Overheating – how can we avoid it? – article from Green Building

This article originally appeared in Green Building, Spring 2014

“Everybody loves the summer time”, as Carole King once sang: everybody that is, except those who are separated from their sweethearts – and those sweltering in stifling buildings that they just can’t get cool.

At its worst, overheating can be a serious – even fatal – health issue, with the very elderly, and babies and small children most vulnerable, and heart attack, stroke, and sudden infant death all possible consequences. But much more commonly it is a discomfort issue, which can affect the usability of buildings, and/or drive people to deploy energy-consuming measures such as artificial cooling.

A building that cannot be cooled down to a comfortable temperature whatever you do is obviously overheating. One that cannot be cooled in a secure and comfortable way (eg, can only be cooled via opening window onto a busy road, or by leaving patio doors open at night), overheats so far as the occupants are concerned. Both are a failure on the part of the design and construction team.

How hot is too hot? The occupant has the last word on this, but designers do need guidance on what ‘most occupants’ can cope with:

As the National Housebuilding Council reports, work by CIBSE and Arup suggests that most people begin to feel ‘warm’ at 25ºC and ‘hot’ at 28ºC. At 35ºC “there is a significant danger of heat stress.” Heat at night bad enough to interfere with sleep seems to compound the danger to health.

In practice, comfort also depends quite a lot on humidity (which determines how readily people can keep themselves cool via sweating) and air movement (ditto) .

In general, it ought to be possible to avoid overheating without sacrificing winter time comfort and energy efficiency. Despite a warming climate we’re still going to want houses (in the UK) warmer inside than out, most of the year.

What causes overheating?

A few years ago a report on some new housing association flats appeared in Inside Housing magazine: “Summertime is not easy if you live in Woods House in south west London. When the sun rises, so does the temperature in the building. And it remains high – even at night. These brand new flats…are so environmentally friendly that they are overheating.”

Inside Housing’s reporter is not the first to blame overheating on modern fabric standards. James Hulme, strategic policy advisor at the House Builders Association, has been reported as saying the “ever exacting standards” of Part L cause overheating by requiring levels of airtightness that are “too high”.

“The regulations need to change and buildings need to be able to breathe,” he is quoted as saying. Insulation is also commonly blamed.

So are houses built to modern fabric standards doomed to overheat? At least one housebuilder seems to think so: “Since starting to build to level four of the code for sustainable homes Frank Haslam Milan has begun installing mechanical cooling to prevent overheating.”

If this analysis were correct, it would suggest that buildings constructed to an even higher standard of insulation and airtightness than Code 4 – such as the proposed zero carbon FEES standard, or indeed Passivhaus, would be even more prone to overheating?

Luckily, the evidence suggests that in fact a well insulated shell can make it easier for occupants to keep their home at a comfortable temperature in the heat, as well as in the cold. But only if that shell is well-designed and well-constructed. Good design and good construction are just as important for keeping cool, as they are for keeping warm.

What do we know?

Not a lot of systematic research has taken place into overheating, particularly in domestic buildings. Good Homes Alliance recently made a step forward by approaching over 1000 housing professionals with a survey about overheating, getting over 100 responses, and going on to look closely at 12 of the cases reported to them. Before that, NHBC had published an investigation into six homes suffering from overheating.

The research into domestic overheating shows that all kinds of buildings are vulnerable; overheating in newbuild flats seems to be emerging as an issue, with a number of poor conversions of older buildings, especially into HMOs (houses in multiple occupation) also inflicting severe overheating on occupants.

HMOs (bedsits), flat conversions, and new modern flats that overheat tend share the problem of lack of cross ventilation – caused by cramming a lot of small residences into a single building, meaning many are only single aspect.

Overheating can be compounded in the top floors and attics of the older buildings by the absence of roof insulation, meaning hot roofs radiate heat down into the dwelling, often well into the night. In the better-insulated modern flats, this is not an issue, but instead, poorly designed and installed communal heating is too often found to be pumping unwanted heat into the flats and access areas 24 hours a day.

Excessive areas of unshaded glazing, especially to the south and west, and inability to make use of door and window ventilation because of noise, pollution or security concerns, also contribute to overheating in many cases.

Glazing and solar gain

As a writer in Building Design remarked: “large expanses of windows or the aspirational floor to ceiling height glazing that is sometimes seen as a benchmark of exclusive city centre residential developments, obviously increase solar gain and therefore make a property far more susceptible to overheating.”

But its not only Grand-Designs-watching punters who favour floor to ceiling glazing; a lot of architects like it too. And on top of that, there’s a common belief that an “eco” building – and, in particular, a Passivhaus building – requires large areas of south glazing, “for solar gain in winter”.

But wait! It turns out that in the UK climate, large glazing areas “to harvest the heat of the sun” are a rather expensive source of free heat. Passivhaus consultant Nick Grant did some calculations on the useful heat gain in winter versus the extra losses at night through extra south-facing glazing – and then worked out how much it cost to harvest the net gain (assuming the windows would last 20 years). It turned out that the extra heat was costing around 80p/kWh.

Nick Grant’s advice is, therefore: “Size glazing for daylight and not winter heating. Unless needed anyway, south glazing is a very expensive way of heating and can lead to summer overheating.” He adds that east and west glazing can also lead to summer comfort issues – they are of course harder to shade, as the sun is lower.

Given that we need windows, we welcome sunshine most of the time, and the net effect of south windows is beneficial, you wouldn’t of course want to avoid all south glazing simply to avoid overheating. Careful calculations and careful use of shading should be able to offer occupants the best of both worlds.

Balconies, though often mentioned, are expensive and present thermal bridging problems, so certainly aren’t a good choice if the main point of them is to shade the room below. But shading that is simply shading is not expensive – and can either be fixed, or moveable under the control of occupants. (Automated shading systems are also available, but are a lot more expensive, and don’t always ask permission before shutting people out of the building, or alternatively, opening to reveal an occupant mid-way though getting dressed.)


shutters at Sulgrave Gardens


Movable wooden shades on the south windows to cut solar gain, on Octavia Housing’s Passivhaus development in West London. These simple louvres run on rails, and are light and easy to operate from inside via an opened window.

Internal gains
Solar gain is not the only source of unwanted heat indoors. All the energy used in the building emits heat into the building. Poorly designed, installed, controlled, used – or chosen – services and appliances lead to unnecessarily high energy bills all year round, and the increased risk of overheating in warm weather.
Consumer appliances should not present too much of an issue, unless the dwelling is very much more densely populated than the designer expects.

Households that do a lot of cooking, or have numerous, large or inefficient freezers, TVs etc may also pay an energy and heat penalty. Halogen downlighters burning down on your head in the kitchen on summer evenings are certainly unpleasant, and could also contribute to overheating.

However, in the examples of overheating examined by GHA and before them the NHBC (in rental properties) in none of the cases did occupant excess (or an excess of occupants, for that matter) seem to be the problem.

What did crop up as an issue in a number of overheating homes, especially in blocks of flats, was excessive heat loss from heating and hot water systems – heating systems are included here because in some buildings the heating system was so badly insulated that it caused year-round overheating.

As reported by Alan Clarke and Nick Grant the losses from (self-contained) domestic hot water systems can be large, calculated to be equivalent to over 10kWh/(m².a) in modern low energy houses. Monitored data from completed low energy houses have shown higher losses even than this.

It appears that community heating is an even more serious offender: as an AECOM report for DCLG put it, after carrying out some research in the housing industry. “Relatively newly built flats, constructed post 2000, were perceived as the dwellings most likely to overheat. The strongest single message is that overheating is occurring as a result of community/district heating systems in apartment buildings, where unintended heat losses due to a lack of insulation is resulting in problems in some parts of some buildings, especially corridors.

“This is of particular interest because it has not been identified at all in the literature review.” [In other words, it was new.]

Community heat sounds great: theoretically efficient, and if using CHP, it is eligible for various “green” subsidies. However, the engineering profession does not seem entirely to have caught up with:
1) the importance of saving energy;
2) how low heat loads really are in low energy buildings; and
3) the extra impact of wasted heat in a low-energy building.

Therefore not only is community heating failing to deliver the promised efficiencies (and cost savings), it is all too often causing overheating as well.

Common problems include oversized systems, high flow temperatures, and systems that don’t shut off when no heat is being drawn: all outdated standards, driven by the need to keep continuous circulation so users all had hot water despite the constant losses – from poor insulation. However as Casey Cole (Managing Director of Guru Systems, who provide smart metering for low carbon energy schemes) has pointed out, it is difficult for the specifying engineers to feel secure changing their ‘good enough for my father, good enough for me’ ways:

“The problem has been compounded by a lack of useful standards and good guidance in the sector. Instead, engineers have had to rely on outdated norms (e.g. flow and return of 82°/71°) and inappropriate or obsolete documents originally intended for more traditional central heating systems, [which] don’t lead to good district heating schemes.

Even a well-designed system however will be let down by poor installation, and a poor one, even more so. The NHBC and GHA research found repeated examples of inadequate insulation meaning that hot pipes were pouring heat into both homes, and communal spaces. (see box)

Though this is often taken as a sign that “we need more ventilation” this is a pretty short-sighted response – only any good until the end of the heat wave in most cases! The heat pouring out of these systems in summer is a clear sign that heat is being wasted (or at best, given away free where it should be being metered) the rest of the year.


IR hot pipes from NHBC

Thermal image of the flow-and-return pipes supplying the heat exchanger in the airing cupboard of an overheating flat. Bright ‘hotspots’ can be seen where insulation has been omitted over anything “awkward” like a joint or valve, but even the lagged areas are hot, suggesting unnecessarily high flow and return temperatures, and low-spec lagging.

As a rough estimate, an increase in flow temperature from 40 degrees above ambient to 60 degrees above ambient (say from 60 to 80) increases heat losses by around 50%. And increasing return temperature from 40 to 70 (still regarded as the ‘norm’) will add more heat loss again.

Missing insulation off the “awkward bits” such as corners, valves and pumps, while insulating the easy straight bits of pipe, will lead to losses of three or four times those from a fully-insulated system.

Image courtesy of NHBC Foundation


Non-domestic buildings
In non-domestic buildings the drivers of overheating take a slightly different form. Excessive solar gain through large areas of East, South or West glazing are certainly a menace, with fully glazed offices almost de rigeur despite the double expense of first of all building them, then dealing with the resulting unwanted losses and gains.

 The ‘hot body count’ in non-domestic buildings can be a lot higher in terms of Watts/square metre than in even the smallest house – this is a trap for the unwary when designing Passivhaus schools in the UK, as PHPP assumes much more generously-sized classrooms than are funded here, which designers need to account for in their calculations.

 Schools and offices often have a dense population of IT and other equipment too. Although it seems usual for office designers to throw in the towel and specify artificial cooling, this is not the only way, and the evidence suggests that people generally prefer to be in control of their own cooling via natural ventilation.


Natural ventilation for cooling does NOT preclude the use of mechanical ventilation to ensure comfort and good air quality all year round. So called ‘mixed mode’ ventilation may sometimes be the best choice for an all-round comfort and healthy, low-impact design.



Insulation stops heat getting in, as well as stopping it getting out. This can be of direct benefit, especially to very lightweight buildings: a resident of a park home who had had external insulation fitted commented: “In previous years, it’s been so hot that candles have melted in the house; this is a massive improvement in comfort level.”

The housing professionals reporting on overheating to the Good Homes Alliance reported that in older properties, overheating is almost always experienced alongside problems with cold in the winter months – for example one told them: “All problem attic flats are the same as the one highlighted and have required enforcement action to remedy – they also suffer excess cold and often dampness and mould due to condensation problems.”

Insulation and airtightness between them will prevent the heat from dissipating so quickly after a hot day; but as we know, a building leaky enough to cool down quickly on a summer evening is going to be expensive, if not impossible, to heat in winter.

This only becomes an overheating problem, however, if poor design means:
• Gains (eg solar, services) are too high and/or
• Provision for ventilation is inadequate.

Generally speaking, good design (and good installation) enables you to keep the gains in any sensible building down to no more than can comfortably be removed by opening the windows – and, should sufficient window opening be unlikely or impossible, it can usually be dealt with by mechanical ventilation instead.

But the proviso is: the design team need to know what they are doing, and they need to control – or be in a position to check – the design, installation and commissioning of the services.

Thermal Mass

Thermal mass does not automatically protect a building from overheating, although people often assume that it does. In the Good Homes Alliance survey more than three quarters of the reported instances of overheating were in ‘heavy weight’, not lightweight, constructions.
In fact thermal mass can make occupants less, not more comfortable, unless the designer knows what they are doing.

Uninsulated thermal mass is of course a liability for almost the whole of the year – it will simply feel cold, and make the building cold. Insulated thermal mass however can be useful for comfort in winter and summer, by smoothing extremes of temperature.

But this will only work if the thermal design as a whole is well thought out – meaning the average temperature is a comfortable one. As the Zero Carbon Hub warns, in hot weather: “when the heat absorbed by building materials is re-emitted into the living space, it can contribute to uncomfortably high internal temperatures. Therefore, high thermal mass in homes can contribute to overheating unless specific measures are undertaken to limit solar gains by shading and remove heat by adequate ventilation.”

To enable thermal mass to contribute to comfort in heat waves, first ensure the mass isn’t offered too much heat to soak up during the day. ‘Night purge’ ventilation is also needed, to cool the thermally heavy materials down – so that they can make themselves useful and absorb heat as temperatures rise the following day.


Draughty buildings guarantee heat loss all year round, but in fact they they don’t actually guarantee good IAQ to the occupants. A more deliberate and controlled approach is needed, for occupant health, energy efficiency, and to enable occupants to control their comfort in cold and warm weather alike.

It is helpful to remember (as Part F reminds designers) that ventilation for year round air quality and ventilation for cooling are different functions and are sometimes best served by different (albeit overlapping) systems.

Ventilation for good internal air quality needs to be continuous, well-matched to the day-to-day needs of occupants and building, and unobtrusive.

‘Purge’ ventilation is needed not only to cool down a hot building, but also to flush away the effects of, say, an unusually large crowd in the building, redecoration, or burnt toast. It should be easy for occupants to control, go up to a high maximum – but still be secure and comfortable if possible. As the NHBC points out, even if mechanical ventilation is used, occupants should be able to open windows adequately and in a secure manner, even if for short periods of time.

Although you’d think this was pretty simple, it is distressing to read about the lack of ventilation provision in some of the dwellings (both old and new) reported to the Good Homes Alliance:

• A student bedsit in “an un-insulated mansard roof. No provision had been made for a window to ventilate the room directly to external air. This room opens onto a rooftop conservatory.”
• “The main bedroom was in a converted roof space, which had no windows.”
• “Poor means for controllable ventilation with only French doors to all rooms on ground floor and no secure windows”
• In one extreme case, which resulted in an almost complete lack of ventilation: the architect did not want opening lights, as it “would have spoiled the aesthetics of the building lines” (although the only view was from the car park opposite).

small windows
Just one of the six small lights in each of these windows would open – unsurprisingly, the building was notorious for overheating. Photo courtesy of Good Homes Alliance (Preventing Overheating, 2014) 

Simply relying on a SAP calculation to predict whether the building openings are adequate to keep the building cool, without using common sense as well, seems to have been behind a lot of the problems. (see ‘design guidance’ below)

Many people, particularly those living in cities – where the need for night purge ventilation is probably highest – are understandably reluctant to leave windows open at night, particularly on the ground floor.

Even during the day, there are commonly problems with pollution and noise, and sadly some occupants, particularly the elderly or vulnerable, have security worries around the clock. Higher up in buildings, windows are sometimes restricted to prevent children from falling out – again, a very real worry, unfortunately. Even in rural areas, some people are extremely reluctant to leave windows open because of their fear of insects.

There are various options for allowing secure ventilation. Windows with restricted opening are better than those which occupants will not open at all, though a narrow-opening window in a deep reveal may offer little useful air flow. Secure grilles can be fitted into opening lights (see picture), though, like window restrictors, they do nothing about noise and pollution.

Secure grilles in a window opening can enable occupants to ventilate ground floor rooms. Architects: Penoyre & Prasad; Photography: David von Sternberg

In small and/or densely occupied buildings where designers have been careful to minimise unnecessary gains, it is usually possible to provide enough night time air flow to cool the building, via the same mechanical ventilation system that would deliver the background ventilation. It may be necessary to specify a fan size up; and running the fan at at full speed in heatwaves clearly has an energy cost (and possible noise implacation). But if you are offering occupants the chance to cool their space without any fear of intrusion, and with the bonus of keeping out most of the noise and a good proportion of the pollution as well, then this is probably a reasonable choice.

If the mechanical ventilation includes heat recovery, it will of course only be any use for night cooling if the heat exchanger can be bypassed so the heat can be expelled. Not all MVHR systems include this ‘summer bypass’ mode – if natural purge ventilation is going to be straightforward, it is often cheaper and simpler to specify an MVHR that does not have this option – the best choice will depend on the specific situation.

(When outdoor daytime temperatures are really high, the occupants of a well-sealed, well-insulated building might opt to revert to ‘heat recovery’ mode in the morning, so the incoming heat can be sent straight back out through the exhaust, without disturbing the restful cool within.)

The role of occupants

However carefully thought out the cooling strategy of a building may be, if the occupants can’t operate it, can’t understand it , or don’t even know about it, it’s pretty well useless.

How much control to “give” occupants appears to be a matter of taste, and often- unspoken assumptions. Experience suggests that in general occupants not only appreciate having control over their own comfort, they will also find the same objective “discomfort” less – well, uncomfortable, if they believe that they can, in theory, do something about it.

What the occupants do will depend on their assumptions, their experience in the current building, previous experience in other buildings (and doubtless that of their parents and grandparents before them) – and on what they have been told.

In an entertaining conversation at slight cross-purposes, during the investigation of overheating (or lack of it) in a Scottish Passivhaus, the academic researcher noted that a comfortable indoor temperature below 25 0C “could only be obtained through occupant intervention through the opening of windows to regulate the internal temperature, despite the use of a mechanical ventilation system” while the occupant appeared pleased to report that “Generally between April and October the windows can be opened.”

The occupant appeared happy opening windows and was not using the summer bypass; as has been noted in other Passivhaus developments, she may not have known it was there. She might however have been pleased to discover it – as she tended to close the windows at night, because “the midges are too bad”.

While this was a pretty benign misunderstanding in a comfortable building, it is not so funny when occupants are blamed for the discomfort they suffer because of faults in the design and/or construction of their building.

In the report about the overheating apartments at Woods House cited above, Inside Housing reports that the developer claimed that “residents had ‘incorrectly assumed that the ventilation system would cool their apartments’ and that cooling should be achieved through ‘conventional opening of windows and shading with blinds or curtains’.”

“The quote from the memo makes no mention of the din from the trains [which run very noisily at night on the tracks immediately below the flats] when windows are open, but [a survey had found] noise levels would breach World Health Organisation limits,” Inside Housing reported.

Communication between designers/managers and occupants needs to go both ways. Who can better teach you about how well you did last time and what you can do to improve next time, than your building’s users – they are in there every day.

One of the comments below the Inside Housing report expressed deep frustration that this simply doesn’t happen: “I lived in a 1990s energy-efficient flat which had small windows and masonry construction. It constantly overheated, and could only be kept cool, winter and summer, by keeping all the windows ajar. Fortunately we were on the first floor. …This was nearly 20 years ago, and I am so angry that there is no still no feedback loop from residents living in new buildings to show what problems there are.”

Thank goodness post-occupancy evaluation has finally started to take off – but what a shame so many buildings are still being occupied without it.

Design guidance

Part L guides designers to use SAP appendix P to check for summer overheating. Yet in 2010, consultancy AECOM warned DCLG that SAP Appendix P “ is a simplistic assessment tool rather than a detailed design tool,” and cited the view of the Zero Carbon Hub that SAP appendix P “can hardly be described as robust – simply leaving windows open ‘50% of the time’ appears to cure most overheating problems, but is a questionable assumption” not least because of people’s fears about security.


patio doors GHA
It should be obvious that people won’t be leaving patio doors open for night ventilation – yet the Good Homes Alliance research found a number of homes where designers had allowed SAP to assume just that – and had gone ahead and had the dwellings built with no other ventilation. photo courtesy of Good Homes Alliance

Depending on SAP alone means designers risk bypassing their own common sense, as the Good Homes Alliance has also warned. Overheating and space cooling are “anticipated for consideration” in the next SAP review. However SAP will never be a substitute for decent modelling – and designers’ own judgement.

Much more realistic and accurate models are available, and should be used. But even the best models are only as good as the information put in to them. Adding realistic gains from sun, services (and occupants) is particularly important with regard to overheating – a wise designer will check the default assumptions of the model they are using (SAP actually has some of the more realistic default assumptions on gains, for use in the UK, as they are based on real-world data).

The wider environment

Cities can be hot places – absorbent dark surfaces, concentrated energy use and lack of evaporative cooling from soil and plants combine to lift the temperature in city centres up to eight degrees above that of the surrounding rural areas.

Climate change is likely to make cities hotter, and buildings being designed or refurbished now will see a fair amount of climate change across their lives. However, simply installing air conditioning is not the answer – even without the concern about energy use – as it merely “pushes warm air outside the building and causes the overall heat levels to rise.”

Busy, noisy, polluted roads also both contribute to the heat (each vehicle is in effect a heater – not such a small one at that); and as we saw above, make it less easy for people to get much needed fresh air into their buildings. Given the documented effects of traffic pollution on health (see Green Building Spring 2014) you might wonder whether it is right to expect people to live in neighbourhoods where the air is so toxic. Certainly any transport measures that that reduce the amount of fuel burned right next to people’s homes would be beneficial in numerous different ways.

Role of landscaping

Given the choice of walking down a baking, leafless city street or heading for the park, we instinctively know which would be cooler. But is there more to this effect than just the shade?

Enthusiastic claims are often made for the cooling, and energy-saving, properties of some of the higher tech forms of ‘landscaping’ such as green walls – without there appearing to be much in the way of evidence to back these claims.

However, a modest amount of science does suggest there can be a genuine cooling benefit from the evapo-transpiration from living plants, over and above the shade they may cast .

Meaningful levels of cooling have been calculated to arise from city-wide green space provision, which could counter the urban ‘’heat island’ effect. An increase in planted area equivalent to 10% of a city centre might have to potential to reduce the temperature peak by around 4 degrees.

At the level of an individual building or project, research and measurements suggest that a young tree growing in ideal conditions (good soil, plenty of water) might offer around 7kW cooling. While this is quite modest (easily outweighed by an idling taxi, for example) and so not any kind of ‘solution’ to overheating, its still nicer to have it than not.

The shade itself is also valuable of course, and where there is no room for trees, growing climbers up fixed structures may still be feasible. A leafy canopy is at least as useful as an artificial one for giving shade, and a leafy courtyard may attract more building occupants to make use of it than would fixed shading.

How worried should we be?

Almost all the sensible steps needed to stop overheating from being a problem in our buildings, are sensible things to do to make building healthy places, and to minimise energy use and energy wastage. Climate change may nudge the balance towards more heat and less cold: but if it brings more wind, and more rain, UK buildings may not end up any warmer.

But designers may not need to second-guess the detail of this. A well-designed and well-constructed building is more comfortable and energy efficient in pretty much any weather, than the shoddier kind of structures discussed above. The place to start is therefore to pay attention to what have been the priorities all along, but with an added justification for doing it.

The need to minimise overheating is one more reason to insist on doing the calculations and delivering the quality control we already know are important.

That way, hopefully no one will feel it might as well rain until September.