation systems [22,28,29,30,31].

Sub-optimal internal conditions are further exacerbated by documented evidence of systemic non-compliance in the installation of retrofit insulation systems. Post-retrofit evaluations have revealed pervasive deficiencies in cavity wall and external wall insulation, with 63% of surveyed dwellings in Northern Ireland failing to conform to established industry standards [32]. In addition, incorrect installation practices are estimated to affect approximately six million properties across the United Kingdom [33]. Such failures not only undermine energy performance objectives but also exacerbate risks of moisture ingress, thermal bridging, and compromised ventilation; factors that further intensify IEQ concerns and highlight systemic gaps in quality assurance and regulatory oversight.

Considering the evolving regulatory landscape and funding structures underpinning the EERP retrofit strategy, the adoption of a staged approach represents a pragmatic response to technical, economic, and policy imperatives. However, as local authorities increasingly pursue BER and cost-optimal targets as primary indicators of retrofit success, it is essential to critically evaluate the broader implications of such interventions on the indoor environment. The presence of unintended consequences underscore the need to ensure that retrofit strategies designed to enhance energy performance do not inadvertently compromise IEQ. Equally critical is the correct and sequential implementation of staged retrofit measures to avoid cascading cause/effect relationships that can lead to fabric failures or energy performance gaps [34,35]. As BER/cost-optimal focused retrofit strategies continue to be implemented, and given prior evidence of unintended consequences, post-retrofit performance monitoring is essential to verify operational efficacy and to identify emergent systemic or occupant-related risks [36,37,38]. By conducting a detailed analysis of post-retrofit performance data, this preliminary study seeks to advance a comprehensive understanding of how current social housing retrofit strategy impacts dwelling performance and the quality of the indoor environment while recognising occupant health and wellbeing outcomes as emergent consequential effects.

2. Materials and Methods

This study employed a high resolution comparative post-retrofit monitoring approach over a six-week period during the heating season (December to January) to assess the IEQ of local authority (municipality) owned dwellings retrofitted under Ireland’s EERP. The research focused on four dwellings: three retrofitted to achieve a minimum BER of B2, and one control dwelling with no retrofit interventions. Each of the monitored dwellings were built between 1980 and 1989, a decade during which Ireland’s local authorities built 47,874 social homes, alongside the construction of 185,508 private dwellings [39], representing c. 11% of current housing stock [40] when there was limited or no requirement for thermal insulation [41,42]. Within this same period, the local authority responsible for the four study dwellings added 4464 homes (including four apartments) to its own social housing register. Each monitored dwelling represents a typical example of the semi-detached or terraced typologies constructed during this era, characterised by uninsulated concrete ground floors, masonry cavity-wall construction, and pitched cold-roof assemblies. The dwellings are comparable in both size and internal configuration, with floor areas of 82.08 m² (TUD001), 80.56 m² (TUD002), 81.37 m² (TUD003), and 81.40 m² (TUD004) in line with the permissible social housing floor area standards in effect at the time (30 m²–125 m²) [43]. Each layout consists of a ground floor with a hallway, kitchen, and living room, and a first floor comprising a hallway, two bedrooms and one bathroom.

Local authority construction of social housing declined sharply during the 1990s and is now provided via approved housing bodies and private developers [44,45]. Consequently, pre-1990 dwellings constitute most of the current local-authority housing stock in Ireland. Therefore, while the present dataset is limited in scale, the monitored dwellings are reflective of the broader thermal and construction characteristics of the national social housing stock. Furthermore, the staged retrofit measures applied in TUD001, TUD002, and TUD003 align with the standard national retrofit strategy adopted for dwellings of this type since 2013 [3]. Phase 1 and/or Phase 2 of the EERP (2013–2020) provided funding targeted at the less intrusive cavity wall/attic insulation and heating upgrade measures and retrofit over 75,000 local authority owned social homes at a total spend of €183 million [46]. Notably, retrofit measures applied under both phases of the EERP prioritise achieving specified BER targets through standardised, repeatable and cost-optimal interventions rather than maximising overall building-performance outcomes. This study aims to evaluate the potential for unintended consequences arising from this unidirectional approach and identify areas requiring further investigation. Although occupancy levels varied among the monitored dwellings, the ventilation rate analysis incorporated adjustments for occupancy loading to facilitate robust and meaningful comparative interpretation of the results.

2.1. Dwelling Typology

This study focused on dwellings retrofitted under the EERP to achieve a minimum BER of B2. Under this scheme each property underwent an individual pre-retrofit assessment to determine the specific measures required to meet this performance threshold.

Table 1. Participant Dwellings—Retrofit Measures.

ID	Type of External Windows and Doors n = new measure e = existing measure dg = double glazed	Type of External Wall Insulation Used IWI—Internal Wall Insulation FFCB—Full Fill Cavity Bead FFMW—Full Fill Mineral Wool EWI—External Wall Insulation N—None e—existing measure	Attic Level Insulation Y/N e = existing	Floor Insulation Y/N	Demand Controlled Ventilation (DCV) Passive Vent (P) n = new measure e = existing measure	Heat Pump (HP) Or Gas Boiler (GB) n = new measure e = existing measure
TUD 001	uPVC—n—dg	FFMW—e	Y—e	N	DCV—n	HP—n
TUD 002	uPVC—n—dg	FFCB—e	Y—e	N	Passive—e	HP—n
TUD 003	uPVC—n—dg	EWI—n	Y—e	N	Passive—e	HP—n
TUD 004	uPVC—e—dg	N	Y—e	N	Passive—e	GB—e

presents the retrofit measures applied across the sample. Several measures were already in place prior to the commencement of the current EERP phase, reflecting the incremental nature of energy upgrades historically undertaken by local authorities.

Table 2. Participant Dwellings—Listed Improvements.

ID	Pre-Retrofit BER	Post-Retrofit BER	Pre-Retrofit Energy Use—kWh/m2/yr	Post-Retrofit Predicted Energy Use—kWh/m2/yr	Pre-Retrofit Airtightness Test result—m3/hr/m2 @ 50 Pa	Post-Retrofit Airtightness Test Result—m3/hr/m2 @ 50 Pa	Occupancy
TUD 001	C2	B1	188.81	79.68	n/a	8.32	2 adults 2 children
TUD 002	D1	B1	252.12	83.92	n/a	6.83	1 adult
TUD 003	C3	B1	212.80	75.61	n/a	12.16	1 adult
TUD 004	D1	n/a	226.87	n/a	9.46	n/a	1 adult 3 children

presents the projected reductions in regulated energy use (space heating, water heating, ventilation and lighting) resulting from the retrofit interventions applied to each dwelling. All retrofitted properties achieved a post-works BER of B1, in accordance with programme requirements stipulating a minimum improvement of 100 kWh in primary energy value [3]. The table also includes the results of post-retrofit airtightness testing conducted as part of this study. While new windows and doors were installed in the three retrofitted dwellings, the level and variance in results suggest that overall airtightness may have received less emphasis, despite its critical role in overall building performance.

2.2. Devices

Environmental data were collected using Airthings Wave Plus (Airthings AS, Oslo, Norway) data loggers, which offer an accuracy of ±30 ppm or ±3% of the measured value within the calibrated range. This level of accuracy is maintained under ambient conditions of 15–35 °C and 0–85% relative humidity, following a minimum calibration period of seven days [47]. To ensure the integrity of measurements, sensors were installed outside the breathing zone to avoid localised concentrations from occupant exhalation. Placement was also optimised to minimise direct influence from ventilation sources such as windows and doors. Participants were not informed of the measurement capabilities of the devices to reduce behavioural bias. Data acquisition was conducted wirelessly at five-minute intervals, continuously over a 24 h cycle in two locations per dwelling (kitchen and master bedroom). During the monitored period, Omega 8 Channel Handheld Thermocouple Thermometer (Model OM-HL-EH-TC; Omega Engineering, Inc., Norwalk, CT, USA) data loggers were utilised to monitor and log the internal surface temperature of external walls. To facilitate placement, a FLIR E6 Pro thermal camera (Teledyne FLIR LLC, Wilsonville, OR, USA) [48] was used to capture thermographic images for the identification of thermal bridging and insulation anomalies within the building envelope.

2.3. Data Analysis Approach

High-frequency IEQ data exhibit strong temporal autocorrelation and typically deviate from normal distributional assumptions. For this reason, summary statistics based on means and error bars were not employed, as these require independence and normality and may therefore misrepresent underlying variability. Instead, distributions, boxplots, and occurrence-based metrics were used to characterise data behaviour in a manner appropriate to the temporal structure of the dataset.

2.4. Ventilation Performance

CO₂ measurements were used to characterise ventilation behaviour across the monitored dwellings. The analyses were undertaken primarily for relative comparison, rather than for evaluating regulatory compliance. CO₂ concentration profiles were interpreted as indicative markers of each dwelling’s inherent ventilation performance and were assessed using two complementary methods, tailored to room function and occupancy patterns. For bedrooms, data collected between 12:00am and 06:00am were analysed using the steady state method, reflecting periods of sustained night-time occupancy. For kitchens, the analysis focused on the period post 12:00am each night, capturing the post-occupancy decay phase and enabling evaluation of ventilation-driven concentration reductions using the decay method. The suitability of this approach for low-ventilation and relatively airtight dwellings is supported by previous studies demonstrating that CO₂ steady-state concentrations and post-occupancy decay profiles remain valid indicators of air-exchange dynamics under restricted airflow conditions [49]. The analysis relies on clearly identifiable steady-state plateaus during sustained occupancy and exponential decay curves during vacancy, both of which were observed in the monitored data.

The bedroom timeframe was selected based on post hoc analysis of the CO₂ data, which indicated that these intervals most consistently aligned with periods of sustained occupancy across the monitored dwellings. This was evidenced by CO₂ concentrations rising above ambient background levels and stabilising at a steady-state equilibrium, reflecting the balance between occupant-generated CO₂ and the prevailing ventilation rate. As the number and physical activity of the occupants is known and the CO₂ generation rate over the monitored period can be calculated [50,51], the steady state method [52] is employed to estimate the air exchange rate per hour. The calculation is based on the following equation.

A_S = 6 × 10⁴ nG_P/[V(C_S − C_R)]

(1)

where n = number of persons in the space; G_P = average CO₂ generation rate per person (L·min⁻¹·person⁻¹); V = volume of the room or space (m³); C_S = steady-state indoor CO₂ concentration (ppm); and C_R = CO₂ concentration in outdoor air (ppm). The average age and average activity level of occupants are used to calculate G_P [51,53] whilst ambient CO₂ levels are set at 414 ppm [54].

For the kitchen areas, ventilation rates are calculated using existing CO₂ as a tracer gas. Analysis of the CO₂ data across the monitored period post 12:00 am allows the identification of the peak CO₂ level, defined as that after which the levels consistently decay toward the ambient levels, signifying zero occupancy conditions. Once no additional CO₂ is introduced, ventilation becomes the dominant process. The rate of CO₂ concentration decay, directly influenced by the operable capacity of the ventilation strategy, is used to calculate the ventilation rate. A single air change is defined as the point at which 63% of airborne contaminants are replaced with outdoor air [55,56,57,58]. Consequently, the ventilation rate can be calculated by measuring the time required to achieve a 63% reduction in excess CO₂ from its peak level. The following equation [55] is used to calculate the ventilation rate based on observed peak CO₂ concentrations:

ACH_T63% = 60/(t₂ − t₁), with t₁ = 0

(2)

where t₁ = initial time when indoor CO₂ is at peak level, t₂ = time (min) when excess CO₂ is reduced by 63%. Indoor CO₂ at peak level (C_S) is the sum of ambient CO₂ (C_R) and excess CO₂ (C_E) generated by the room occupants. CO₂ measurement starts at peak level, therefore t₁ is always 0. The time that is needed to remove 63% C_E (t₂) is the time point where the indoor CO₂ level is C_63%E = C_S–63% C_E where C_E = C_S–C_R. Ambient CO₂ levels are set at 414 ppm [54].

2.5. Internal Temperature and Relative Humidity

Internal temperature and Relative Humidity (RH) were continuously monitored to evaluate the IEQ of each dwelling. These parameters were selected due to their critical role in maintaining comfort, considering ventilation effectiveness, and preventing moisture-related risks such as condensation and mould growth. The collected data enabled analysis of thermal stability and internal moisture load, supporting the calculation of Vapour Pressure Excess (VPX) under standardised and modelled external conditions. RH data is used to evaluate the performance of ventilation systems in relation to occupancy-driven moisture generation, highlighting the synergistic relationship that exists between ventilation strategies and indoor environmental outcomes.

2.6. Moisture Load and Vapour Pressure Excess

Excess vapour pressure arises when the internal vapour pressure within a building exceeds that of the external environment, establishing a gradient that drives moisture migration toward or through the building envelope. This movement occurs as water vapour naturally diffuses from regions of higher vapour pressure to lower, seeking equilibrium. If the building envelope is not adequately designed to manage vapour diffusion, this differential can result in condensation, moisture accumulation, and long-term material degradation [59]. From a design perspective, it is essential to ensure that during the coldest month, the average RH at internal surfaces does not exceed 80%, which is widely recognised as the threshold for mould growth [60]. Maintaining humidity below this limit is critical for preserving indoor air quality and preventing biological contamination.

Ridley et al. [61] demonstrated a linear relationship between external temperature and VPX, leading to the definition of Standardised Vapour Pressure Excess (SVPX), a reference condition where outdoor temperature is set at 5 °C and RH at 80%, facilitating comparative analysis across different building scenarios and climates. SVPX is calculated using the Tetens equation [62], the monitored internal and standardised external data:

$$\mathrm{VPX} = [{\displaystyle \frac{\mathrm{R}\mathrm{H}\mathrm{i}\mathrm{n}}{100}} \times 610.78 \times e^{({\displaystyle \frac{17.27 \times \mathrm{T}\mathrm{i}\mathrm{n}}{\mathrm{T}\mathrm{i}\mathrm{n} + 237.3}})}] - [{\displaystyle \frac{\mathrm{R}\mathrm{H}\mathrm{o}\mathrm{u}\mathrm{t}}{273}} \times 610.78 \times e^{({\displaystyle \frac{17.27 \times \mathrm{T}\mathrm{o}\mathrm{u}\mathrm{t}}{T\mathrm{o}\mathrm{u}\mathrm{t} + 237.3}})}]$$

(3)

where T_in = indoor temperature in °C, RH_in = indoor relative humidity in %, T_out = outdoor temperature in °C, RH_out = outdoor relative humidity in % and VPX = vapour pressure excess in pascals (Pa).

2.7. Thermal Performance and Surface Temperature Analysis

Internal surface temperatures of external walls were monitored to identify thermal anomalies and potential thermal bridging, which are indicative of insulation deficiencies and heat loss pathways. Comparative analysis was undertaken to evaluate the thermal performance of the utilised insulation strategies in maintaining stable internal surface temperatures. Attention is paid to identifying cold spots and temperature depressions that may signal incomplete insulation fill, material settlement, or workmanship issues with thermal irregularities cross-referenced with RH and SVPX data to assess the risk of surface condensation and mould growth.

2.8. Limitations

While the sample size (4n) restricts the statistical generalisability of the findings, it does not diminish their technical relevance. The study was designed to explore the performance of retrofit strategies in depth rather than breadth, and the insights gained are grounded in well-established principles of building physics. As such, the observed relationships between ventilation rates, thermal performance, and moisture dynamics reflect repeatable physical phenomena that are likely to manifest similarly in comparable contexts. Efforts were made to minimise behavioural bias, such as withholding device capabilities from participants, however occupant behaviour remains a variable that could influence outcomes, particularly in relation to ventilation and moisture generation. Additionally, the study was conducted during the heating season, and while this period is critical for assessing condensation risk and thermal comfort, it does not capture seasonal variations that may affect IEQ. As such, the findings reflect conditions under cold-weather boundary scenarios and should not be interpreted as representative of full-year performance. The study employed high-accuracy sensors and followed established IEQ monitoring guidelines and standardised placement protocols [63], however spatial variability within rooms and temporal fluctuations may still influence measurements. CO₂ concentrations, for example, can vary based on micro-zones of occupancy and airflow, and surface temperature readings may not fully capture thermal anomalies across entire wall sections. Furthermore, as single-point measurements cannot capture all intra-room variability, the data reflect conditions in the monitored micro-zone rather than the full spatial field. This limitation is common to in-use residential monitoring and should be considered when interpreting absolute values. Nonetheless, it does not affect the comparative analyses conducted across dwellings, which rely on consistent sensor positioning and equivalent deployment protocols.

While the limited sample size constrains generalisability, the repeatability of the findings is reinforced using building physics-based methods. Techniques such as the steady-state and decay methods for ventilation assessment, vapour pressure excess calculations, and thermographic analysis are grounded in physical laws and standardised procedures. Ventilation rate analysis incorporated adjustments for occupancy loading to enable robust comparative interpretation. These methods are replicable across different dwellings and contexts and provide a robust framework for future studies and policy evaluation. Furthermore, the variable use cases examined reflect patterns commonly observed in the wider dwelling stock [64].

3. Results and Discussion

3.1. Indoor Environment

Figure 1 presents the recorded CO₂ concentrations over the monitoring period. Data from TUD001, representing a household of two adults and two children, indicates a significant ventilation deficiency. Only 8% of kitchen readings and 2% of bedroom readings fall below 700 ppm, a threshold commonly associated with adequate ventilation. Notably, 73% of kitchen and 54% of bedroom measurements lie within the 1000–2000 ppm range, with an additional 36% of bedroom readings exceeding 2000 ppm. The bedroom in TUD001 is occupied by two adults, and the kitchen is potentially at times used by all four household members, which may partially account for the elevated CO₂ levels particularly when considering the additional infiltration associated with the dwellings sub-optimal airtightness (8.32 m³/hr/m² @ 50 Pa). Although occupancy represents a critical variable that warrants consideration [65], CO₂ concentrations exceeding 1000 ppm reflect reduced dilution capacity, thereby intensifying the accumulation of contaminants originating from both occupant-related and non-occupant sources [66].

In contrast, TUD002, occupied by a single elderly resident, indicates ventilation performance strongly influenced by occupancy. The consistently low CO₂ concentrations (with 99% of kitchen and 82% of bedroom measurements below 700 ppm, and all remaining values under 1000 ppm) suggest a combination of effective ventilation and minimal occupant-generated emissions. This outcome reflects the significant role of occupancy in determining indoor air quality. Similarly, TUD003, which is also singly occupied, exhibits comparable performance, with 91% of kitchen measurements below 1000 ppm and the remainder below 1500 ppm. These findings collectively highlight that, while airtightness and ventilation systems are critical, occupancy remains a dominant factor influencing internal CO₂ levels.

The control property, TUD004, was not retrofitted at the time of monitoring. Despite being occupied by one adult and three children, all recorded CO₂ measurements remained below 1500 ppm. While higher occupancy would typically result in increased CO₂ generation, this effect may have been mitigated by elevated uncontrolled infiltration, likely due to its relatively weak airtightness (9.46 m³/hr/m² @ 50 Pa). Notably, TUD001 and TUD004 exhibit broadly similar airtightness values (8.32 and 9.46 m³/hr/m² @ 50 Pa), yet their ventilation outcomes differ significantly. This discrepancy can be attributed to both ventilation strategy and infiltration dynamics. TUD001, being retrofitted, relies on a demand controlled ventilation system designed for airtight envelopes. If this system underperforms or becomes imbalanced, CO₂ will accumulate during periods of high occupancy along with other occupant and non-occupant generated pollutants that depend on effective ventilation for dilution. In contrast, TUD004 may experience greater uncontrolled infiltration due to its non-retrofitted state and leakage characteristics. Even with similar airtightness metrics, the distribution and nature of leakage paths can influence air exchange [67], enabling more effective dilution of indoor air in TUD004. TUD001’s envelope may have limited uncontrolled infiltration compared to TUD004, where leakage is likely more dispersed and effective for dilution, as retrofit-driven sealing typically results in tighter and more uniform building envelopes which substantially reduces natural air exchange and consequently increases the need for well-designed and effective mechanical ventilation systems to maintain IEQ [68].

3.2. Ventilation Performance

TUD001 employs a constant-pressure, DCV system capable of delivering a maximum airflow of 100 m³/h at 100 Pa. The system is activated through two primary mechanisms: (a) variations in indoor humidity levels within habitable and wet rooms, which trigger humidity-sensitive air inlets, and (b) presence detection via passive infrared sensors in wet rooms, bathrooms, utility spaces, and kitchens, which activate the extract units. Humidity-sensitive air inlets, equipped with insect filters, are installed in all habitable rooms. These inlets contain a polyamide strip that expands in response to increased humidity, thereby opening one or more shutters to regulate the inflow of external air. The degree of shutter opening is proportional to the relative humidity (40% humidity results in 40% shutter opening, 50% humidity results in 50% opening) allowing for dynamic adjustment of air intake. Likewise, a humidity-controlled strip and/or presence detector allow shutters within air extract units to open in wet rooms/bathrooms/utility and kitchens, resulting in the removal of stale moist air from these zones. The resulting negative pressure is balanced by the inflow of fresh air through the habitable room inlets, maintaining indoor air quality and pressure equilibrium.

In contrast, TUD002, TUD003, and TUD004 utilise a natural ventilation strategy. These dwellings rely on the passive movement of external air through purpose-designed openings, such as windows and vents. The air-change rate in these units is influenced by external environmental factors, including temperature (stack effect) and wind speed, as well as occupant behaviour such as window operation. Additionally, all dwellings are subject to infiltration via uncontrolled air leakage through unintended gaps in the building envelope. While infiltration may enhance pollutant dilution, it simultaneously undermines energy efficiency by increasing regulated energy use. Ventilation strategies, whether natural or mechanically driven, depend on the building fabric being appropriately airtight, requiring a good practice standard of 5 m³/hr/m² @ 50 Pa [69].

Figure 2 details the Air Exchange Rate (AER) during the defined time windows. The median AER in the kitchen areas assessed via the decay method during zero occupancy conditions varies from 0.20 to 0.40 h⁻¹, which is lower than other studies (albeit A-rated BER retrofits) in an Irish context [22,29]. The results from dwelling TUD001 are of particular interest, as it is equipped with a demand-controlled ventilation system. However, previous research has demonstrated that the presence of mechanical ventilation does not, in itself, guarantee adequate AER, with system performance potentially influenced by factors such as user interaction, system configuration, control logic, and operational deficiencies [70,71]. It is assumed that occupant behaviours (such as window opening) that may positively affect the air exchange rate during daytime hours are not engaged during the post 12:00am (night-time) assessed period. Consequently, the median AER observed during this timeframe is considered to reflect the inherent performance capacity of the employed ventilation strategies, independent of occupant intervention. Although independent ventilation validation is statutorily required to ensure that the system has been installed, balanced and commissioned to meet the minimum requirements of Technical Guidance Document F—Ventilation [72] to the Irish Building Regulations [73,74], the findings should be understood within the context of possible interacting influences, including possible system inbalance and/or sensor responsiveness which may affect the observed decay characteristics.

The median AER in the bedrooms assessed via the steady state method range from 0.25 h⁻¹ (TUD001) to 0.64, 0.79 and 0.78 h⁻¹ (TUD002, TUD003, TUD004, respectively). Whilst TUD001 is again low, the remaining dwellings align with or exceed rates achieved in the previously mentioned studies [22,29]. The superior performance of passive ventilation systems within this sample raises a concern, given that demand-controlled ventilation is intended to operate independently of natural driving forces and their variability [75], but highlights the previously identified and critical need for ongoing maintenance of ventilation systems [23].

3.3. Indoor Temperatures

Figure 3 illustrates the range of indoor temperatures recorded across the dataset during the monitoring period. The comfort associated with retrofit interventions in dwellings TUD001, TUD002, and TUD003 are readily observable when compared to TUD004. Each retrofit dwelling is equipped with a heat pump system connected to a permanent live power supply and regulated via room thermostats. Consequently, the heat pumps are technically capable of continuous operation; however, in practice, their activation is governed by thermostat signals, which initiate or terminate operation based on set indoor temperature thresholds. This control strategy not only enhances energy efficiency (albeit based on achieving required airtightness levels) but also contributes to maintaining occupant comfort within acceptable limits. The observed temperature profiles underscore the pivotal influence of retrofit interventions in regulating indoor environmental conditions. In contrast, TUD004, which lacks these enhancements, exhibits greater temperature variability, leading to increased energy consumption during times of operation and potentially, reduced occupant comfort.

3.4. Relative Humidity

Figure 4 presents the recorded RH profiles recorded across the monitored dwellings, revealing substantial variability primarily driven by differences in air AER and occupancy levels. Dwellings TUD001 and TUD004, each housing four occupants, exhibit consistently higher humidity levels compared to TUD002 and TUD003, which are occupied by single elderly residents. This disparity highlights the influence of internal moisture generation, which scales with occupancy. None of the monitored dwellings are equipped with dedicated clothes-drying facilities, aside from outdoor washing lines located in rear gardens. Consequently, moisture generated from daily activities, such as cooking, cleaning, showering, clothes washing and human respiration, accumulates within the indoor environment. This effect is amplified in higher-occupancy dwellings, where the cumulative moisture load is substantially greater, contributing to elevated RH levels.

The demonstrated interaction between occupancy-driven moisture generation and ventilation effectiveness is critical. In dwellings with lower AER or sub-optimal ventilation control, excess humidity may persist, increasing the risk of condensation, mould growth, and occupant discomfort. Conversely, dwellings with adequate ventilation are better equipped to dilute and remove moisture, maintaining healthier indoor conditions. These findings highlight the necessity of tailoring ventilation strategies to occupancy profiles, ensuring not only improved IEQ, but also energy-efficient operation.

3.5. Vapour Pressure Excess

It is accepted that as external temperatures fall, VPX will rise as ventilation rates fall [60]. Building on this relationship, Ridley et al. [61] through an analysis of 1604 dwellings, proposed a classification system based on VPX values. According to their findings, a VPX of up to 300 Pa is indicative of a dry indoor environment, values between 300 and 550 Pa represent average condition, and VPX levels exceeding 550 Pa are characteristic of a wet internal environment, with increased risk of condensation and mould growth as the excess pressure attempts to equalise across rooms and external barriers. The SVPX analysis presented constitutes a scenario-based sensitivity assessment, not a predictive model of indoor moisture behaviour. To isolate the influence of external climatic severity, internal temperature, humidity, and occupancy were deliberately held constant across scenarios. This approach is intended to illustrate each dwelling’s relative sensitivity to changes in external boundary conditions, and the resulting SVPX values should therefore be interpreted as indicative rather than as forecasts of actual indoor conditions.

Figure 5 illustrates the internal excess moisture load across the monitored dwellings, calculated using monitored internal data and under standardised external conditions of 5 °C and 80% RH. The indoor environment in TUD001 is notably moisture-laden, a condition attributed to reduced ventilation rates and prevailing wintertime external conditions. In contrast, TUD002 and TUD003 consistently fall within dry and average moisture zones under these reference conditions, once again indicative of the slightly increased ventilation performance and the reduced occupant density. Interestingly, TUD004 demonstrates relatively favourable performance despite its occupant density and absence of retrofit measures. This performance is noteworthy given the expected moisture generation associated with multiple occupants. The outcome can be attributed to the previously mentioned ventilation strategy and infiltration dynamics leading to elevated uncontrolled infiltration, which facilitates moisture dilution and prevents excessive vapour pressure accumulation. While this infiltration-driven ventilation may appear beneficial in mitigating condensation risk, it is inherently energy-inefficient and highly variable, depending on wind speed and temperature gradients. Consequently, reliance on uncontrolled air leakage as a moisture management mechanism is unsustainable in the context of energy performance targets and climate resilience. Despite the exploratory scope of this study, it remains essential that retrofit strategies balance improvements in airtightness with robust, demand-responsive ventilation systems. Such integration is necessary to maintain vapour pressure within safe limits, thereby safeguarding IEQ while meeting energy-efficiency objectives.

Although the precise dwelling locations remain confidential to preserve anonymity, the regional external conditions during December and January typically range from minus 2.7 °C to plus 7 °C, with an average relative humidity of approximately 83%. These prevailing winter conditions impose a significant moisture load on building envelopes and ventilation systems, complicating efforts to maintain IEQ. As external temperatures fall, relative humidity typically rises, reducing the potential for passive drying and increasing the likelihood of condensation in inadequately ventilated spaces.

To further investigate this relationship, the dataset was further analysed under two additional external scenarios: 2.5 °C at 85% RH and 0 °C at 90% RH. Figure 6 and Figure 7 illustrate the corresponding rise in internal moisture load across the dwellings under these conditions, assuming no change in internal temperature, humidity, or occupancy levels. These results indicate the sensitivity of indoor moisture dynamics to external climatic variations, particularly in dwellings with limited ventilation capacity. It remains essential that ventilation systems within retrofit strategies are designed to account for climatic variability, thereby ensuring adequate air exchange and effective moisture control. This is critical to prevent the occurrence of surface or interstitial condensation formation and mould growth, and to safeguard occupant health.

3.6. Internal Surface Temperature

The thermal images in Figure 8 and Figure 9 illustrate the temperature distribution across the external facade of the un-retrofitted TUD004. The absence of thermal insulation in the external walls along with the infiltration of cooler external air results in consistently low surface temperatures, indicative of significant heat loss. Notably, Figure 9 highlights localised thermal anomalies on the right-hand side, corresponding to the gas boiler and its flue, which exhibit elevated temperatures due to concentrated heat emission. These observations align with the measured surface temperature data recorded over the monitoring period, as illustrated in Figure 10. The persistently low external surface temperatures contribute to a reduced thermal gradient across the dwellings external wall sections, ultimately lowering the internal surface temperature. This condition increases the risk of surface condensation, particularly under elevated indoor RH levels, and may have implications for occupant comfort and health and building fabric integrity.

Previous investigations into retrofit cavity wall insulation have identified substantial performance issues that challenge the assumed efficacy of these interventions [32,33]. Voids and gaps, frequently resulting from poor workmanship and physical obstructions within the cavity, have been shown to inhibit complete insulation fill, thereby undermining thermal continuity [76]. Thermographic analysis has proven instrumental in corroborating these deficiencies, revealing cold spots, uninsulated zones, and instances of thermal bridging, each of which contributes to elevated heat loss and increased risk of condensation within the building envelope [77]. These findings are further substantiated by internal surface temperature data for dwellings TUD001 (full-fill mineral wool) and TUD002 (full-fill cavity bead), as presented in Figure 11 and Figure 12. The temperature profiles of these retrofitted dwellings exhibit localised depressions in surface temperature that are comparable to those observed in the un-retrofitted control dwelling TUD004. This alignment with prior research suggests that despite the application of full-fill insulation systems, areas of the walls remain thermally vulnerable, likely due to incomplete fill or post-installation settlement. Consequently, these areas are subject to the same thermal inefficiencies and moisture-related risks typically associated with uninsulated cavity walls.

In contrast to the variable thermal performance observed in cavity-insulated dwellings, the externally insulated dwelling TUD003 (Figure 13) is demonstrative of a notably stable internal surface temperature profile. This stability underscores the efficacy of correctly applied external insulation systems in mitigating thermal irregularities. By encapsulating the building envelope within a continuous insulating layer, such systems effectively eliminate thermal bridges and cold spots, thereby substantially reducing the risk of interstitial and internal surface condensation.

These exploratory findings suggest that surface condensation remains a persistent challenge, particularly in retrofit scenarios where insulation strategies fail to positively alter the hygrothermal dynamics of the building envelope. Empirical and simulation-based studies have consistently demonstrated that when indoor relative humidity exceeds 80%, the risk of condensation on cooler internal surfaces increases significantly [78]. This threshold is widely recognised in building science as a critical point for the onset of mould growth and material degradation [60]. Furthermore, it has previously been demonstrated in an Irish social housing context [79] that 80% relative humidity or more is a critical factor for mould growth, especially in poorly ventilated and/or thermally bridged areas, both factors demonstrated to exist in the dwellings under review. Figure 14 illustrates the relationship between the recorded room-level RH and the internal surface temperatures across all datasets, showing how localised cooling elevates RH adjacent to cooler surfaces. The analysis demonstrates that when air at room conditions is exposed to cool zones at thermal bridges, its temperature can drop to the point where RH rises towards saturation without any additional moisture being introduced. As surface temperatures fall, RH can surpass the critical 80% threshold, initiating condensation formation and creating conditions conducive to mould growth. This highlights the susceptibility of hygrothermal performance to material discontinuities and construction detailing on and within the building envelope.

The comparative data align with previous research and highlight the inherent limitations of cavity insulation retrofits in delivering consistent thermal performance, particularly in existing buildings with complex or obstructed cavity structures. In contrast, external insulation systems, by virtue of their continuous placement around the building envelope, provide a more robust solution for reducing heat loss and mitigating internal surface condensation risk. Where cavity insulation is considered, the potential for thermal bridging and incomplete fill must be rigorously assessed, as these conditions can create localised cool zones that elevate moisture-related risks. Conversely, external insulation systems maintain stable surface temperatures, reducing condensation risk while improving occupant comfort and energy efficiency. By ensuring uniform thermal performance, these systems minimise localised heat loss and enhance overall envelope integrity. These findings reinforce the importance of comprehensive insulation strategies in retrofit programs, where external insulation can deliver superior hygrothermal resilience compared to partial or cavity-based approaches and aligns with best practice aimed at preventing surface mould growth by design [80]. While exploratory in scope, the findings suggest that local authority retrofit policy and practice may benefit from giving greater consideration to the use of external insulation in situations where cavity wall integrity cannot be reliably assessed or assured. Prioritising such approaches has the potential to improve indoor thermal conditions, reduce energy demand, and mitigate moisture-related risks, thereby contributing to enhanced indoor environmental quality and long-term building performance.

4. Conclusions

This study undertook a detailed evaluation of indoor environmental quality outcomes following retrofit interventions in local authority owned dwellings using high-resolution post-retrofit monitoring. The dwellings examined are broadly representative of the wider local authority housing stock and reflect prevailing retrofit practices under Ireland’s EERP. Although all retrofitted properties exceeded the targeted BER, the monitored data indicate that improvements in energy efficiency do not necessarily correspond to consistent enhancements in indoor environmental conditions. Given the long lifespan of retrofit measures, these findings raise concerns regarding the potential for today’s interventions to create the ‘hard-to-treat’ dwellings of the future if performance limitations remain unaddressed.

Across the monitored dwellings, several interacting mechanisms shaped IEQ outcomes, including occupancy-driven moisture generation, ventilation system responsiveness, building-fabric characteristics, and external climatic conditions. Sub-optimal ventilation performance was evident and contributed to elevated CO₂, RH, and VPX values. These observations suggest that the existing retrofit strategy may not reliably achieve the intended synergy between airtightness improvements and ventilation provision and may struggle to accommodate the ventilation loads associated with typical daily living.

Thermal performance data further highlight the role of envelope continuity in mitigating thermal irregularities. Localised thermal anomalies were observed where insulation systems lacked continuity, whereas externally applied, uninterrupted insulation exhibited more stable internal surface temperatures. These patterns reflect underlying hygrothermal mechanisms rather than system type alone as continuity of the thermal envelope plays a central role in moderating heat flow paths and reducing cold-surface formation. The discussion therefore centres on drivers of performance, namely thermal bridging, airflow pathways, vapour-pressure dynamics, and occupancy interactions, rather than on categorical comparisons of insulation system types.

Moisture-related outcomes demonstrated strong dependence on ventilation effectiveness. Elevated RH and VPX were recorded in dwellings with greater occupancy and sub-optimal ventilation, increasing the risk of condensation and mould growth. These findings highlight the importance of integrating ventilation system design and insulation measures in a manner that responds to dwelling typology, occupancy patterns, and climatic conditions. Potential strategies include the use of demand-responsive ventilation with appropriate sensing and modulation capabilities, and the prioritisation of ventilation upgrades early within staged retrofit pathways to ensure stable IEQ during and after retrofit works.

Although exploratory, the value of the high-resolution approach undertaken lies in its ability to illuminate performance patterns that are often obscured within large scale or aggregated datasets, thereby offering insights that can inform, rather than substitute, broader empirical evidence. The observations should be interpreted as indicative signals rather than definitive conclusions. They point to areas where additional empirical evidence is needed to assess the prevalence, significance, and generalisability of the mechanisms identified. Such research would help clarify potential economic impacts, including retrofit cost-effectiveness, healthcare implications, and long-term maintenance burdens, should similar patterns be observed at scale.

From a regulatory perspective, the findings highlight the value of grounding national and international standards in in-use performance evidence, particularly regarding ventilation sizing, commissioning, control behaviour under winter conditions, and envelope continuity. However, determining whether regulatory adjustments are warranted requires broader datasets and further targeted investigation.

Overall, the study underscores the importance of continued research that integrates building-physics analysis with real-world monitoring. As larger-scale evidence becomes available, it can inform the development of retrofit strategies that more reliably balance energy efficiency objectives with indoor environmental quality and occupant health considerations.