Monday, 1 May 2017

How much can we reduce heating demand, realistically?

Around 40% of our energy use is for heating in buildings, both space and water heating, and a large proportion of this is fuelled by gas with associated carbon emissions. We can reduce this either by switching fuel or by reducing demand or both. According to Zero Carbon Britain (ZCB) it is feasible to reduce the demand for domestic heating by 60%. However the Committee on Climate Change disagrees - they have assumed 17% in their scenarios. Which is right? Also, how much progress have we made so far?

Overall heating demand is important because low carbon supply is limited.
This is important because whichever way you switch fuels there are practical limitations to supply. We can switch to biogas from waste, which is limited by how much waste we have (CCC reckons 5%), or we can grow biomass crops, but then we are limited by how much land is available. Or we can switch to electricity, which would mean much more generating capacity - even more wind turbines, or more nuclear power stations, depending on your view.

ZCB view: 60% improvement possible from a range of measures.
This figure shows how the 60% reduction proposed by ZCB comes from: 40% from insulation, another 10% by fixing the draughts and a final 10% by better heating controls leading to a lower average temperature.
The impact of measures that reduce a building's heat demand, Figure 3.7 from [1]
ZCB's analysis was based on what we knew in 2010.
These figures are based on analysis for the 2050 Pathways project published in 2010 [3], reflecting what was considered achievable at the time. For example, they quote solid wall insulation reducing heat loss by 84%, cavity wall insulation achieves 78% and draught proofing reducing air leakage by two thirds. Their level 4 (Apollo mission level effort) scenario achieves a 50% drop in average heat loss coefficient, in line with ZCB's figures.

In practice, 50% drop in heating energy is hard to achieve. According to national figures (for example from the National Energy Efficiency Database [4]), savings are generally much less. There are a number of reasons for this.

Uninsulated solid walls and some cavity walls are not quite as bad as we used to think.
One reason is that the 2010 analysis was based on the best available knowledge at the time for things like average heat loss through solid walls. Since then, field measurements have meant we have updated our assumptions [5]. For example, solid walls were assumed to have a U-value of about 2.2 W/m2/K. Now we think the average is more like 1.5 W/m2/K [5] though some solid brick walls have been measured as 0.8 or less. Unfilled cavity walls were assumed to be about 1.6 but current models use lower levels for homes built in the last 30 years: 1.0 or even 0.6. The lower U-values mean the heat loss was not as bad as we thought before, and adding insulation makes less difference.

Retrofitting insulation is awkward and expensive to do well.
Another reason is that insulation is not always fitted as carefully as it could be. This is especially true for solid wall insulation. Most builders installing external wall insulation will not take the trouble to move an electricity or gas meter on the wall (which can cost £500), and are likely to work around soil pipes. The fiddly bits take time and add to the costs. Also they may leave cold bridges around windows and doors, floors and on the roof line. For example, external wall insulation usually stops above the damp proof course, even if this is above floor level. This leaves a massive cold bridge. On the other hand, if you install insulation on the inside it is almost impossible to get to the bit between floors - so there is another large cold bridge. All these bits add up so that after insulation the improvement is not as good as it might have been.

Many homes are underheated before insulation.
The biggest issue is probably comfort taking, and here I think our current research may be pessimistic. A lot of so-called comfort taking is due to previous under-heating and this is worse in fuel poor homes. Since many government initiatives providing insulation have been targeted at exactly those households, the reduction in bills we have seen so far is less than you would expect from the stock as a whole.

50% improvement is achievable - we did it in our house.
Speaking for myself, in our Victorian detached house we have achieved 50% reduction in energy use from fabric improvements and a new boiler - and we are warmer than before (see A retrofit experience - the savings). So 50% reduction is not ridiculous. However, this was a big effort, we were starting from a low base (solid walls) and we spent a lot of money. For example, where we have used external insulation we went down below the damp course to foundations level. The ZCB target might be theoretically achievable but no-one said it would be easy.

The level of demand reduction achievable depends on the effort deployed.
So what is a plausible level of demand reduction? Clearly that depends on how much effort (and cash) you are prepared to throw at the problem. The really big savings come from innovative retrofit techniques like Beattie's Tcosy system, which wraps the building in an outer insulation layer, achieving EnerPHit energy efficiency standards and, in at least one case, 80% decrease in heating energy. Based on this example, 50% is certainly achievable but perhaps not in all cases. Lets say on average you might get 40%.

Over 2005 to 2015, gas consumption relative to heating demand decreased 23%
How much progress have we made so far? Lots of loft insulation and other measures have been installed through grant schemes such as CESP, CERT and the Green Deal. The chart below shows total domestic gas consumption, plotted against degree days which is a measure of heating demand - gas consumption is mostly for space heating and varies from year to year according to the weather. As you can see from this chart gas consumption relative to heating demand has made some progress. Between 2005 and 2015 gas consumption relative to heating demand fell by 23%. However most of this happened prior to about 2011. CESP and CERT closed in 2012 and since then we have only had the Green Deal. Also a lot of the easy savings like loft insulation and new boilers have already been achieved.

Domestic gas consumption [7] and heating degree days [8]

Taking into account the increasing number of homes, the improvement is 30%
That chart shows total consumption and fails to take into account increasing population. Since my measure is domestic gas consumption I have added the number of domestic gas meters into the mix. Over the 10 years from 2005 to 2015 the number of meters increased 8.3% [9]. (This is slightly faster than the increase in population which was 7.8% [1]) The next graph shows gas consumption normalised by both degree days and gas meters. By this measure, efficiency has reduced consumption 30% and is still improving albeit slowly.
Domestic gas consumption [7], normalised by degree days [8] and number of meters [9]

So, it looks like we did really well over 2005 to 2010 but after that our efficiency improvements are only just keeping step with demographics. Unfortunately, ZCBs baseline is 2010 and so from their points of view we have made very little progress.

Subsidies on insulation work come back to the Treasury in income tax and benefit savings.
Most people struggle to afford insulation without subsidy, and that makes it look very expensive to the Treasury, but it is not as bad as it looks at first sight. Insulation materials are actually quite cheap (at least the mainstream ones are) and a lot of the cost is due to labour. This means that even if the work is subsidised by grants, a lot of that money comes back to the Treasury in income tax and savings on unemployment benefit. In practice, the real cost is at most half the subsidy. (See The cost to the taxpayer of Green Deal subsidies is less than you think.)

The biggest barrier is aesthetics.
The biggest barrier to energy saving in our homes is probably not cost but aesthetics and cultural heritage. External wall insulation improves the aesthetics of homes where they are ugly and dilapidated to start with and this can be a big selling feature. However, many people living in traditional homes like Cambridge's Victorian terraces are fond of the interesting features you see on the outside: patterned brickwork, door pediments, decorated roof lines and so on. They don't want to see these covered up.

The choice - more modern home exteriors or more wind/nuclear power.
I like these too - but I also like to live in a comfortable house and a sustainable lifestyle. So we have a choice. If we don't want to wrap up our traditional houses in a modern exterior we can generate even more low carbon energy: the forms that are least constrained by fundamental limits are wind and nuclear power.  Or we can compromise and do a bit of both.

[1] ZCB Rethinking the Future (Zero Carbon Britain) July 2013
[2] Next steps for UK heat policy (Committee on Climate Change) October 2016
[3] 2050 Pathway Analysis (DECC) July 2010
[4] National Energy Efficiency Database (
[5] Francis G. N. Li, A.Z.P. Smith, Phillip Biddulph, Ian G. Hamilton, Robert Lowe, Anna Mavrogianni, Eleni Oikonomou, Rokia Raslan, Samuel Stamp, Andrew Ston, A.J. Summerfield, David Veitch, Virginia Gori & Tadj Oreszczyn (2015) Solid-wall U-values: heat flux measurements compared with standard assumptions Building Research and Information
[6] Caroline Rye & Cameron Scott, THE SPAB RESEARCH REPORT 1. U-VALUE REPORT (Society for the Protection Ancient Buildings) 2012
[7] Energy Consumption in the UK (
[8] Degree Days: DUKES table 1.1.8 (
[9] Regional and local authority gas consumption statistics: 2005 to 2015 (
[10] Population Estimates for UK, England and Wales, Scotland and Northern Ireland

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