Earth Notes: Towards a Real LZC (Low/Zero-Carbon) UK Home

Can we really go zero-carbon or negative-carbon in an existing home near London? (2007)
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(Article written/started 2007/12/08; we became become a SuperHome in early 2012 with more than 60% energy/carbon savings over an unimproved home.)

Various carbon calculators already rated our 4-person household as pretty good (only just over 1t of CO2 per person per year compared with a national average nearer 2.7t according to the Google UK Carbon Footprint Project tool as of December 2007) and our mains/grid electricity (7kWh/day) and natural gas (10kWh/day for DHW, ie hot water, and in winter an additional 20kWh/day CH, ie space heating) seem also to be comfortably below averages reported by the government, etc.

We have already taken steps to reduce waste of electricity and gas, since conservation is the cheapest and simplest measure to start with.


We've had an extra layer of insulation (~10cm) put in the loft (December 2007) which should keep upstairs a little warmer and reduce wasteful heat loss. "Part L" building regs suggest a minimum of 27cm, and we come up to that level in June 2008, part subsidised by our gas suppliers, if current plans pan out.

We're also on an 'all green' electricity supply possibly saving 1.1t CO2/year if it is really is all from zero-CO2 sources every hour of every day.

We have a little more we can do in the way of heat insulation (eg behind radiators on outside walls) but we've probably got to actively generate/collect some RE (Renewable Energy) to get much lower if we don't want to sit in the dark and cold singing mournful whalesong...

The possible targets that I have in mind are:

  1. To get our house electricity-neutral year-round (eg so that we export as much electricity as we import), almost certainly solar PV.
  2. To cover most of our DHW (hot water) demand from locally-captured RE, probably solar thermal.
  3. To cover all or almost all our heating (DHW/CH) demand from locally-captured RE, probably solar, possibly with a seasonal thermal store and/or GSHP.
  4. To get our house grid-independent year-round (eg we export on all but the darkest mid-winter days) and can run critical loads such as lighting and heating controls/pumps even if the grid is down.
  5. To export enough energy that we compensate for some or all of our energy consumption away from the home, eg at work.

We may not be in our current house for very long, but if we are, there are a few problems with improving its energy performance. For example:

  • The roof is pitched facing east and west and is slighly shaded by some large trees nearby.
  • The property is small (roof area is probably <50m^2, though I have not measured it.
  • A small mains gas combi 'instant' DHW/CH system was recently fitted, and is not terribly suitable to integrate with (say) solar thermal, so might have to be discarded.
  • Though we have some garden space, eg for GSHP (heat-pump) collectors, the area is quite small and is riddled with local sewers, and it is also not clear how low the water table is given that we're close-ish to the Thames and another small river (the Hogsmill).
  • We're not rolling in money!

It seems to me that whatever we do it should be modular, so that we can build the system piece-wise when we have resources, even though this implies some extra expense in scaffolding etc, from multiple visits to fit equipment.

Also, the current UK RE regime does not include many significant/realistic financial inducements to install microgeneration such as a feed-in tariff, so achieving the above targets by degrees allows the possibility of putting off parts that might get better support in future.

Whatever we do, it will be for the benefit of the environment rather than to line our pockets.

Year-Round Zero-Electricity

Almost certainly the easiest step independent of the others would be to go electricity-neutral year-round, ie make as much electricity over the course of a year as we use. This probably isn't zero-carbon for various reasons, but it is a good start. The maximum that a simple (single-phase) grid-tied solar PV G83/1 system can export is 3.7kW (16A) without special arrangement with the local DNO (Distribution Network Operator, ie the owner of the local power cables), so we could probably usefully put up a ~4kWp system.

(Some islanding/battery capacity could ensure that we avoid drawing power from the grid during the 4pm-to-8pm mid-winter peak when generation is most carbon-intense and is competing for gas with domestic heating, as well as maintaining our lighting and other essentials in a power-cut.)

Energy Analysis

This ~10kWh/day year-round RE average would be the equivalent of about 37% of our 2007 average daily grid energy imports (27kWh/d electricity/gas year-round average).

CO2 Analysis

The most favourable/optimistic analysis of the CO2 saving this would make is to (a) ignore the time for the equipment to pay back manufacturing CO2, and (b) to assume that the grid acts as a perfect storage medium such that importing/drawing 1kWh (eg on a winter's evening to power lighting) is exactly balanced by exporting 1kWh at a different time (eg at summer noon), and that exporting excess defers use of carbon fuels for another grid user. Under this analysis we can assume that every kWh of electricity saved/exported avoids production of ~0.43kg of CO2 and that we can ignore ignore other transmission (etc) costs. In London, 1kWp of PV might produce a year-round average of ~9.9kWh of power (assuming 2.47Wh/Wp/day from south-facing optimally-inclined fixed panels). Our reduction in CO2 per day is therefore 9.9kWh*0.43kg/kWh, ie ~4.25kg/day or 1555kg/year, taking us to a net negative CO2 for electricity of 452kg/year, ie we are saving nearly half a tonne per year of CO2 that would otherwise have been produced generating power for our neighbours.

A more pessimistic view of the CO2 savings would start by discounting the manufacturing energy costs from the total, ie maybe 5--6 years amortised over a useful 25--30 year average equipment life (panels often have 20+ year warrantied, grid-tie inverters more like 5, and lead-acid batteries may need replacement every 3--10 years for example). So, from the ~9.9kWh/day leave ~8kWh/day energy after manufacturing.

Then, conservatively, we should assume that an exported kWh may displace only the most efficient (natural gas) grid generation sources, maybe at a little over 0.2kg/kWh (not allowing for transmission losses). Thus, a light on on a mid-winter's evening when not powered by our PV costs maybe twice the CO2 that would be saved by powering that same light from PV instead of using minimally-carbon-intense non-baseload daytime grid power. And except at noon on bright non-winter days, a 4kWp array is unlikely to be able to cover the instantaneous power draw of the heating elements of appliances such as our kettle, washing machine and dishwasher. Let us assume for the moment that the year-round average mismatch of generation and consumption like this is (say) ~50%, ie that we will have to import 50% of all the units we use, even though that leaves more units to export each day.

We'll ignore transmission losses on exported units, assuming that near neighbours will consume whatever we export, especially since we did not allow them in our 'least-intense-displaced' figure.

Instead of importing all our 2.6MWh/year at 0.43kg/kWh (1.1t CO2), we import half (producing 0.55t C02) and have 4.5kWh/day (1.6MWh/year) to export saving generation of maybe 0.33t CO2 on behalf of our neighbours, so a total CO2 saving of ~0.88t CO2 per year.

In principle, given that we are already on Ecotricity's 'all green' traffic, our average kgCO2/kWh figure for imported power should be lower, even zero, if we're really getting 'green' power round the clock (eg solar/wind/hydro). That would mean that our 'saving' would only be the figure that we save our neighbours by making their electricity more green. That would be our entire average 8kWh/day (2.9MWh/year) at ~0.2kg/kWh, ie ~0.58t CO2 output avoided each year.


ItemBenefitCost £/otherFuel Savings/Earnings £patCO2pa saved min/max
4kWp Solar PV Takes us electricity neutral (zero-energy, not zero-carbon). £28k (ie ~£7/Wp) + ~28m^2 of south-facing roof space £600pa from reduced import and selling excess power and ROCs 0.58/1.55
1 day's 'essentials-only' battery backup (~5kWh) Minimises power draw at peak grid demand and thus carbon intensity, and gives power-cut protection, eg for lighting and other essentials. Might also allow fast/frequency support to National Grid. £1k to be replaced every 5 years approx ? support fees. ?

Solar DHW (SDHW) and some CH Support

The next logical step would be to put in an oversized solar thermal system to cover most DHW (hot water) demand year-round, even in winter, with overheat protection in summer, possibly using a drainback system, (near) vertical panels, and some overhang shading for summer. Normally these systems are designed to just provide all the DHW in summer and thus maybe only 20% to 30% in winter; I would like a higher winter fraction and thus summer overheat protection would need to be better than usual. The oversized system should include provision to attach the CH radiators and/or future underfloor radiant heating and other heat sources such as GSHP. (It would be unfortunate if we could not retain our existing reasonably-new and efficient gas combi as topup/backup for DHW and CH for the winter.) One datapoint suggests that with care maybe 67% of year-round DHW, and some CH support) could be provided by solar thermal this way.

I would prefer the solar thermal pumps to be driven by directly-attached PV, both to eliminate any parasitic mains electricity use, and also to ensure that the system need not stop if there is a power outage.

Note that if we wanted to expand this later to CH (central/space heating) then we'd need to be able to add more capture area on our small roof and so we'd need to ensure on the initial installation that there is space left (given possible competition with space for solar PV too) and that the water volume in the collectors remains sufficiently small to allow efficient energy capture in winter; both of these suggest selecting evacuated tube solar collectors from the start.

Energy Analysis

This ~10kWh/day year-round average RE average would be the equivalent of about 37% (without a seasonal thermal store excess energy in summer is not used) of our 2007 average daily grid energy imports (27kWh electricity/gas year-round average).

CO2 Analysis

If this were simply to cover all DHW heating (~10kWh/day) year-round, which should be possible with 13m^2 at somewhere between 70* and vertical, that would avoid production of ~0.19kg of CO2 per kWh of heat from burning mains gas, ie ~0.69t CO2 per year.

Conceivably this might also contribute to CH (space heating) in autumn/spring, which might save another 10kWh/day for 3 months, bringing us to 0.86t/year.

In this case we can assume that the manufacturing energy cost is paid back very quickly, in a couple of years typically, and so can be neglected.

ItemBenefitCost £/otherFuel Savings/Earnings £patCO2pa saved min/max
Solar Thermal DHW+ Saves ~50% of mains gas use for heat ignoring cooking (reduced carbon footprint), covering DHW all year round. Maybe £10k + 13m^2 of south-facing vertical wall £120+pa reduced gas bill at 2007 prices 0.69/0.86

Space Heating: GSHP and Seasonal Thermal Store

In theory, and ignoring cost, we can safely (ie no overheating problems) have as much solar PV as we like, and export or simply not use any excess. Supposing that our solar thermal generates enough on average per mid-winter day to cover DHW (10kWh/day), then we could cover the balance (CH at 20kWh/day) with a heat-pump (GSHP and from a moderate seasonal store) of CoP>3 and ~6kWp of extra solar PV (generating a mid-winter 6kWh/day).

If we wanted to export all or most of the excess electricity in summer then we'd probably need to upgrade to a 3-phase supply, at which point a G83/1 grid-tie inverter can export a little over 11kW maximum. A little more might be available by negotation with the DNO.

If we're covering ~67% of year-round DHW/CH demand directly from solar thermal then we might be able to size our thermal store to maybe 25% of the total, allowing for some greywater recovery, and using the area around the water tank and an extension of the thermal store. That implies maybe 20kl (20t) of tank. That's almost small enough to go in a cellar in the house, and a slow leak of heat back into the house in winter might be welcome providing it can be minimised in summer.

It would be better to use underfloor radiant heating (UFH) than radiators in conjunction with the heat-pump to keep the CoP as high as possible.

Energy Analysis

For 6 cold months this would replace ~20kWh/day of imported gas for CH demand, and for the remainder this would probably export ~20kWh+/day of electricity. This ~20kWh/day year-round average RE average would be the equivalent of about 75% of our 2007 average daily grid energy imports (27kWh electricity/gas year-round average).

CO2 Analysis

The CO2 savings on the the gas side are fairly simple: if the system works as intended then it should eliminate the need for mains gas for space heating (except possibly for a few exceptionally bad days). That would therefore save 20kWh/day * 0.19kg/kWh * 180 days = 0.68t. We must be careful not to double-count any of the savings from CH-assist from the SDHW 'module'.

If we assume that exported power (when CH is not needed) is only displacing minimally-intense grid generation then we have might save (say) 20kWh/day exported * 0.20kg/kWh * 180 days = 0.72t. That figure does not explicitly discount manufacturing energy costs, etc.

ItemBenefitCost £/otherFuel Savings/Earnings £patCO2pa saved min/max
Additional 6kWp Solar PV to Drive Heat-Pump Eliminates grid gas use for heating (takes us zero carbon). £42k (ie ~£7/Wp) + 3-phase connection + ~42m^2 of south-facing roof space or south-facing ground, though possibly less if angled at 70°+ for maximal winter efficiency. £500pa ~4MWh in summer exports at ~£0.13/kWh. 0.72
GSHP Multiplies heat gain from solar PV (takes us zero carbon). £7k + maybe £3k for collector pipework and civils £120+pa reduced gas bill at 2007 prices 0.34
20kl Seasonal Thermal Store and Greywater Heat Recovery Improves CoP, possibly to 5 or better (takes us zero carbon). Maybe £10k including tank and equipment and civil engineering. Improves performance of GSHP above. 0.34

Going Electricity-Negative

Adding a final 4kWp of solar PV to the original 4kWp that took us electricity-neutral, plus enough battery store for maybe a week, should ensure that we never needed to draw electricity from the grid (except in the the most long, gloomy and unpleasant stretches of weather) even mid-winter, so that we would become pure exporters (or zero) all the time.

Energy Analysis

This ~10kWh/day year-round average RE average would be the equivalent of about 37% of our 2007 average daily grid energy imports (27kWh electricity/gas year-round average).

CO2 Analysis

Assuming the same assumed energy output per year from 4kWp (2.9MWh/year) as for our first 4kWp, but assuming that all of this is exported (and used by near neighbours, so with minimal transmission losses), then our low CO2 saving figure is based on our assumed displaced least-intense 0.2kg/kWh figure, and our highest estimate at the standard 0.43kg/kWh, ie in the approximate range 0.58t to 1.3t.

Using our battery bank to ensure that we never need to import even mid-winter when grid demand is highest and generation probably most carbon-intense (providing that our inverter can handle the highest (heating) loads), allows us in principle to claim the full CO2 savings from our first 4kWp too. If we assume that any imports on our 'all green' tariff are already zero-carbon, the gain is just that of minimally-intense generation from all our PV power, which is already the minimim saving claimed.

Note that using energy via batteries, ie charging and discharging them, itself wastes some energy, maybe as much as 20% of that used in charging. Thus 8kWp of panel, after battery losses, may only just carry 7kWh/day load, leaving nothing at all to export mid-winter.


ItemBenefitCost £/otherFuel Savings/Earnings £patCO2pa saved min/max
Additional 4kWp Solar PV Eliminates electricity import from grid (takes us negative-carbon). £28k (ie ~£7/Wp) + ~28m^2 of south-facing roof space £600pa eliminated imports and ~3.6MWh in exports at ~£0.13/kWh. 0.58/1.3
1 week's battery backup for bad winter weather (~50kWh) for above Ensures power use is maximally netted, and gives power-cut protection. Might also allow fast/frequency support to National Grid. £10k to be replaced every 5 years approx Possible £1000+pa in support fees and/or ability to export at peak times in future for a better price per kWh. ?

Doing It All

So what would be the capital cost (and other resources) of doing it all, and essentially going off the electricity and gas grids for heating?

ItemBenefitCost £/otherFuel Savings/Earnings £patCO2pa saved min/max
Solar DHW, CH, and PV Going carbon negative by ~8MWh of electricity (3.4t of CO2) per year. £130k+ (made up of £100k solar PV (+100m^2 of south-facing roof and/or unshaded ground) + £10k solar thermal (+13m^2 of south-facing wall) + £20k GSHP and seasonal thermal store + £2k/year in batteries) £1k in savings and fees at current rates. 3.2

At current retail (and reselling) energy prices, and without external incentives such as government grants, this is clearly not financially viable given typical 25 year equipment life, but that is not the motivation.

If this scheme is viable, and once it has recouped its embodied energy, this would be taking our CO2 footprint at home from +1t/year to -1t/year each, ie firmly negative, and might well cover some of our other activities.

Other than the capital cost (ie upfront cash), the main limitiation is probably the collector area needed, which needs to be mainly unshaded and south-facing in order to maximise efficiency.

Energy Analysis

We'd go from an import of ~27kWh/day to an export of >~20kWh/day on average.

CO2 Analysis

If we applied all the above mechanisms we might reduce our household CO2 output each year by a little over 3t by a reasonably pessimistic analysis.

It is fairly clear that the most cost-effective component to reduce CO2 emissions per £ spent is probably the solar DHW, so possibly that should in fact be the first 'module' to be implemented. It can be constructed so as to continue to produce hot water even in the face of failure of mains electricity (and gas), ie be completely autonomous with pumps driven by small dedicated solar PV, and depending on the details of the construction and plumbing might mean that we could divert a little heat to keep the worst of the chill off the house which could be particularly valuable with children.

Location, Location, Transport

We would like to move to somewhere close to a major train station with a fast London connection. To get enough space for the above LZC scheme we may have to move a little further away from public transport links/termini, and so we might need a zero-emission vehicle such as an electric scooter or car to use for commuting, local shopping and general transport, if no longer possible on foot as now.

The costs (and charging) of such a scooter/car should be part of the scheme. For example, a new G-Wiz i at December 2007 prices is about £10k with about 40 miles range on a little under 10kWh, so would be tough but not impossible to accommodate within the above scheme mid-winter (a full charge is more than 1 day's electricity or DHW energy) but would probably be a breeze in the summer with solar PV though G-Wiz's charger "needs up to 12Amps available to work properly" according to their engineers, so still might force some power import. The G-Wiz can accommodate 2 adults plus a child and (for example) shopping. (A second-hand G-Wiz is ~£5k.)

There are parking and charging points in many of the main places that I would go to on business around London. From my home to my main client is ~16 miles, well within the car's range, especially given the availability of a charge point at the client's site.

Another electric car is the [defunct: ""] MEGA City with a similar range for a similar price, and there's an [defunct: ""] electric scooter with a 70 mile range for about £7k. So there is already a bit of choice and infrastructure available in London. I did a test drive.

Since the car's batteries represent a little over 1 day's home electricity, it is just conceivable that they could be part of the scheme's battery bank, though G-Wiz has said that direct access to the 48V is not approved of:

"If someone accessed the 48V under the back seat, they would void the warranty and you couldn't monitor if they pull too much current out of the batteries and damage them."


Water is another limited resource, and takes energy to treat and bring to us, drinkable, via the mains, and take away again as waste. (The carbon footprint of 1l (one litre) of UK potable mains water is estimated at 0.298g of CO2.)

Using uSwitch's calculator I estimated our total household consumption (2008/02/11) as ~125m^3/year (including ~33m^3/year each in bathroom and kitchen). That may be a significant under-estimate at ~340l/day, but I'm guessing that the biggest elements are my bath (~75l/day), other showering/baths (~75l/day), washing machine (~60l/day), the dishwasher (~20l/day), and loo flushing (~20l/day), totallng 250l/day, so it's plausible. The Thames Water meter tariff as of 2008/02/11 has fixed charges of £67/year for water and waste, and then usage charges of £1.48/m^3. Compared to our current £280/year bill we could be 20% better off metered. In other words, I suspect that we're already reasonably efficient.


Done and To-Do

While it may not be possible to go completely zero-carbon, there are things that we have already done and more that can be done in our existing small property in suburban London. (Note that staying small and improving an existing building is quite 'green'.)

Some energy-saving things already done:

  • 2007/07: replaced inefficient computer servers (~16kWh/day+ electricity saved).
  • ~2007/08: Tuned server software to minimise power use when on-grid (<0.5kWh/day + ~0.2kWh/day for the DSL modem).
  • 2008/08: upgraded to A+ fridge/freezer (~1kWh/day electricity saved); ~£530.
  • 2008/02/27: installed 1.29kWp grid-tie solar PV (~2kWh/day mean electricity generated).
  • 2007/10/01: switched to 'all renewable' Ecotricity electricity supply at ~5% premium over local supplier's 'standard' rates.
  • Replaced all lighting with CFL or LED as lamps needed replacing.
  • ~2007/07: Took some lighting entirely (and other loads partially) off-grid with solar PV.
  • Made behavioural changes (turning lights off when leaving a room, turning things off at the wall and avoiding standby, avoiding using the tumble dryer, etc).
  • Kept the central heating thermostat down, usually <18°C, and off entirely ~May--Sep.
  • Good (obsessive?) monitoring of energy usage with bills (free!), power meters, etc.
  • 2007/12: some draught exclusion, extra loft insulation, etc; ~£300.
  • ~2008/04: some lagging of CH/DHW hot pipes; ~£5 c/o Wickes!
  • 2008/04/21: relocated existing 12V off-grid system components and wiring to reduce air leaks and to be available in kitchen
  • 2008/05/24: added some cheap (amorphous) PV panels on east of house.
  • ~2008/05: Fitted nylon-brush letterbox draught excluder for front door; £4 c/o Wickes.
  • 2008/06/20: added an extra 20cm of loft insulation; £200 after subsidy from our gas supplier.
  • 2008/07/18: replaced/repaired the two oldest/leakiest/cracked windows (bathroom and one bedroom, already double-glazed) with low-emissivity glass (Pilkington K Glass), argon-filled, with a 'warm edge' spacer bar; ~£200.
  • 2008/08/05: curtain for single-glazed front door and side window (with scheme to deal with letterbox and cat-flap in door); <£45 c/o John Lewis and Wilkinson.
  • 2008/09/29: had thermostatic valves fitted on two radiators (and a boiler tune-up/service done); ~£140. (For comparison, when a minor boiler component failed about two weeks later, the full repair cost was similar.)
  • 2008/09/29: created a (gas) heating energy budget equivalent to ~£30/month at our current gas prices or about 8.9MWh for the year (down on last year), which left us ~2MWh for Oct/Nov/Dec and which we achieved with some effort.
  • 2008/10/11: put net curtain on our living-room's entrance into the hall to help keep the room's warmth in during winter; <£14 c/o Wilkinson. We may yet put in a door too.
  • 2008/11/07: thermostatic valves (TRVs) fitted to all the remaining candidate radiators; a little over ~£400 (whoops: ~30% budget overrun due to various complications with the microbore pipework).
  • Draught exclusion on one openable window where we've replaced the sealed unit (which was cracked) but where the gasket at the edge of the window has not recovered from previous neglect; <£2 c/o Wilkinson.
  • 2009/02/10: installed additional 2.58kWp of grid-tie solar PV (~4kWh/day mean electricity generated), which should make us a slight net exporter/generator.
  • 2009/03/21: replaced dead washer/dryer with more efficient model that can wash in cold water and has a timer/delay to allow it to run overnight when electricity carbon intensity is lowest. The old ZWD1260W consumed 0.91kWh for a 60°C wash of towels with a 13°C mains inlet temperature (the manual specifies 1.10kWh, 63l and 145m for that wash, ie 0.22kWh and 12.6l per kg of washing) our replacement ZWD14581W specifies 0.17kWh and 8l per kg and is A-rated for wash efficiency. A full wash in cold water with suitable detergent uses <0.1kWh, ie ~90% saving! (We also considered the £50 cheaper ZWD16270 with a faster spin (1600rpm vs 1400rpm, the dead machine having been 1200rpm) and thus nominally-better energy rating, but reduced ability to wash in colder/unheated water, smaller maximum load, and no delayed-start timer.) Note that the Energy Saving Trust doesn't suggest any washer/dryers at all, which is blinkered IMHO, since it could at least recommend them on the basis of wash efficiency where most washing will be line-dried, given that it does recommend some tumble dryers. Cost ~£470.
  • 2009/03/30 and 2009/04/06: did whole-house air-leak testing ("pressure test" to verify a dwelling meets parts F and L of the UK building regulations) with smoke-pencil tests to highlight problem areas. Air permeability (at 50Pa underpressure) measured to be 7.1--7.2m/h (ie ~7m^3 per m^2 of wall+floor+ceiling area per hour), and is more than twenty times Passivhaus standard for example. Cost of one/cheapest permeability test £170+VAT.
  • 2009/04/18: used expanding foam filler (fire-rated to 4 hours per BS 476 Part 20, lurid pink, in aerosol canister) for first attempt to block some of the most egregious air leaks identified during pressure testing; £13 c/o Wickes.
  • 2009/07/20: re-fitted a living-room door to keep the room's warmth in during winter. The central-heating system thermostat is in the living room. £145 including fitting.
  • 2009/07/20: replaced the front-door and side-panel with double-glazed/insulated units as the only single-glazed and single-plywood sheet areas of the house. Only a small proportion of the cost can be attributed to energy-saving (~£235); most (£1000) is nominally redecoration/security.
  • 2009/08/29: bought some more push-on foam water pipe insulation so as to be better able to insulate the hot pipe that passes under the bath (on the water to the kitchen for example) and wherever else I fancy. Wickes stopped carrying stock for a couple of months inexplicably, so a little under £6 in Homebase.
  • 2009/09/13: tidied up loft insulation disturbed by electrician fitting our last round of PV and loft light.
  • 2009/09/14: re-fitted side of bath, partially insulating the hot pipe under it and filling most egregious gaps that may cause air-leaks, though allowing some venting to prevent build-up from any condensation, etc; not at all air-tight to the room currently, which we may resolve when we provide a cosmetic finish.
  • ~2009/09/25: replaced 'laptop' server with SheevaPlug that uses about 4W and runs almost entirely from the off-grid solar PV system, saving about another 0.5kWh/day of grid electricity.
  • 2009/10/25: put up closer-fitting lined curtains in the living-room; ~£100.
  • 2009/11/02: replaced the outer seal/gasket (and fitted a "weather bar") on our living-room double-glazed doors, mainly to keep out leaks from heavy rain but partly to reduce draughts, ~£80.
  • 2009/11/19: deployed thermal (tog ~2.3) underlay under living-room carpet, £50 in total more than if we'd used the typical underlay that we are using elsewhere.
  • 2009/12: insulated a little more pipework, especially the central heating flow and return pipes at the boiler, which is near the fridge/freezer, in the hope of a double benefit!
  • 2009/12/15: replacement AAA-rated dishwasher which should give a better wash in less water and for about the 0.8kWh that we were using before.
  • 2010/04/26: extra grid-tied PV to bring total generation to ~4MWh/y which brings us very close to zero-carbon for gas and electricity.
  • 2010/06/06: hung solar-reflective blackout linings on curtains in one (child's) room to help keep out excessive light and heat in summer and keep the room warmer in winter, ~£23 including P&P for 66"w/72"drop from The Linen Depot.
  • 2010/07/14: internally superinsulated (dry-lined) the living-room external (north and west) walls to help make the room the warm focus of the house in winter with Spacetherm-P (40mm aerogel) to reduce the U value to <0.3 for current building regs and best practice (CE189) which also suggests improving air-tightness to 5m^3/h/m^2 at 50Pa so special attention was paid to sealing possible leaks, eg with expanding foam. Ceiling insulated with 100mm glass wool above new plasterboard (and Xtratherm foil-backed rigid foam against the noggins). Target was to save at least 500kWh/year/£1000 spent (thus >100kgCO2/year/£1000capex out of current ~1.7tCO2/year gas emissions) to match possible emissions savings from solar DHW but I hope to actually save ~1MWh/y gas (~0.2tCO2/y) and the total cost of project including Spacetherm (and radiator reconfiguration) is approaching £6000, but experimentation with new materials, comfort and redecoration were considerations also. A practical target is that even if our heating fails on a cold winter's day, that room should stay comfortably warm with 3 or 4 people in it.
  • 2010/08/27: moved living-room radiator to internal wall (to reduce heat losses through exterior wall, and replaced with more efficient (W/deltaT) design but physically much smaller.
  • 2010/09/04: fitted 5m of push-on foam insulation to the most exposed (mainly flow, but some return) new pipework to minimise stray heating of the kitchen, £2.49, and the priceless assistance of my not-quite-5 daughter including a discussion of the properties of trapped-air insulation!
  • 2010/09/18: hung solar-reflective blackout linings for living-room for extra insulation in winter intially; £42+P&P (two pairs of 90"x66" from The Linen Depot) plus about £1.50 for curtain hooks.
  • 2010/09/25: replaced our ancient sub-2l slow cooker with a 3.5l model so that we can do more 'low-energy' hot meals, especially in winter; £9.99 Argos "Cookworks" model on offer.
  • 2010/10/18: sealed gap round gas-boiler flue through outer wall with cement (while builders were fixing the guttering, no extra charge).
  • 2011/02/02: borrowed a thermal-imaging camera to look for thermal bridging and other weak spots that need fixing, but saw none glaring; free!
  • 2011/02/26: took delivery of another 170mm (rockwool) loft insulation to try to ensure that we exceed building regs wherever possible; £30.
  • 2011/03/06: had a professional thermal imaging survey done to double-check my own efforts; found several things I'd missed including evidence of air infiltation up to first transverse (gable-to-party-wall joist) in all rooms, and poor sealing at bottom of front door, and heat leaks round top of windows at back of house, for example. £195 all-in.
  • (The previous aim stands: halving space-heat energy demand or ~≥2MWh/y saving in a cold winter... Remedial works may include resealing round windows/doors/pipes, thermal carpet underlay especially downstairs, thermal blanket under upstairs wooden floors, thermal insulation of inside (eg aerogel and/or foil plus more plasterboard) of exterior walls including in the loft, some rot-safe (for wood-framed house) insulation in the existing wall cavity, moving radiators to internal walls from exterior walls, etc. Aim to save at least 500kWh/year/£1000 spent (thus >100kgCO2/year/£1000capex out of current ~1.7tCO2/year gas emissions) to match possible emissions savings from solar DHW. The probably-unreachable target would be for 'passive' house standards of ~15kWh/m^2/year space heating (cf 2008/2009 ~80kWh at 76m^2 floorspace, mild 2011 <30kW/m^2). We have to be mindful of possible use of asbestos in the building fabric for this late-60s building.)
  • 2011/04/18: fixed flaws in existing loft insulation discovered in thermal survey, replacing existing loft boards with insulated boards, and topping up to ~40cm with the rockwool (or at least exceeding regs throughout), ~£250 along with other works.
  • 2011/05/11: replaced broken patio door with Ug=1.0 double-glazed Solaglas Climacontrol warm-edge argon-filled 4(Planitherm)/16/4 french doors (target was Uw=1.0, but difficult to do quickly and triple-glazed would be heavy); ~£1500.
  • 2011/10/04: another experimental dry-lining round in bedrooms this time with a different formulation/brand of boards (Magnaline 9mm magnesium board + 30mm of aerogel), less thick because the rooms are used/heated less and to a lower temperature, and this time paying more attention to thermal bridging in various places, for subsequent re-inspection with thermal imaging. Some air-infiltation through the west-facing wall was reduced in the process.
  • 2011/11/07: improved insulation on back/top of plywood loft-hatch with strips of EPS (polystyrene packing) and an offcut of aerogel-backed plasterboard, wrapped in some cardboard packing to try to protect it from knocks, though that may not be enough. (Was in loft to take some iButton readings and measure actual depth of loft insulation: measured/estimated to vary between about 240mm and 300mm.)
  • 2011/12/21: MHRV (Vent-Axia HR25H) installed in bathroom, to see if it helps keep air fresh upstairs and reduce condensation in bedrooms with less/no opening of windows and thus heat loss. ~£300 to buy + 4hrs of builder's time.
  • 2012/03/05: removed cardboard from behind master bedroom radiator (in place at least a couple of winters) to check for signs of mould growth: none seen. Replaced in part with sheet of EPS from some solar-panel packaging!
  • 2012/03/12: glaziers came back to attempt to stop air paths around lock in new patio doors apparently causing draughts, with application of lots of silicone sealant behind exterior plate.
  • 2012/08/07: (badly) shifted remaining small amount of unused mineral wool loft insulation directly above where I sit upstairs much of the day (free). I think that it's unlikely to make much difference, and a grown-up could probably improve the insulation of that part much more, but having the insulation totally unused was annoying me. Put shirt and trousers in cold wash to remove covering of fibres, probably requiring ~0.2kWh!
  • 2012/08/17: had surveyor check the loft: insulation is fine (could put a little more above where I work) but it was the hottest and most humid this year in the loft while he was there and he suggested improving ventilation. He also suggested a free fire inspection by the fire brigade/service with a view to checking the safety of the partition between us and the neighbour in the loft for example.
  • 2012/09/11: replaced all old/leaky/blown double-glazing (2G) with Passivhaus-compliant argon-filled low-E triple-glazing (3G), which should improve air-tightness as well as reducing heat-loss and condensation on the windows; £4220 (including disposal/fitting/VAT) or approx £440/m^2. Assuming existing 2G is ~2.8W/Km^2 and ~10m^2 and average temperate drop across the window is ~10K (7°C external, 17°C internal) and given an average heating load across 6 months of ~4MWh (minimum) or ~900W then at most the windows could drop that by ~200W, but 100W or ~10% might be a more reasonable hope, so ~400kWh of gas or ~76kg CO2 per year.
  • 2012/10/02: on the long transom in the living room the handle was catching and tearing the inner gasket when pulled closed, so Warmlite moved the handle up a couple of mm and we will try harder to pull it horizontally to close and minimise pulling down. If we want the gasket replaced that's fine, but as it is welded in we'll see how we get on without.
  • 2012/10/02: BBA inspector visited today on behalf of FENSA; checked windows for fire egress (open wide enough), low-E coating on exterior side of inner pane (checks for reflection, and non-conduction of house side of inner pane), and achieving at least energy band C (U ~1.6W/Km^2). Interestingly there does not seem to be any reliable way of checking that the inert-gas fill is correct in situ, even with expensive test equipment ~£7k+.
  • 2012/10/11: bought a couple of these sub-£5 Neewer Digital LCD Thermometer / Humidity Meters in the house to help keep conditions comfortable, efficiently.
  • 2012/10/12: had gas boiler serviced; £96.
  • 2012/10/12: attempted to block direct flow of any warm air from new central-heating pipework up behind fridge/freezer heat-dump coils and around compressor (etc) at base; partially insulated return in particular. Should monitor fridge/freezer energy consumption once heating is in use.
  • 2012/11/13: bought 26" LED-backlit LCD LG 26LS3500 'A'-rated TV (29W in use) to replace failing 20-year-old 13" Sony CRT using ~60W (plus a little more consumed by a SCART/analogue converter); ~£214.
  • 2012/11/15: installed i30 i-temp electronic stand-alone TRV for girl's room claiming 30% saving over normal TRVs in a field trial; £25. See how it worked for us.
  • 2012/11/26: installed Lo-carbon Tempra P MHRV fan for the kitchen (ordered 2012/11/12 from DEALEC), partly to be able to disipate excess humidity (from cooking and drying) with less energy cost in cold weather, and partly to improve ventilation generally; just under £180. I was not impressed by Vent-Axia refusing to sell me the unit directly given that I bought the HR25H directly from VA before with no trouble, nor in the poor customer service of the supervisor failing to call me back to talk to me about this; I may escalate.
  • 2013/02/27: (re)installing TRV in living room and starting roll-out soft-zoning (room-by-room open-source electronic TRV control and calls for heat from the boiler) rather than just a single house thermostat.
  • 2013/05: installing 40mm of aerogel IWI on most of exterior wall surface during kitchen refurb. Exchanging gas stove for induction hob and fan electric oven should help reduce carbon footprint as grid gets greener. (Having the (re-arranged) DHW pipework reasonably-well lagged and boxed may have reduced losses from ~60W/m to ~20W/m, thus maybe as much as 400W saved from 24kW max output while a bathroom hot tap is running.)
  • 2013/11: no longer using old house thermostat at all with everything zoned per room with OpenTRV tech; maybe 10% improvement in heating efficiency to ~1.5kWh/HDD12 over last November.
  • 2014/02/17: replaced boiler primary heat exchanger full of many years' worth of scale; £240.
  • 2014/03/14: fitted additional external bypass as boiler internal one working but not really up to the job with rejuvenated exchanger (and had been having to leave hall rad on as bypass); £240.
  • 2014/07/04: analysis of the last ~5 years of 16WW HDD data suggests that a base temperature of 11°C gives the best fit, better than the current 12°C that I have been using, though probably not by much; calendar year-by-year the optimal value changes and is about 12°C±2.
  • 2015/11: north wall aerogel IWI and redecoration of master bedroom and girl's bedroom; ~£10.5k.
  • 2016/08/01: successively upgraded LED kitchen lighting with more efficient devices (60--100W/lm) to two separately-switched bars of 20W, ie 40W max total, mainly warm-white (~2700K) but some cool white near the window.
  • 2016/08: in the process of adding ~500Wp of off-grid solar to try to take 0.5kWh/d load off-grid even mid-winter, now that second-hand PV is cheap.

Non-energy things done:
  • 2010/10/18: installed roof downpipe/downspout with 200l+ water butt for watering garden (slightly complicated by the fact that all our roof gutters drain to the neighbouring house). Primary motivation was to stop gutter again blocking and overflowing and further damaging the facia board and other things... Total cost ~£400 including cementing the flue. Run-off is to garden which should very slightly reduce strain on drains during a rain storm. (Just about empty as of mid-May 2011 though covering all our watering needs until then, following very dry April. Heavy-ish rain overnight 2011/06/05 more than half-filled it... Took until 2012/08/04 to empty the following year after very wet Apr/May/Jun! Lasted until empty 2015/06/21.)
  • 2011/03/12: installed little foot Water Widget (arrived free) to aerate water entering the showerhead, with a claim of being able to reduce shower water use by up to 70% (with an energy saving in heating the water too).
  • 2011/12/02: installed Fluidmaster dual-flush valve while sorting out results of cistern leaking into our electrics a week or two ago, and attempting to improve the ability of the flush to clear the pan, with the aim of using less water overall.
  • 2012/08/22: preparing to plant a (manageable) fruit tree rather than our unproductive and out-of-control bay tree, removed today! A Cox should fruit even with a short summer for example, and the kids like apples.
  • 2012/09/04: called an asbestos surveyor to get an idea of how much a survey would cost given my assumptions about use in exterior roof soffits and old flue: £375+VAT including two samples.
  • 2012/09/12: had fire-safety visit, and the LFB people brought two new piercingly-loud ionisation smoke alarms (without removable batteries) and installed them free of charge. The loft partition did not overly concern them and is now recorded on their database as not firewalled so that they know on any call-out. They claim to be 7 minutes away once called. We will be prompted periodically to have our smoke alarms replaced. (Note: one alarm turned out to be faulty and the other is seemly poorly fixed, so overall not good; complaint was lodged during follow-up 'customer service' call, and LFB turned away on subsequent unannounced visit as I didn't want them breaking the working alarm.)
  • 2012/12/20: received from Thames Water four free water-saving items: Flowpoint water-saving showerhead, kitchen swivel tap head, 4-minute shower timer, and two sets of stickers for the children (I still remember my Thames Water stickers from nearly 40 years ago). 2012/12/23: only one set of stickers was sent; I didn't feel able to put on the kitchen tap widget with the tools to hand; I had to take out the shower water widget (that lets air into the stream) to put on the new shower head because water was simply squirting out sideways possibly due to higher flow resistance. (2012/12/29: the shower timer keeps me focussed and I can be done comfortably in 4 minutes!)
  • 2016/06/08: repair of garden brick wall weakened by frost.

Some possible further efficiency improvements to make, highest-priority/easiest/in-hand first:

  • 2012/10/23: I ran the Energy Saving Trust's online Home Energy Check and got a report which suggests insulating solid floors at ground level (£900) and getting a condensing gas boiler (£2300) for savings of £78 and 308kgCO2 per year. I'll think again about floor insulation when we're doing the kitchen...
  • Reduce air infiltration above ceilings at front and back (and heat leaks above windows) as discovered on 2011/03/06 thermal survey. (Any such fix has to be done in such a way as to avoid encouraging moisture build-up and rot, but we did a little of this under the floor of the boy's bedroom when drylining to reducing the noticeable breeze and temperature drop as detected by the iButton sensor.)
  • Consider building a lean-to greenhouse along our south-facing garden wall to capture as much solar energy as possible in winter, possibly with integrated solar PV or thermal for the house.
  • Consider a heat-recovery unit in kitchen (such as Ecocent) or loft to help pre-heat water with or instead of solar thermal in summer (could also help cool house)
  • Consider pelmets or tracks for some or all of (heavier, lined) curtains to reduce heat loss from convection against cold windows.
  • Fit 'setback' room central-heating thermostat, possibly with timer and/or PIR (Passive InfraRed) movement detector (or other smart heating controls), such as the Danlers PIR thermostat for Heating, to automatically reduce heating when not needed.
  • Replacement larger and more efficient radiators to allow the CH to run at a somewhat lower temperature (and moved to interior walls), possibly assisting with transition to heat-pump.
  • Replacement of appliances such as dishwasher with ultra-energy-efficient (and water-efficient) models as they fail. (Eg tumble dryer using clever heat-pump to warm the air and cool the condensing water and/or "dynamic-demand" grid-friendly features.)
  • Glaze in the porch to reduce heat loss and draughts through the front-door and make a little extra secure storage space, likely to be upwards of £1200 (~2009 prices).
  • Installation of oversized solar DHW for year-round contribution and possible export of some waste/excess energy in summer as grid-tie electricity, or install PV/T for generation of both hot water and electricity from the same panels.
  • Replacement of current <80%-efficient combi gas CH/DHW with winter CoP≥3 air-source heat-pump combi with possible solar-thermal (eg PV/T) assist to reduce overall CO2 emissions from water/space heating. Winter CoP≥2.5 given current UK grid CO2 intensity ensures lower CO2 emissions than direct gas heating.
  • More solar PV to scavenge extra power for an expanded 12V DC system and/or grid-tie, eg placed on porch roof, in garden on south wall (~1kWp potential max).
  • If air-tightness improves enough, add whole-house forced ventilation with heat-exchange (mechanical heat-recovery ventilators, MHRV), or deal with more individual rooms.

Non-energy things that we might do include:

  • Eves and/or felt/tile vents to improve roof ventilation.
  • Install and use another water butt at the front of the house for watering and as a (partial) source of water for our WC cistern with solar-powered pump, with run-off to our garden rather than the public drains.
  • Replace pre-zone-1 bathroom light fitting with zone-1-compliant version.
  • Get a water meter installed so that we can measure what we're using, and therefore manage it. (As of ~2015, Thames Water lets you revert to an unmetered tariff within 12 months of having a meter fitted voluntarily if metering would work out more expensive.)

(UK per-capita annual water consumption is ~150l circa 2009, with the government's strategy to reduce this to ~120l/p/y by 2030, using efficiency measures, metering and tariffs. Roughly one third of UK households are currently metered for water. The south-east of the UK, eg around London, is significantly water-stressed. 1% of all UK CO2 emissions can be attributed to the water industry.)