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Article

Assessing the Effect of Insulation Materials Used for Energy Conservation in Buildings on Indoor Radon—The Scale Model Room Approach

by
Ilaria Rocchetti
1,
Manuela Portaro
1,
Paola Tuccimei
1,*,
Gianfranco Galli
2,
Michele Soligo
1,
Cristina Longoni
3 and
Dino Vasquez
3
1
Dipartimento di Scienze, Università Roma Tre, 00146 Roma, Italy
2
Istituto Nazionale Geofisica e Vulcanologia, Sezione Roma 1, 00143 Roma, Italy
3
Mapei S.p.A., Waterproofing Line, 20158 Milano, Italy
*
Author to whom correspondence should be addressed.
Appl. Sci. 2025, 15(22), 12106; https://doi.org/10.3390/app152212106
Submission received: 25 September 2025 / Revised: 7 November 2025 / Accepted: 13 November 2025 / Published: 14 November 2025
(This article belongs to the Section Environmental Sciences)
Browse Figures
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Abstract

This study investigates how external insulation materials used for energy efficiency affect indoor radon accumulation, using a scale model room built with ignimbrite, a highly radon-emitting volcanic rock. Two insulation materials—mineral wool (open-cell, 98% porosity) and extruded polystyrene (XPS, closed-cell, >95%)—were applied to the outer walls of the model room. Their effects were tested in combination with three internal radon barriers (silane-terminated membrane, silicone sealant, bitumen membrane) and under varying ventilation rates (0.11 h−1 and 0.44 h−1). Radon concentrations were measured using calibrated detectors over five experimental phases. Without ventilation, XPS increased indoor radon by up to +351%, while mineral wool showed a milder effect (+26%). The silicone sealant reduced radon by up to 90%, outperforming other barriers. Ventilation significantly lowered radon levels, simulating the “flushing” effect of wind. The combination of impermeable insulation and lack of air exchange led to the highest radon accumulation. High-performance insulation can compromise indoor air quality by trapping radon, especially in buildings with high geogenic radon potential. Effective mitigation requires pairing insulation with high-performing radon barriers and adequate ventilation. These findings highlight the need to balance energy efficiency with indoor environmental safety.

1. Introduction

Thermal renovation of existing homes is a highly effective method for a short term reduction in CO2 emissions. As such, retrofitting the existing building stock has become more pressing and state grants and subsidies have been promoted all over the world [1,2,3]. Research on the housing supply in Europe indicated that enhancing the structure of current and new residential buildings offers a significant opportunity for saving energy, potentially reaching around 90 Mtoe by 2030 across all EU nations [4]. Retrofitting measures should reduce energy consumption as well as improve the indoor climate while still being cost-effective. This can be challenging in European cold climates [5].
Research indicated that energy conservation (thermal insulation, transparent components, airtightness) and efficient energy use (heating and cooling systems, heat pumps, on-site energy sources, ventilation, heat recovery) are the most effective approaches [5]. Renovating the building fabric and windows can lower the need for heating and cooling by installing thermal insulation panels and multiple glazing windows [3]. New high-performing insulation layers are all addressed to increase the thermal behaviour of the building envelope [5,6].
Unfortunately, energy-saving measures are boosting the concept of strongly insulated structures to avoid dispersion and air exchange between inside and outside. This approach greatly increases the risk of accumulation of indoor natural and domestic pollutants, such as radon, a very dangerous agent which is considered one of the most significant contributors to lung cancer after smoking [7,8,9]. Radon can seep into indoor spaces through small cracks and openings where the building contacts the ground or through emission from building material. If the building is isolated and the air exchange is reduced, radon accumulates, and mitigation actions must be implemented. Active and passive techniques are usually employed, alone or in combination, depending on the radon sources and the building structure. Among them, the use of waterproof membranes in the crawl space of a building, or on the internal walls of edifices constructed with high radon-emitting materials, is widespread [10,11]. To ensure adequate air exchange and reduce the level of indoor pollutants, supply, balanced and energy recovery ventilation systems are also used.
A recent review of twenty-three exposure studies completed between 2010 and 2020 [12] highlighted concerns with increased radon, presence of mould and increased concentrations of other chemicals in indoor air, post-retrofit [13]. Higher concentrations of radon were detected post energy-efficiency refurbishment in homes located in radon prone areas [14], particularly where the restoration did not include mechanical ventilation [15,16]. Multiple studies [17,18,19] now recommend adapting the retrofit strategy to the local radon emissions levels [13,20].
To investigate the impact of energy efficiency measures on indoor radon, we utilized a model room [21,22,23,24] constructed from an ignimbrite known for its significant radon emissions, externally insulated with two varieties of thermal coatings to evaluate Relative changes in Indoor Radon (RIR). The model room is a reduced-scale representation of the reference room suggested by the European Commission to monitor and safeguard against radiation exposure from building materials [25]. It is 62 cm long, 50 cm wide and 35 cm high, with a volume of 0.1 m3. The selected insulation materials, mineral wool and extruded polystyrene (XPS), were chosen for their specific structures and porosities, expected to trap indoor radon in varying ways. In addition to that, the impact and possible counterbalancing effects of 3 anti-radon barriers with different abilities to reduce indoor radon in the room were also considered: a silane-terminated polymer membrane, a silicone sealant (samples B14 and B15 in [24] and a bitumen membrane (labelled as PEMA, tested for the first time in this work). They were placed on the room’s internal walls, and their effect on indoor radon was assessed with and without thermal coatings and air exchange at varying levels.
The three membranes were selected because they showed different radon reductions of −66% and −90% (respectively, for the silane-terminated membrane and the silicone sealant) and a possible value of −37% for the bitumen membrane, with characteristics very similar to a product tested in [24] (sample A2). The experiments were carried out in the Environmental and Isotopic Geochemistry Laboratory of Roma Tre University where a Heating, Ventilation and Air Conditioning (HVAC) system is active during the weekdays, but not on the weekends. This situation made it possible to simulate the “flushing” effect of wind on indoor radon, and the consequent decrease in radon entry rates from the walls of a building constructed with high radon-emitting materials [26].
This paper, which investigates for the first time the impact of thermal coating on the indoor radon concentration in the model room, is structured as follows. Section 2 outlines the methodology, detailing the materials, the experimental configuration and the approach to test design. Then, the results for each experiment are presented individually in Section 3, and a comparative discussion is provided in Section 4. This paragraph also evaluates the role of laboratory ventilation on indoor radon and its impact on the entry rates from the model room walls, simulating the “flushing” effect of wind on radon. Lastly, general conclusions are drawn, emphasizing the contribution of this paper to the issue of the impact of energy efficiency improvements on indoor radon, as well as on other domestic pollutants.

2. Materials and Methods

This section describes the thermal coatings, the radon barrier materials and the model room approach used to study the effect of energy efficiency measures on indoor radon.

2.1. Materials

Mineral wool and polystyrene are among the most widely available and sold thermal insulators on the market [6,27,28]. Because thermal insulation requirements of building envelopes vary according to regional regulations for different building uses, most insulator types are available at different thicknesses [28]. In this study, we tested the commonly used eight-centimetre-thick insulation materials (mineral wool and extruded polystyrene) characterized by different porosity and structure. In temperate climate, characterized by progressively milder winters, a target thickness of 8 cm is considered suitable for balancing thermal insulation, energy savings and payback period [29,30]. The insulation boards were placed over the outdoor walls of the model room according to various settings: with and without an inner barrier (a silane-terminated polymer membrane, or a silicone sealant, or a bitumen membrane), with and without room ventilation at different rates.
Mineral wool (labelled as C1) is a product of inorganic mineral origin, which is obtained through a weave of filaments bound by a fireproof resin. It is characterized by an open cell porosity of the order of 98%, with a thermal conductivity of 0.034 W m−1 k−1. This product is used for thermal and acoustic insulation on vertical and horizontal surfaces in total safety in case of fire [30].
Extruded Polystyrene (XPS, labelled as C2) with graphite additives has a thermal conductivity of 0.031 W m−1 k−1 and a content of closed cells higher than 95%. The last parameter is the amount of polymer vesicles completely closed, as these cells determine the insulation capacities of rigid foams commonly used in the thermal insulation of housing. Thanks to its structure, it has excellent thermal insulating power, high resistance to compression and excellent water-repellent performance (preventing the growth of harmful mould and fungi).The silane-terminated polymer membrane (MAPEI S.p.A, Milano, Italy) applied over the internal walls of the room (sample B14 in [24] was previously tested with the model room approach, achieving a reduction in indoor radon of −66% [24]. It was spread on 11 mm thick plasterboard panels according to current operative procedures in the construction sector and then fixed with silicone on the walls of the room.
The silicone sealant (MAPEI S.p.A, Milano, Italy) is a neutral, solvent-free product designed to seal joints between sheet metal parts and details in the waterproofing works (sample B15 in [24]. It was spread on 11 mm thick plasterboard and analyzed with the same procedure used for the previous membrane, obtaining a radon reduction of −90%.
The bitumen membrane (MAPEI S.p.A, Milano, Italy), labelled as PEMA, was not tested in [23,24]. Its performance is evaluated in this work according to the same procedure used for the other materials. It is a self-adhesive bitumen material coupled with a HDPE film with high mechanical resistance. It was chosen for these experiments based on its similarity to a product analyzed in [23,24], sample A2, which produced a reduction indoors of −37%.

2.2. Experimental Configurations

The experimental setup is thoroughly detailed in [23,24]. Although the description of the instrumentation has been condensed, the key aspects have been retained; for further details, see references [23,24]. The setup includes the model room and two kinds of radon detectors, two flowmeters, and vinyl tubing (Figure 1).
The model room is a scale reproduction of a chamber [25], designed to manage and safeguard against natural radiation exposure. It is constructed from an ignimbrite, characterized by elevated radon emission and significant porosity [21,22,23,24]. The ceiling and the ground of the room are Plexiglas sheets. Two faucets on the upper panel facilitates the intake and expulsion of air to a RAD7 (Durridge Company Inc., Billerica, MA, USA) radon detector. An additional device, the AER PLUS (Algade Instrumentation, Bessines-sur-Gartempe, France), is situated in the model room for the detection of radon. The circuit is completed by two flowmeters, one for measurement and the other for control, which manage the air flow into the chamber.
The building material serves as a source of radon, while the RAD7’s integrated pump is utilized to bring in outside air at exchange rates of 0.11 h−1 and 0.44 h−1, adjusted via the flowmeters, in line with the experimental protocol. This air movement mimics supply ventilation systems commonly used today to enhance the health and comfort of indoor occupants by elevating the quality of the air they inhale. The average standard deviation of radon measurements is generally 5%. RAD7 and AER PLUS are solid-state radon detectors calibrated against the INGV (Istituto Nazionale di Geofisica e Vulcanologia) radon chamber. Additional information regarding this calibration can be found in [23,24].
Another RAD7 was used to assess the background radon levels in the lab, and two RHT50 dataloggers (EXTECH Instruments, Waltham, MA, USA) were positioned close to the model room to track ambient humidity, temperature, and pressure throughout the test.
During the weekend, the Heating, Ventilation and Air Conditioning (HVAC) system in the laboratory, used to regulate and move heated or cooled air throughout the room, was turned off. The closest vent was in the ceiling, one metre away from the model room.

2.3. Experiments

Nine different experiments were carried out (Table 1). Each was organized into five phases. Initially, no panels (neither the radon barrier membranes nor the thermal coatings) were applied over the walls of the room (part 1); thereafter, the membrane (either with or without the outer thermal layer) was secured to the internal surfaces of the chamber (Section 2), based on the specific experimental configuration (Table 1). Subsequently, outdoor air was brought into the room at exchange rates of 0.11 h−1 (part 3) and 0.44 h−1 (part 4). Finally, the membrane and air circulation were removed, and partial segments of insulations covers were applied to ensure effective coverage of the external walls (part 5). After the ventilation period between the individual experimental phases, back-diffusion or re-emission of radon from internal wall surfaces was not considered, as it was strongly reduced by the presence of radon barrier materials.
Equilibrium radon levels in the room were measured using the RAD7 and AER PLUS radon detectors. In parts 1, 2, and 5, the two devices were used, whereas only AER PLUS was utilized in parts 3 and 4, with RAD7 functioning solely as a pump (refer to Table 1 and Figure 1) to bring in external air to the chamber that was isolated from the external environment. The measurement interval was set to 1 h for both devices. The equilibrium between the radon emitted from the construction materials and the decaying radon was attained in a minimum of 24 h, influenced by the system’s airtightness of single experimental parts. Each experimental part lasted not less than 3 days. The procedure employed to verify that equilibrium conditions were effectively attained is detailed in Appendix A. This was confirmed by examining the average and the standard deviation from the mean of radon data across sequential time intervals [23], resulting in a duration of at least 3–4 days for every experimental phase. All equilibrium radon measurements were adjusted to 23 °C, the average temperature recorded in the laboratory during the experiments.
The Relative change in Indoor Radon (RIR, %) of experimental phases 2, 3, 4 and 5 (RnB) with reference to part 1 (RnA), was calculated following Equation (1).
RIR = (RnB − RnA)/RnA × 100

3. Results

Table 2 reports radon data from the nine experiments. See Table 2 for an explanation.
In this section, experimental data are presented individually, while in Section 4, a complete and comparative discussion of the results is given. Relative changes in Indoor Radon (RIR) of experiments 1, 2 and 3 are reported in Figure 2. Experiment 1 corresponds to the test conducted on sample B14 as descibed in [24]. The radon reduction (RIR) observed in part 2 was substantial (−66%), and further reductions were achieved in parts 3 and 4, with air exchange rates (ACH) of 0.11 and 0.44 h−1, respectively, resulting in radon decreases of −80% and −89%. In Experiment 2, where mineral wool was externally applied to the walls, part 2 showed a comparable radon reduction (−70%) to that of Experiment 1, and slightly greater reductions in parts 3 and 4 (−86% and −94%, respectively). Upon removal of the B14 membrane and air exchange in part 5, indoor radon levels increased, reaching values like those in part 1 within the margin of error (RIR of −9%). Experiment 3 showed only a modest increase in radon concentration (+1%) in part 2, while parts 3 and 4 yielded RIR values of −75% and −89%, respectively. Part 5 was marked by a pronounced radon increase of +161%.
Figure 3 presents the RIR results from experiments 4, 5, and 6 (Table 2), all conducted using the best-performing material: silicone sealant. Experiment 4 corresponds to the test on sample B15 described in [24]. The outcome in part 2 was excellent, with a radon reduction of −90%, which was not further enhanced by the introduction of ventilation at 0.11 and 0.44 h−1 in parts 3 and 4. In experiment 5, the application of mineral wool to the external walls produced minimal changes in radon levels, both without ventilation (part 2) and with ventilation (parts 3 and 4). However, the removal of the radon barrier in part 5 led to an increase in indoor radon, reaching a RIR of +26%. In experiment 6, the use of a more impermeable coating resulted in a moderate radon reduction of −52% in part 2. Subsequent ventilation further decreased radon levels to −72% and −86% in parts 3 and 4, respectively. The exclusion of both the silicone sealant and air exchange in part 5 caused a substantial rise in indoor radon concentration, with a RIR of +259%.
Figure 4 and Table 2 present the results of experiments 7, 8, and 9. Experiment 7 investigated the effectiveness of PEMA (a bitumen-based membrane) in limiting radon ingress through the room’s walls. Its performance was comparable to that of material A2 previously tested in [23,24], yielding a modest RIR of −46% in part 2, followed by progressively improved reductions in parts 3 and 4 (−77% and −91%, respectively). In experiments 8 and 9, the addition of thermal coatings proved less effective than in earlier tests involving higher-performing membranes. Experiment 9 showed increased radon levels in parts 2 and 5, with RIR values of +40% and +351%, respectively. Nevertheless, parts 3 and 4 of the same experiment demonstrated a clear downward trend in radon concentration, with reductions of −60% and −78%, consistent with the patterns observed in previous experiments. A similar, though less pronounced, trend was observed in experiment 8, where the application of mineral wool provided only moderate sealing of the room, resulting in RIRs of −22% and +58% in parts 2 and 5, respectively.
Since each experiment lasted more than a week, laboratory conditions varied throughout the test, with air vents closed during the weekends. This influenced the radon levels in the model room by simulating the “flushing” effect of the wind on indoor radon with consequences on the entry rates from the walls of model room. To better investigate this effect, some experimental phases (part 1 of experiment 1, part 2 of experiment 3 and part 5 of experiment 2) were extended up to four weeks. These data are presented and discussed in the next section.

4. Discussion

4.1. The Effect of Thermal Insulation on Indoor Radon

This section addresses the impact of building exterior insulation on indoor radon levels and the compensatory effect of radon barriers placed on the interior walls of the model room, as well as the influence of air exchange.
The results of experiments 1, 4, and 7 serve as reference points for evaluating the impact of the two thermal coatings when membranes with varying radon barrier properties are applied to protect indoor environments (Figure 5). Focusing specifically on part 2 of these experiments—which isolates the effect of the membrane applied to the internal walls of the model room—the following RIR values were observed, listed from highest to lowest: −90% for the silicone sealant, −66% for the silane-terminated membrane, and −46% for the bitumen membrane.
Introducing room ventilation at different air exchange rates (parts 3 and 4 of experiments 1, 4, and 7) consistently led to a reduction in indoor radon levels. The magnitude of this effect varied depending on the membrane used: it was most pronounced with the least effective membrane (PEMA), minimal with the best-performing product (B15), and moderate with the intermediate membrane (B14). These findings suggest that highly efficient membranes may eliminate the need for costly mechanical ventilation systems, as previously noted in [23,24].
This general trend remains unchanged when thermal coatings are applied to the external walls, as demonstrated in experiments 2, 3, 5, 6, 8, and 9 (Figure 5). These results reinforce the conclusion that mechanical ventilation in newly constructed airtight buildings is effective in reducing indoor radon concentrations, in agreement with findings reported in [8]. Based on this consistent pattern, experimental parts 3 and 4 will not be discussed further.
The influence of thermal insulation on indoor radon levels is particularly evident in parts 2 and 5 of the experiments (Figure 5). Focusing first on the effect of the coatings alone (experimental part 5), both materials tend to either increase radon concentrations or leave them largely unchanged. However, the XPS coating (C2), being the more impermeable of the two, seals the system more effectively than mineral wool (C1). This is reflected in the RIR values: +161% versus −9% for sample B14 (with the negative RIR falling within the experimental error range), +259% compared to +26% for sample B15, and +351% versus +58% for the PEMA membrane. The limited air exchange associated with XPS—due to its closed-cell content exceeding 95%—appears to significantly contribute to the rise in indoor radon levels. In contrast, mineral wool, with an open-cell porosity of 98%, allows for greater ventilation and thus mitigates radon accumulation more effectively. This difference warrants careful consideration when selecting insulation materials for radon-prone environments.
An analysis of part 2 data, where a radon barrier membrane is installed within the model room (Figure 5), highlights its compensatory effect on indoor radon levels, which are consistently lower than those recorded in the corresponding part 5. Notably, the most effective membrane (B15) continues to reduce radon concentrations even in the presence of thermal insulation, achieving reductions of 84% and 52% with coatings C1 and C2, respectively. In contrast, when the membrane is absent, radon levels increase by 26% and 259%, respectively. Similar trends are observed for membranes B14 and PEMA, reinforcing the notion that the more efficient the membrane, the greater its ability to counteract the radon-enhancing effects of thermal insulation (Table 2 and Figure 5).
To quantify this compensatory effect, a new parameter—Compensatory Effect (CE)—was introduced, calculated as: ( R n 2 − R n 5 ) / R n 5 × 100 , where R n 2 and R n 5 represent the equilibrium radon concentrations in parts 2 and 5, respectively, under the C2 coating condition. CE values range from −86% for B15 to −62% for B14, further illustrating the mitigating role of high-performance membranes in controlling indoor radon levels.

4.2. Simulation of the “Flushing” Effect of the Wind on Radon Level at the Boundary Between the Room Walls and the Outside

The influence of air vents in the laboratory on radon levels during weekdays mimics the impact of wind on the removal of radon emitted in the boundary layer between the building walls, made of high radon emitting materials, and the outside.
As reported previously, during the weekend the Heating, Ventilation and Air Conditioning (HVAC) system in the laboratory, used to regulate and move heated or cooled air throughout the room, was turned off. During these periods we recorded an increase in equilibrium radon activity concentration in the model room, when membranes and/or external coatings were applied to the walls.
Figure 6, Figure 7 and Figure 8 illustrate the variations in radon concentration within the model room during part 1 of experiment 1, part 2 of experiment 3, and part 5 of experiment 2, respectively. These specific datasets were selected to highlight the differing impacts of the HVAC system under three distinct conditions: in the absence of a permeable envelope (Figure 6), with both internal and external protective layers (Figure 7), and with external cladding only (Figure 8).
Figure 6 displays indoor radon concentrations and temperature data from part 1 of experiment 1, both in the scale model room and in the laboratory. Radon levels in the model room rose during the initial 24 h, eventually stabilizing at equilibrium [23], and subsequently exhibited daily fluctuations, accompanied by a gradual decline attributed to decreasing temperatures. This temperature drop slightly reduces radon exhalation from the building material—ignimbrite—and consequently lowers indoor radon concentrations.
Due to the high porosity of the rock (43%, [21,22]), the system responded rapidly to changes in ventilation conditions. As a result, deactivating the HVAC system over weekends had no measurable impact on radon levels in either the laboratory or the model room. In this configuration, the absence of an impermeable barrier allowed radon emitted from the model room walls to disperse freely into the laboratory, even during periods of reduced ventilation. Comparable effects on radon transport have been modelled by [31], who demonstrated that wind exerts only a minor to moderate influence on radon concentrations and entry rates from materials with medium to high permeability
Figure 7 illustrates the radon concentration trends observed in part 2 of experiment 3, where a radon barrier membrane was applied to the inner walls of a model room coated with XPS. This configuration effectively isolated the room, allowing the influence of the HVAC system on indoor radon levels to be clearly identified. The nearest ventilation outlet was in the ceiling, approximately one metre from the model room.
Radon concentrations required approximately two days to stabilize once the HVAC system was activated. A recurring pattern was then observed: radon levels increased over the weekend when ventilation was turned off and subsequently returned to equilibrium during the following workweek. This cycle persisted throughout the duration of the experiment.
We hypothesize that laboratory ventilation contributed to lowering radon concentrations in the outer layers of the XPS coating by removing the high-radon zone at the interface with the room, thereby promoting outward airflow. In contrast, the absence of ventilation during weekends likely allowed the formation of a radon-rich layer at the room/coating interface, resulting in radon retention within the material and a subsequent rise in indoor radon levels. Temperature was not a controlling factor in this experiment, as it remained constant throughout the testing period.
Figure 8 reports radon levels during part 5 of experiment 2 when C1 coating was applied to the inner walls of the room. As shown in Figure 7, radon stabilized during weekdays when the air vents removed the high concentration layer at the interface with the room and increased on the weekends when the high concentration layer was re-established. This demonstrates that insulation materials can affect how radon moves through the building envelope, potentially trapping it or altering its pathways [8].
In the absence of wind, the natural ventilation of radon gas from the walls of buildings constructed with high-emitting materials is significantly reduced, leading to radon accumulation at the interface between the indoor environment and the exterior [32]. This buildup increases the likelihood of radon infiltration through existing entry points, particularly in dwellings located above ground level [33], and limits the release of radon to the outdoor air. Moreover, during periods of calm weather, the temperature and pressure gradients that typically facilitate the dispersion of exhaled radon are diminished, potentially resulting in elevated indoor radon concentrations [26].
Finally, developing a numerical or analytical model to interpret radon transport mechanisms through the wall–insulation system would be an interesting direction for future work and could represent a significant enhancement to this study.

5. Conclusions

Insulation materials, applied to the outside walls of a building to improve its thermal efficiency, can increase indoor radon concentrations, particularly if they are not permeable. It is recommended not to use these products in buildings located in areas characterized by high geogenic radon potential or made of high emissive materials.
The simultaneous installation of radon barriers on the inner walls or floor of constructions, along with appropriate air exchange, minimizes radon buildup. This attenuation is more accentuated for best performing membranes.
Additionally, radon accumulates in buildings, not only when the air exchange with the outer environment is reduced, but also in the absence of winds, creating a high radon concentration boundary layer that restricts radon exhalation from the building walls, especially when they are made of high radon-emitting materials.
In conclusion, as buildings become more airtight to save energy, with consequent accumulation of radon or other internally generated contaminants, enhancing indoor air quality becomes crucial. This can be achieved by using a passive anti-radon membrane on the walls or floors of buildings or by increasing ventilation, through supply, balanced or energy recovery ventilation systems.

Author Contributions

M.P., I.R., P.T., M.S. and G.G. were equally involved in conceptualization, methodology, formal analysis, investigation and writing. C.L. and D.V. participated in the investigation and validation. All authors have read and agreed to the published version of the manuscript.

Funding

This research was carried out in the frame of two PhD projects. The research also benefitted from funding by Dipartimento di Scienze, Università Roma Tre (MIUR—Italy, Dipartimento di Eccellenza, Articolo 1, Commi 314–337 Legge 232/2016). C.L. and D.V. provided us with the products to be tested.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors wish to thank Alessandro Rosati for his participation in the experimental part. Cristina Longoni and Dino Vasquez are acknowledged for supplying materials for the tests.

Conflicts of Interest

Authors Cristina Longoni (C.I.) and Dino Vasquez (D.V.) were employed by the company Mapei S.p.A. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Appendix A

To demonstrate that equilibrium conditions were generally achieved within 24 h—depending on the system’s tightness of single experimental parts—we applied a standardized data analysis procedure across all experiments. As an illustrative case, we used Experiment 7, analyzing the standard deviation from the mean across different segments of individual experimental phases. This example confirms that, according to our methodology, equilibrium in radon activity concentration was consistently reached and verified within the acceptable error range (Table A1).
Table A1. The mean and standard deviation of radon activity concentrations during the selected equilibrium periods of Experiment 7 (Table 2 and Figure 4) were compared with those calculated over consecutive 12 h intervals, obtained by subdividing the entire time span. This comparison demonstrated that the data remained consistent within the defined error margins.
Table A1. The mean and standard deviation of radon activity concentrations during the selected equilibrium periods of Experiment 7 (Table 2 and Figure 4) were compared with those calculated over consecutive 12 h intervals, obtained by subdividing the entire time span. This comparison demonstrated that the data remained consistent within the defined error margins.
Average and Standard Deviation of Radon Data
Part 1From 25 to 67 hFrom 25 to 36 hFrom 37 to 48 hFrom 49 to 67-
1127 ± 1241042 ± 1171158 ± 821160 ± 130
Part 2From 63 to 114 hFrom 63 to 74 hFrom 75 to 86 hFrom 87 to 98 hFrom 98 to 114 h
633 ± 57590 ± 45643 ± 56 658 ± 43624 ± 68
Part 3From 26 to 90 hFrom 26 to 37 hFrom 38 to 49 hFrom 50 to 61 hFrom 61 to 90 h
226 ± 86234 ± 101212 ± 70210 ± 72237 ± 92
Part 4From 26 to 77 hFrom 26 to 37 hFrom 38 to 49 hFrom 50 to 61 hFrom 61 to 77 h
102 ± 6265 ± 32138 ± 6798 ± 44106 ± 65

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Figure 1. Experimental configuration during phases 1, 2 and 5 (a) and 3 and 4 (b). Modified from [24]. See text for explanation. The model room is 62 cm long, 50 cm wide and 35 cm high, with a volume of 0.1 m3.
Figure 1. Experimental configuration during phases 1, 2 and 5 (a) and 3 and 4 (b). Modified from [24]. See text for explanation. The model room is 62 cm long, 50 cm wide and 35 cm high, with a volume of 0.1 m3.
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Figure 2. Indoor radon change (RIR, %) in the scale model room during the five parts of experiments 1, 2 and 3. Experiment 1 with the silane-terminated membrane alone, experiment 2 with the addition of the mineral wool coating (C1), experiment 3 with the addition of the XPS coating (C2). In all parts except phases 1 and 5, the silane-terminated polymer membrane was applied internally to the walls of the model room.
Figure 2. Indoor radon change (RIR, %) in the scale model room during the five parts of experiments 1, 2 and 3. Experiment 1 with the silane-terminated membrane alone, experiment 2 with the addition of the mineral wool coating (C1), experiment 3 with the addition of the XPS coating (C2). In all parts except phases 1 and 5, the silane-terminated polymer membrane was applied internally to the walls of the model room.
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Figure 3. Indoor radon change (RIR, %) in the scale model room during the five parts of experiments 4, 5 and 6. Experiment 4 with the silicone sealant membrane alone, experiment 5 with the addition of the mineral wool coating (C1), experiment 6 with the addition of the XPS coating (C2). In all parts except phases 1 and 5, the silicone sealant membrane was applied internally to the walls of the model room.
Figure 3. Indoor radon change (RIR, %) in the scale model room during the five parts of experiments 4, 5 and 6. Experiment 4 with the silicone sealant membrane alone, experiment 5 with the addition of the mineral wool coating (C1), experiment 6 with the addition of the XPS coating (C2). In all parts except phases 1 and 5, the silicone sealant membrane was applied internally to the walls of the model room.
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Figure 4. Indoor radon change (RIR, %) in the scale model room during the five parts of experiments 7, 8 and 9. Experiment 7 with the bitumen membrane alone, experiment 8 with the addition of the mineral wool coating (C1), experiment 9 with the addition of the XPS coating (C2). In all parts except phases 1 and 5, the bitumen membrane was applied internally to the walls of the model room.
Figure 4. Indoor radon change (RIR, %) in the scale model room during the five parts of experiments 7, 8 and 9. Experiment 7 with the bitumen membrane alone, experiment 8 with the addition of the mineral wool coating (C1), experiment 9 with the addition of the XPS coating (C2). In all parts except phases 1 and 5, the bitumen membrane was applied internally to the walls of the model room.
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Figure 5. Indoor radon change (RIR, %) in the scale model room recorded in the nine experiments. To make the graph easier to read, only RIR values for parts 2 and 5 are shown as they are the most affected by thermal coatings. See text for explanation. It is worth mentioning that experiments 1, 4 and 7 are reference tests where the three different membranes were evaluated without the coatings, therefore without part 5.
Figure 5. Indoor radon change (RIR, %) in the scale model room recorded in the nine experiments. To make the graph easier to read, only RIR values for parts 2 and 5 are shown as they are the most affected by thermal coatings. See text for explanation. It is worth mentioning that experiments 1, 4 and 7 are reference tests where the three different membranes were evaluated without the coatings, therefore without part 5.
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Figure 6. Indoor radon levels in the scale model room and the laboratory during part 1 of experiment 1. Similar daily fluctuations were recorded during weekdays and weekends when the room envelope was permeable (not covered by any type of material). The HVAC system does not affect the radon trend.
Figure 6. Indoor radon levels in the scale model room and the laboratory during part 1 of experiment 1. Similar daily fluctuations were recorded during weekdays and weekends when the room envelope was permeable (not covered by any type of material). The HVAC system does not affect the radon trend.
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Figure 7. Indoor radon levels in the scale model room during part 2 of experiment 3 (with the silane-terminated membrane and the XPS coatings). The HVAC system influenced radon levels differently during weekdays and weekends (see text for explanation).
Figure 7. Indoor radon levels in the scale model room during part 2 of experiment 3 (with the silane-terminated membrane and the XPS coatings). The HVAC system influenced radon levels differently during weekdays and weekends (see text for explanation).
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Figure 8. Indoor radon levels in the scale model room and laboratory during part 5 of experiment 2 (with the mineral wool coating). HVAC system influenced radon levels differently during weekdays and weekends (see text for explanation).
Figure 8. Indoor radon levels in the scale model room and laboratory during part 5 of experiment 2 (with the mineral wool coating). HVAC system influenced radon levels differently during weekdays and weekends (see text for explanation).
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Table 1. Experimental procedure with indication of the radon monitors used to detect indoor radon in the different parts of the tests.
Table 1. Experimental procedure with indication of the radon monitors used to detect indoor radon in the different parts of the tests.
Experiment NumberPart 1
(RAD7 and AER)		Part 2
(RAD7 and AER)		Part 3
(AER)		Part 4
(AER)		Part 5
(RAD7 and AER)
1No panelswith B14 membraneas part 2 with ACH of 0.11 h−1as part 2 with ACH of 0.44 h−1-
2No panelswith B14 membrane and C1 coatingas part 2 with ACH of 0.11 h−1as part 2 with ACH of 0.44 h−1only C1 coating and no ACH
3No panelswith B14 membrane and C2 coatingas part 2 with ACH of 0.11 h−1as part 2 with ACH of 0.44 h−1only C2 coating and no ACH
4No panelswith B15 membrane as part 2 with ACH of 0.11 h−1as part 2 with ACH of 0.44 h−1-
5No panelswith B15 membrane and C1 coatingas part 2 with ACH of 0.11 h−1as part 2 with ACH of 0.44 h−1only C1 coating and no ACH
6No panelswith B15 membrane and C2 coatingas part 2 with ACH of 0.11 h−1as part 2 with ACH of 0.44 h−1only C2 coating and no ACH
7No panelswith PEMA membrane as part 2 with ACH of 0.11 h−1as part 2 with ACH of 0.44 h−1-
8No panelswith PEMA membrane and C1 coatingas part 2 with ACH of 0.11 h−1as part 2 with ACH of 0.44 h−1only C1 coating and no ACH
9No panelswith PEMA membrane and C2 coatingas part 2 with ACH of 0.11 h−1as part 2 with ACH of 0.44 h−1only C2 coating and no ACH
B14 and B15 [24] are a silane-terminated polymer membrane and a silicone sealant, respectively; PEMA is a bitumen membrane; C1 and C2 are the mineral wool and the extruded polystyrene thermal coatings; ACH stands for air changes per hour.
Table 2. Equilibrium 222Rn and RIR values (%, in parenthesis) achieved in the scale model room in the different phases of experiments: 1, 2 and 3 with the silane-terminated polymer membrane (sample B14 in [24]); 4, 5 and 6 with the silicone sealant membrane (sample B15 in [24]); 7, 8 and 9 with the bitumen membrane (labelled as PEMA).
Table 2. Equilibrium 222Rn and RIR values (%, in parenthesis) achieved in the scale model room in the different phases of experiments: 1, 2 and 3 with the silane-terminated polymer membrane (sample B14 in [24]); 4, 5 and 6 with the silicone sealant membrane (sample B15 in [24]); 7, 8 and 9 with the bitumen membrane (labelled as PEMA).
ExperimentEquilibrium 222Rn
*Bq m−3
Part 1Part 2Part 3Part 4Part 5
11955 ± 122671 ± 122 (−66)382 ± 156 (−80)212 ± 130 (−89)-
21955 ± 122581 ± 71 (−70)266 ± 82 (−86)125 ± 67 (−94)1788 ± 114 (−9)
31955 ± 1221965 ± 189 (+1)492 ± 117 (−75)214 ± 99 (−89)5105 ± 990 (+161)
41423 ± 172148 ± 31 (−90)196 ± 76 (−86)127 ± 49 (−91)-
51423 ± 172232 ± 55 (−84)223 ± 70 (−84)169 ± 71 (−88)1788 ± 114 (+26)
61423 ± 172690 ± 120 (−52)392 ± 154 (−72)195 ± 113 (−86)5105 ± 990 (+259)
71132 ± 125614 ± 56 (−46)259 ± 104 (−77)107 ± 64 (−91)-
81132 ± 125884 ± 175 (−22)309 ± 83 (−73)190 ± 99 (−83)1788 ± 234 (+58)
91132 ± 1251582 ± 220 (+40)450 ± 114 (−60)254 ± 88 (−78)5105 ± 990 (+351)
* For details on the experiments, see Table 1. C1 and C2 are the mineral wool and the XPS thermal coatings, respectively. All radon data have been reported to a reference temperature of 23 °C. Errors are quoted as one standard deviation. The numbers in brackets are the relative changes in indoor radon (RIR, %) detected in the second, third, fourth or fifth part of each test with respect to the reference radon level in the first part.
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MDPI and ACS Style

Rocchetti, I.; Portaro, M.; Tuccimei, P.; Galli, G.; Soligo, M.; Longoni, C.; Vasquez, D. Assessing the Effect of Insulation Materials Used for Energy Conservation in Buildings on Indoor Radon—The Scale Model Room Approach. Appl. Sci. 2025, 15, 12106. https://doi.org/10.3390/app152212106

AMA Style

Rocchetti I, Portaro M, Tuccimei P, Galli G, Soligo M, Longoni C, Vasquez D. Assessing the Effect of Insulation Materials Used for Energy Conservation in Buildings on Indoor Radon—The Scale Model Room Approach. Applied Sciences. 2025; 15(22):12106. https://doi.org/10.3390/app152212106

Chicago/Turabian Style

Rocchetti, Ilaria, Manuela Portaro, Paola Tuccimei, Gianfranco Galli, Michele Soligo, Cristina Longoni, and Dino Vasquez. 2025. "Assessing the Effect of Insulation Materials Used for Energy Conservation in Buildings on Indoor Radon—The Scale Model Room Approach" Applied Sciences 15, no. 22: 12106. https://doi.org/10.3390/app152212106

APA Style

Rocchetti, I., Portaro, M., Tuccimei, P., Galli, G., Soligo, M., Longoni, C., & Vasquez, D. (2025). Assessing the Effect of Insulation Materials Used for Energy Conservation in Buildings on Indoor Radon—The Scale Model Room Approach. Applied Sciences, 15(22), 12106. https://doi.org/10.3390/app152212106

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