During a heatwave, many people assume that hot air outside is the main reason their building becomes uncomfortable. In reality, most overheating problems are caused by a combination of heat gains from several different sources, all adding heat faster than the building can remove it.
The challenge is that buildings behave a bit like a thermal battery. Throughout the day they absorb heat from the sun, occupants, lighting and appliances. If this heat cannot escape overnight, the building starts the next day already warm and temperatures continue to climb.
Understanding overheating therefore comes down to two simple questions:
1. Where does the heat come from?
2. Why can’t it get out?
Part 1: Heating – Where Does the Heat Come From?
Solar Gain: The Biggest Contributor (50-70%)
In most homes, solar gain through glazing is by far the largest source of overheating during a heatwave. Sunlight enters through windows and is absorbed by walls, floors and furniture. These surfaces then re-radiate heat into the room, creating the familiar greenhouse effect.
A typical south or west-facing window can admit hundreds of watts of heat during sunny periods. In many modern well-insulated buildings, solar gain can account for well over half of the total heat entering the building on a hot day. This is why external shading often provides more overheating benefit than additional insulation.
Building Fabric (10-20%)
Roofs, walls and windows also absorb heat directly from the outside environment. Dark roofs can reach temperatures of 60-80°C on sunny days, transferring some of this heat into the occupied spaces below. Poorly insulated lofts and roof spaces are particularly prone to this effect.
However, contrary to popular belief, heat transfer through walls is often a smaller contributor than direct sunlight through glazing. Often it’s uninsulated roofs that provide large heat gains in the building fabric
Occupants (5-10%)
People are effectively small heaters.
An adult sitting quietly typically emits around 70-100 watts of heat. A family occupying a house may contribute the equivalent of several hundred watts continuously, particularly during evenings when windows are often closed for security or noise reasons.
Appliances and Lighting (10-20%)
Every unit of electricity used inside a building eventually becomes heat. Computers, televisions, cooking appliances, hot water systems and lighting all contribute. A gaming PC can release several hundred watts. An oven can release several kilowatts while cooking. Even efficient LED lighting ultimately converts its energy into heat within the room.
These gains are often overlooked because they are less obvious than sunshine through a window.
Ventilation and Hot Outdoor Air (5-15%)
Opening windows during the hottest part of the day can make overheating worse.
If outside air is warmer than the internal air temperature, ventilation becomes a source of heat gain rather than cooling. Buildings therefore need carefully timed ventilation rather than simply opening every window whenever it feels warm even if the strong breeze makes things feel much more comfortable.
Part 2: Cooling – How Does the Heat Escape?
Getting heat out of a building relies on surprisingly few mechanisms.
Ventilation
The most effective cooling method available to most buildings is moving cooler outside air through the building. When night-time temperatures fall, opening windows or using mechanical ventilation allows stored heat to be carried away by the cooler air.
The greater the airflow, the faster cooling occurs. It’s this overnight airflow that has the greater impact in cooling buildings. Ideally looking to be capable of 5+ air changes per hour which is a large continuous draught.
Unfortunately, during still nights there may be very little natural air movement, so simply opening windows does not always provide the cooling people expect as the temperature or humidity difference alone is not enough for the air to flow at a significant speed.
Night Purge Cooling
The goal of night ventilation is not primarily to cool the air.
Instead, it cools the building fabric itself. Walls, floors and ceilings often store far more heat than the air in the room. If these surfaces can be cooled overnight, they effectively reset the building ready for the next day.
This explains why a house can still feel hot at midnight even after outside temperatures have fallen. The goal of night purge cooling is to flush the building with as much cooler air as possible. Often this requires fans near windows to help draw the air through the building.
Radiation to the Night Sky
Buildings also lose heat through radiation. Roofs, walls and external surfaces continuously emit heat to their surroundings. On clear nights they can become surprisingly effective at shedding heat directly to the sky.
While invisible to occupants, this process contributes to overnight cooling and is particularly important for lightweight roofs.
Thermal Mass
Thermal mass often receives mixed publicity because it can both help and hinder overheating.
Heavy materials such as concrete, stone and brick absorb heat during the day and slow temperature rises. However, if the building cannot cool at night, that stored heat remains in the structure and may contribute to overheating for several days.
Thermal mass can therefore be really helpful as it can slow the building heating up but needs a lot of cooling air flow to draw the heat out of the building fabric.
Active Cooling
When natural ventilation and shading are insufficient, active cooling may be required. With more frequent heat waves we are getting more nights where the outdoor temperature doesn’t drop below 20 degrees C and thus air needs to be moved much faster through the building overnight to cool it.
Air conditioning removes heat directly from the building and rejects it outdoors. While highly effective, it should generally be considered after first reducing solar gains and improving ventilation opportunities to reduce the amount of cooling required.
A number of people with heat pumps and underfloor heating are switching their heat pumps into cooling mode which can give a very significant cooling effect by removing heat from the floor slab. Whilst effective, care should be taken with this as cold surfaces can create condensation and lead to building issues.
A surprising amount of overheating can often be solved without installing significant cooling equipment.
How Woohoo Can Help
At Woohoo, we help clients understand exactly why their building overheats as part of their wider energy and sustainability strategies. Every building behaves differently, and measures that work well in one property can sometimes have little effect in another.
By assessing heat gains, ventilation pathways and building fabric performance, we can identify where the heat is coming from, where it is getting trapped, and which improvements will provide the greatest benefit.
The goal is simple: how can we make buildings more comfortable during increasingly frequent heatwaves while avoiding unnecessary energy use and cost.