Charlie explains why, when looking to create a low-energy, sustainable building, it pays to take the time to research and get the insulation right.

When people are approached on the subject of insulation, it’s rarely met with enthusiasm. It’s seen as dull and unsexy, and I am often gently mocked for my nerdish interest in the stuff. But (and it’s a big but), insulation is the single most crucial thing in sustainable architecture in the UK. For many self-builders it is the low-carbon technology that beeps and sparkles with flashing dials that gets them excited. Heat pumps, rainwater harvesting and the like often command a disproportionate amount of their interest, design time and budget. However, in our climate, it is predominantly high levels of insulation that make low-energy buildings really work — the other stuff is just the cherry on the cake.

I have seen projects where the solar panels are on the roof, the rainwater harvester is in the ground, and the mechanical ventilation heat recovery system is threaded lovingly through the structure, but the walls are about as insulated as a northern lass on a Saturday night out! The sad fact is that, despite what the homeowners believe, this kind of set-up will never be a truly sustainable home – let alone comfortable – and while you can replace and update all those glitzy technologies in the future, the insulation is there for good.

If you are building a house and want to do it properly, then you need to get interested in insulation. The types available vary greatly and the choice you make will have a huge impact on your home. If you consider that we spend around 90 per cent of our lives inside buildings, then the quality of the internal air becomes a priority. Many types of insulation are made from petrochemicals and exotic gasses, and the long-term impact these have on human health is not clear.

Performance is another area to consider, as the effectiveness of one insulation compared to another varies enormously. High-performance insulation is often higher in price than the simple, lower-performance stuff, but going the cheaper, thicker route can provide the same level of insulation at no extra cost. The level of insulation (U value) quoted for a given construction is a theoretical measure and is always a best-case scenario. So if you are relying on a thin layer of super-duper foam to achieve your super-warm house, then any gaps or issues with degradation of the insulation will, over time, have a significant effect. Any house built now will, at the current rate of house renewal, stand for over 2,000 years, and there are concerns about the long-term performance of some types of insulation as they slump, settle, degrade, off-gas and get eaten by insects or even vermin.

And what about installation? Rigid insulation in solid sheet form can be hard to work with and needs to be carefully cut and fitted to the gaps in the structure of your house. If this is done badly you can end up with thermal bridging — heat going straight through the open joints. If you look at many houses with a thermal imaging camera, you will see multiple spots with little or no insulation where heat is being lost, leading to higher bills as well as the potential for mould, which will bring with it significant health impacts. Try and use multiple layers of rigid insulation and stagger the joists to reduce the chance of this occurring. In many cases, using flexible or blown insulation is better as it is more likely to fill all the little gaps and spaces — I am increasingly using blown insulation on my projects for its gap-filling properties.

Another issue around installation is keeping it dry during construction. This is critical, as getting it wet will significantly reduce the performance, and even when it dries out the effectiveness will be decreased.

My final point may seem esoteric, but I believe it reaches the very heart of sustainable building: the construction of a house produces a huge amount of CO2 even before someone has moved in and turned on the lights and heating. This is called embodied energy and is the energy used to manufacture, transport and erect a building. It needs to be considered in any project, as most types of insulation use a lot of chemicals and energy in their manufacture. Wood and cellulose fibre insulations, however, do not, and using them actually locks in the carbon from which they are made. Burning wood in a stove or boiler puts the carbon recently sequestered from the atmosphere back into the air (bad for global warming), so making it into useful insulation and sticking it in a house for hundreds of years is infinitely more preferable. The result is carbon-positive buildings that actually reduce the CO2 in the atmosphere as a result of their construction. With this way of thinking, thick walls of wood or cellulose fibre make the most sense in terms of lower bills, healthier homes and a healthier planet.

The world of insulation is very complicated and you need to do your own research, but if you are tackling a building project then you need to engage in this seemingly uninspiring area. Looks can be deceiving.