Staying Cool: Managing Overheating in a Passive House

Comfort through a warming summer

Cornish summers are changing. Heatwaves that once felt rare now arrive most years, and homes built for a cooler climate are struggling to keep pace. For us, this is not a future problem. It shapes decisions we make today, on every project, from the first sketch.

Overheating is often treated as a technical risk to be modelled and signed off. We see it differently. It is about comfort. About being able to sleep on a warm July night without a fan running, or a bedroom door propped open for air that never quite arrives.

Much of this risk is self-inflicted. Modern homes are highly insulated built to hold onto heat, with large windows to make the most of a view. Left unmanaged, the same fabric and glazing that keeps a home warm in winter does exactly the same job in summer: letting heat in, then keeping it there.

Passive measures first

Our starting point is always passive. Before any mechanical system is considered, we ask how the building itself can manage heat, through its form, its orientation and the way it is shaded.

South-facing glazing is set back beneath deep, overhanging eaves. In summer, when the sun sits high, the eaves cast enough shadow to keep rooms comfortable. In winter, when the sun is low, the same glazing lets light and warmth flood in. It is a simple piece of geometry, calculated for the specific latitude and orientation of each site.

East and west elevations are harder to shade this way, because low morning and evening sun comes in almost horizontally. Here we turn to shutters and louvres: fixed or operable screens that filter direct sun while still allowing light and views through. They give occupants a degree of control, so a room can be closed down on the hottest afternoons and opened up again once the sun has moved round.

Plinth House has a deep recess casting shadow over the bedroom windows to keep them cool. Photography by Form Photography. Designed while at KAST Architects.

Cooling the home at night

Cornwall has one advantage most of the country does not: cool air moving in off the sea on most summer evenings. We design to make use of it.

Effective night cooling depends on cross ventilation, openable windows on opposite sides of the building, so air can move through rather than simply in and out of one opening. Rooflights positioned over stairwells are particularly useful. Warm air rises and escapes through the top of the house, drawing cooler air in at low level and setting up a gentle, steady flow through the rooms.

Done well, this purge ventilation can bring a home back down to a comfortable temperature overnight, ready for the next day, without any mechanical input at all.

Ready for more, without needing it yet

Even with careful passive design, we do not assume a home will never need active cooling. Climate projections point to hotter summers becoming more frequent across the South West, and we would rather plan for that now than retrofit later.

Where underfloor heating is installed, the same pipework can often be run in reverse to provide a gentle cooling effect. Ventilation systems can be specified with the capacity to add post-cooling to an MVHR unit at a later date, without stripping out ductwork or replanning the system. Nothing is installed unless it is needed. The home is simply left ready, so that if a client's needs change, or the climate makes it necessary, the option exists without disruption to the fabric or the finished spaces.

Valley House: tested against a real site

Valley House sits on a steep, west-facing plot, a site where overheating was a risk from the outset rather than an afterthought. Low western sun in the late afternoon and evening is difficult to shade using building form alone, and the site's own topography added another layer of complexity.

We modelled the shading cast by the valley itself, along with the seasonal shading provided by mature trees on the slope above the house, to understand how much solar gain the building would see through the year. That modelling shaped the size and position of the west-facing glazing from an early stage.

Even with this care, some risk remained on the hottest days. In response, we introduced external blinds to selected west-facing windows, a further, adjustable layer of shading that gives the occupants direct control when the sun is at its most intense. It is a good example of passive design and a simple mechanical response working together on a genuinely difficult site.

With these measures in place, the model now shows no overheating risk. As further reassurance, we specified an air source heat pump capable of running in reverse, paired with floor sensors, so the underfloor heating can also provide gentle cooling. The additional cost was minimal, and it means the clients can sleep easy on the warmest nights.

Valley House, a Passive House on a west-facing valley site in Cornwall, where overheating risk was managed from the earliest design stages. Project delivered in collaboration with KAST Architects.

A quiet kind of comfort

None of this is about chasing a number on a certificate. It is about a home that stays calm and comfortable through the year, whatever the weather does. A bedroom cool enough to sleep in on a warm night. A living space that does not need blinds drawn all afternoon just to stay bearable.

Getting there takes care from the first conversation: understanding a site's orientation, its exposure, its relationship to sun and wind, and designing around those realities rather than against them. It is what fabric-first, Passive House-informed design means in practice, a home that works quietly, in every season.

If you’re considering a new home or renovation, you can read more about our approach to design here.

Or if you’d like to talk through your own project, feel free to get in touch.

Written by David Jones, RIBA Chartered Architect and Passive House Designer, based in Cornwall.