Keeping your house cool without air conditioning really is possible, and the difference is bigger than you think. With the right approach you keep your home 5 to 8 degrees cooler than outside. It comes down to three things: keeping the sun out before it gets in, venting the heat at night, and limiting the heat you make yourself.
That order matters. By far the biggest gain is in sun shading. Exterior shading keeps out 80 to 95 percent of the sun’s heat. Once the heat has come in through your glass, you have a very hard time getting it back out. So prevention is everything.
After that comes the night strategy: when it’s cooler outside than inside, you throw everything open and flush the heat out. In practice that’s worth up to 5 degrees on its own. And finally you limit the heat you produce inside: oven, dryer, appliances on standby, bright lamps.
Do these three things together and it stays pleasant inside while it’s baking outside. No air conditioning, no big power bill. Below I explain exactly what gives what, which windows you open when, which tips are old wives’ tales, and what to watch out for when there are vulnerable people in the house.
How do you keep your house 5 to 8 degrees cooler without AC?
There is no single magic trick. It’s the combination that does the work. But you hold back the biggest chunk of heat with one thing: making sure the sun doesn’t get into your house.
The sun shining through your windows is by far your biggest source of heat in summer. Glass works like a greenhouse. The light comes in, turns into heat, and that heat stays trapped inside. A room facing south or west can easily get 5 to 10 degrees warmer than outside because of this.
That’s why the number one measure is: keep the sun out, on the outside of the glass. After that you ventilate smartly during the cool hours, and finally you limit your own heat production.
Here’s what the complete approach looks like:
- Keep the sun out with exterior shading, closed curtains or window film. Biggest effect.
- Vent at night and early in the morning when it’s cooler outside than inside.
- Keep everything shut during the day, windows and shading, as soon as it gets warmer outside than inside.
- Limit internal heat: oven off, dryer off, appliances off standby, bright lamps off.
- Cooling on the skin with a fan (used sensibly) and a damp cloth.
I go through each of these steps below with concrete numbers, so you know what gives the most and where to put your energy.
Why is sun shading the most important thing against heat?
Because you hold back heat before it gets into your house. And everything you keep out, you don’t have to deal with inside anymore.
Quick but important distinction, because this is where it often goes wrong. Interior shading and exterior shading are not the same, and the difference is enormous.
Exterior shading hangs on the outside of the glass. Think of screens, roller shutters, exterior blinds, awnings or shutters. The sun is stopped before it hits the glass. This keeps out 80 to 95 percent of the sun’s heat. This is by far the most effective measure there is.
Interior shading hangs on the inside: curtains, roller blinds, slats. The problem: the sun has already come through the glass and the heat is already inside. Blackout curtains still keep out 35 to 59 percent of the heat. Better than nothing, but nowhere near as good as exterior shading.
The rule is simple: the further out you stop the sun, the cooler it stays inside. A closed screen outside beats the finest blackout curtain inside with ease.
Don’t have exterior shading and don’t want to invest right away? Then solar window film is a cheaper interim solution. You stick it on the glass and it holds back part of the heat radiation. Or go for solar control glass when you replace: then check the g-value, which tells you how much solar heat comes through. A g-value of 0.4 or lower means at most 40 percent of the solar heat gets in.
And don’t forget the simplest thing: just close your curtains on the sunny side during the day. That alone is worth 2 to 3 degrees.
Which windows do you open when (and shut)?
This is where most people get it wrong. They open the windows during the day to let in “fresh air”. But during the day the air outside isn’t fresh, it’s hot. You’re letting the oven in.
The rule is rock hard: windows open when it’s cooler outside than inside, windows shut when it’s warmer outside than inside. That’s all there is to it.
In practice that means:
At night and early in the morning (roughly 10 pm to 8 am): windows wide open. Now the outside air is cooler than your house, and you flush out yesterday’s heat. Open windows on opposite sides so you get a draught, right through the house. That’s called cross ventilation and works ten times better than one window on a crack. Fully open, not on the tilt setting.
During the day (roughly 8 am to 10 pm): everything shut. Windows shut, shading down. Your house is now a closed, cool cave. Every time you open a window for the “fresh air”, you let heat in.
A clever detail: warm air rises. At night, open the windows upstairs especially, and downstairs on the other side. Then the warm air up top draws out and the cool air down below gets sucked in. That gives you a natural chimney effect through your whole house.
And the loft deserves special attention. Heat collects right at the top. If you have a skylight or a hatch in the loft, open it at night so the hot air there can escape. Shut and shaded during the day.
How much does night ventilation really save?
More than you think. Good night ventilation lowers the indoor temperature by up to 5 degrees. On its own already.
The idea is that you let the stored heat run out of your house at night, so you start cool in the morning. The cooler your house is in the morning, the longer it takes to heat back up during the day.
This works best in houses with a lot of mass: thick stone walls, concrete floors. That mass takes on the coolness at night and gives it off slowly during the day. More on that below under thermal mass.
A few things that make night ventilation better:
- Make a real draught. One window open does little. Two windows on different sides of the house: then the air flows.
- Use the height difference. Window open downstairs plus a window or hatch upstairs, and the warm air draws up and out by itself.
- Fan as a helper. Put a fan in front of an open window blowing outwards, and you actively push the warm air out. Or pointed inwards to draw cool night air in.
- Shut in time. As soon as it gets warmer outside than inside in the morning, everything shut. That moment is often around 8 or 9. Don’t wait too long.
Mind your safety: open windows at night can be a burglary risk. Tilt setting with a bolt or window restrictor, or only windows upstairs, keeps it safer.
What is thermal mass and why does it keep you cool longer?
Thermal mass is your house’s ability to store heat and give it off slowly. And it’s exactly why an old, thick-walled stone farmhouse stays cool while a light, new home heats up within a day.
Think of it as a sponge for heat. Thick, heavy materials (brick, concrete, clay) take on the heat slowly during the day and only give it off again in the evening and at night. Light materials (plasterboard, wood, thin walls) heat up fast and cool down fast.
For summer this means: a house with a lot of mass lags behind the outside temperature. If it’s 35 degrees outside at three in the afternoon, that peak only arrives inside hours later, when it’s already cooling down outside. And then you vent it out again at night.
This is called phase shift: the time heat needs to work its way through something. With insulation material this makes a big difference:
- Wood fibre or cellulose: phase shift of 10 hours or more. The midday heat only arrives in the middle of the night.
- PIR or polystyrene: phase shift of 3 to 6 hours. The heat comes through in the afternoon already.
For winter it insulates just as well, but in summer a heavy, slow insulation material is much more pleasant. If you’re going to insulate your loft or roof, keep this in mind.
Important to know: thermal mass only works if you keep the direct sun out. As soon as the sun shines in through your window and falls on that mass, it heats it up instead. Mass and sun shading belong together.
What does a fan do and not do (and when does it get dangerous)?
Let’s clear up a misconception: a fan does not cool the air. Not a single degree. What it does is blow air past your skin, and that makes your sweat evaporate faster. That feels cooler. For you, not for the room.
That means two things. First: leaving a fan running in an empty room is pointless. It cools nothing. Switch it off when you leave, that saves power.
Second, and this is important: above 35 degrees inside a fan can become dangerous. Then it blows hot air against you, speeds up your dehydration instead of cooling you, and works against you. In extreme heat the fan is not your friend. Then it’s better to wet your skin and rest.
The well-known ice trick does work, but modestly. Put a bowl of ice cubes or a frozen bottle of water in front of the fan, and the air passing by cools a little. The difference is small and local, fine for right next to your sofa or bed, but it doesn’t cool a whole room. Don’t expect miracles from it.
Smarter is the damp cloth. Hang a wet towel in front of an open window where a breeze passes by, and the evaporating moisture cools the incoming air. Or lay a cool, damp cloth on your neck or your wrists. That works directly on your body, and that’s what it’s about.
If you go for a fan, choose a quiet one with a DC motor. That uses up to 90 percent less power than air conditioning and makes less noise, nice when it runs at night.
Why should you dry the washing outside and leave the oven off?
Because every appliance that makes heat helps heat up your house. And in a house kept shut, that heat stays trapped.
Just count up what you produce in heat indoors on a hot day:
- The oven heats up your whole kitchen and warms the surrounding rooms too. On hot days eat cold: salad, bread, dishes without oven or hob.
- The tumble dryer literally blows warm, moist air into your house. On hot days just hang your washing outside in the sun, that dries faster and for free.
- Appliances on standby make a bit of heat all day long. TV box, games console, chargers, the router. Together it adds up. Switching appliances fully off also saves 110 to 160 euros a year in phantom load.
- Lighting. An old-fashioned incandescent bulb puts 90 percent of its energy into heat, an LED only about 10 percent. Not on LED yet? On hot days that’s an extra reason.
A detail that’s often forgotten: hanging your washing inside to dry. That does cool things a tiny bit, because the moisture evaporates and evaporation costs heat. Count on 2 to 3 litres of water per laundry basket going into the air. But watch out: it also raises your humidity, and there’s a catch in that.
Does humidity play a role in heat in the house?
Definitely. And it’s an underrated factor. The more humid the air, the warmer it feels, and the less well your body can cool itself.
Your body cools by sweating. That sweat evaporates off your skin and takes heat with it. But if the air is already full of moisture, your sweat no longer evaporates well. You stay clammy and warm, no matter how fast your fan spins.
That’s why:
- Switch off humidifiers during a heatwave. You don’t want to make the air more humid.
- Be careful with wet washing inside and evaporation tricks. They cool a few tenths of a degree but raise the humidity. In a clammy house that works against you. Drying outside is almost always better.
- The ideal indoor humidity is between 40 and 60 percent. If you’re well above that, 28 degrees feels like 32.
An overly humid house is bad for your health too, by the way: dust mites and mould thrive in it. Another reason to keep the humidity low on hot, sticky days.
How do you keep your bedroom cool at night?
The bedroom is often the hardest, because heat rises and bedrooms are usually upstairs. Sleeping badly because of heat hits you hard the next day. A few concrete things:
- Don’t ventilate during the day, do at night. Keep it shut and dark during the day, wide open at night for the cool air.
- Use light, cotton sheets. Cotton breathes and wicks moisture away. Synthetic material sticks and traps heat.
- Heat sources out. No charging phones, no appliances on standby giving off a bit of heat all night.
- Cool your body, not the room. A cool shower just before sleeping, or a damp cloth on your neck. Some people put their fitted sheet or pillowcase in the freezer for a bit, that gives a few minutes of cooling as you drop off.
- Sleep downstairs if you have to. If upstairs is really unbearable, a mattress in the coolest room downstairs is not a bad idea during the hottest nights.
What helps in the longer term: greenery, roof and insulation
The tips above work straight away. But if you want to make your house structurally heat-resistant, these are the big levers.
Greenery against and around your house. A mature tree on the south or west side gives shade and evaporates water through its leaves, which cools the surroundings. The cooling power of one large tree equals several air conditioners. A patio under a tree is a good 5 degrees cooler. Choose a deciduous tree: shade in summer, and in winter it lets the sunlight through. Climbing plants against a facade (Virginia creeper, wisteria) cool the wall and keep it out of the direct sun.
Your roof. The roof catches the most sun, and a black roof gets scorching hot in summer, up to 70 degrees. A green roof with sedum stays around 35 degrees, a difference of 35 degrees. White gravel or a light, reflective roof coating helps too: a white roof reflects up to 70 percent of the solar radiation. On an uninsulated flat roof that’s worth 20 to 30 degrees of roof surface.
Insulation. Good insulation works both ways: in winter it keeps the heat in, in summer it keeps it out. Cavity wall insulation can reduce the heat flow through your wall by 70 percent. And as said: for loft and roof choose a heavy, slow material like wood fibre, which holds back the heat for hours.
These are investments, but they pay off in comfort and in a lower energy bill all year round.
Table: what does each measure give?
| Measure | Effect on temperature | Cost / effort |
|---|---|---|
| Exterior shading (screen, roller shutter) | Keeps out 80 to 95% solar heat, up to 5-6°C cooler | Investment, big effect |
| Curtains shut during the day | 2 to 3°C | Free, immediate |
| Solar control glass or window film | Up to 60% heat out | Medium to high |
| Night ventilation with cross ventilation | Up to 5°C cooler | Free, every evening |
| Oven/dryer off, appliances off standby | Prevents extra heat inside | Free, saves power too |
| Fan (on the skin) | Feels cooler, doesn’t cool air | Low |
| Damp cloth in front of open window | Small, local effect | Free |
| Green roof or white roof coating | Roof surface 20 to 35°C cooler | Investment |
| Tree on south/west facade | Patio up to 5°C cooler | Long term |
| Cavity wall and roof insulation | Heat flow up to 70% less | Investment, year-round effect |
The numbers are guidelines and depend on your house, the orientation and how you combine them. But the order holds: exterior shading and smart ventilation do the most work, and that costs you the least.
Which tips are old wives’ tales?
A lot of nonsense gets sold about cooling down. Let’s clear up:
“Put a bowl of ice in front of the fan, then you’ve got free air conditioning.” Half true. It cools a little spot right in front of the fan, no more. You don’t cool a whole room with it. Fine next to your chair, no substitute for air conditioning.
“Windows open during the day for fresh air.” Wrong, and it makes things worse. During the day the outside air is hotter than inside. You’re letting the heat in. Everything shut during the day.
“Drink hot tea, then you cool down.” This only works in very dry air, where you sweat more and that sweat evaporates. In clammy northern European summer weather it doesn’t work. Just drink cool water.
“A fan cools the room.” No. A fan cools you, not the air. Empty room? Switch it off.
“Blackout curtains are just as good as exterior shading.” No. Once the sun is through the glass, the heat is already inside. Curtains do help, but exterior shading is many times better.
Small tip. Big difference: put your energy into the things that really work, and not into the gimmicks.
Watch out for vulnerable people during a heatwave
This bit gets skipped in most articles, but it’s the most important of all. Heat isn’t just uncomfortable, it’s downright dangerous for some people.
At extra risk are: the elderly, babies and young children, people with a chronic illness (heart, lungs, diabetes) and people who live alone. Their body regulates temperature less well, and they sometimes don’t feel thirst and heat coming on properly.
What to watch out for, in yourself and in others:
- Warning signs: headache, dizziness, confusion, nausea, a racing heartbeat, muscle cramps. Take this seriously, it could be overheating.
- Keep drinking, even if you’re not thirsty. Water, no alcohol, no large amounts of caffeine.
- Seek out the coolest room in the house during the hottest part of the day, usually downstairs or on the north side.
- Check on the elderly and people living alone. A call or a visit costs you nothing and can make a big difference.
During prolonged heat, health authorities issue heat-health alerts. That’s a signal to look out for each other extra. If someone becomes confused, stops sweating or their skin gets hot and dry: that’s an emergency, call 999 (or 112).
So a cool house isn’t just comfort. For some people it’s health care.
Step-by-step: how to keep your house cool this heatwave
The whole approach at a glance, so you can get started tomorrow.
- Early in the morning (5 to 8): windows wide open, cross ventilation on, draw in cool night air.
- Around 8 or 9, as it warms up outside: everything shut. Windows shut, shading down, curtains shut on the sunny side.
- During the day: keep the house shut and dark. No windows open for “fresh air”.
- During the day: oven off, dry washing outside, appliances off standby, bright lamps off.
- Don’t heat yourself up: eat cold, drink cool water, a damp cloth on your neck.
- Fan only when you’re in the room, and not above 35 degrees inside.
- In the evening, as soon as it gets cooler outside than inside: everything open again, vent the heat out.
- Watch out for the vulnerable people in and around your house. Drinking, cool, and an eye kept on them.
Do this consistently for a few days and you notice the difference straight away. While the neighbours sit inside puffing, at yours it just stays bearable.
Frequently asked questions
How much cooler can I keep my house without AC?
With the right combination of measures you keep your house 5 to 8 degrees cooler than outside. The biggest gain is in exterior shading (up to 5-6 degrees) and good night ventilation (up to 5 degrees). Together that adds up nicely.
Should I open or shut my windows during the day in the heat?
Shut. During the day the outside air is warmer than inside, so with open windows you let the heat in. Keep everything shut during the day and the shading down. Windows only go open in the evening and at night, when it’s cooler outside than inside.
Does a fan really work against the heat?
A fan doesn’t cool the air, it only cools you because your sweat evaporates faster. In an empty room it makes no sense then. And above 35 degrees inside it can even work against you, because it blows hot air against you.
Is interior shading just as good as exterior shading?
No. Exterior shading keeps out 80 to 95 percent of the solar heat, because the sun doesn’t even reach the glass. Blackout curtains inside keep out 35 to 59 percent, because by then the heat is already in the house. Outside is always better.
Does hanging up wet washing or a bowl of ice really help?
A tiny bit. Evaporating moisture cools the air a few tenths of a degree. But it also raises your humidity, and clammy air actually feels warmer. A bowl of ice in front of the fan only cools a spot right in front of it. Don’t expect air conditioning from it.
What do I do if someone overheats?
Watch for headache, dizziness, confusion and a fast heartbeat. Move the person to a cool place, get them to drink and cool the skin with damp cloths. If someone stops sweating, their skin gets hot and dry, or they become confused: call 999 (or 112) right away.
A cool house in summer and a warm house in winter come from the same corner: good insulation, smart sun shading and no waste you don’t see. Whoever makes their house heat-resistant often lowers their energy bill all year round too.
I’ve put together everything I know about bringing your bill down and making your house smarter on the Energy Pulveriser. Concrete steps, no vague waffle. Have a look around: energy-pulveriser.fredsdiyplans.com