Is your home overheating in London? Here’s the reason why…

Have you spent this summer struggling to sleep in the heat, unable to choose between the security and noise issues that come with opening your windows at night, or closing them and lying in bed sweating?

You’re not alone.

We often hear about how dangerous cold weather is for those suffering from fuel poverty in winter, but overheating in summer kills as many if not more people in the hottest climes, and shouldn’t be disregarded as a vital issue in future building design.

Potentially thousands of new build apartments and houses are affected by serious overheating during the summer months, particularly those which face south, with only a single facade of opening windows and at high level (so under the sun all day).

This is because there are are a number of serious errors in the heart of the overheating analysis provided by the architects and energy consultants during the planning stages of a new build development that are seriously underestimating the risk of overheating.

Why is my home so hot and airless in summer?

If you live in a new build flat in London, or indeed any UK city, chances are you’ve been finding it unbearably hot to live indoors once summer starts. In the worst case scenarios, I’ve visited flats with indoor temperatures at 30 Deg C in January.

Generally, we start feeling discomfort above 25 Deg C, and this is the temperature that the Passivhaus Planning Package (PHPP) will alert the building designer to a serious risk of overheating. If the modelling of the property indicates the home will be above 25 Deg C for more than a few weeks of the year, the building will fail and need to be re-designed.

There are four main ways to cool a building, in order of low to high energy usage:

External shading

Trees, parasols, balconies, deep window reveals, external blinds or shutters – if you can keep the sun’s rays from hitting the glazing, you’ll stop the most intense heat from entering the home. Nearly all modern homes are built with very high levels of insulation, which compounds the overheating problem – once the heat is in, the insulation (great in winter) keeps it in all day and into the night.

Passive ventilation

Opening windows, preferably at both ends of the house to allow air to pass through and blow/purge away the day’s heat. The problem with this solution is that it doesn’t work on calm, windless days, and orientation, security and noise issues can make it impossible to open windows comfortably in practice.

Active ventilation

An MVHR system can work in helping with overheating by removing stale, humid, warm air from inside the home and exhausting it straight outside via the summer bypass mode. In the evenings and at night it will then bring in cool air from outside, bringing the temperature down. An MVHR also solves the problem of guaranteeing ventilation and air movement even on noisy, windless nights (as the windows can be locked tight).

Active cooling

The nuclear option for cooling a home in the UK is to install air-conditioning, but this should be a last resort for only the most severe overheating risks. Normally we only see it in rooms with lots of south-facing glazing or in loft spaces. Both of these situations have solutions to mitigating overheating, but sometimes it’s impossible to put in external shading – eg, if you’re apartment is on the 20th floor, or in a conservation area.

If air conditioning is the only solution, a plug-in type such as this portable AC on Amazon might help, but remember that AC systems by their nature tend to be noisy, very expensive to run and not particularly comfortable or healthy for long periods. Long term, this type of unit is a really inefficient way to cool a home, so long term solutions such as external shading should still be pursued.

Luckily though, air conditioning will be useful for about six weeks of the year in the UK in the average home – we really don’t have that hot a summer for that long a time. But when you can’t sleep because it’s too hot, any cooling will feel heaven-sent.

Air conditioning units scatter the exterior of a building

The sun is not the only issue – dealing with internal heat gains

Unlagged hot water pipes, audio-visual equipment and computers, tumble dryers, washing machines and structural pieces such as steel beams can all contribute to overheating. If you’ve ever been in the corridor of a new build apartment building you’ll probably have felt how insanely hot it is. That’s because of the hot water in the secondary circulation pipes, which sits there so as to provide instantaneous hot water to each flat, but unfortunately leaks out this heat into the corridors – all year round.

What’s specifically making new build apartments overheat?

A new build flat is at the highest risk for overheating because it tends to struggle to achieve any or all of the above mitigating factors.

Flats can be at high level (which means there are no buildings or trees to overshadow them), they can have only a single facade of openable windows (which means crossflow ventilation isn’t possible), and they can be too small internally for air-conditioning or good quality ventilation systems.

This is being borne out in the experience of urban dwellers living in high-rise homes across cities in the UK. On some days these flats are unbearably, and even dangerously, hot.

Why isn’t the planning and design stage picking up the overheating risk?

Take a look at this screenshot from the overheating analysis survey of a proposed new build development scheduled for construction in 2020. What do you notice about the flats they chose to model?:

Screenshot from overheating analysis report of modern London new build flat

What I notice is that they’ve chosen the easiest apartments on the south-facing orientation to pass the overheating analysis model. They’re at low level and therefore potentially shaded by other buildings (not shown in the model), and they’re above ground level so there’s no security risks with leaving the windows open.

What’s worse, the overheating analysis makes this assumption about the behaviour of occupants:

“Windows are modelled as open when both internal dry bulb temperature exceeds 22 Deg C and the room is occupied.”

The reason that line is significant is because opening the windows fully is an easy way to limit or remove an overheating risk in a theoretical design model. But I don’t think that in reality people are really opening the windows fully as soon as the temperature hits 22 Deg C – that’s a cool temperature for anyone. And plus, it’s dangerous to assume that windows can be opened, with the security and noise issues it carries.

What’s the solution for overheating in UK homes?

For the homes yet to be built, we need firmer guidance on which apartments in a block or homes in a development are being tested. It should be insisted that the homes most at risk from overheating are analysed, and there should be rules on how this is assessed and monitored.

For homes that are already built, possible overheating solutions include placing solar film on the windows, designing with less windows generally (and I personally hate floor-to-ceiling windows), fitting external shading (where possible), fine tuning or boosting the ventilation system or adding in a small air-to-air heat pump to provide limited cooling. All of these options have drawbacks, which is why good design is so important in the first place.

Please leave a comment if you’re affected by summer overheating in your home, or any other building design issue you’d like to discuss. The link to the portable AC above is an affiliate link.





  1. I’ve recently moved into a new build flat and I’m struggling with this – it’s been 14 degrees outside today but 44 degrees inside my flat.

    Not sure where I stand with building regulations and whether there’s any onus on the builder to help rectify it?

    • Hi Daisy,

      That is excessively high, and very dangerous for long term occupancy. It sounds like you may have some of the issues I mention in my article.

      Please can you send me a direct email with more information to Patrick [@] and I’ll endeavour to help you. If you have plans or drawings, that would be helpful, as would any information about the heating, hot water and ventilation systems.

      Best wishes,


  2. Hi, we moved into a new build home last year which has a large amount of south facing glass. When the sun is out we get temperatures of up to 40 degrees inside when it is much cooler outside and heat cannot escape.
    The developers are saying they carried out mitigation tests such as SAP test & the results were medium suggestion air cooling is recommended but not compulsory. Obviously what we experience over the months from April to September is unbearable and we wonder where we stand with this.

    Any help or advice would be hugely appreciated

    • Hi Reuben,

      If you purchased a year ago you may still be under the builder’s warranty, or the ten year NHBC warranty (or similar), which might give you some bargaining power. Purchasing some cheap humidity/temperature data loggers will also help. You can put these in a few areas around the home and they’ll record the temperature every 15 minutes for years, and you can download the data and see temperature/humidity over the year. This way you can build evidence of sustained overheating problems.

      All new homes will have had an overheating assessment, but they are fundamentally flawed and understate the overheating risk significantly – which leads to the problems you are suffering. SAP is a compliance tool for energy efficiency, not a design tool for overheating.

      Stay persistent with the builder to conduct remedial action (external shading, solar film, increased ventilation, or even AC).

      A home has failed if it is too uncomfortable to live in for half of the year. Architects and building developers need to understand that too much south-facing glazing in a home is causing massive overheating problems for thousands of homes across the country, and these professional services have seemingly no idea that what they are designing and building is completely inappropriate.

      Please email me with more details and I’ll see if I can help with more guidance/strategy: Patrick [@]

      Best wishes,


  3. Hi,
    I have also been struggling with this and I don’t know where to turn. The housing developer has refused to engage with me to assess let alone address the problem- they do not care. My flat has reached 47.4 degrees recently, the bedroom temperature rarely drops below 29 degrees on warm nights, often remains in the 30’s throughout the night. It is so oppressive and impacts every aspect of my daily life, I dread using the oven or hob, or having to blow dry my hair, and I’m lucky if I get a few hours sleep.
    Any advice on how to take this forward and get something done would be so appreciated.

    • Hi Vicki,

      I’m sorry to hear of your issues, it’s maddening that the housing developer is not helping you. When was your home built? If it is still under the builder’s warranty then significant overheating is something they must remedy to your satisfaction. If not there may still be some redress – please can you email me at patrick [@] with more details and we’ll discuss.

      Best wishes,


  4. A ground floor flat in a New Barkley home dev. Uncomfortably hot all year round, especially in summer.

    Only have doors to patio, no window that opens, so unable to leave open as is a security risk. Also have a cat who will get out.

    Finally getting a rep. for Berkley to visit tomorrow – we’ll see what they will/can do.

    • Hi Greg,

      Yes, one of the errors when noise / overheating assessments are calculated at design stage is how realistic it is for occupants to open their windows to purge the warm air from their home. Security, pets and a general feeling of not wanting strangers to look inside are not considered when deciding what to set a “typical” window-opening strategy in summer – sometimes the overheating design will assume windows are open fully for 18 hours a day, which isn’t normal, comfortable or safe.

      Let me know how the visit goes, and whether they offer any solutions.

      Best wishes,


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