Summer comfort is decided at design stage, not after fitting the air conditioning

After several heatwave summers, many clients ask us the same question at our very first meeting: how to avoid overheating in their future home. Our answer sometimes comes as a surprise: the essentials are decided before the windows are even chosen. In this article, we explain why summer comfort is an architectural decision, and how we make it together with you.

Why air conditioning arrives too late

A house overheats for two simple reasons. In summer, a house overheats mainly because the sun enters through the glazing and heat builds up. Once the walls are up and the windows fitted, you can only treat the symptoms.

Air conditioning treats the consequence, not the cause. It consumes energy, makes noise and needs maintenance. It also expels heat outdoors. Passive solutions, by contrast, aim to limit heat gain, temporarily store excess heat, or help it escape. They need no compressor, no refrigerant and no significant mechanical energy.

Above all, it's a question of cost. Moving the living room on the plan at sketch stage costs nothing. Adding blind boxes to a finished façade, or reworking a roof to create an overhang, is expensive and rarely gives a good result.

Room orientation is decided at sketch stage

This is the first lever, and the cheapest. Orientation is one of the main factors in summer comfort. Here, at a latitude of around 50 degrees, the sun behaves very differently depending on the façade:

• To the south, the summer sun sits high in the sky at midday (around 63 degrees at the solstice). It is therefore easy to block with an overhang or canopy. In winter it stays low (around 16 degrees) and reaches deep into the house. This is the ideal façade for large living room windows.
• To the west, the late afternoon sun is low and hot. It arrives almost horizontally, just when the house has already built up the day's heat. This is the trickiest façade in summer.
• To the east, the morning sun is gentler. This is a good orientation for a kitchen or bedroom.
• To the north, light is stable and overheating-free. This is perfect for a study, workshop or utility rooms.

In practice, we place living spaces to the south, keep bedrooms away from the west wherever possible, and group together on the north side whatever doesn't need sunlight. On a poorly oriented plot, in a housing estate or a narrow street, this requires more finesse: shifting a volume, opening up a courtyard, playing with the section. This is where the architect's craft really comes into its own.

Catching the winter sun without suffering the summer one

A large south-facing window is an asset in winter, provided it is protected in summer. Shading must be designed together with the façade:

• Fixed shading: roof overhangs, canopies, balconies, pergolas. For a 2.20 m high south-facing window, an overhang of around one metre is enough to shade it almost entirely at midday in high summer, while still letting in the low December sun.
• Movable external shading: screens, adjustable louvres, shutters. These are essential to the east and west, where an overhang isn't enough.
• Planting: a deciduous tree provides shade in summer and lets sunlight through in winter.

One key point: shading must be external. External solar shading (blinds, screens) blocks around 80% of heat before it enters, compared with only 20% for internal shading. An internal blind protects against glare, not heat.

To integrate them properly, screen boxes are designed within the thickness of the wall and insulation, above the lintels. Planned from the outset, they disappear into the façade. Added afterwards, they stand out and often create thermal bridges.

Thermal mass, a reserve of coolness placed in the right spot

Thermal mass is the ability of heavy materials to store heat and release it slowly. Thermal mass makes it possible to store night-time coolness and release it during the day. Concrete slabs, screeds, tiles, masonry partition walls: this mass dampens heat peaks.

But this mass still needs to stay in contact with the room's air. A few design choices make all the difference:

• Avoid hiding a heavy slab behind a suspended ceiling when it isn't necessary.
• Use masonry internal walls rather than lightweight partitions in exposed rooms.
• In timber frame construction, compensate with a heavy screed, a raw earth wall or solid brick.

In renovation, this is often a hidden asset. The thick stone or brick walls of a farmhouse or old house in the Gembloux area are real coolness batteries. If you insulate from the outside, you keep this thermal mass on the inside. Internal insulation, sometimes required when a façade must be preserved, partly cuts it off. This is a trade-off we discuss with you on a case by case basis.

A house ventilated from end to end

Thermal mass is useless if the stored heat cannot escape at night. Natural ventilation must therefore be designed, not improvised. Designing continuous air paths, from the coolest side towards the outlet, releases coolness at night and limits the temperature rise during the day.

In practical terms, at plan stage, this means:

• Openable windows on two opposite façades in the main rooms (cross ventilation).
• A staircase or double height space acting as a chimney: cool air enters at the bottom, warm air escapes at the top through a roof window or a high opening.
• Openings that can safely be left open at night: tilt and turn windows, grilles, louvred shutters.
• Internal doors that don't block the flow of air.

In a presentation by the municipality of Wanze on EPB (PEB) requirements, the measures suggested to limit overheating point the same way: an openable area of more than 6.4% of the floor area for bedrooms and living rooms, cross ventilation or a chimney effect, solar glazing and shading.

What EPB checks, and what it doesn't see

For new builds and renovations requiring planning permission, the EPB (PEB) calculation includes an overheating indicator. According to the Energie de la Wallonie website, alongside the K-level and U-values, there is the overheating risk S. Regulations set a maximum value for each of these indicators. The exact thresholds depend on the region and the date the permit is submitted: your EPB assessor checks them from the preliminary design stage onwards.

Meeting EPB requirements does not, however, guarantee a comfortable house during a heatwave. This indicator is a simplified simulation. It is based on a stationary monthly method, historical climate data that does not (yet) account for climate change, and standardised occupancy assumptions. It doesn't, for example, pick up a west-facing bedroom under the roof. This is why we go beyond the regulatory calculation, room by room, with support from Optibim for EPB and special techniques.

One last pitfall: the EPB certificate for an existing house, the one required for selling or letting, tells you nothing on this point. The EPB certificate contains no numerical indicator of summer comfort. If you're buying to renovate, have the orientation, glazing and roof checked before signing.

Let's talk about your project

Are you building or renovating a house in Wallonia or Brussels and want it to stay cool without air conditioning? The best time to talk about it is before the first sketch. The STARC team, based in Gembloux, studies your plot, orientation and lifestyle with you, then translates all of this into plans. Get in touch with us for an initial meeting.

Frédéric Odent