Passivhaus carbon emissions

There is a perception that Passivhaus buildings use hardly any energy because of all of the insulation they contain. Whilst this construction standard provides unrivalled comfort and massively reduces the heat load required it does not necessarily mean the house uses much less energy for anything else.

In a conventional house the heat losses during the winter months are such that you need a significant amount of heat input to maintain a sensible temperature internally. Even when it’s not that cold outside the strong winds that we’ve been having mean that lots of heat is lost from drafts. Additional gains from the sun and waste heat from appliances are relatively insignificant and so are mostly ignored.

At Passivhaus levels of insulation and airtightness the amount of additional heat energy required is minimal (so far this winter the Silverton Passivhaus has used 260kWh of heat or about £13 worth of gas). However, this is not to say that no heating is required at all as you do still need to heat and you still need energy for appliances. It is simply that everything else done inside a building that produces heat becomes the heating instead.

From one angle this is obviously a big benefit as the carbon emissions from each dwelling can be reduced enormously but with a large caveat. Gas is widely used for heating and, in a modern condensing boiler, has the lowest carbon emissions per unit of heat produced of any of the fuels.

In the move towards ever more energy efficient buildings we are becoming more and more dependent on electricity as an energy source. Modern homes are ever more packed with electrical devices for security, ventilation and environmental controls, all of which demand ever more electricity and create a much higher base load for the building. We are at risk of substituting low carbon heating energy for much ‘dirtier’ (from a carbon perspective) electricity in the drive to lower our heating bills.

Grid electricity currently produces around 0.55kg of CO2 per kWh and natural gas produces around 0.185kg per kWh. To illustrate the point, the average UK home consumes 16,900 kWh of gas and 4000 kWh for electricity. This would give a combined CO2 production of 5326 kg. Whilst me and my family may not represent an ‘average’ household our gas usage has reduced to around 7000 kWh but our electricity is around 10,000 kWh. This means that our energy usage produces 6795 kg of CO2.

Our heating/water heating requirement now costs about 75% less than our old cob house (which was also 1/3 the size of the new Passivhaus) but our electricity bill is slightly higher, even though we have LED lights everywhere and A+++ rated appliances. I assume this to be because of the higher base load from ventilation and control gear which we hadn’t anticipated.

To summarise, Passivhaus and other low energy standards provide un-rivalled levels of comfort and vast reductions in heating requirement but they do not necessarily provide the reductions in CO2 emissions that one might expect.  Unless the design is kept simple and the temptation to install the (often vast) array of controls and systems that are stuffed into ‘Low Energy Buildings’ is resisted we may not be reducing our impact as much as we think. Until grid electricity becomes cleaner and lower carbon we should rely on it rather less to achieve our goals.

Chris Brookman
Chris Brookman