Performance Testing of Ventilated Windows in a Nordic Living Lab

Ventilated window systems offer a promising way to improve indoor comfort while saving energy. This study compares two systems tested in a Norwegian living lab: a basic design with PCM plates, and an advanced version combining PCM plates with an omniphobic coating and a 3D-printed acoustic absorber. Results show how design choices affect airflow, heat recovery, and comfort, with fan speed emerging as a key factor in reducing draft risk.

Keywords: Ventilated window, Living lab, Low-cost monitoring system, Acoustic absorber, Draft risk, PCM

Abstract

This study assesses the thermal, acoustic, and ventilation performance of two ventilated window systems tested in Norway’s ZEB living lab. A basic ventilated window is compared to an advanced design with an omniphobic coating and 3D-printed acoustic absorber. Using a low-cost monitoring system, results show that the advanced ventilated window lowers airflow and post-PCM thermal efficiency. Both systems demonstrate effective preheating, with a peak efficiency of 55%-70%, with fan speed identified as a key factor for minimizing draft risk and ensuring occupant comfort.

Background

The drive for nearly zero-energy buildings (nZEB) prioritizes ventilation solutions that harmonize energy efficiency with indoor air quality. Among emerging strategies, ventilated windows offer a decentralized approach by introducing outdoor air directly through the building envelope, leveraging passive solar gain and possibly phase change material (PCM) storage to precondition air and reduce reliance on active HVAC systems [1].

Adoption of such systems requires a detailed understanding of real-world performance across thermal, airflow, and acoustic domains, including their resilience under varying environmental conditions. While prior studies have explored PCM-enhanced and adaptive façades [2][3], limited research addresses the coupled behaviour across different domains introduced by fan-assisted ventilated windows.

This study investigates two ventilated window prototypes, basic and advanced, installed at the ZEB living lab in Trondheim, Norway. Performance is evaluated using a low-cost, field-deployable monitoring system, capturing continuous airflow, temperature, sound, and light data. The system, built on a Raspberry Pi platform and calibrated low-cost sensors, follows the design methodology proposed by ref. [4]

Ventilated window design

Both window types utilize a dual-fan configuration to introduce outdoor air via the glazing cavity and preheat it before delivery indoors. In the basic ventilated window (VW), lower fans draw air into the cavity, while upper fans propel it through a phase change material (PCM) module housed in an upper compartment, enabling passive thermal gain. The advanced ventilated window (AVW) builds upon this with two key enhancements:

·         Omniphobic coating on the external glazing improves resilience to rain, dust, and environmental degradation.

·         3D-printed acoustic absorber within the PCM box reduces fan noise, addressing a common complaint associated with mechanical ventilation systems.

Figure 1 illustrates the configuration differences, underlining how functional upgrades contribute to thermal efficiency and acoustic performance.

Figure 1. Comparison of basic (VW) and advanced ventilated window (AVW) designs.

Methodology

Monitoring system architecture

To monitor the performance of the ventilated window prototypes, a cost-effective and scalable data acquisition system was deployed, cantered around a Raspberry Pi microcontroller. The system incorporated the following sensor suite:

·         OMRON D6F velocity sensors to capture air speed through the window cavity.

·         DFRobot SEN0232 analogue sound level sensor to measure noise emissions associated with fan operation.

·         MCP9808 digital temperature sensors to monitor inlet, cavity, and supply air temperatures.

·         BME280 digital temperature sensors to monitor indoor and outdoor air temperatures.

·         TSL2591 light sensors to assess incident solar radiation by measuring illuminance, with a calibration procedure established to correlate these quantities.

All sensors were linked to the Raspberry Pi for continuous data logging and real-time monitoring, with custom Python scripts handling acquisition, timestamping, and storage. All sensors were calibrated against laboratory-grade reference instruments before deployment to assure measurement accuracy. The Testo 400 IAQ Kit was used to benchmark airflow velocity, ambient air temperature, and light intensity readings, ensuring consistency across environmental conditions. For acoustic validation, the Nor145 sound level meter was employed to verify the sound measurements obtained from the low-cost analogue sensor.

Experimental protocol

Performance testing involved controlled variation of fan speeds (25%, 50%, 75%, and 100%) in both bottom and top fans, resulting in 16 configurations per window unit. Simultaneous airflow and noise measurements enabled comprehensive performance mapping, including the AVW’s acoustic enhancements. Additionally, several days of monitoring were conducted to evaluate real-time thermal efficiency during preheating air supply mode with 100% fan speed. From these, one representative day, characterized by stable outdoor temperatures between 10°C and 15°C, was selected for detailed analysis due to its optimal conditions for assessing preheating performance. Preheating efficiency was calculated before and after the PCM box using the following formulations [5]:

Furthermore, a smoke test was conducted to visualize the air jet’s trajectory into the room, while a set of anemometers was used to measure airflow velocity at occupant level, helping to assess potential draft risk and occupant comfort.

Results and discussions

Airflow rate performance

Figure 2 presents a comparative analysis of airflow rates and sound levels for the basic ventilated window (VW) and the advanced version (AVW) across four fan speed settings. As expected, the VW consistently delivered higher airflow, reaching over 140 m³/h at full speed, compared to approximately 70 m³/h for the AVW. This reduction in airflow is primarily attributed to the added flow resistance introduced by the 3D-printed acoustic absorber in the AVW. Given that the floor area the air is delivered to is 23.2 m², with 100% fan intensity, the ventilation rates are 0.815 l/(s·m²) for the AVW and 1.713 l/(s·m²) for the VW, which align with Category II for the AVW and Category I for the VW, according to the design ventilation rate standards for low-polluting buildings.

Despite this trade-off, the AVW exhibits better acoustic performance. Across all fan speeds, the AVW maintained noticeably lower sound levels, with a reduction of approximately 2-3 dB compared to the VW at each operating point. This confirms the absorber’s effectiveness in noise attenuation.

Figure 2. Airflow rates and noise level for VW vs. AVW at varying fan speeds.

Thermal performance over preheating mode

Figure 3 illustrates solar irradiance and corresponding air temperature profiles over a full-day period, selected for its favourable outdoor conditions (primarily between 10-15°C) to evaluate preheating performance of the ventilated window systems. In the left plot, both windows exhibit similar irradiance transmission patterns, suggesting that the omniphobic coating on the AVW has a negligible effect on solar gain under these conditions. The right plot compares indoor (T_in), outdoor (T_out), and supply air temperatures (T_supply) for both window types. Both VW and AVW demonstrate effective preheating behaviour, with their supply air temperatures exceeding outdoor levels during peak solar periods. However, the VW consistently achieves slightly higher supply temperatures than the AVW during the daytime.

Figure 3. Variation of solar irradiance and air temperature over a one-day measurement period.

Figure 4 illustrates the preheating efficiency (η_PH) of both ventilated window systems over a full day, calculated at two points in the airflow path, before and after the PCM box. The left plot reflects the effect of solar irradiance on the cavity air. Both systems follow similar efficiency trends in this region, with the AVW exhibiting slightly higher values during peak solar hours, suggesting a marginally greater heat capture in the cavity. The preheating efficiency of AVW and VW systems may reach approximately 55% before PCM box. The right plot, showing post-PCM box efficiency, reveals more pronounced differences. The VW system achieves higher preheating efficiency most of times, particularly during daytime hours. This trend likely results from the reduced airflow rate in the AVW, caused by the inclusion of the acoustic absorber, which in turn limits the volume of air interacting with the PCM plates and consequently lowers the heat transfer coefficients between plates and passing airflow. After PCM box, the preheating efficiency of AVW and VW systems may reach approximately 70%. While the AVW improves acoustic performance, this design trade-off appears to compromise heat exchange capacity after the PCM stage, resulting in lower thermal efficiency.

Figure 4. Preheating efficiency of AVW and VW systems over a one-day measurement period.

Draft risk assessment

This section evaluates the draft risk associated with the AVW system at varying fan speeds (25%, 50%, and 100%). The left side of Table 1 displays smoke test images visualizing the air jet penetration across the room.

Table 1. Visual inspection of air jet length and measured supply air velocity (m/s) for AVW.

As expected, higher fan speeds result in longer jet trajectories, indicating deeper air delivery into the occupied space. On the right, measured air velocities are presented in tabular form at different distances from the supply diffuser and at multiple heights above the floor. At 170 cm, representative of the occupied zone height, air velocity exceeds 0.2 m/s at 100% fan speed, as highlighted in red, suggesting a potential for draft discomfort. In contrast, at reduced fan speeds, the velocities reduced, thereby minimizing draft risk and enhancing thermal comfort for occupants.

Conclusion

This study investigated the performance of two ventilated window prototypes, basic ventilated window (VW) and advanced ventilated window (AVW), installed in a Norwegian living lab, focusing on airflow rate, thermal preheating efficiency, and draft risk. The AVW, equipped with an acoustic absorber and omniphobic coating, demonstrated superior noise reduction but at the cost of reduced airflow and post-PCM thermal efficiency. While both systems effectively utilized solar gain for preheating, the VW consistently delivered higher supply temperatures and preheating efficiency due to greater airflow. The preheating efficiency of AVW and VW systems may reach approximately 55% before PCM box, while the preheating efficiency may be increased to about 70% after the PCM box. Draft risk analysis further highlighted the need to optimize fan speeds, as higher settings may compromise occupant comfort. These findings underscore the practical trade-offs in ventilated window design and provide valuable guidance for balancing energy performance and indoor comfort in low-energy building applications.

Acknowledgement

The European Union's Horizon 2020 Research and Innovation Programme, under the iClimaBuilt Project, Grant Agreement No: 952886, has contributed to the funding for this experimental study.

References

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