People + Place Architects

Sustainable Architects in Kent

8 Tips for your house extension

and how to make it more sustainable.

  1. How big does your extension really need to be?

Is the problem you have with your existing home just due to a lack of space, or is much of the issue created by the existing space being badly arranged? Is it more a question of poor connection, between the garden and the kitchen say, or is a key room awkward or gloomy?

If you consider these issues before you start, it will help ensure you do not extend beyond what than you need, providing maximum benefit to you, whilst minimizing the damage to your wallet – and the environment.


2. Consider the orientation & shading of your new extension

A north facing window will lose a lot more heat than a south facing window (in the northern hemisphere at least). We all know this intuitively, but this poses a particularly tricky problem when adding an extension to your house, because the existing home is already in place. However, with careful design you can still add light and space and improve the feel of your home, without resorting to large expanses of environmentally damaging northern glazing.

Equally, overheating is increasingly becoming a problem and one that is only going to get worse as average temperatures rise. So large southerly windows, or even west facing glazing needs to be carefully considered to avoid baking in the summer.

You want to aim to allow in the low winter sun and benefit from the ‘free’ heat it offers, but keep out the scorching rays of the high summer. This can be achieved through external overhangs or brize soleil or by using external elements such deciduous trees and other planting.


3. Insulation, insulation, insulation

You will already appreciate the benefits of insulating your house, so I am not going to repeat what is now general knowledge. But there are two key issues, often not discussed when it comes to insulation, that you will want to consider for your house extension.

Firstly, always follow the mantra, “no insulation without ventilation”. The whole point of insulation is to maintain a temperature difference between the inside and the outside – you want your house to stay warm inside when (baby) it’s cold outside. But this means every time you insulate you are creating a situation where condensation can form. If this condensation gets in your structure and cannot get out, the structure will deteriorate and become damaged. There are systems that rely on blocking all moisture, but it is beneficial – and opens the door to more sustainable choices – if a ventilation path is considered and the moisture allowed out naturally.

Secondly, not all insulation is environmentally equal. One of the most common, readily available insulations, is something called PIR – or polyisocyanurate. It is a very high performance material and there are occasions when its use is justified, but it produces about 17 times more CO2 during manufacture than a renewable insulation such as sheeps wool.

There are so many different options for sustainable insulation that it needs an article in its own right, so I cannot cover it all here, but your insulation is a key consideration if you want to make your house extension as sustainable as possible – even if it does mean you have to source materials beyond what is in stock at your local builders merchant.


4. Watch your extension’s manners – avoid a big carbon burp

The carbon produced by making and transporting the materials to site and the energy used in construction add a huge amount to a building’s carbon footprint. With the climate emergency becoming increasingly urgent, reducing your energy output by having a well insulated extension, will have less of an impact if it takes you decades to recover the carbon emitted during the build. So imagine all that CO2 from manufacture being belched out at once. This is your house extension’s carbon burp.

The way to reduce this is to use low impact materials. A timber frame will lock in the CO2 sequestered during growth and is always a good place to start. If you need to use bricks for whatever reason, consider reclaimed bricks from a source as close to you as possible.

Reclaimed materials can be sourced and used for many building elements from the foundations to the roof – another article in its own right – but if you start thinking about your proposed aesthetic by considering the impact of the materials first, then this will be more likely to lead you down a sustainable path than trying to find environmentally friendly versions of items you have seen on websites or social media.

For instance you can repurpose old, or second hand kitchens, by adding a fresh coat of paint (plenty of low VOC sustainable options available) and then source a mixture of door handles from reclamation yards and second hand shops.


5. Window efficiency is about more than just double or triple glazing

How a window is installed is as important as the performance of the window itself. A poor arrangement can reduce the effectiveness of even the highest quality Passive House components.

Of the two images shown, the junction with the window located within and surrounded by insulation performs 30 times better than the other, more standard, window installation.

For a house extension, you will not be able to achieve such great details, but you want to aim to create a continuous thermal envelope and make sure the insulation overlaps the edge of the window frame by as much as possible, either externally or internally.

Also watch out for manufacturers claiming good U Values but only listing the value for the centre of the pane. This is no use if the spacer around the edge and the frame perform poorly. The whole window U value needs to be considered with an installation that is equally robust.


6. Be wary of eco-bling

Solar panels are becoming increasingly efficient and they do have an important place in the eco-mix. But as the greatest energy requirement in a house is the heating, you will save a lot more money by reducing your heating bills than you will gain by generating your own electricity. So PV should only be considered after the thermal performance of your extension has been optimised – and the performance of the rest of your house improved.

Also, green roofs should not be seen as the default for an eco house extension, as some feel. They do not magically improve the thermal performance of your house extension, not least because much of the year they are wet, and in some instances they can also require additional structure to support them.

Where they are most effective is in providing biodiversity corridors in urban environments and helping to reduce the heat island effect. Their benefit – environmentally at least – is less clear cut in a more rural location, though there are still aesthetic reasons to want a green roof.

If you do install a green roof, there are many good proprietary systems out there, but make sure you use one that has a decent sub-base, allowing a good 100-150mm of rooting medium in addition to the drainage and other elements. This will allow you to have a properly diverse wildflower roof that can be a haven for insects.


7. Use an architect

Well I would say that wouldn’t I? But research has shown that good design can add value to your home and considering the investment you are likely to be making, it will be worthwhile to work with someone who can optimize those resources to give you the feel you want.

The Royal Institute of British Architects has produced a guide on ‘Working with an architect for your home’ and if you follow its guidance, and find the architect that best matches you, your house extension can then take the step up to the next level.


8. Make sure you’re going to love your house extension

This may in some ways seem obvious, but if you are going to put all that money (and embodied carbon) into your home, the best way to ensure it is sustainable is to make sure it is well used and cared for.

The parts of my home that I love the most aren’t those that would look good on Instagram. I love the way the light falls through a rooflight at the bottom of the stairs when I come down in the morning. I love the glimpse of a barn that I can see between the trees from the kitchen table. To me these are the moments that make our house a home.

With careful and deliberate consideration of these elements you can create somewhere you love and will have a beautiful, sustainable extension that becomes a vital part of your home for years to come.


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