Sunday, 15 April 2012

A retrofit experience - the savings

It's nearly time for this year's Open Eco Homes when people like us who have modified our homes to use less energy give guided tours to interested members of the public. (This is in Cambridge, organised by Cambridge Carbon Footprint but there are similar schemes in other towns around the country). When we were first invited two years ago we'd only finished the first phase of upgrades and this time last year we had to warn people not to fall in the gaps in the floor where the builders were still at it. (It's a big house). This year finally we are done and at last we have a reasonable idea of what the changes have achieved. Our annual gas usage is down by about half, which is less than we had hoped but we are also a lot warmer than before. 

Using my simple heat loss calculator, the insulation and draught proofing we put in should have reduced our heating requirements by 50%, but our gas bills should be down even more because we have a new condensing boiler too - that should be another 20% savings. Of course we also get hot water and cook with gas which the model does not account for. Our cooking consumption is probably only 5 kWh/day judging by our summer time bills. The hot water panel probably provides half our hot water on average through the year. Here is a more complete list of what we did:
  • Our walls are solid brick but we added insulation throughout, mostly with 100mm of rigid foam.  Theoretically this reduced heat loss through the walls by a factor of 10.
  • We put insulation under the suspended floor downstairs
  • We topped up the insulation in the roof space which we use for office use
  • The new insulation was supposed to be draught proof so we installed mechanical ventilation with heat recovery. This means there is forced ventilation in every room and the air coming in from outside is warmed up by the stale air going out which should recover 80-90% of that heat.
  • We have a new condensing gas boiler
  • We have a solar hot water panel
Our winter time gas usage is down from 135 kWh/day (2011) down to 80 kWh/day (for 2012) which is great but not quite as much as we expected. There are several reasons for this:
  • We are warmer than before. Previously there were parts of the house that we hardly used in winter because they were too cold. The old heating system simply couldn't put out enough heat. Now the whole house is warmer by 1 or 2 degrees and some parts even more.
  • The house is not as draught proof as it is supposed to be, but this is difficult to fix. The worst areas are firstly somewhere in the kitchen, probably distributed all over the roof, and secondly upstairs somewhere under the eaves where there isn't enough space to crawl in.
  • We do have a lot more window than we had before. At night time we use insulated shutters but during the day it seems a shame not to enjoy the natural daylight. (This 3 year building work wasn't just about adding insulation - it started with remodelling half the front of the house, and all the roof space...)
  • We aren't using the wood stove nearly as often as we used to because we don't need to. We used to burn wood every evening through the winter. Now we rarely use the stove unless the temperature is below zero outside and even then we don't really need it. We used to burn say 5kg of wood each night which would add another 20kWh of heat, not on the gas bills.
Judging by both anecdote and research literature, our experience is not unusual. The difference between the expected savings and the actual savings from building improvements, often called the reduction factor (or rebound effect*) basically has two parts:
  • A comfort factor, from heating the house to a warmer temperature than before. This can take back half the savings, if the house was very cold before.
  • An additional factor due to features not being installed correctly - like our remaining draughts, or lack of insulation in places which are difficult to get at.
Plus there is always some uncertainty because houses vary enormously and it is not possible to model each house accurately. Even if you had the time to make the measurements you would still find yourself guessing at structures which are hidden from view.

To complicate things even more, in our case we made other changes at the same time which added more glazing area, and we changed our behaviour so as to use the wood stove much less.

Government policy needs to take into account the reduction factor in estimating what savings are achievable by retrofitting old houses. The Green Deal, which should start this autumn, is designed to help people upgrade their homes by providing a loan with no up-front costs and instituting a 'golden rule' - that the works done must overall cost less than the savings in energy bills, over the loan period. However, to get an accurate picture of savings you need to predict the reduction factor with some certainty, which is difficult. If you are thinking of a retrofit yourself, do make sure you ask your adviser about the reduction factor and take that into account when you are estimating what savings you are will achieve.

* The term rebound effect is also used more generally to describe the fact that when you make savings in energy use you also save money which you can then spend on other things - perhaps a little holiday, or a new mobile phone. But that holiday means spending more energy for travel and so on, and the phone takes a lot of energy in manufacture, so at least some of the original savings are offset by new energy use. If your motivation to save energy is saving carbon emissions then this can be a bit of a downer, but you can choose less energy intensive luxuries than plane travel for your treats.

This article was updated in 2017 to reflect updates to the home heat loss model, affecting the modelled savings.

You won't be able to get the full text of these from the internet but you can at least read the abstracts.

Hugo Hens , Wout Parijs, Mieke Deurinck (2010) Energy consumption for heating and rebound effects Energy and Buildings 42 (2012) 105-110

Geofrey Milne, Brenda Boardman (2000) Making cold homes warmer: the effect of energy efficiency improvements in low-income homes Energy Policy 28 (2000) 411-424

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