How to keep cool in summer

Some people say that modern energy-efficient homes are more prone to over-heating in summertime but in fact over-heating can be a problem with any age of house. Over-heating is worse in the city partly because towns are generally warmer than the surrounding area (the heat island effect) and partly because they are often noisy which is annoying if we open the windows.

What will work for you depends on you and your home. Here are some tips for how to keep cool.

Key strategies

  • Minimise heat generated inside the building e.g. turn off unnecessary appliances; when cooking use an extractor
  • Minimise heat coming in from outside e.g. block off windows from the sun with blinds or shutters; insulate roofs and walls; apply reflective paint to dark flat roofs
  • Ventilate to remove the heat e.g. open windows to create a through draught (but only when it is cooler outside than in)
  • Cool yourself. Loose clothing and low power fans will help you lose heat by sweating. Iced drinks will cool you inside.
  • Make use of thermal inertia of the building fabric by ventilating when it is cool and closing windows during the day

I like to wear light clothes in the summer like these. They are 'adventure' clothing and they wick really well so I don't feel sweaty. Also here you can see our apple tree that shades the south-facing kitchen window.

Yourself

Dress for the weather

Dress for the weather! Modern synthetic fabrics, especially those designed for sports or adventure, can be even better than cotton. The material must be loose to allow air flow and/or breathable to allow sweat to evaporate. This the main way our bodies adapt to the heat, though some people sweat more than others and older people often sweat less. Use a gentle fan if necessary to create air flow to assist sweating.

Cool water for inside and out

Take cool drinks - you need to replace fluid lost by sweating. Also, dipping your hands and wrists in cool water is a quick way to lose heat and very refreshing.

Use ice or a damp towel

It will not look cool but if you find you are really suffering from the heat try wrapping some ice cubes (or a small pack of frozen peas) in a tea towel and applying that as an ice pack for the head. Or, we know someone who uses damp towels overnight to keep their feet cool. This works by evaporation, the same principle as sweating.

Avoid exercise

Check the weather forecast and try to avoid exercise - including house work - when it is hot. Can you even take a siesta and work earlier or later instead?

Your garden

Cooling plants

All green plants (grass, shrubs and deciduous trees) help keep the air cool because water evaporates from their leaves. A leafy climber on a wall will help keep the wall itself cool (and protect it from wet weather too). You should plant a climber a little way away from the wall so that the roots do not interfere with your foundations. Wisteria is popular here. Grape vines are good for a pergola or arbour.

Trees for shade against windows

Windows facing east or west get morning and evening sun which is low and shines straight through. These windows can be even worse for solar gain than south facing. So shrubs or trees shading these windows make a big difference. Ideally use deciduous plants because then in winter time when you need more light you will still get it. Fruit trees are a nice size, but it is best to choose a drought tolerant one such as a fig.

Green roofs

Green roofs are good on flat or shallow roofs as they absorb less heat than dark tiles or bitumen. A sedum roof should not need any watering and only occasional weeding. It also supports biodiversity. See here for advice from the RHS.


This is a lightweight green roof growing a variety of sedum species which are very drought tolerant.

Appliances and cooking

Leave the washing and hoovering until later

Any electricity you use - from TV, dishwasher or whatever - puts heat into the building. Even 50W (a small TV) can make a noticeable difference in a room. So keep an eye on your energy use, turn things off when they are not needed, and try to avoid running energy hogs such as hoovers or tumble dryers when it is hot. If it has to be done, do it in the early morning when you can dump the heat more easily because the temperature outside is lower.

If you are not sure whether something is using energy, see if it is warm to the touch. Use your smart meter to see how much you are using in total, or a smart plug for individual appliances.

When cooking, use an extractor

Older ovens that are not well insulated are terrible for heating up the kitchen so avoid using the oven if you can. If you are cooking on the hob make sure you use your extractor to remove the hot steam.

Use efficient appliances

Fridges and freezers have to work harder in hot weather, which generates more heat which means the room gets hotter which means they have to work harder ... so be sure to ventilate the room where they are. Also an inefficient appliance generates more heat (and pushes your bills up) so look at the energy ratings when you buy.

Lag hot water pipes

In some homes, usually flats with communal heating, hot water pipes stay warm most of the day. Make sure these are well lagged so that they do not act as unwanted radiators.

Solar panels, batteries and inverters

If you have solar panels you doubtless have at least an inverter and possibly a battery too. These are never perfectly efficient so they generate heat as well. Suppose your inverter is 98% efficient. If your panels are generating 4 kW you are getting 80W of heat. This is fine if it is outside the house, or in an unheated loft. Otherwise, well you have probably already been advised to keep it well ventilated. The solar panels themselves generate a lot of heat and can get quite hot but this is not a problem unless they are integrated into the roof.


We have external blinds on the Velux windows on the south side. They keep the direct sun off and by opening the windows here at the top of the house, as well as in the kitchen below, we get a good draught all the way through.
Brise soleil shade patio doors to keep the room cool. From Cambridge Carbon Footprint Open Eco Homes

Windows

Close the blinds to keep the sun out

Windows turn your house into a greenhouse, trapping heat from the sun. South, East and West facing windows are the worst. Cover windows with blinds or shutters when the sun is on that side. Curtains are a lot better than nothing, especially if they are light coloured on the outside, but reflective blinds or shutters are better. Internal blinds are almost as effective as external blinds as long as they are shiny and reflect the sun back out through the window - the window itself does not absorb much heat.

Roof windows pointing towards the sun are the worst. Velux windows can be fitted with external blinds, operated by a string or electric (as shown in the picture).

For more examples of ways to shade windows, including roller shutters (either manual or electrically operated) and use of gazebos and awnings - see Tips, tricks, do’s and DON’Ts to keep a home cool in summer (GetEnergySavvy).

Eaves, awnings and brise soleils

Rather than taking the trouble to open and close blinds and shutters during the day you can have fixed shading. Trees can be very effective - deciduous trees will let the sun in during the winter. Also deep eaves keep the high sun out on south-facing upper floors - they are not so effective on east and west sides where the sun shines in from low down. Where you haven't got eaves you can fit awnings - you can do this DIY if you are handy - CCF tell you how in this video here . Awnings need to be taken down in bad weather. Ideally you would have a winding mechanism for this. Or you can fit brise soleil which are permanent slatted shades, usually made of aluminium.

Low-G windows and solar film

You can apply solar film to your windows to reduce the amount of solar gains. This can be done DIY: video instructions here. The film will also affect the transparency and colour of the window slightly. It is not a panacea but it does help. Sticking space blanket material over the window can be used as a temporary measure (the film is slightly transparent so you will still get some light). If you are buying a new window you can also select a low-G glass. The G-value is a measure of how much radiant heat from the sun is transmitted directly through the glass (not to be confused with the U-value which is how fast heat passes through by conduction). Low-G coatings and film reduce solar gain in winter too so you have to choose a balance between heating in winter and cooling in summer.

Cooling with windows

Hot air rises. The best way to create a natural draught through the house is to open windows at the top and bottom of the house. When this works you are losing hot air and gaining cooler air so this helps to cool the house directly. The air has to have somewhere to come in as well as somewhere to go out so you need more than one window open. There is almost always a bit of a breeze so you can also get good ventilation on one floor if you have windows open on two opposite sides, especially if they line up. If the natural ventilation path leaves some rooms stuffy, you can use fans to direct the air flow where you want it.

When it is warmer outside than in, opening the windows can do more harm than good because it lets the warmth in. Unless you need the fresh air, keep windows closed during the day when it is warm. If you have mechanical ventilation with heat recovery, the heat exchanger will help keep the heat out.

The coolest time of day is normally the early morning so you want the windows open then.

You may not want to open windows for a number of reasons.

  • Security - consider a window restrictor that prevents the window from opening too much.
  • Insects - you could fit a fly screen which is basically a thin mesh. You can get removable ones with magnetic clips.
  • Noise or air pollution - you may have to resort to air conditioning.

A couple of different sorts of window restrictor. The one on the left unlocks with a key. The one on the right unlocks with a careful flick.

Walls and roofs

Insulation

Insulation keeps heat out as well as in. Flat roofs and room-in-roof insulation are often rather thin and this is a problem as, in this country at least, the roof covering is often dark and absorbs heat very well. So you need to stop the heat coming inside. The easiest way to check the insulation is with a thermal camera.

If you are insulating your walls, insulation on the outside rather than the inside increases the thermal inertia in your house. This makes night time cooling more effective.

Light/reflective paint

Dark surfaces absorb heat so another way to reduce heat gain is to use a reflective surface such as brilliant white paint. This is particularly useful for flat roofs where insulation is difficult and the standard bitumen covering is appalling for heat gain. For walls, brilliant white can be a bit harsh on the eyes so look for anti-glare shading paint.

Thermal mass and night time cooling

It is usually cooler at night and most homes take a while to warm up the next day. It is a good idea to get the house as cool as possible overnight by ventilating. Hopefully it will stay cool through most of the day.

If your heating controls have an overnight setback temperature, make sure this is too low to come on in the night - below 10°C would be safe. Also, if you have mechanical ventilation with heat recovery, be sure to activate the summer bypass so that it does not keep the house warm at night.

To store lots of coolness you need materials with a high thermal mass such as brick or stone. Thick stone walls can absorb an amazing amount of heat and stay cool for weeks. If you have solid walls and you want to insulate them, external insulation will keep your thermal mass on the inside. If you put insulation on the inside then there is no way for coolness stored in the walls to connect with the room.

Some modern constructions are low in thermal mass. One way to fix this is by laying ceramic floor tiles or wall tiles.

Use underground coolness such as cellars

If you have an underground cellar you will probably find it is cool even during hot days. You can use this as a source of cool air and circulate it around the house. However it is important that the area is clean. You do not want to circulate dust or mould spores at the same time.

Fans and air conditioning

Low powered fans can direct air flow efficiently, either to cool you directly or to cool the fabric of the home. For example you might use one to force air flow through particular rooms for overnight cooling. If the weather is not too humid fans can be effective with very little power. If you have high ceilings, a central fan in a bedroom can keep you cool at night by helping to evaporate your sweat. However, fitting them is a hassle.

Fans do not need to run all night if the outside temperature gets cold enough. A plug with a built in thermostat switch can turn the fan off automatically.

Air conditioning is effective but contributes to the heat island effect (except ground source, see below). A/C works like a fridge, cooling the room instead of the fridge. However the rejected heat is dumped into the air which makes the problem worse! For this reason A/C should only be a last resort. Air conditioners are a form of heat pump. They are usually air to air heat pumps and many are reversible so they can be used for heating or cooling.

If you are installing an air to water heat pump as a heating system with normal (hydronic) radiators, you can consider getting one that is reversible for cooling as well. For example the Vaillant AroTherm works for heating or cooling. One of our energy group members is considering this. There is a danger with this type of system that you get condensation on pipework in awkward places where it can lead to damp and mould. To avoid condensation you need a control system which ensures that the cooling water is always a few degrees warmer than the dew point.

Ground source heat pumps are more expensive than air source but they can also be reversible and have the major advantage that this does not contribute to the heat island effect because in cooling mode they dump heat into the ground not the air. This also helps to warm the ground up for the winter. You are using the ground as a huge heat battery.

Lessons from hot countries

Think of the architecture and customs in places such as Spain and the Far East where the climate is much hotter than here. There are tips we can use.

  • White or light coloured walls and roofs to reflect the heat.
  • External shutters.
  • Thick walls, often of stone, for high thermal inertia.
  • Shady courtyards with lots of green plants and fountains for evaporative cooling (only in dry climates).
  • Windcatchers to increase natural ventilation. Fundamentally this is similar to a chimney cowl, designed to increase the draught up the chimney.
  • Working hours start early and finish late with a long siesta in the middle.

Houses in Colmar, France, with awnings and shutters over the windows.

More information

From the WHO: Keep Cool (Suffering in hot weather and worried about your health?)

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