How to prevent overheating in homes as summers get hotter in the UK

Overheating in homes has been the subject of many a headline this year, with the UK experiencing hotter, drier summers and heatwaves.

In our temperate climate, and to comply with Building Regulations, we focus on heat retention when building new homes and renovating existing houses. However, this means that we often overlook another major issue, overheating, and how to mitigate it in the summer.

Fortunately, overheating has now been more widely recognised in the Building Regulations for new homes with the introduction of Part O. This only applies to new builds, however, not the millions of existing homes within the UK.

Here, we explore the mitigation measures which can be put in place when designing a new house or extension, and in some instances, when renovating an existing home, to help reduce the possibility and impacts overheating in the home.  

What causes overheating in homes?

With average ambient temperatures predicted to increase due to climate change, and homes becoming more efficient and built closer together, overheating has become a problem that can lead to health issues.

Overheating is not just limited to the hottest days of summer. It occurs whenever the temperature inside a building becomes uncomfortable for the occupants for a long time.

It can be caused by:

• inadequate or absent central heating controls
• poor ventilation
• warm outside air
• large areas of glazing (where mitigation measures are not considered)
• it is exacerbated by high humidity, either internally or externally.

Overheating can be caused by a number of factors, and often exacerbated by poor design (i.e. it was considered from the outset). 

How to prevent overheating

There are a number of ways to help prevent and alleviate overheating in our homes. In basic terms, some of these mitigations measures which can add those looking to find out how to keep a house cool in summer include:

• designing in the correct use of thermal mass
• a good insulation and airtightness strategy
• installing external shading
• maximising natural external spaces
• providing the correct cross ventilation regime.

Ultimately, many modern heat mitigation methods will only be partly effective. The best way to avoid overheating is to consider and plan for it with your designer and relevant consultants at the design stage when building a new home or extension. "And importantly, considering both the house and possible mitigation methods holistically," adds Claire Lloyd, Editor of Homebuilding & Renovating.

Building a new home or extension provides an opportunity to consider overheating from the outset. In fact, Part O of the Building Regulations makes it a requirement to do so for new homes.

Building Regulations Part O now sets out two main ways to calculate and mitigate against overheating in new homes. The first, perhaps more straightforward methodology, aptly named The Simplified Method, looks, at minimising unwanted solar gain (by limiting glazing), and removing excess heat through cross ventilation. Introducing solar shading in high-risk areas is also outlined.

While Part O only applies to new homes, it could prove useful to those designing extensions.

Here, we drill down into the key ways you can help prevent overheating in your home.

1. Introduce external shading

Designing in shading around large areas of glazing, particularly southerly or westerly-facing glazing is key, and the some of the main ways of doing so include:

• Specially designed shading such as roof overhangs or a brise soleil can keep the sun out in summer and let it in during the winter
• Deciduous trees can be strategically positioned to block the sun in the hottest months but let the light through in winter
• External shading such as shutters can reduce the effect of the sun’s heat
• Retractable awnings above glazing.

"Internal blinds do not work very well for keeping radiated heat out, as they are on the wrong side of the glass," says energy-efficiency expert and contributor to Homebuilding & Renovating, Tim Pullen. (Although this can work when looking for quick ways to cool down a room.)

"Blinds between the panes of glass work better, as some of the heat is trapped between the panes, but ultimately nothing works as well as external shade," says Tim.

"You can introduce an element of shading from a roof overhang, designed to take account of the fact that the summer sun tracks across the sky at a very high angle. A correctly positioned overhang has the potential to shade much or all of a south-facing window in summer, yet absorb all of the low winter sun," says Mark Brinkley, self build expert and author of The Housebuilder's Bible. 

Another form of solar shading which can also be used to make an architectural statement is a brise soleil. "A brise soleil is typically a horizontal projection that extends over windows on a building’s southerly side. It is usually made up of angled louvred blades or fins, which allow the lower winter sun through, but block the direct summer sun," explains architect and self-build expert Allan Corfield.

2. Insulate properly

It's a misnomer that high levels of insulation and airtightness cause overheating. In well-designed homes, where heat loss in winter and overheating in summer are considered at the outset, such as those built to Passivhaus standards, insulation will serve to keep homes cooler in the summer months.

However, we can also take a closer look at the insulation we use. 

The heat from the sun is defined as radiant heat and when it strikes a solid object it is absorbed and re-emitted at a different frequency, either as conductive or convective heat.

So we really need to look at the types of wall and roof insulation used, particularly in lightweight construction such as timber frame. Much of it is designed to stop the transfer of conductive and convective heat and allows the radiant type heat to pass through and transfer into the home.

It therefore makes sense to design in the use of different types of insulation, reflective membranes and trapped air products together, to cover the widest spectrum.

3. Build in thermal mass

It takes around four times more energy to heat up a solid than it does to heat up air. By including some solid thermal mass in the building (in the floor screed and walls, for example) we create a buffer zone to level out the heat peaks. The thermal mass absorbs the heat during the peak periods during the day and releases it when the air is cooler.

"Thermal mass, in the form of stone and masonry, is useful for storing solar heat energy and in doing so, it helps prevent overheating," adds Tim Pullen. "It also flattens the peaks and troughs in the heating cycle, allowing the heating system to operate more efficiently."

Other ideas to introduce thermal mass could include:

• Adding some thermal mass to studwork walls in the form of thicker (around 25mm) and denser plasterboard, such as Fermacell
• Polished concrete can also add to the thermal mass.

Thermal mass must be designed appropriately. Including too much will slow down the reaction time and could cause the home to take too long to heat up (or cool down). It could also be a continuous heat sink.

4. Add in phase change materials to lightweight construction systems

Lightweight construction systems such as timber frame ideally need some thermal mass included in the design. This may take the form of masonry cladding or polished concrete flooring, but there is another alternative worth considering.

"The ideal material for providing thermal mass in lightweight houses has been kicking around for decades — phase change material (PCM)," begins Tim Pullen. "These materials typically contains tiny pellets of a form wax, that melt as they absorb heat. As ambient the temperature falls, the pellets then solidify as they cool, releasing that heat back into the room."

"Good phase change materials can store 14 times as much energy as concrete. However, despite decades of development, they have not yet made it into mainstream architecture. 

"If we think about thermal mass at all it is still in terms of a big lump of concrete or stone. PCM products could be used in place of plasterboards to form the ceiling or walls. 

"They effectively bridge the gap between when free solar energy is available and when it is needed, and unite the ideas behind ‘light and tight’ and ‘mass and glass’." 

5. Calculate overheating risk from the outset

Whilst Part O sets out a methodology for mitigating overheating, there are a number of other tools which can be called upon to aid with this process.

"If you want to be more accurate, you can use 3D modelling software, so your designer can track the solar shading in and around the building," says Allan Corfield.

"This can be complemented with a PHPP (Passive House Planning Package) calculation and report, to establish the amount of overheating.

"Both options should be used as design tools during the early design stages to help the designers make changes to the building (eg reduce window sizes, increase overhangs or increase brise soleil) to improve or minimise the overheating issues."

6. Combat the urban heat island effect

If the air inside the property does get too warm, one of the best ways to cool it is to cross ventilate with cooler air. A single window is not enough if the air cannot move from another opening.

Home ventilation is good in areas with lots of outside space, but in more suburban areas the air outside could easily be as warm, if not warmer, than the inside air. The temperature in cities can be up to around 9°C warmer than surrounding rural environments.

This effect is known as an ‘urban heat island’ and is caused by hardstandings and solid structures absorbing the energy from the sun during the day and releasing it during the night. This leaves no opportunity for cool air to enter the property to lower the air temperature.

These conditions can be managed by:

• planting foliage and vegetation
• having open water to enhance evaporative cooling
• using lighter colours on hard surfaces — avoiding large areas of patio in dark colours, adjacent to glazing, for instance.

It's worth noting that air conditioning is a large-scale solution in urban areas and if you have photovoltaic panels on your roof the cooling could be seen as ‘free’. 

However, the result of cooling inside is that the waste heat is exhausted to the outside. The air temperature then rises and adds to the ‘urban heat island’ effect.

7. Design in adequate ventilation 

"Visit the Mediterranean and you can’t fail to notice the use of shutters on windows. Shutters here serve a dual purpose: they keep the sun out of the house, but they also allow cross ventilation, using a concept known as night-time purging," says Mark Brinkley, a self build expert.

"This is where the windows are left open all night while the shutters are closed, but open vented which allows a throughput of air across the house, removing much of the heat stored within the walls.

"All too often ventilation is overlooked, particularly in new developer homes. Designing in proper cross ventilation and adequate means of purging hot air (rooflights can be particularly effective here) when outdoor temperatures drop at night and in the early morning can help to cool down homes in the summer — and is now a requirement in new homes under Part O of the Building Regs," says Claire Lloyd, editor of Homebuilding & Renovating. 

"Also bear in mind that mechanical ventilation system will also be required if a new house is built to superior levels of thermal performance," she adds.

"A well-planned ventilation system will be capable of removing much of the unwanted heat and make the house much more comfortable to live in during the summer months," adds Mark.

8. Consider solar control glass

Different types of glass can help block out the sun — more specifically solar control glass features a metallic coating which allows light to pass through the window whilst also stopping certain frequencies of sunlight entering the internal space. This can, in turn, help prevent overheating in homes.

It's not cheap solution, but a good idea if you're considering the inclusion of large areas of southerly or westerly-facing glazing.

It can not be easily retrofitted, as the coating is applied when the glazing unit is manufactured. However, there are inexpensive films available which can go some way to aiding on existing windows.