What is an Eco-House?
When planning the renovation of your home, many people ask how they can make it as eco-friendly as possible. So, we’ve put together our thoughts on what actually makes an eco-house.
So what is an eco-house? An eco-house is a home designed to be energy efficient and to have a minimal carbon footprint. Most eco-houses use modern technology and sustainable materials to reduce their environmental impact and create high-quality, healthy living spaces.
The term 'eco-house' is actually a bit of a catch-all, and there are many different ways of reducing the impact our homes have on the planet. Many people think that an eco-house has to be made of recycled tyres and mud, and look like a spaceship. There's actually a lot more to choosing the right approach for an eco-house, and it's quite possible that refurbishing an existing (and conventional looking) building will be the best way to go.
Not all eco houses have to be green.
Anatomy of an Eco-House
There are three key things that combine to make an eco-house:
Building fabric. It's important that the walls, the roof, and the floor are well insulated and have excellent airtightness – you don't want to let your heat out or the cold in.
Renewable energy and ventilation. We can reduce dependence on external energy and water sources by generating electricity and harvesting rainwater. This will also help to power the lungs of our building: air circulation (MVHR) that takes the stale air out and pumps fresh air in, without wasting our heat.
Embodied energy and life cycle. This is the energy that has been used to produce and transport the materials that are used to build with. To help keep this down we can choose to use recycled materials, or to use materials that can easily be recycled at the end of the building's life.
Eco-House Building Fabric
An eco-house needs to be all wrapped up to make sure it can keep itself warm or cool without using too much energy. This requires a combined approach: making sure we have enough insulation, using good quality double or triple glazed windows, getting the house as airtight as possible, and using thermal mass to maintain a steady temperature.
The insulating ability of each element is quantified by a U-Value. The lower the U-Value, the better it can insulate and the less energy it will lose. The exact number that you'll want to achieve depends on your climate, budget, and design (amongst other things) but in the UK it’s a good idea to aim for something like the following:
Floor, walls, and roof: 0.15W/m2K
Windows & doors: 1.0W/m2K
To put this into perspective, the walls of a typical Victorian terraced house (solid brick) will have a U-Value of around 2.0W/m2K; and if we add 100mm insulation to the outside it will improve to 0.18W/m2K.
Spending time and money getting our insulation improved to this level can be made even more worthwhile if we keep the building as airtight as possible. Letting warm air out and cold draughts in is a major contributor to increased energy usage. Airtightness is measured by blowing air into the house and measuring how much is lost over time; we should really be aiming for less than 3m3/h/m2.
For a new build, air tightness is as much about the design and construction as it is about 'filling gaps' and needs to be considered early on. With existing houses it's a little more difficult, but a starting point might be sealing gaps around windows and doors or draught proofing a loft hatch.
Renewable Energy and Ventilation
To keep a healthy living environment in an eco-house with good airtightness, we need to ventilate. This can be achieved with an MVHR (mechanical ventilation and heat recovery) unit. This takes warm stale air out of kitchens and bathrooms, uses it to heat incoming fresh air, and then pumps the resulting warm fresh air into living spaces (and the cold stale air is returned to the outside).
Mechanical ventilation is a really important aspect of any eco home as it creates a healthier living environment. The benefits are numerous:
Provides a constant supply of fresh air, not just when windows are open
Filters incoming air, reducing pollen and particulates (great for allergy sufferers)
Prevents a build up of air pollution like dust or radon gas
Actively dehumidifies in the colder months and helps prevent mould and mildew
MVHR units suitable for a typical residential project draw between 20w and 50w of electricity, so around the same as a lightbulb. In order to offset the power use, an eco house should incorporate PV (photovoltaic) panels, commonly referred to as solar panels.
These panels convert sunlight to electricity, and are best used in conjunction with batteries to reduce our reliance on the national grid. The more light they receive, the more electricity that they can generate – in the UK they are typically found on south-facing roof slopes for the greatest efficiency.
It's important to note that they are not all equal, and some are able to convert energy more effectively. We can compare them by looking at their kilowatt peak (or kWp) which gives us an indication of the maximum output in ideal conditions (in this case, higher is better).
Embodied Energy & Life Cycle
For a house to truly have a minimal impact we need to take into account the energy that has been used to build it, not just the energy that we use to maintain it. The more a material is processed and transported, the greater its embodied energy.
For example, the embodied energy of metal is usually fairly high – steel requires iron ore to be mined and heated to around 2,000 degrees Celsius before it can even be processed into the raw material and transported to site. Timber, on the other hand, can have a very low embodied energy as it can be grown locally and requires minimal processing to be useful.
It isn't always quite so simple, as some components in a building can be reused and recycled, spreading their embodied energy across decades. This is why we also have to consider the lifecycle of an eco-house. It's important to plan your project with the future in mind: what happens when the building reaches the end of its life and has to be demolished? Could the bricks or tiles be reused for another building, or do they have to be crushed and put in the ground?
When you're planning your eco-house, there are a number of things you can do to reduce the embodied energy:
Make sure your plans enable the building to be used for a long time
Make it easily adaptable, and use durable materials
Avoid making it bigger than you really need it to be
Use locally sourced materials where possible
Choose materials that can be reused or recycled at the end of their life
Refurbish an existing building – you can literally upcycle a house!
Related Questions
How much does it cost to heat an eco-house?
As the term 'eco-house' is a bit of a catch-all, we will use the Passivhaus criteria to establish a baseline. A typical 3-bed eco-house house (around 100m²) would cost around £70 year to heat. They need no more than 15kWh/m² in a year, so our 3-bed would use 1,500kWh – with an air-source heat pump (300% efficiency; and electricity at 14.4p/kWh) this would cost us £72.
How do I plan an eco-house project?
The best place to start planning an eco-house project is by getting in touch with an architect. It's a good idea to find a someone with a passion for sustainable design as the shared interest in building better and healthier homes will really drive your project.