CODE FOR SUSTAINABLE HOMES ’ZERO CARBON’ BIAS
Figure 3: Stamp duty savings for different house prices 6000
4000 2000 0
-2000 -4000 -6000
50 100 150 200 250 300 350 400 450 500 Dwelling size, m2
CARBON EMISSIONS: METRICS AND TARGETS
This highlights wider issues regarding carbon emissions assessment. Should carbon emissions be measured by improvement over a building of the same size (the current method), on a per-floor-area basis (the ‘zero carbon’ Hub proposed approach), or by total emissions? The current system permits huge dwellings with very high emissions, since the target is based on a building of the same size. Imagine if the argument that ‘it’s bigger, therefore it must have a higher target’ were applied to cars: small city cars would find it much harder to achieve proportional carbon savings than large off-road vehicles. It is also worth considering whether ‘zero carbon’ emissions are the most appropriate target. The most cost-effective way of reducing emissions is not through renewable energy, but through passive measures. There are many ‘quick wins’ that reduce emissions for minimal cost, but eliminating them completely is much more expensive. It costs more to build one ‘zero carbon’ dwelling than to halve the emissions of two dwellings, achieving the same overall reduction. The majority of the housing stock must be brought up to a reasonable standard, particularly by ensuring that all new dwellings require much less energy to heat. Only then will it be worth reducing emissions to zero by incorporating renewables.
more electricity it might be necessary to add PV to non-south facing roofs, or to use very expensive high-efficiency PV cells. The table on page xx outlines the calculation for a small and a large dwelling. For a house 20 times larger, the increase in occupancy is just over double. The larger house uses six times more energy for internal appliances in total, but only a third as much if measured by floor area. To generate enough electricity, the small dwelling must use nearly half the roof for PV; if it was part of a two-storey block, there wouldn’t be enough south-facing roof. Small dwellings suffer a financial premium for ‘zero carbon’ status, due to the higher proportional energy target and the difficulty of incorporating renewables. If the sales price increase available through attaining ‘zero carbon’ status is a percentage of the total, large ‘zero carbon’ homes become more financially viable
£/m2
2000 3000 4000 5000
to build. This also affects the reduction in stamp duty available for ‘zero carbon’ homes3
, although this is less clear-cut due
to the banding of stamp duty and the cap on the zero-rating at £500,000. The graph in Figure 3 shows the stamp duty saved through ‘zero carbon’ status, accounting for the installation of sufficient PV to meet only internal energy use (in reality this will be slightly higher to cover other electricity use). There are no savings for house prices of £2,000/m2
is comparatively too expensive. There are no savings for dwellings less than 100m2 £3,000/m2
dwellings (up to 250m2
or lower, as PV at
, but there are savings for larger ). The dwelling size
for peak savings drops as prices rise, so the biggest benefit is for small dwellings in very expensive areas. The current means of assessing ‘zero
carbon’ homes is biased towards larger dwellings. This contradicts the drive towards denser urban developments, penalises the less well-off (who live in smaller houses), and provides no incentive to reduce overall emissions. This is not in line with the stated aim of the Code for Sustainable Homes: to ‘reduce our carbon emissions and create homes that are more sustainable’. CJ
Reference
1 The case for space: The size of England’s new homes, Rebecca Roberts-Hughes, RIBA, September 2011
2
www.planningportal.gov.uk/buildingregulations/ greenerbuildings/sustainablehomes
3 Zero stamp duty for zero-carbon dwellings up to £500,000; reduced by £15,000 for those over £500,000.
DR RALPH EVINS is a research engineer in sustainability and building physics at Buro Happold
Should energy for appliances and cooking be related to occupancy or floor area?
It is obvious that bigger houses need more energy for space heating. However, for internal energy uses, the correlation is less clear since there are so many factors to consider. In large houses with few occupants (as assumed by SAP), if lights and appliances are turned off when not in use, energy use could be the same as a small dwelling with the same number of people. Conversely if lots of ‘per room’ equipment is left on regardless of occupancy, and if there are more luxury goods that consume a lot of power, energy use could be much higher than would be proportional. Should energy for appliances and cooking be
30 CIBSE Journal June 2013
related to occupancy or floor area? Energy used by appliances is assumed in SAP to be roughly equal to the square root of floor area multiplied by occupancy.
Lighting energy is calculated in the same way. This implies that lighting and appliance energy use scales equally with both occupancy and dwelling size. There is no means of accounting for low-energy appliances (though there is for low energy lighting). Cooking energy is taken as proportional to occupancy, plus a large constant. This implies that energy for cooking will be fairly constant, regardless of the number of people.
www.cibsejournal.com
Saving, £
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