Part 3 I'm pleased to say that what with the relatively warm 2011 and our conservation efforts we had the lowest consumption of electricity and gas at home of any year yet, a bit over 1,500kWh ('units') of electricity and under 4,000kWh of gas. (A typical UK household is nearer 3,300kWh 'leccy and 18,000kWh gas.) With our solar PV exports we were just carbon-negative for power by my calculations.
Even allowing for the warmer year with Heating Degree Days (about 820 in 2011 for us compared to a more typical 1,100, and a whopping 1,481 in 2010), I could see a small further improvement in heating efficiency last year. Some will have come from the aerogel insulation in my boy's room, but one more gadget had not yet had time to contribute by this year end...
MHRV: Warm story
To save energy at home, you need not just better insulation, but also better air-tightness to avoid leaking warm air to the outside, especially in an icy gale.
Somehow at the same time it is important to maintain decent ventilation to avoid condensation, stuffiness and frankly a build up of pong (my youngest's nappies were all the evidence you needed).
But this ventilation cannot just be 'unplanned' or 'accidental' via leaky walls or constantly open windows in the winter, so efficient buildings often use mechanical heat-recovery ventilation (MHRV) which forces stale air out, pulls fresh air in, and transfers the heat from the outgoing to the incoming air to get the best of both worlds. The same principle of countercurrent heat exchange allows penguins to stand on the ice and yet not have blood return to their hearts icy cold!
Because our house will probably always be too leaky for a whole-house MHRV system like the big boys have, and because I'm kinda cheap, I have installed a single-room MHRV (a Vent-Axia HR25H) in our bathroom. It does seem to keep the air upstairs fresh without opening windows, and air is brought in at remarkably close to room temperature, but it is not helping as much as I'd hoped to keep humidity and condensation down overnight with all of us snoring away (up to 80 per cent relative humidity at night: recommended levels are 30%RH--60%RH).
Partly I'm tackling the humidity coming from the kitchen by putting on a portable dehumidifier in there (for example when laundry is drying) which is a lot more energy-efficient than the tumble dryier and also provides heat somewhere between electric and gas heating in terms of CO2 per kWh to the room.
It may also be time very soon to replace all or nearly all of our ageing double-glazing with triple glazing to take away the cold surfaces for the condesation to form, plus continuing to superinsulate a bit at a time to make sure damp does not form on (or in) the exterior walls.
In summer we'll just turn the MHRV off and have windows open as usual!
I continue to monitor the temperature with iButton sensors (read via OWFS on Linux on my SheevaPlug) in and around the boy's room to keep an eye on insulation performance. Late January's graph:
shows the heating down when we were away a few days over Christmas. Also, after Christmas, it shows a higher daily minimum temperature and more stability since we are no longer throwing open windows in the morning to vent the fog now that we have the MHRV purring away 24x7.