Having spent about half of my professional time over the last decade modelling overheating risks in buildings, I feel compelled to share my perspective on the ‘topic of the moment’ across much of Europe after the record-breaking heat wave we’ve just suffered in June.
I’m also from a hot Mediterranean region and have lived most of my life without air conditioning at home, so my experience with overheating mitigation isn’t from computer modelling only.
Much is being said and will be said about the causes of the unbearable heat inside many houses and buildings, and about potential solutions (including in those built in recent years when the need for overheating mitigation was already on the table):
— Is having more insulation good or bad?
— Does thermal mass help or not?
— Is it the lack of cross-ventilation?
— Should we now install external shutters everywhere?
— Is it time to loosen restrictions on cooling for homes in northern Europe?
However, I think that, as in many aspects of the construction sector, the problem is less a lack of technical solutions than a lack of genuine commitment from the different parties involved to address the issue.
In my view, it comes from two main reasons you probably won’t hear about out there: complacency and oversimplification.
Complacency
The risk of overheating in naturally ventilated buildings, and in dwellings in particular, is being discussed in the sector for over a decade now. In the UK, there are standards to prevent overheating by design through specialised modelling since 2013. And a the specific standard for dwellings was released in 2017.
However, even when these standards have been applied, the matter has been treated pretty much the same way as energy efficiency in buildings — just looking for a pass in a report, but changing the predefined designs as little as possible.
The result: little progress has been made and new buildings that pass the standard are almost as prone to overheating as before. The infamous performance gap observed in energy performance is now also often found in overheating prevention.
But there’s a fundamental difference: nobody is hurt (directly at least) by buildings consuming more energy than targeted, but overheating in homes can become a serious health threat in the coming years.
If we want to prevent this risk, we need to treat it like that — a risk. Not an abstract figure on a table or a box to tick. A real, tangible risk for real people that will live in those homes.
This means designing proper mitigation measures and being thorough analysing and testing them against climate change scenarios.
A revision of the UK overheating assessment methodology (TM59) is due this year. But, having looked at the new Design Summer Year weather datasets already released, I have the feeling that passing requirements might be watered-down rather than being more demanding. Trying to avoid “overengineering” the designs in the complicated regions, which basically means reducing costs to developers and buyers.
Oversimplification
Overheating risk is a complex matter, with many variables involved. Analysing it properly takes time, technical understanding, and specialised skills.
However, it’s often discussed from rather narrow and biased perspectives, which often just try to impose preconceived ideas instead of investigating properly the causes and solutions in each specific case.
And even when we carry out technical overheating risk analyses, using specialised software, we rely on standard assumptions for the main variables that are rarely challenged. Among other reasons, because assessors don’t have any incentives to do it differently.
Without going into all the details, the following are some of the variables involved in overheating risk, and the uncertainties or limitations involved when it comes to making design or retrofit decisions:
The weather
It’s always unpredictable, but now it’s even more due to climate change. So, even if there are specific datasets to demonstrate compliance with overheating risk standards, we shouldn’t confuse models with reality. The wisest approach would be always testing designs and mitigation measures using several datasets, to check if they are robust against potential climate variability.
Occupancy patterns
The occupancy patterns of a home can have a significant effect on the risk of overheating. We normally assume that the dwellings will be occupied at all times, and occupants will operate windows or shading elements when required, but that isn’t often the case in reality. Many people are not always home, or they don’t behave like robots with a thermostat operating elements.
Window openings
Without air conditioning, opening windows is the only cooling method we can use. But it’s not a very reliable or predictable one since it is a manual operation by occupants. Nor very effective in certain conditions (with high outside temperatures or little wind).
We may also assume in overheating models that, as per the standards, windows start to open when internal temperature reaches 22°C. But this is actually comfort temperature for most people, and in reality they may not open windows until it reaches 24°C or 25°C. Making rooms less prepared (already hotter) when the heat wave hits than we had assumed.
Window openings are also often limited by security concerns, outside noise, insects, etc.
Thermal mass
The role of thermal mass in overheating can be complex, therefore it is controversial. Some prefer to ignore or dismiss its importance, others elevate it to the most important design feature.
I’d need a full article to explain it properly, so I’ll just say that yes, it can have an important role. But its performance depends on many other factors: the local average temperatures and daily swings, the characteristics of the heat wave, the position of the mass relative to the insulation, if there are shading elements or not, the natural ventilation capacity, the airtightness level, the occupancy patterns…
Movable shading
Movable external shading elements are effective for reducing solar gains, yes, but it depends on how they are used and that someone is there to keep an eye. And for external roller shutters, beyond aesthetic considerations, there are several additional factors to take into account. They can obstruct airflow, require inward-opening windows, create significant thermal bridges and air leakage if not carefully installed, and, in flats, may also be subject to fire safety restrictions.
Air conditioning
Local authorities in northern Europe are generally not in favour of the widespread use of full air conditioning in dwellings, as it contributes to the urban heat island effect, places further strain on the electricity grid, and contributes to further global warming through additional carbon emissions and refrigerant leakage.
And, until now, it has been considered unnecessary in temperate climates if sufficient passive cooling measures are applied. However, this position may become increasingly debatable if temperatures continue to rise.
“Light” cooling
An option that is increasingly being considered, and accepted by planning authorities in some regions, is the use of “light cooling” through the mechanical ventilation system. This consists of a very small heat pump incorporated in the ventilation system that cools down the ventilation air supply. It doesn’t require an outdoor unit or additional ducting. However, the cooling capacity of this systems is limited by their small size and the air flow rate of the ventilation system, which is much lower than in a dedicated cooling system.
Solutions
So, with all this in mind, what can we do if we want to be more serious about overheating risk mitigation in new build and retrofit projects?
In my view, the most important change is that design teams focus on proposing and testing mitigation measures, investigating and learning new approaches to construction, rather than waiting for someone else to validate their usual designs with minimal changes against a standard.
And we should all, designers and assessors, bear in mind that we are dealing with a potentially serious health risk in the near future. This is not good-looking “green credentials” or energy-saving “good-to-have” features. This is a very tangible risk that could get locked into buildings for decades.
After all, a building that does not protect its occupants from all reasonably expected adverse conditions is not fulfilling its primary purpose.
With this in mind, I believe the wisest approach would be to put greater effort into testing a range of potential scenarios and developing a more comprehensive map of the risk. This would help us better determine what is acceptable and what is not.
If I were part of the team developing overheating standards, I would propose expanding the compliance criteria to cover not only thermal comfort under standard assumptions, but also thermal safety under more demanding, non-standard conditions.
Let’s hope for the best, but prepare for the worst.
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