Ventilative cooling can be very effective to reduce the cooling energy demand in buildings in summer or mid-season conditions. Although energy simulation tools can be used to assess ventilative cooling potential, finding the appropriate trade-off between accuracy and simplicity for regulatory Energy Performance methods remains very challenging.

Ventilative cooling is the use of natural or mechanical ventilation strategies to cool indoor spaces. Together with solar shading and thermal mass utilisation, it gives a range of options to address the overheating risk and avoid energy and aesthetics issues associated with the installation of active cooling units in existing buildings. Despite its effectiveness, there are significant barriers to consider this technology in regulatory energy performance assessment methods.

To address these concerns, venticool, IEA Annex 62, AIVC and QUALICHeCK supported the organisation of a 90-minute workshop during the Clima 2016 conference. Discussion was based on the analysis of answers to prepared questions asked to the audience, preceded by a series of short presentations to introduce those questions. Answers were collected instantaneously with a voting system and are briefly summarised below.

Outcomes of a survey amongst 9 countries

The major outcomes of a survey performed in September 2015 on the ability of EP-regulations to consider ventilative cooling in 9 European member states (AT, BE, DK, FI, FR, GR, IE, IT, UK) were presented. Respondents were experts contacted through the IEA Annex 62 and venticool networks. At the time of the survey, two countries (DK, IT) did not have thermal comfort criteria in their energy performance assessment method; three (IT, IE, UK) did not account for any ventilative cooling strategy in residential buildings; five had monthly calculation methods, three (AT, FI, UK) had simplified hourly methods, one (FR) had a full-year hourly method. Most countries did not have airflow models based on building parameters such as window sizes, vent characteristics, etc.

The participants confirmed their concerns about these key aspects to properly consider ventilative cooling in EP-regulations. All participants agreed that monthly methods cannot fairly consider ventilative cooling; a full-year hourly method is necessary according to 75% of the participants.

Some major elements missing

Considering ventilative cooling in an energy simulation entails interactions between physical models or input data accounting for: climatic data; building characteristics; building occupancy and operating conditions; control options and algorithms; airflow rates, heat transfer by infiltration and ventilation; and building energy need (see FprCEN TR 16798-10).

Although several standards cover all of these aspects, the participants agreed that some key elements are missing, in particular, control algorithms for airing. The majority (57%) thought worthwhile including a pick list of about 10 identifiers to define major control types and associated algorithms, similarly to what is done in the French regulation. 60% considered appropriate to use temperature limits using adaptive comfort to contain overheating risks, but the subsequent discussion suggested this should be combined with some form of penalty (e.g., increased energy use).

The participants also discussed the absence of methods to consider the draught rate and humidity but did not have clear views on how to tackle these problems.

About two thirds the participants thought that the new set of EPBD standards would at least partially be used in their country to consider ventilative cooling in the EP-regulation.

Airflow rates through airing

Part of this workshop focussed on ventilative cooling achieved with natural airflow rates through windows. The participants discussed the changes included in FprEN 16798-7 (which will replace EN 15242 in the new set of EPBD standards). The presenters gave results of cross-comparisons between models (both in cross and single-sided ventilation) and with experimental data (in single-sided ventilation). The analyses show very limited risks of overestimating airflow rates to remain conservative with ventilative cooling potential assessment.

Despite these developments, only 50% of the participants thought that their country would use FprEN 16798-7 to consider ventilative cooling through airing in their EP-regulation.

Standard or guideline developments

All participants agreed that there is a need to develop guidance for natural ventilative cooling systems, but it was not clear whether this should be done at national or European level, or whether standard or guidelines should be developed to fill this gap. Most participants (86%) also identified a clear need for trainings to improve the skills of designers to implement ventilative cooling systems.

Conclusions and perspectives

In summary, ventilative cooling is poorly accounted for in most EP-regulations in Europe. There is an array of methods in existing standards or in the new set of EPBD standards that could overcome barriers often cited, for instance, to calculate airflow rates through window airing. Nevertheless, there remain key challenges, in particular with control strategies, which need to be addressed.

 Presentations and voting results of this session are available on the REHVA website.