The new European Technical Specification FprCEN/TS 18335:2026 defines a nine-step design process for natural, mechanical and hybrid ventilative cooling, prioritising prevention, modulation and free cooling before considering any form of active cooling.
Annamaria Belleri, Eurac Research | LinkedIn profile
Christoffer Plesner, VELUX A/S | LinkedIn profile
(Note: Opinions in the articles are of the authors only and do not necessarily reflect the opinion of the European Union)
Buildings across Europe are getting hotter, and not just in summer. A survey involving over 27,000 Europeans found that four out of five respondents had experienced at least one climate-related impact in the last five years. Heat was the most common of them: nearly half of the respondents felt too hot at home, and over 60% felt too hot outdoors in their neighbourhoods. More than one-third of respondents said they could not afford to keep their home adequately cool in summer, rising to two-thirds among those already struggling financially. Yet the same survey found no single household thermal resilience measure — from solar shading to insulation to air conditioning — in place in more than half of the homes surveyed, pointing to a clear gap between rising exposure to heat and the pace of climate change adaptation.
The usual fallback, air conditioning, comes with a cost: UNEP's 2025–2026 Global Status Report for Buildings and Construction notes that energy demand for space cooling in buildings has already grown 70% since 2015, and warns that passive cooling measures remain ‘critically underrepresented’ in building legislation and standards worldwide. Of the countries it reviewed, only 19 countries mandate the use of solar shading on windows, which is a key overheating mitigation strategy for preventing solar heat gains from outside.
Ventilative cooling, using ambient (outdoor) air, driven naturally, mechanically or through a hybrid of the two (e.g., using openable automated windows, without any mechanical pre-cooling), is one of the most accessible passive cooling strategies. It is a renewable source for cooling using ambient air, where Delegated Regulation (EU) 2022/759 establishes a precise and legally binding boundary for what counts as renewable cooling from ventilation systems. Yet, it has so far lacked a dedicated European design standard or specification, which is a real regulatory gap. A 2026 AIVC survey of building codes across 19 countries found that fewer than half explicitly include ventilative cooling as an overheating mitigation strategy. Only two countries use the term ‘ventilative cooling’ at all in their national legislation. Just five countries explicitly include resilience checks for overheating.
FprCEN/TS 18335:2026 ‘Ventilative cooling systems – Design’, developed by CEN/TC 156/WG21 (European Committee for Standardization), fills that gap. It has now been out for a formal vote for three months across European countries, with a conclusive vote, meaning the specification will be published in Q1 2027. This article introduces the new European Technical Specification, explaining its scope, its practical relevance for architects, engineers and building energy consultants, and how its methodologies are fully aligned with the Energy Performance of Buildings Directive (EPBD), facilitating seamless integration into national legislation and compliance tools.
At the heart of the new European specification sits the cooling ladder, a design ethos that structures the entire document. Rather than treating ventilative cooling as one option among many, the cooling ladder sequences the designer's decisions into four steps: first, prevent and protect against unwanted heat gains; then modulate them; then dissipate the remaining heat load (with ventilative cooling as the primary dissipation technique); and only afterwards, turn to supplementary free cooling (evaporative or ground cooling, for instance), and, as a last resort, active mechanical cooling. This logic mirrors the cooling hierarchy already adopted in the Greater London Authority's energy assessment guidance or in the sustainable cooling hierarchy in UNEP’s Global Cooling Watch 2025. It gives designers a concrete decision sequence.
Figure 1. The cooling ladder sequence. Source: chart created by the authors with the assistance of AI.
For designers, this matters because it moves ventilative cooling upstream in the design process. Orientation, glazing ratios, solar shading, thermal mass, and building form — all decisions typically made at the conceptual design stage, long before an engineer is appointed — directly determine how much of the cooling load can be handled by ventilative cooling. This European specification is explicit that prevention and modulation measures should be revisited iteratively whenever the ventilative cooling potential falls short of comfort targets, before any supplementary or active system is added to the design.
The European specification organises the whole design process into two stages — feasibility and detailed design — separated by a mandatory thermal resilience check. The thermal resilience check draws on REHVA's resilient cooling design guideline, asking not only whether a building is comfortable today, but whether it will still perform during future heatwaves and power outages. The building design is tested under heat-stress conditions, which might be short-term (specific heatwaves) or long-term (typical meteorological years for future weather scenarios).
In the feasibility stage, the designer works with limited information (building type, use, location, general envelope characteristics) and applies a simplified evaluation method to establish how far ventilative cooling alone can go. Once a conceptual design is agreed upon, but before detailed engineering begins, the specification requires a dedicated thermal resilience assessment: a bridging step that checks whether the chosen cooling strategy could fail under more extreme or future weather conditions before those choices become too expensive to reverse. Only then does the process move into detailed design, where dynamic building energy simulations analyses support the sizing of the final ventilative cooling system.
This structure is one of the more consequential contributions of the new European specification for engineers and consultants: it makes thermal resilience an explicit, auditable checkpoint rather than an afterthought bolted on at the commissioning stage. The document defines thermal resilience through four criteria:
Each of these can be evaluated using multi-year, future-weather-file assessments. The specification points designers to a set of associated thermal resilience and comfort KPIs to support the check, where inspiration was taken from IEA EBC Annex 80 on Resilient cooling. For consultants advising clients on climate-proofing a portfolio, this offers a structured, standardised way to demonstrate — rather than merely assert — that a building's cooling strategy will still perform in the 2050’s heatwave scenarios.
Underpinning both stages are nine key design steps that any design process for natural, mechanical, and hybrid ventilative cooling systems in both new and existing, residential and non-residential buildings is expected to work through:
Figure 2. Nine key design steps for feasibility assessment of ventilative cooling strategies. Source: FprCEN/TS 18335: 2026.
Following the nine key design steps produces a well-founded cooling concept that can later be verified with detailed simulation — but the real value lies in reaching a design methodology that is both credible enough to trust and simple enough to fold into legislation and compliance tools.
The European specification also formalises the ventilative cooling potential method (VCPM), a simplified early-design-stage tool for estimating ventilative cooling capacity and design airflow rates for ventilative cooling systems. The ventilative cooling potential analysis is based on a methodology developed within the International Energy Agency (IEA) Annex 62 project with the aim of assessing in the early design stages the potential effectiveness of ventilative cooling strategies by considering building envelope thermal properties, occupancy patterns, internal gains and ventilation needs. The calculation methodology is integrated into Annex H of the European Technical Specification and is available to designers in the venticoolpy Python library and in a free online tool, both developed by Eurac Research, Italy.
VCPM is a simplified, hourly calculation method built around a lumped-parameter (resistance-capacitance) model of a reference thermal zone, aligned with the dynamic calculation approach of EN ISO 52016-1. It requires only the kind of information available early in a project — envelope thermal transmittance, thermal mass, shading presence, occupancy patterns and required IEQ comfort category — and returns, hour by hour, whether ventilative cooling can meet thermal comfort needs, what airflow rates are required to keep indoor temperatures within the comfort ranges, and the resulting sensible cooling energy needs with and without ventilative cooling applied.
Figure 3. Output example of the venticoolpy library showing the distribution of ventilation mode over the year for an Irish school classroom. Mode 0 – No ventilative cooling required, because heating is needed; Mode 1 – Natural ventilation can be exploited at minimum rates, i.e. just to meet the minimum IAQ ventilation rate (per EN 16798 1); Mode 2 – Ventilative cooling is needed and effective: increased ventilation rates (beyond the IAQ minimum) can be used to maintain indoor conditions within the adaptive comfort range; Mode 3 – Ventilative cooling is not useful during occupied hours (outdoor air can't offset the load, e.g. too warm/humid or insufficient driving force). Source: venticoolpy library.
A worked example of the method in practice is documented in the Ventilative Cooling Guide developed within the HeriTACE project, which applied the ventilative cooling potential method to a historic townhouse in Mantua, Italy. The ventilative cooling potential analysis resulted in a design airflow of around 830 m³/h in the building's most overheating-prone room, an upper-floor kitchen. This airflow was used to size the outlet of a historic wind tower sitting above the building's central staircase, turning a disused architectural feature into a stack-ventilation exhaust without altering the street-facing façade. A subsequent coupled thermal–airflow-network simulation validated the VCPM-based design: the intended airflow direction — inflow at the occupied rooms, exhaust at the tower — held for essentially the entire ventilative cooling season, and the target airflow rate was met for the large majority of night-time hours and a solid majority of daytime hours, meaningfully cutting the building's overheating exposure. For the full worked example, including the opening-sizing method and detailed simulation results, please refer to the linked document.
The European technical specification's methodology aligns closely with the direction of the 2024 recast of the Energy Performance of Buildings Directive (EPBD), which looks beyond emissions reduction to occupant well-being, resilience against risks like heatwaves, and reduced energy dependency through adaptive design. Passive solutions — orientation, shading, thermal mass, natural ventilation — take on a strategic role in this framework, mitigating cooling demand as temperatures rise, with active systems reserved for when these measures fall short.
The technical specification also explicitly maps its own dependencies against five categories of related EPB standards: indoor environmental quality criteria (EN 16798-1), building energy needs calculations (EN ISO 52016-1), building automation and control (EN ISO 52120-1), airflow rate calculation (EN 16798-7) and ventilation energy calculations (the EN 16798-5 series for mechanical and hybrid systems).
Ventilative cooling can, in principle, be plugged into existing EPBD-derived calculation chains. Finally, the document addresses how the additional airflow used for ventilative cooling should be treated under the EU's renewable energy accounting rules. Following the 2022 amendment to Annex VII of the Renewable Energy Directive, the ambient air used for cooling above standard ventilation rates is recognised as an ambient renewable energy source, provided the system is deliberately and intentionally operated (whether by schedule or sensor control) rather than incidentally. A dedicated Annex K sets out the calculation methodology for quantifying this renewable cooling contribution, while flagging that each Member State will still need to define its own national EPBD calculation approach for accounting for the additional airflow. For consultants working on nZEB or renewable-energy-share compliance calculations, this is a direct, quantifiable link between a passive design measure and a regulatory renewable energy target — something that has, until now, been largely absent from national compliance tools.
It is worth being precise about the European specification's boundaries. It does not set normative requirements for supplementary free cooling systems or active cooling systems — these are addressed only informatively, through their interfaces with ventilative cooling. It covers temperature control only, explicitly excluding moisture control and heat loss calculations. It treats acoustic comfort, draught, outdoor air quality, safety, security and maintenance as design aspects to be considered rather than as normative criteria with defined thresholds. The specification sets a clear design framework, supporting the passive-first approach in which a holistic building design can be made with clear evaluation points along the design process, increasing the chance of making buildings less reliant on energy-intensive active cooling systems, while having a low carbon footprint and embodied carbon.
With Europe's building stock ageing, largely uncooled and increasingly exposed to more frequent and severe heatwaves, the absence of a common European reference for designing ventilative cooling has long been a gap in the standards landscape, filled until now by a patchwork of national guidance, research projects and individual practice. This CEN/TS closes much of that gap. It gives architects an early-stage design logic through the cooling ladder; it gives engineers a validated, lightweight calculation tool for ventilative cooling potential alongside detailed sizing guidance for natural, mechanical and hybrid ventilative cooling systems; and a resilience-checking framework and a direct route to quantifying renewable cooling contributions under EU rules.
The specification is designed from the outset to slot into national EPBD transposition and compliance software. Finally, the methodologies and design steps from the specification can quite easily be adopted into national standards, guidelines, and legislation, as the framework is quite forward-looking and fully compliant with the EPBD.
