Passivhaus Principles
The UK is seeing more extreme weather on the back of human-caused climate change. Winters bring flooding. Summers, excessive heat. And trying to keep your house cool in 30 degrees plus isn’t always easy!
When heatwaves strike, the temptation is to panic. Buy an air conditioning unit, dig out an electric fan, or save up and spend thousands on retrofit insulation.
One of the world’s most rigorous sustainable construction standards – Passivhaus – suggests another option.
This innovative system, uses smart, passive principles to keep houses cool, rather than energy hungry gadgets. Passivhaus buildings maintain stable, comfortable indoor temperatures all year-round by controlling thermal energy flow, managing solar radiation, and maximising ventilation.
The good news? You don’t have to spend a small fortune on a retrofit programme to keep your house cool. Just follow these Passivhaus principles and you’ll get some of the way there.
With a basic understanding of heat transfer and zero budget, you can hack your current home to keep it cool over the hot summer months.
1. External Shading Hacks: Stop the Sun Before It Hits the Glass
The core rule of Passivhaus summer comfort is external solar shading. Once sunlight gets through your windows, the solar radiation converts into heat inside your room. Trapping it like a greenhouse. Closing interior blinds and curtains helps, but the heat is inside your home.
To achieve zero-cost Passivhaus-style shading:
- The External Shower Curtain Trick: Take a cheap or spare shower curtain, an old light-coloured bedsheet, or a reflective space blanket, and hang it on the outside of your south- or west-facing windows or glass doors. Securing it on the exterior stops up to 80% of solar radiation from ever touching the glass.
- Cardboard Shuttering: You can also tape light-coloured cardboard panels to the outside frames – to create an instant thermal barrier.
2. Master “Night Purging” & Dynamic Ventilation
In a certified Passivhaus, a Mechanical Ventilation with Heat Recovery (MVHR) system manages air movement. Without an MVHR, you can replicate air exchanges manually through Purge Ventilation. But you need to know exactly when to seal your home and when to dump hot air.
Daylight Lock-Down:
- When it’s warmer outside, close everything down: The moment the outdoor temperature rises above your indoor temperature (usually about 9.00 am), shut every window, exterior door, and trickle vent. Seal the cool air inside.
The Night-Time Purge:
- Open up when the temperature drops: Once the evening air is colder than indoors, open windows on opposite sides of the house. This creates cross-ventilation pulling the hot air out.
- Harness stack ventilation (the chimney effect): If you live in a multi-floor home or flat, open windows on the ground floor and top floor at the same time. Warm air rises and escapes through the top windows, drawing cooler air through from the bottom of the house.
3. Minimise Internal Heat Gains
A Passivhaus is so well-insulated that internal heat sources – appliances, body heat, and electronics – play a major role in room temperature. During a heatwave, eliminate internal heat:
- Ditch the oven and hob: Cooking creates massive internal heat gains. Opt for raw meals, cold salads, or use a microwave (which generates far less ambient heat).
- Unplug phantom electronics: TVs, game consoles, laptops, and chargers emit continuous radiant heat when plugged in on standby. Turn them off at the wall.
- Drying clothes: Avoid running tumble dryers if you possibly can. Instead, hang damp laundry indoors near open evening windows – the evaporating water creates a natural evaporative cooling effect, dropping local air temperatures slightly.
4. Manage Thermal Mass and Interior Zoning
Traditional UK homes – especially Victorian brick terrace blocks – contain high thermal mass (materials like brick, concrete, and heavy plaster that absorb and store heat).
- Protect mass during the day: Keep internal doors closed between hot, sunlit rooms and shaded, cooler rooms. This prevents heat from migrating across the floor.
- Cool the mass at night: Leaving internal doors open only during night purging allows the cool night air to flush out the heat stored in your internal walls and floors overnight.
Small Shifts, Huge Impact
Applying passive cooling isn’t just about surviving a heatwave in comfort – it’s a practical lesson in understanding energy. By replacing energy-intensive appliances with natural physics, you cut your carbon footprint, eliminate electricity, and make your home significantly more resilient.
Want some other hacks for keeping your house cool? Our founder, Anne Snelson, posted these tips on Linkedin.
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