NET ZERO | RETROFIT TARGETING RETROFITS The energy demand of our homes must be reduced significantly to make a net-zero future viable, but for a high proportion of existing buildings this will be challenging. Passivhaus Trusts John Palmer says upcoming LETI guidance will show professionals what is possible R etrofit and refurbishment difficult, expensive, messy. Do we really need to do this? Maybe we can get away with a bit more loft insulation, turn our thermostats down by a degree and fit some heat pumps. Wouldnt that be good enough? The problem is, our homes in the UK are some of the oldest in Europe, so are poorly insulated and draughty. Overall, they use 480TWh of energy annually, with the vast majority of this being thermal energy for heating and hot water, provided by natural gas. This is a huge amount of heat and much of it is wasted. In a net-zero future, we simply wont be able to combust all that natural gas to produce heat. So, what does our future net-zero building look like? This is a complicated question, but looking at operational emissions only we need to match the energy use associated with the building with the same amount of renewable energy. The generation of this renewable energy doesnt necessarily need to happen within the footprint of the building, but it needs to happen somewhere we cant just hope that theres enough out there to make it all work. And this is where it gets interesting. Renewable energy is almost infinite enough solar energy reaches Earth every hour to power humanity for a year. Similarly, there is 40 times more wind energy in the atmosphere than our current global energy use. However, the practicalities of deploying technologies to harvest this energy mean that it is actually finite. Wind farms, solar arrays and tidal barrages cost money, result in their own embodied carbon emissions and will, typically, need to be replaced every 20 years. How much renewable energy we will have is difficult to predict it depends heavily on future government policy and long-term investment. The most optimistic figures from the National Grids Future Energy Scenarios, however, are showing that we might have around 580TWh of renewable generation by 2050. Thats a lot of energy but it will also, by that point, be our main source of energy. This means that all sectors industry, transport, agriculture and buildings will want their fair share of renewable energy. As we move our buildings away from natural gas to electrified heating, the National Grids figures show that we will need to reduce the thermal energy demand of our homes by up to 75% to make the demand-v-supply equation balance (see Figure 1). Thats a big change. Just to put it into context, adding 300mm of loft insulation to a previously uninsulated loft will result in only a 7% reduction in heat demand. There is some good news, in that we will get a significant reduction from the coefficient of performance that heat pumps offer. However, this wont get us all the way and we need to remember that heat pumps will only be viable in most of our existing homes if the buildings fabric is upgraded to reduce the heat demand to a level that can be delivered at the lower flow temperatures needed by heat pumps to work efficiently. But what about hydrogen? Surely a repurposed gas grid and hydrogen boilers in every home would mean we can carry on being as wasteful as before and not bother trying to reduce demand? Whether hydrogen could become the primary energy supply for domestic buildings is a big question. One way of doing this would be to produce hydrogen from natural The guidance will set out a target for a series of building types, alongside some practical guidance about how to achieve a best-practice retrofit 40 January 2021 www.cibsejournal.com CIBSE Jan21 pp40-41 LETI retrofit.indd 40 18/12/2020 17:08