The Breath of Efficiency: Ventilation in Passivhaus Design

23 January 2024

In the realm of Passivhaus design, achieving optimal ventilation is not just a luxury but a fundamental necessity. In two insightful seminars featuring experts Andrew Clapton of Total Home Environment and Paul McGurk of the Green Building Store, the critical role of ventilation in Passivhaus construction emerged as a cornerstone for both energy efficiency and occupant comfort.

There is an intricate dance between ventilation and heat recovery in Passivhaus design, which emphasises the need for careful consideration in house design and domestic hot water requirements, for which heat recovery ventilation (MVHR) plays a pivotal role. However, precaution must be taken to avoid the "cling film effect" – a paradox where good air tightness can lead to poor indoor air quality. When key Passivhaus principles are achieved: air tightness, good insulation and adding well sealed triple glazed windows, the risk is that lack of air input leads to poor air quality.  A mechanical ventilation system, then, is critical and a system that pre-heats incoming air is sensible.

The MVHR system, with its internal heat exchanger, is capable of extracting stale air at 21°C and reintroducing fresh air at 17°C, ensuring a comfortable and consistent indoor environment. To further enhance efficiency, one can introduce the concept of Heat Pump Ventilation (HPV) with multiple supply zones, not only providing heating but also serving hot water needs. The air-to-water heat pump's ability to supply water at temperatures ranging from 45 to 50°C adds a layer of versatility to Passivhaus systems.

Addressing the importance of summertime comfort cooling, the systems are able to reduce the temperature of the air supply to a range between 10 and 15°C, ensuring a pleasant living environment. Practical considerations, such as the use of metal ducting for its robust mouse-proof and antibacterial properties, along with the strategic placement of open-web joist floors, contribute to effective ventilation design.

Designing for optimal ventilation involves considering various factors, including house orientation, prevailing wind, and the placement of intake and extract vents. Overhangs, and shading should be considered to mitigate solar overheating. Trickle vents and extract fans should be omitted and replaced with circulating cooker hoods plus 10mm undercuts to internal doors to ensure effective air circulation.

A fabric-first approach is crucial, requiring meticulous planning. This involves incorporating a plant room in the design, conducting room-by-room heat loss appraisals, and utilising 3D CAD ventilation designs. To streamline the construction process, early ordering and collaboration with other trades are essential and highlighted through Gantt charts to illustrate the sequential requirements of supplies.

Michael Holliday of Palmer & Partners echoed the importance of a fabric-first approach while delving into the economic considerations of Passivhaus design. By omitting certain elements, such as underfloor heating systems, upfront costs can be reduced, allowing for an approximate 10% increase in the initial build budget. Holliday emphasised the site-dependent nature of construction methods, urging a shift away from purely aesthetic considerations.

Creating a comfortable living space involves recognising subtle nuances, like a two-degree temperature difference between the air around the neck and the feet, which can result in discomfort due to air movement. Here, a heat pump ventilation (HPV) unit emerges as a versatile solution, combining the functions of a heat pump and MVHR, while also providing hot water.

In conclusion, the importance of ventilation in Passivhaus design cannot be overstated. A well-designed ventilation system not only ensures optimal indoor air quality but also plays a pivotal role in achieving energy efficiency and substantial long-term savings. As architects and builders embrace these principles, Passivhaus evolves into more than just an architectural standard—it becomes a mechanism where all the component parts work together in harmony to create a symbiotic, comfortable, sustainable living environment.