Tuesday, 21 January 2014

Zero Carbon Homes Hierarchy

The Trouble with Triangles

The governments Zero Carbon Homes (ZCH) policy was launched in 2007 with the aim of reducing the carbon emissions from new homes built in the UK to 'near zero'.  I've written in the past about how this policy has been successively watered down and why the argument put forward in support of 'easing the cost burden on house-builders' is fundamentally flawed.

Recently, however, I started to wonder about the way the proposed structure of the ZCH policy was presented.  No government pronouncement on ZCH is complete without a pictorial representation of the ZCH Hierarchy.

The misleading qualities of geometry?

This image, reproduced from the recent DCLG consultation on Allowable Solutions, is shown above.  I have carefully scaled the triangle from this document for reasons that will become apparent when you read on...

The base of the triangle represents the carbon emissions from a dwelling built to 2006 Building Regulations, and the top of the triangle represents the 'pinnacle of achievement' that is a Zero Carbon Home.  The layers of the triangle represent different parts of the proposed policy. 

Starting at the bottom, we have the Fabric Energy Efficiency Standard (FEES).  The idea is that developers should 'build-in' efficiency gains to the property first through higher levels of insulation, lower draughtiness and installing an efficient heating system.  Government plans to legislate a minimum standard the home must achieve with such measures.  This concept was recently implemented into Building Regulations for the first time in the 2013 version.

The next layer up the triangle  is called "Carbon Compliance".  It represents the reduction of carbon emissions on-site through the use of low and zero carbon technologies such as solar panels.  Government plans to set a minimum level of carbon reduction that must be achieved on site.

Finally, for that difficult to achieve 'last little bit' the developer can buy their way out of building the energy saving into the house and instead pay into a fund - details to be confirmed.

The shape of the triangle produces a reassuring impression that the heavy lifting is going to be done by measures actually on the property, and that only the last little bit will be simply 'bought'.

Hang on.  Let's have a look at the actual figures.  Government is favouring proposals from the Zero Carbon Hub.  In their report, "Fabric Energy Efficiency for Zero Carbon Homes", the proportion of carbon reduction from each type of measure can be derived from the figure on the last page.  I have tabulated the values below:

In every case, the bought-in Allowable Solutions represents the largest part of the savings.  If the Zero Carbon Triangle were to be re-drawn so that the area of each section represented their relative contributions, it would look very different.

How it ought to look
Of course, if developers use the Allowable Solutions mechanism then the energy bills for householders purchasing a so-called 'Zero Carbon' house will be much higher than if developers choose to meet the target wholly with fabric improvements and renewable energy on the building itself.

Let's at least not kid ourselves about how far away from delivering zero carbon on the ground we might be by accepting misleading graphics at face value.


  1. I had a look at a supposedly Code 6 new build recently of some massive houses.

    They were aiming to achieve code 6 via the use of a massive pellet boiler and 10kWp of solar PV.

    3 of these houses were being built next to each other, but nobody had considered the need to ask the DNO about the potential for actually installing 30kWp of solar PV in 3 houses right next to each other, and 2 of them already had the panels installed, with the 3rd just about to go on when I did a survey prior to quoting for the PV (which we walked away from).

    I suggested solar thermal to reduce the actual bills in the houses and stop the huge pellet boilers being run for water heating alone in summer, but apparently this would have made no difference at all to the Code assessment as burning wood is viewed as being as green as solar water heating under the code assessment.

    The insulation levels were better than building regs, but nothing that special for supposedly the top level zero carbon homes - certainly nothing like passive house standards, and the insulation detailing was pretty poor..

    I didn't even know anything about the offsetting payments you refer to, so thanks for this blog... something has gone seriously wrong with this scheme from what I can see.

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