Andy Mitchell, Managing Director of 21 Degrees, explains what factors influence human comfort and how we have tried to make our homes more comfortable through the years. Embracing building physics has been a turning point and while we have the knowledge and skills to create healthy and comfortable homes today, it’s still not the default approach. So Andy outlines the options available to get yourself into a better home.
Interview with Andy Mitchell
Andy Mitchell is the Managing Director of 21 Degrees. Engineering runs in the blood as he comes from a family of engineers and he has always been fascinated by houses and buildings, even considering architecture as a possible career path at one stage. Studying product design at A-Level he then became interested in company structure and business. His first significant role in sustainability was at a company called Natural Building Technologies in the early 2000s, where there was a focus on healthy bio-based materials. Today he heads up 21 Degrees, a company which supplies key technology to homeowners and architects enabling them to build more efficiently, and generate and store energy.
In this podcast Andy explores the concept of comfort and explains why we can do much better when it comes to creating a comfortable and healthy home in which to thrive.
We can define a comfortable temperature
While we might think that we are all different and therefore prefer to live at different temperatures, that is not true. We can define a comfortable temperature range because of our physiology.
“Any human being, no matter where they are on the planet is most comfortable between 20°C to 22°C. The reason is simple. If the temperature is higher, your body has to work harder to keep you cool; and if it’s colder, your body has to work harder to keep warm.”
The body doesn’t cope well under 15°C
Below 15°C humans have to work hard to keep warm. At 9°C or less, it’s a matter of survival.
How active we are, alters what we perceive as a comfortable temperature
If we’re engaging in a lot of activity, such as cleaning our homes, we are generating heat and keeping ourselves warm, so may desire a cooler temperature. Alternatively, if we are working at a desk and sedentary then the reverse is true and we may want it to be warmer. Generally speaking the impact of this is plus or minus 2°C.
Putting on clothes is another dynamic
You only need to look to the Inuit in parts of Greenland to understand what an impact clothing can have. They could survive in subzero temperatures. The downside is that their clothing was bulky and would limit the activities they could perform. Any clothing will go some way to making us feel warmer because it’s trapping the body’s heat and slowing it down from moving away from the body. It would be the same effect sleeping under a duvet. The duvet is insulating your body heat.
Relative humidity levels impact our health and comfort
Humidity, or the level of moisture that’s in the air relative to the temperature, has an impact on comfort. The warmer the air, the more moisture the air can hold. And generally, humans are most comfortable at 50%, plus or minus 5%. It’s quite hard to fix humidity levels but between 40-60% is seen as a healthy range. If the levels exceed 70% regularly then these conditions can lead to damp and mould, which are damaging to human health. Below 40% is not good either and could cause respiratory issues.
Lowering the temperature to save money will create an unhealthy environment
Andy explains that it may be a false economy to turn down the thermostat in winter to save money or reduce fossil fuel usage. This is because as you drop the temperature the relative humidity goes up.
“If you had a temperature of 22°C and the relative humidity was 50%, and you dial the temperature down to 18°C, you’ll actually increase the relative humidity to 64%… You’ve made yourself slightly more uncomfortable because you’ve increased the relative humidity.”
With poor ventilation, which is common in many British homes, the relative humidity may have been closer to 55% anyway.
“If we’re closer to 55% relative humidity that really starts to get interesting, because if you dial down from 22°C to 18°C, you’re at 73% relative humidity and that starts affecting your health. For example, dust mites proliferate above 70%, eczema and asthma are a direct consequence of allergic reactions to dust mite poop. You’ve also got issues with mould growth.”
Radiant heat also plays its part in comfort
In addition to temperature and humidity, radiant heat plays a significant role in how comfortable we feel. Andy shares a couple of examples of when you might have experienced this.
If you’re taking a stroll on a sunny day and a cloud goes in front of the sun, you might notice you suddenly feel a bit cold. The air temperature hasn’t changed. What’s happened is the radiant heat that’s been hitting your body has dropped because it’s been obscured by the cloud.
Radiant heat is also absorbed and emitted by surfaces in our homes such as furnishings, walls, windows, etc. So the temperature of those surfaces will have a significant impact on your comfort levels.
Our homes have got better over time!
Over the centuries we have turned to fires to keep us warm. In Victorian times there were open fires, which were extremely inefficient because most of the heat went straight up the chimney. However, the rooms felt more comfortable because of the radiant heat being absorbed and then given off from walls and furnishings, etc.
“The big, winged armchairs that we perhaps thought were to stop granddad falling out of his chair when he fell asleep were actually to keep the radiant heat in… It captured that heat that was being radiated out from the fireplace in the room.”
With the advent of cheap energy, such as access to North Sea gas in the UK, we could pipe fuel straight into our houses. Burning this in boilers, heating water and then piping it around radiators was a big leap forwards. Although fossil fuels are energy dense and have been an easy source of fuel, their emissions have contributed to global warming and we have nearly exhausted the supply in a few decades.
Indoor temperatures have been increasing
Back in the 1970s, the average temperature in a British house in winter was 12°C – 13°C. These days, it’s probably closer to 18°C – 19°C. Andy says that figure could be impacted by a number of people who are in genuine fuel poverty. In other words, they find it unaffordable to heat their homes. One reason for this is that the UK has a large amount of old and leaky properties.
Leaky homes are just about tolerable in a mild climate
Andy says that the UK has the most inefficient housing stock in Europe. While Germany is on a latitude further South and might face temperatures of -10°C, a British winter is milder and more akin with Italy. Therefore with a few token measures, such as double glazing, and cheap energy, we have got by.
“Sadly, I think we focus more on basic survival and then toleration, rather than what makes for a great home? What really does give us that extra level above survival, so that we can thrive? Again, to me, this is very much what comfort brings… It means you can be more productive in terms of your work. You’ve got higher levels of energy, because you’re not using it to keep yourself warm or cooler.”
Sensing a ‘muggy’ environment indicates pollutants have built up
As we live in our homes the indoor air is changing. We exhale CO2 and generate moisture from our activities. Relative humidity builds up and there may even be VOCs given off from furniture, paints, etc. Our instinct is to open a window, which Andy says will solve the problem.
“By opening the window, you purge the air to the outside, because generally warm air will move to the outside, particularly if there’s a bit of cross ventilation, and as a result, arguably, the temperature will be dropping a bit. But if you don’t leave that window open for too long you’ll have a complete air change, and the air that’s come into that room, because the walls and the fabric are warm, to some extent, that air will warm up. And what was outside air, which was probably around about 70% relative humidity, would have dropped down to around about 50% relative humidity, and suddenly you feel much better again!”
The problem is this purge is needed regularly and is hard to manage. More often than not there will be a drop in internal temperature.
In winter another source of discomfort is draughts, where you’re sensing changes in air temperature. For example, if you stand near windows that are single or double glazed, they are colder than the internal air temperature so you will feel the heat moving away from your body.
You may also experience stratification of the air and this heat differential causes air movement, which in turn can make you feel uncomfortable. For example, your feet can feel colder than your torso. You may not even be aware of it.
We have got to grips with the physics of buildings and know how to build better homes
In the past we have simply put our buildings together without worrying too much about how effective they are, focusing more on aesthetics. However, there is an approach that works and has been around for over thirty years. Dr Wolfgang Feist and the team at the Passive House Institute decided to look at things from the first principles and get a holistic picture. This led to the development of the Passivhaus standard, which is a formula that anyone can follow to make a building that works. Core principles include insulation, airtightness, high performance components such as windows and doors, reducing or eliminating thermal bridges, introducing mechanical ventilation with heat recovery, and orientating the building towards the sun.
Andy points out some of the benefits. For example, in a Passivhaus the temperature of the surfaces will be plus or minus 3°C of the air temperature. It will be draught-free and the heat loss will have been reduced significantly. The introduction of mechanical ventilation with heat recovery also means there’s constant fresh air coming into the building, but the heat from the outgoing stale air is retained. And this is a way of controlling the relative humidity, too.
So building or buying a certified Passivhaus is one way of ensuring you have a comfortable and healthy home.
Buying a new home doesn’t necessarily mean it’ll be efficient or healthy
Although the Passivhaus standard was developed decades ago, the learnings and methodology have not been incorporated into the building regulations. Therefore there is still a performance gap in most new buildings, where the design intent is never reached in the completed building.
We are all responsible for the status quo
Not only do we as homeowners tend to accept the status quo rather than demanding higher standards, but architects, designers and contractors struggle to adopt new strategies. So sometimes the drawings fail to give enough detail, particularly around things like airtightness or thermal bridging, which is just where you’ve got surfaces or materials joining together, and you can get additional heat loss that you wouldn’t do through the principal wall or roof or floor areas. It can be the contractors on site, who don’t fully understand the goals of the project and might cut through a membrane not realising the impact of doing this.
Then there’s the building standards themselves. The reality is the building standards stop at the point of design. Currently there is no ‘as-built’ check of the performance.
“So the performance standard stops at the design stage because you demonstrate through something called SAP, Standard Assessment Procedure, what is the heat loss from this building based on the building fabric. It’s pushed through a calculation. Effectively, it says yes this fabric will pass, and that’s where the testing ends. There is nobody from Building Control actually measuring the heat loss of the building once it’s occupied.”
The choices are getting better
Andy believes that building to a performance standard above and beyond building regulations is becoming more common. We’ve mentioned the Passivhaus standard but there’s also the retrofit standard EnerPHit. The AECB New Build standard is another option, which is slightly easier to reach, but has the same rigorous building physics approach.
Building your own home allows you to get what you want
When people are building or commissioning their own homes they almost always strive for higher standards. And doing a self-build project is not quite as onerous as it might seem, with more and more help available. Andy says we build about 5% of our homes in the UK this way, or 15,000 homes a year, which is equivalent to the output of the fourth largest volume house-builder in the UK. This is a good start but to put it into perspective Germany deliver around 50% of their homes as self-build. So maybe it’s not surprising that these standards have taken time to be adopted.
It is possible to improve the performance of an existing property
If building new or buying from the open market is not proving successful, then there is another option. Existing homes can be retrofitted to a performance standard. It is often harder, and some homes will be more challenging than others. For example, how the building is orientated is something that is hard to change. There are also significant costs for a deep retrofit.
“The more complex a building is in its form, often the more challenging it is to reduce the heating requirement, the heat load. You can still go a long way, though, to improve airtightness and therefore comfort levels but you might not achieve the same low energy results that other more simpler forms of building can.”
On a phased retrofit, improve the ventilation first
Doing a retrofit in one hit often makes life easier, but not everybody is in a position to do this. So another option is to do a phased retrofit. The most important part is to have a comprehensive whole house plan at the beginning.
Andy says that a good first step is to improve the ventilation of a property because that will give you cleaner, fresher air, with more controlled levels of relative humidity. While it may cost a bit more in heating initially until you have reduced your heat loss, you can immediately benefit and also reduce the risk of damp and mould building up in places where you have improved the fabric but have not improved the ventilation yet.
The average family home produces up to as much as 15 litres of moisture a day that goes into the air. This evaporates from our bodies when we’re sleeping or walking around the house. Moisture is also generated when we’re cooking, taking a shower or doing the washing, and it has to go somewhere.
So after the ventilation has been addressed, the next phase might be looking at improving the airtightness and replacing the windows.
Adding insulation changes the physics of the building
When you apply products to your building, you are changing the physics of the building. So if you apply a piece of insulation to a part of your property, you are changing what was there before, and if it hasn’t been well thought through you could end up with unintended consequences. Andy gives an example of fitting closed cell insulation internally which is principally a bad idea because you are likely to trap moisture in the wall. This interstitial condensation could ultimately lead to the building fabric being damaged or breaking down, or potentially creating mould pockets, which then can affect the quality of the air in the room.
Mechanical ventilation must be designed well
There are various ducted ventilation systems on the market but they have to be designed well to be effective.
“The ducting design, for example, in MVHR is far more important than the actual unit that you plug into the end of it. A decent fan and heat exchanger is important, but you can have the best MVHR unit on the market and you plug it into poor ducting design, and you’ll just have a poor system. Whereas you could have an average MVHR unit and plug it into some effectively designed ducting, and you’d have actually a very good system.”
MVHR can be useful if the outdoor air is not clean
If the external air quality is poor, such as on a busy road, or if there’s a lot of pollen that is having a particularly bad effect on a hayfever sufferer then a fully vented, ducted system often takes the air through filters. So you can filter particulates from outside and stop them coming into the house.
Making a home more healthy and comfortable shouldn’t be about payback
When people go on holiday they don’t look for the payback on their holiday! People are also prepared to spend a lot of money on a kitchen. However, when you tackle a big project that will improve health and save money on bills, they suddenly believe it should pay for itself.
Treat homes as homes, not as assets
It would be good if we moved away from viewing houses as assets, and actually looked at them as a way to help us live effectively.
Andy is tackling his own retrofit
We are all on a journey to creating better homes for ourselves and our families. And that includes Andy! He is making improvements to his Victorian home with a phased approach.
“You do a measure to your own property, and you still find yourself being quite impressed! So we live in a Victorian property, and half of the floor of the property is a cellar. The other half is solid floor, and we insulated in the joists of effectively the cellar part, and that made a significant improvement. And then the following year, we had an airtightness membrane installed underneath the joist and the insulation, and that made as significant, if not more significant an improvement in both the thermal performance of that room.”
Andy believes that most of us become complacent with our homes when we actually have the opportunity to enhance our lives.
“As human beings, we have this incredible gift of adaptation. The problem is it’s also a curse, and we just tolerate an awful lot of rubbish. If our awareness was increased and we could begin to learn what is possible, particularly in terms of how we can live within our built environment I think the change for our welfare, both as individuals and society in general, would be significantly impactful.”
Find out more
Why MVHR maintenance and servicing is important
What makes a well designed MVHR system?
How do you specify high performance windows?
Two radical retrofits that use an internal wall insulation strategy
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