Tips for homes that are energy-efficient, comfortable, carbon-neutral with low or zero energy bills
By Richard Keech 2018-05-19
Based on a talk given on Sustainable House Day 2017 at The Cape (Cape Paterson)
Scope excludes water efficiency and materials issues such as embodied energy. Much of this applies to both improving existing homes and new builds.
Caveat. This is not intended to be comprehensive or a structured introduction to the topic – simply my own perspective, in temperate southern Australia, on a few things that are of particular importance or interest to me. So apologies in advance if I fail to mention things that are, perhaps, already well-understood, outside my expertise, or simply overlooked.
Firstly and most obviously it’s important to design to make best use of the sun. Configure the home to admit the winter sun, block the summer sun. This is easier said than done because of trade offs. An example of poor contemporary design is the so-called ‘al fresco’ areas which provide large, under-cover outdoor space at the expense of solar access.
Passive efficiency beats active efficiency
Passive measures work just by their presence – ie no mechanism. Measures include:
- Draught proofing,
- glazing improvements,
- shading and window coverings.
No operational energy is required to make passive measures work, so zero running costs. To a degree there can be a trade off between active air conditioning and insulation – less of one means more of the other. Passive measures are generally more intrinsically reliable and durable. If things go wrong (blackouts etc), passive measures will keep on working.
Second-order effects: improved insulation double benefit
The first and obvious benefit of insulation is less thermal power to keep at the same set point. Beyond this, there is a less obvious second-order benefit to better insulation. This is a more relaxed air conditioning/heating set point. So insulation has a greater positive impact on thermal comfort than generally supposed. So, heating/cooling systems don’t need to work so hard to achieve comfort.
For more on this see my article in ReNew here.
Passive efficiency: Draught proofing is the low-handing fruit
Draught-proofing is often done wrong or incompletely. Common issues are found with:
- exhaust fans;
- fire places;
- gaps and cracks;
- warped doors.
It’s worth getting a specialist to assess draught proofing.
Efficiency beats solar PV
There’s a saying that ‘negawatts beats megawatts‘ [attributed to Amory Lovins]. This means that demand reduction is more important than generation. Efficiency directly makes you more comfortable and reduces both bills and emissions. Efficiency measures can give resilience, whereas solar panels just reduce bills and emissions.
Solar PV beats solar thermal
Electricity is much more valuable and versatile than low-grade heat. Solar hot-water systems generally have high winter-time boost energy requirements. Solar hot-water systems cannot make use of any excess collected energy. Solar hot water isn’t as good as PV plus heat pump. The exception is in a hot climate where solar hot water works year-round.
Solar PV beats batteries
On-site generation of energy is more important than storage of energy. Batteries are still expensive. Batteries good at reducing or eliminating peak demand. Batteries don’t automatically provide blackout protection. Consider batteries once efficiency and solar are dealt with. Staying grid-connected is important.
Economics of PV are excellent
Solar PV economics are better than often supposed. Financial benefit increases with increasing self consumption. However PV will still pay for itself with current feed-in tariffs even if all energy is exported.
Rule of thumb (for Melbourne): Lifetime cost per kWh ~ $/kWcapacity divided by 300.
So $4800 for 4kW = $1200/kW means lifetime energy costs are about 4c/kWh (see The Energy Freedom Home, Chapter 9).
Winter solar more important than summer solar
PV yield varies greatly with season assuming normal panel configuration. There is an energy mismatch in that in winter we have the lowest generation and the highest energy needs. On the other hand, where there is lot’s of PV, in future it will probably be necessary to curtail summertime generation. So, it’s appropriate to optimise for winter generation by having panels mounted steeply. This even includes mounting vertically on facades. So in future I think we won’t be so concerned about maximising annual generation, but in maximising the time that there is sufficient generation.
Insulation beats thermal mass
The common view of thermal mass it that it slows down the change of temperature. This is a two-edged sword. Thermal mass is a good slave but a bad master.
Thermal response time varies with both thermal mass (M) and insulation (I). The way thermal mass and insulation work together is as a function of M times I. So halving thermal mass and doubling insulation have leave thermal response unchanged. Where an active system for heating and cooling is available then a light-weight, highly insulated structure is probably best because it will be able to respond more quickly when needed.
Possible issues with slab-based thermal mass
Concrete is an unforgiving surface – things (including people) that fall on it are prone to break. Concrete brings maintainability issues – pipes are entombed. Access in and under is very difficult for inspection, modification, repair.
If the slab temperature gets outside comfort range then it’s hard to budge.
Embodied energy is high, even with most eco concrete mixes;
In a world where flooding and flash flooding is an increasing risk, it makes sense to get up off the ground, which is harder with a slab.
(See more on this here)
Double-glazed windows not good insulators
Single glazing gives terrible thermal performance. Per unit area, single-glazed windows are about a factor of about 36 worse than a ceiling/roof in permitting uncontrolled heat flow. Double-glazed is much better by a ratio of 2:1, but from a low base. Performance is merely ‘bad’, not ‘terrible’.
- Lesson 1: Double glazing is necessary but not sufficient;
- Lesson 2: Avoid massive windows, even if double glazed;
- Lesson 3: Good window coverings and shading can add significant performance.
I hope that in a decade double glazing will be ubiquitous, and we’ll be talking about triple glazing like we talk about double glazing today.
Indoor air quality is often overlooked
Traditionally there is a trade off between ventilation and energy efficiency, ie closing up tight helps impede heat loss at the expense of air quality. Indoor carbon dioxide, VOCs (volatile organic compounds), and spores are poor for human health and comfort. Air-tight homes need a way of keeping air fresh. So there is an onus on occupiers to ventilate except when necessary. Heat-exchanging ventilators give the best of both worlds but have on-going energy needs. I predict that indoor air quality will become a hot-button issue in the next decade.
Homes that don’t need gas have a number of advantages:
- Avoiding gas costs, high and rapidly increasing;
- Synergy of having a single service change for energy ~$400/annum saving;
- Avoided connection charge for new homes;
- Gas redundant because electric can do the job better in every case;
- Synergy between solar PV and all-electric home improves economics of PV;
- Gas is not actually a low-emission fuel;
- Environmental impact of new CSG is considerable.
Avoid ducted systems for heating and cooling
Ducted configurations are a poor choice because:
- Heat loss in ducts is considerable;
- Less flexible because whole-house circulation;
- Imbalances arising from closed doors of conditioned rooms;
- Configurations for heating with both heating vents and return air vent at ceiling level doesn’t work because heated air can’t mix down to floor level. This leads to stratification.
Hot-water pipe insulation
Code minimum is 13mm foam insulation on internal, and 25mm for external hotwater pipes (for Vic, excluding alpine). Code compliance is rare but simple. For more on improving hot-water systems see here.