Monday, 22 June 2015

Why are we building new homes based on carbon emission standards of yesterday?

The UK regulations for new buildings are based on predicted carbon emissions. On the face of it that sounds sensible, as it is carbon emissions that matter for mitigating climate change. However, the current carbon standards are based on historic data from power generation and in practice this is likely to change dramatically over the next few years. At the moment, generating 1 kWh of electricity produces 2.4 times as much carbon as 1 kWh of gas [1] but in three out of the four scenarios modelled by National Grid, (the ones that hit our 2020 renewable energy target) the intensity of electricity will be as good as or better than gas by 2025/2026 [2].

National Grid's projections for carbon emissions from electricity show that electricity will probably be better than gas from sometime between 2020 and 2026 [2]

Using today's carbon factors it is easier to meet the carbon standards required with gas based heating than with electric heating such as heat pumps, even though heat pumps use 2.5 times less energy than gas for the same amount of heat delivered. However, gas heating will never be low carbon and once you've got a gas heating system it will probably last you at least 15 years, after which you will probably buy another gas boiler because it isn't straightforward to change over from gas central heating to heat pumps (see Do heat pumps deliver?). Or you might switch to a biomass boiler because it can feed the same radiators, but our potential for growing biomass is limited because biomass takes a lot more land than low carbon electricity (see Biomass, wind and solar, how our heating choices affect our landscape). By basing our building standards on today's carbon intensities rather than tomorrow's, we are building new homes locked into a high carbon pathway.

Building regulations are changing over the next few years but sometime late next year our government expects to bring in the Zero Carbon Standard, ultimately based on carbon emissions. The EU has a slightly different policy - the Nearly Zero Energy Buildings standard - based on energy use rather than carbon, but this is based on primary energy (energy used in generation) not delivered energy [3]. It takes at least 2 units of heat energy to make 1 unit of electricity in a gas power station, whereas renewables such as wind and solar are 1:1 so the EU standard still depends on the energy generation fuel mix and so it suffers from the same problem.

The Passivhaus standard for energy use is defined purely in terms of energy used, regardless of fuel. This means that the Passivhaus standard does not change over time. If a home meets the standard today it will still meet the standard tomorrow [4]. However, there are vested interests in the traditional building industry that don't like the Passivhaus standard because it requires changes to building methods. Our government no longer allows local authorities to set energy performance standards higher than the national standard (though they can opt for higher standards in water efficiency, disabled access and minimum space [5]). Also in Ireland, central government is leaning on local authorities not to adopt it on the grounds of cost [6] but not everyone agrees that Passivhaus is more expensive. In fact Passivhaus methods could even cost less rather than more because they rely on more pre-fabricated components, and they don't need so much plumbing for heating.

I am reminded of my recent post on overheating, where I pointed out that building regulations are based on historic climate data, not what we expect in the future (see Why are we still designing buildings for yesterday's climate?). This is the same issue in a different guise - if we must use a carbon emissions standard, let's use values that represent the future, not the past.

[1] Using carbon factors from Digest UK Energy statistics 2014, electricity (overall generation mix) generates 454 tCO2/MWh supplied while gas is 185 tCO2/MWh
[2] UK Future Energy Scenarios 2014 (National Grid) reports carbon factors for electricity generated, not as supplied. Their figure for 2013 is 403 tCO2/MWh compared to DUKES 454 tCO2/MWh for supply. So I have adjusted their projections for carbon emissions to 2035 in the same ratio.
[3] ZERO CARBON HOMES AND NEARLY ZERO ENERGY BUILDINGS UK Building Regulations and EU Directives (Zero Carbon Hub)
[4] The Passivhaus Standard
[5] Housing - optional technical standards (
[6] Irish government in row over passivhaus eco building regulations (Guardian) June 17th 2015


  1. The catch is, we know with certainty how carbon-intensive today's electricity is, but we're only guessing about tomorrow's. And most predictions of future carbon emissions from electricity are hopelessly optimistic. The Government's new Energy Bill, announced in the Queen's Speech, will also make it MUCH harder to build any more onshore wind turbines - the most economical form of zero-carbon electricity available.
    As for Passive House, no-one who knows what's involved on site would argue that it could ever be cheaper than alternatives. Sadly, the history of pre-fab components suggest they almost always push costs UP, not down.

    1. I won't argue with you on the costs of Passivhaus - time will tell, I expect. However, I disagree with you on guessing about the carbon emissions. Firstly, we are supposedly 'on target' despite this government's inconsistent policies. Secondly, if we are a bit late with the targets, it doesn't really matter. The balance between emissions from heat pumps and gas central heating is fairly close even now and it doesn't take a lot of shift to make the heat pumps better. But more importantly, by using the future prediction we can avoid the lock-in to gas which will in the long run produce much higher emissions. Remember it is the total emitted over time that matters, rather than the current rate.

    2. Jason, I too have been skeptical that passivhaus will actually be cheaper than 'building regs minimum', but a builder has an article in the current issue of passivhause+ mag arguing that, (at least in Ireland, where the building regs are somewhat better than ours), there is now no cost premium for PH - indeed it is slightly cheaper. Detailed costings of the various build components are given.

      This probably isn't quite true in the UK yet, but the premium is shrinking all the time, and PHs have been built here by those being careful, which cmoe in well under normal build costs.