Cooling an Empty House in Summer: Heat, Humidity and Energy

A house with nobody in it still lives through the summer. The sun heats the roof and glass, appliances sit plugged in, and stored belongings cope with the temperature. If you run cooling, you pay while no one benefits. If you switch it all off, you may return to a sauna that is also damp.

This page covers the risks of heat in an empty home, passive measures such as shade and ventilation, how public energy bodies suggest thinking about thermostat settings, and some fire and electrical cautions. The guidance quoted comes mainly from the United States and the United Kingdom.

What heat does to an empty home

The risks fall into three groups: the building, the equipment and the contents. Appliances left on standby keep making a little heat of their own, and stored items such as medicines, paints, batteries and food often carry manufacturer storage limits. Read those labels before a hot season, since no single public source covers every item.

Humidity is the less obvious risk. Our guide to damp, mould and humidity in empty homes explains how the pieces fit.

Stop the heat getting in first

The Energy Saving Trust, a UK organisation that gives public energy advice, puts it simply: it is usually easier to stop heat coming into the home than to cool it down again. Its suggestion is to close blinds and curtains when the sun is shining through a window, and to open them again only when the sun has moved away. It also says that adding solar film to windows reduces the heating effect of sunlight, and that if you cannot treat every window you should prioritise those that get the most sun, such as south-facing ones (Energy Saving Trust: how to keep your home cool in hot weather).

Shutters do a similar job to blinds and curtains, and many holiday homes already have them. The sources we read speak of blinds and curtains rather than shutters, so treat that as our reading of the principle: block the sun at the window, ideally before it reaches the glass. Closed shutters also help with security, covered in security for empty homes.

Ventilation: helpful, but harder when no one is home

The Energy Saving Trust advises opening windows when it is cooler outside than inside, which is mostly at night or early morning, and closing them when the outside air is warmer. It notes that a house absorbs heat during the day and releases it slowly, so opening windows at night helps remove the extra heat "if it's safe to do so."

That last phrase carries more weight for an empty home. Windows left open overnight in a house nobody is in have obvious security and weather consequences. Many owners therefore rely on other approaches: secure ventilation openings, trickle vents or louvres built for the purpose, or a neighbour or manager who can air the house during a visit.

Fans mainly move air over skin rather than cool a room, according to the Energy Saving Trust, so they do little in an empty house. The US Department of Energy (DOE) advises turning a ceiling fan off when you leave the room (U.S. DOE: Top 11 things you didn't know about saving energy at home, summer edition), and the same logic extends to leaving the house.

Setting cooling for an empty house

Should the air conditioning stay on? Public bodies do not give a single answer for empty second homes, so what follows is a way to think about the trade-offs rather than a rule.

The DOE says the smaller the difference between indoor and outdoor temperatures, the lower the cooling bill, so it suggests setting the thermostat as high as possible in summer. It states that running air conditioning at 78°F rather than 72°F can save between 6 and 18 percent on a cooling bill. That article dates from 2012 and is a general household tip, not guidance for unoccupied homes, but the principle is clear. It also recommends a programmable thermostat that can turn cooling off when you are not home. For an owner, that points to a middle path: a higher set point rather than full-power cooling for an empty house.

Switching cooling off completely saves the most energy but carries a humidity risk. The EPA's mould guidance names air conditioners and dehumidifiers as ways to reduce moisture in hot, humid climates, while warning that in some hot regions air conditioners that cool too quickly may not run long enough to remove excess moisture, and that evaporative coolers can encourage mould growth in drier regions (U.S. EPA: main ways to control moisture in your home). The EPA's general target is indoor humidity below 60 percent, ideally 30 to 50 percent, which comes from its brief guide to mold, moisture and your home.

Together these show the trade-off.

Cooling choices for an empty home and what the sources say

Approach	Energy side	Humidity side
Cooling off	Lowest energy use	Warm, humid air may build up; EPA names air conditioning and dehumidifying as ways to cut moisture in humid climates
Higher thermostat set point	DOE: smaller indoor and outdoor difference means lower cooling cost	Some cooling continues, but EPA warns that short cycling may not dry the air
Evaporative cooler	Energy Saving Trust: lower energy use than portable air conditioning	Adds moisture to the air, so the Energy Saving Trust says keep rooms well ventilated; EPA flags mould risk

No row wins everywhere. Humid and dry climates lead to different answers, and a humidity meter you can check helps more than any rule of thumb. We leave out running costs, which depend on your tariff. How your bills are handled while you are away is covered in utilities and bills for absent owners.

Fire and electrical cautions

Heat and electrics are a combination that fire services take seriously. Mid and West Wales Fire and Rescue Service, in its checklist for people going away, advises turning off and unplugging electrical devices that are not needed, turning off gas appliances that do not need to run, and having working smoke alarms so that neighbours are alerted if a fire starts (Mid and West Wales Fire and Rescue Service: while you are away). The same checklist suggests closing internal doors so that any fire is better contained.

For cooling in particular, the sensible reading is that a portable air conditioner or fan left running should not sit on a chain of extension leads in a home nobody is checking. Your own fire and rescue service will have local advice. If you intend to leave any cooling equipment running unattended, check the manufacturer's guidance first and, where electrics are involved, ask a qualified electrician about the installation. We cannot assess your wiring from here.

Country differences and honest limits

Several things change from one place to the next: how hot summers get, whether humidity or dry heat is the main problem, what electrical standards apply, whether air conditioning is common, and what your insurer or local authority expects of an unoccupied property. The sources above reflect UK and US conditions and may not match yours.

The DOE tip we quoted is old and general, and the Energy Saving Trust writes for people living in their homes. Neither addresses a second home visited a few times a year. In extreme heat or very high humidity, a local professional will give better answers than a general guide.

For a wider look at running an unoccupied property, including who checks on it, see looking after a home you are not living in.

The short version

Block the sun before it enters, ventilate only in ways that are secure, and treat any cooling you leave running as a balance between energy use and humidity. Fire services favour unplugging what you do not need. Your climate, insurer and local rules decide the details, and a qualified professional should look at anything involving your wiring.