What's so special about the SOLARHOME?

When a car manufacturer releases a revolutionary new model that uses absolutely no petrol, gas, or oil, never has to be plugged in for charging, has an unlimited range, and simply requires a new battery every twenty years, it will be considered a game changer.  This is essentially what the SOLARHOME is.  These days there are plenty of 'sustainable/eco/net-zero-carbon' houses around, but look closely and you'll find that they are all either connected to and dependent on a fossil-fuel-powered grid (which  is increasingly unreliable and expensive, and certainly not emission free) and/or use diesel, petrol, gas, coal, or wood fires for heating, cooking, hot water, and backup generators (none of which are emission free).  Greens Beach SOLARHOME is all-electric and has all the comfort, conveniences, and appliances expected of a modern home, yet is powered 100% by fully recyclable solar-charged batteries 100% of the time, throughout the year, even during a cold Tasmanian winter.  It has zero emissions (direct or indirect), zero utility bills, and requires zero maintenance.  Most 'experts' will tell you this can't be done with current technology, but the SOLARHOME is living proof that with good design and very few compromises, this could be (and hopefully will be) how all houses are built in the not-too-distant future.  It's a commercially-viable  off-grid house design for people who want to live a normal modern life without having to constantly worry about generators, gas bottles, wood fires ... or their quarterly power bills.

Will the power in the SOLARHOME drop out after a few days with no sunshine, and will we have to continually worry about turning off lights and appliances?

The off-grid power system in the SOLARHOME is actually far more reliable than most electricity grids, and no, you don't have to continually monitor power usage.  You won't even notice any difference to a normal grid-connected house (except that the electricity supply is actually more even, without the usual voltage drops you often get with the grid).  The system has enough storage to last almost a week of normal usage with no solar input at all (which would never happen anyway, since the solar panels are efficient enough to have a significant input on overcast days). The batteries can safely cycle down to 30% state of charge, but in practice rarely drop below 80%, so are expected to last at least twenty years. Even if something drastic was done (like leaving the oven or an iron on for several days) to flatten the batteries, the system will protect itself by shutting off power points while leaving lights and other essential items (the fridge and water pressure pump) still functioning.  It's a very robust, maintenance free, virtually fool-proof system.

What is the payback period of the solar power system?

It's interesting that no one ever asks what the payback period is for a new kitchen, bathroom, car, the clothes they buy, their favorite charity, or for that matter the money they spent on their last overseas holiday.  Like everything other than a business investment, it's all just money they choose to spend on improving their quality of life, peace of mind, and perhaps even helping improve the world in some way.  The power system for Greens Beach SOLARHOME was designed and installed by award winning Tasmanian company  MODE ELECTRICAL  at a cost of around AU$50,000 (which incidentally, is less than the cost of the driveway and garage at Greens Beach, and less than many people spend purchasing their car, which is unlikely to be worth anything in fifteen years).  The system includes an 8 kW solar array (25 panels) with a 66 kWh battery bank and is expected to last about twenty years with no maintenance whatsoever.

What do you mean by 'off-grid'?

Many people interpret the term 'off-grid' in a broad sense; to be self-sufficient, providing your own power and water, treating your own waste, and growing your own food.  Taken to its extreme, it means living totally independently in an isolated environment, disconnected from the outside world.  That's a lifestyle choice and even a strongly held personal philosophy for some, although a necessity for others.  Of course the SOLARHOME would be ideal for such a situation.  But that's not what is meant here by 'off-grid'.  The purpose of the SOLARHOME design is to simply allow a house to be independent from the electricity grid, based on the belief that a large interconnected national electricity grid is too inefficient, expensive, and leaky to ever be compatible enough with renewable energy to create a totally emission-free power source, no matter how 'smart' they can make it.  (This is discussed in more detail below, but is not a political philosophy or ideology, it is simply a pragmatic response to an obviously looming problem).  The grid also suffers from a tendency known  as 'the tragedy of the commons', where shared resources tend to be far more heavily used than if there is more personal ownership and responsibility.  After all, to a consumer, electricity from the main grid is apparently limitless.  And there's plenty of evidence to support the belief that the grid will continue to become more and more expensive and increasingly unreliable, and will always require fossil fuel (or nuclear) powered base-load backup, no matter how many wind turbines and solar arrays are feeding into it, and regardless of how cheap they become.  Plenty of apparently well-informed experts believe otherwise, although their arguments seem to be driven more by ideology than reality.  Time will tell if they're right or not.  In defense of the 'off-grid solution', at this moment, the off-grid SOLARHOME at Greens Beach works well and is totally emission free, while in most places the grid is not working that well, and is no more emission free than it was twenty years ago.

But if more and more people do disconnect from the grid, won't that make it even more expensive for those who can't afford their own off-grid system?

Connecting intermittent renewable energy sources (solar panels and wind turbines) to the grid is what's forcing electricity prices up (see below for more details about why).  Power companies will be able to operate the grid far more efficiently and reliably if they don't have to handle the sudden demand from households when their grid-connected solar panels aren't supplying any power.

How much energy does the SOLARHOME use?

Greens Beach SOLARHOME uses an average of less than 10 kWh per day (when occupied) throughout the year for all electricity (which is provided 100% of the time by the off-grid solar/battery system, powering all heating, air conditioning, hot water, water pressure pumps, waste water treatment system, lighting, cooking, and all the usual appliances expected of a modern home, including a 24 hour WIFI modem.  No other source of energy is required;  no wood or coal fire, gas, oil, or generator).  This equates to 43 kWh/m² per annum (based on an internal floor area of 84 m²), which is significantly less than the theoretical primary energy demand required for German Passivhaus certification, by far the strictest energy efficiency standard in the world.

Could I go off-grid with my existing house?

Most modern houses have way too much poorly orientated glazing, nowhere near enough insulation, and are far too leaky to ever be truly energy efficient in regards to heating and cooling.  They're also generally much bigger than necessary.  However, if gas (or a wood fire if you live in the country) is used for cooking and heating, it's not that difficult to go totally off-grid with current technology.

Why use solar panels and batteries only, what about all the other types of renewable energy technology?

Solar panels and most modern batteries – unlike all other known sources of electricity generation and storage, including wind turbines and hydro – have no moving parts, are robust, silent, emission free, and virtually maintenance free.  And as with computers for the past few decades, solar panels and batteries are technologies with enormous potential for exponential advancement in performance and cost reduction.

Why not just use a backup generator and gas for cooking and heating, like all other off-grid houses?

A generator (which is usually diesel or petrol powered) complicates a system significantly – the power system is no longer simple, silent, and  maintenance free.  It not only emits pollutants, but also needs regular servicing, refueling, and replacement.  Most, if not all of the problems encountered with an off-grid power system will be related to the generator.  And who wants to be constantly replacing and paying for gas bottles if possibly avoided.

Aren't batteries dangerous, toxic, and bad for the environment?

Greens Beach SOLARHOME uses sealed gel-type lead-acid batteries to store electricity so that it is available when the solar panels aren’t producing any.  Properties of lead-acid batteries should not be confused with lithium-ion batteries, which have been more recently developed to provide lighter-weight storage for mobile devices and cars, and contain hazardous materials which are expensive and difficult to recycle or dispose.  Lead-acid batteries are predominantly just lead and water enclosed in a plastic container.  They are recyclable, and because of the value of lead, 99% of them are in fact recycled.  In comparison, most other recyclable materials have an actual recycled rate far lower because of their low value.  Gel lead-acid batteries are sealed, maintenance free, cannot leak, even if damaged, have no gaseous emissions, and are classified as non-hazardous for transport.  There's more lead in the roof flashing and windows of an average old church (where it is actually exposed to the environment) than in a bank of household batteries.  And despite claims to the contrary, the new Tesla Powerwall batteries are not (yet) suitable for a totally off-grid application;  as with all lithium-ion home storage batteries currently available, they are designed to work with the grid.

I've heard that batteries would be lucky to last 10 years, how can you claim that the Greens Beach SOLARHOME batteries will last 20 years?

The lifespan of most types of batteries depends on how deeply they are regularly discharged.  The batteries at Greens Beach SOLARHOME rarely drop below 85% state of charge, and are mostly above 90%, so will have no trouble lasting 20 years.  And the way battery technology is progressing, in twenty years time the replacement batteries will be a fraction of the size and cost of the current ones.

With all the recent news of power blackouts in South Australia, it's becoming obvious that renewable energy just might not be economically or practically viable, so why bother with solar panels?

Critics of the current foray into 'renewable energy' claim that it is achieving nothing other than forcing up electricity prices and creating an unreliable electricity grid.  They are quite possibly right.  There is plenty of evidence to suggest that the trillions of dollars spent on grid-connected solar panels and wind turbines in the past 20 years throughout the world has not only forced up electricity prices and created an unreliable grid, but has also done absolutely nothing to reduce actual (as distinct from theoretical) fossil fuel consumption.  So it's certainly debatable whether installing grid-connected solar panels will do anything towards 'saving the planet'.  And, unless heavily government subsidised, they probably won't save you any money either, because the cost of staying connected to the grid will continue to rise as grid-connected renewables increase (because the power companies sales volumes decrease, and their cost base remains the same or increases no matter how cheap solar panels and wind turbines become).   Even if you install enough solar panels to be a net exporter to the grid, those big fossil-fuel-powered generators have to keep running so that electricity is available when you do suddenly require it.  But there is a big difference between grid-connected renewables and off-grid renewables.  It's a complex issue, and there's no shortage of wildly differing opinions from thousands of zealously confident online 'experts', but surely if enough houses (and businesses) do go totally off-grid like the SOLARHOME, then large fossil-fuel-powered generators can actually be permanently shut down forever, and this will definitely have a significant effect on air pollution.   One of the main issues of going off-grid is that it virtually forces you to become extremely energy efficient to make it viable, and you have a limited daily supply.  When connected to the grid you can always keep increasing your consumption, and in practice, use as much electricity as you like (something that does actually happen when people have convinced themselves that all their electricity is 'green').  If you are interested in reading more about this complex and controversial issue, you can download my university Masters thesis here:  Hot Air, Smoke & Mirrors  which proposes that the most likely outcome will be a move to very efficient decentralized (off-grid) solar-generated electricity supply systems consisting of smaller autonomous units that include battery storage.  Because in practice, an interconnected national electricity grid which is huge, grossly inefficient, expensive to maintain, unsightly, dangerous (probably causing up to 50% of Australia's annual bushfires), and incredibly leaky (with transmission inefficiencies possibly as high as 50%), is simply incompatible with battery storage, which requires extreme energy efficiency to be functionally and financially viable.  The only thing that grid-connected batteries can achieve is to stabilise the grid by evening out the massive input fluctuations from renewables.  They will not provide enough storage (for the grid) to be of any significance.   For battery storage to create a pollution-free grid, a quick calculation indicates that Australia's national grid demand of 35,000 MW would require more than 5,000 battery banks equivalent to the 100 MWh system Tesla has recently proposed for South Australia.  At an estimated cost of $240 million each, the total cost would be $1,260 billion, for batteries alone, not counting the cost of solar panels and wind turbines capable of producing at least 35,000 MW continuously for at least eight hours a day.  And you'd have to replace those batteries every 15 years.  So that's about $630 billion every 15 years just to make up for transmission losses.  If we really do want a zero carbon electricity grid, it's obviously not going to happen that way.  And it's ironic that the same people that denigrate 'climate deniers' for their ideological blindness in not seeing the 'inconvenient truth' of human-induced global warming, refuse to accept the 'inconvenient truth' of the limitations of grid-connected renewables.

How is the SOLARHOME heated?

With such a high level of insulation (R8 throughout), triple-glazed windows, and airtight construction, the SOLARHOME needs very little heating, even during a cold Tasmanian winter.  In fact most of it comes from internal heat gains (from occupants, the fridge, electronic equipment, cooking etc.).  But an airtight building also needs fresh air for healthy air quality and to prevent internal condensation.  This is provided by a unique ventilation system which preheats incoming fresh air with stored hot water.  So the air remains fresh, with a subtle but constant movement which is unnoticeable.  Combined with a small heat pump type of reverse cycle air conditioner, the internal temperature is kept at a constant 20°C, even when it's close to zero outside.

So how does it stay cool in summer then?

The highly insulated walls and ceiling, exclusion of direct sunlight by having no windows facing east or west, a deep eave overhang over the north-facing glazing, and continuous extraction of warm air through roof vents (driven by the heat stack effect of rising hot air), allows the house to remain less than 26°C, even when it's well over 30°C outside, without the need of an air conditioner or fan.

How do you build a house as energy efficient as the SOLARHOME?

To create a house energy-efficient enough to be all-electric and totally off-grid requires a combination of super insulation (R8 metric with no thermal bridging) in walls, ceiling, and floor, airtight construction, triple glazing, a passive ventilation system with preheated fresh air, a minimal internal volume, no more than 25% glazing to floor area ratio, no east or west facing windows, and adequate shading over north facing windows.  Which is all fairly straight forward, but virtually impossible to achieve with most house designs using standard construction detailing.

What type of roof does the SOLARHOME have?

Greens Beach SOLARHOME has a flat concrete roof (with a small gradient to the centre and one end for rainwater collection).  This has nothing to do with energy conservation, but everything to do with longevity and safety.  When combined with the masonry walls, the concrete roof forms part of a robust external skin which creates a house that could potentially last hundreds of years, while being cyclone proof, fire proof, and virtually maintenance free.  Not even a large falling tree would cause any significant damage.  If you look at photos of any storm or fire-ravaged suburbs or towns, you'll notice that no matter how well built the houses may be, the roofs are nearly always severely damaged or completely gone.  The SOLARHOME roof has a waterproof membrane under the concrete slab, protecting the membrane from solar radiation or mechanical damage,  so that the roof requires no flashing, gutters, painting, or external waterproofing, and any solar panel or other mountings can be bolted into the concrete without compromising the water tightness of the roof.  The flat concrete roof also gives the SOLARHOME a very pure, uncluttered  modernist style, which can also easily accommodate a roof garden, roof deck, or even an additional storey in the future.  However, if people prefer a different style, a timber-framed pitched-roof of tile, slate, shingle, or sheet metal can easily be added to create any style they wish.

What if we design and build our own eco-house?

The strategies used to create what is generally perceived as being an 'eco-house' can generally be divided into one of two categories:   (a) low-tech, locally-produced, natural or recycled materials and techniques including mud brick, rammed earth, adobe, straw bale, recycled materials, and green roofs.  (b) high-tech materials and equipment such as heat pumps, mechanical ventilation units with heat exchange, solar panels, wind turbines, special multi-pane glazing, synthetic insulation, low-watt lighting, and smart meters.  The low-tech strategies are based on building techniques that have been around for thousands of years, and they tend to be very labour intensive.   Which  in the past was okay when cheap labour was available, and nowadays is okay if you’re either very wealthy or have more time than money and plenty of willing friends to help you build your own home.  Also, it's difficult to monitor and control the properties and performance of these ‘home-made’ materials.  All modern building materials are designed and manufactured to meet internationally recognised standards.  It’s hard to know how strong and robust a straw bale or mud brick house may actually be, so there’s always going to be resistance to these techniques from local authorities, banks, and insurance companies.  It may be a lot of fun (at least for the first few months anyway) to build your own home this way, but for the above reasons, none of these  low-tech strategies have been adopted to any significant degree by the mainstream building industry, nor are they likely to be in the future.   On the other hand, high-tech materials and equipment have been widely adopted.   It's relatively easy (and commonly done) to simply add some solar panels, water tanks, and dual-flush toilets to any house design and call it 'sustainable'.  But a 200kg wrestler isn't a ballerina just because he's wearing a pink tutu and slippers. It takes a lot more than any – or even all – of these features combined to create a genuine low-energy house, especially one that’s actually affordable.  The harsh reality is that most modern eco-houses are difficult to build, very expensive, and often quite large.  And many of them use just as much energy as a 'normal' house to operate.  Few, if any, are ever scientifically tested after completion. In fact designers generally don’t want their buildings critically scrutinised.  Recent university Post Occupancy Evaluation (POE) studies of many award winning ‘sustainable’ buildings have discovered that most of them use significantly more energy than old buildings which make no claims to be ‘green’.   A genuine low energy home needs to focus on minimizing the energy required to heat and cool it – which is where the majority of energy is consumed (often more than 80%) – while keeping it as simple and affordable as possible.

What is the size of Greens Beach SOLARHOME, and how much did it cost to build?

The two-bedroom Greens Beach SOLARHOME  has a total footprint of 170m² (including the 43m² covered veranda), with an 84m² internal floor area.  It is built to be fireproof, cyclone proof, and virtually maintenance free (and to last possibly hundreds of years), with a very high quality fit-out and finish.  It cost almost AU$400,000 to build, excluding the cost of land and site works (excavation and footings on a difficult sloping site,  water tanks, septic system, carport, shed, concrete driveway, and landscaping), but including the power system, which will provide free electricity and heating for twenty years.  A much cheaper version of the SOLARHOME could be built so that it had similar energy efficiency, but to significantly reduce costs it would have to be on a flat site, and of timber-framed construction, with lower quality finish, windows, doors, and fit-out, and without the longevity, strength, and maintenance-free simplicity of the concrete roof.

What materials are sustainable?

It’s commonly accepted that a ‘sustainable’ material is one that is not only renewable, but also has a low embodied energy (the energy required to produce it – discussed in detail below).  But many environmentalists are now starting to acknowledge that, firstly, although some materials may not be renewable they are virtually inexhaustible (rock and sand for example), while the production of some easily renewable materials can be quite harmful to the environment (replacing rainforest with energy- and chemical-intensive sugarcane plantations to produce ethanol for example).  And secondly, while considering the embodied energy of a material is important, it’s an incomplete assessment without considering the useful lifespan of the material.  For example, a concrete driveway for a new home on a steep site should last, maintenance free, for hundreds of years.  The much cheaper alternative, with considerably lower embodied energy would be a gravel driveway.  However, the gravel would have had to be re-laid and rolled after every wet winter and summer storm.  At the end of its useful life, the monetary and energy cost of the concrete driveway would be far less than that of the gravel driveway.  Not everyone will agree, but it's certainly arguable that in many circumstances concrete is more sustainable than many materials which are claimed to be sustainable but have a very short usable lifespan.  As for recycled timber, it's a nice idea, but it will always be in limited supply, you can't buy it off-the-shelf, and it takes a lot of time and energy to source and prepare, so it's only really feasible for people with either a lot of money or a lot of time.  It will never be a mainstream building material.

Please discuss embodied energy in detail.

Embodied energy is the total amount of energy required to produce something, including material extraction, processing, manufacture, packaging, transport, and assembly.  So materials with low embodied energy are claimed to be 'sustainable'.  But just how far you take this calculation is difficult to determine, and consistent figures to compare materials are impossible to find, mainly because there are so many known and unknown variables.  For instance, wood from a hand-sawn locally-grown thirty-year-old pine tree has a vastly different embodied energy than wood from a two-hundred-year-old, machine-felled, kiln-dried, chemically-treated rainforest tree transported halfway around the world.  And the more the issue of embodied energy is examined, the more complex and uncertain it becomes.  A product manufactured entirely with electricity produced directly from solar panels theoretically has very little embodied carbon … unless of course the embodied energy/carbon of the solar panels – which may be manufactured with coal-fired generators and imported from the other side of the world – is taken into account.  Additionally, embodied energy is usually expressed as tonnes of carbon dioxide per kilogram of the material - which can be problematic when comparing materials, because, for example, steel may have a much higher embodied energy than concrete, but much less of it may be required to build a similar structure.   And the claim that a material is environmentally friendly because it is recyclable is also questionable.  Almost everything is recyclable to some extent, but it may take more energy, water, and money to recycle a material than to use a similar 'new' material.  In practice, only expensive materials tend to be reliably recycled.  Lead, for instance, which few people would claim to be environmentally friendly, is almost 100% recycled, simply because it is way too valuable to discard and is easily melted and re-formed. Whereas only a very small percentage of paper is actually recycled – it's just not valuable enough.   So when it comes to deciding which materials to use for a sustainable building, it's probably best not to get too bogged down in the embodied energy debate or limit your choice to 'renewable' materials only.  In practice, any material which has a long serviceable lifespan and can contribute significantly towards reducing the operating energy-consumption of a building is preferable, regardless of its apparent embodied energy or renewability.  And of course if you want good indoor air quality, any material that emits toxins should be avoided. Untreated, unprocessed, natural materials are always the safest bet.  But perhaps the most sustainable thing we can do is build well-designed, stylish (as distinct from fashionable), high quality buildings made from durable, non-flammable materials which will last hundreds of years.

Please explain insulation in detail.

Insulation is one of the most important components of low-energy building, and is often the most misunderstood. For a start, there’s a big difference between thermal insulation and acoustic insulation.  Thermal insulation prevents heat transfer through the external fabric of a building, which will help it to be warmer when it’s cold outside and cooler when it’s hot. In general, the denser a material, the less effective it is as thermal insulation (as distinct from acoustic insulation, which requires the opposite, high density - hence lead-lined sound studios). So if you want a wall that has good thermal and acoustic insulation, it needs to have separate layers of materials with very different characteristics. Therefor a double-skin masonry wall with an insulation-filled cavity is an ideal combination.   Metal, glass, concrete, masonry, and even most types of wood are not particularly good thermal insulators.  Materials like mineral wool (rock and glass wool), cellulose, and synthetic foams (polystyrene etc.) are the most effective thermal insulation as they contain many small pockets of air. Unprocessed, untreated natural materials generally have less embodied energy and toxicity than synthetic materials (although not always, as they often have to be chemically treated with vermin and fire retardants), but are often not as effective or durable.   Thermal conductivity of a material is expressed as its lambda value (W/mK), sometimes called  'k value'.  The lower the lambda value, the better the insulation.  Good insulation materials have a lambda value less than 0.05 W/mK.  Insulation with a lambda value of 0.02 will only need to be half as thick as insulation of 0.04 to be equally as effective.  So, for instance, rammed earth (1.51 W/mK) would need to be over 7 metres thick to have the same insulating effect as only 100 mm of polyurethane.  A structure will usually be made up of several different materials with varying lambda values and the insulating value of the structure can be expressed as its U-value or R-value, which takes the total thickness into account. So the lambda-value is a measure of a material, while the R- or U-value is a measure of a building structure (a wall, floor, or ceiling). Thermal transmittance (U-value) is the reciprocal of thermal resistance (R-value). So U = ¹/R. Ultra-low energy buildings generally aim to have external structures with U-values less than 0.12 W/m²K (= R8 metric). For Americans, using imperial units, the R-value is a multiple of approximately 6 times the metric R-value. R8 metric is about as high as you need to go because the benefit starts to plateau around there.   Despite common misconceptions, insulation can play an important role in keeping a house cool.  It may not seem as significant as it is in keeping a house warm on a cold day (where the insulation may help to keep the inside a comfortable 20°C when it's only 5°C outside), but it can mean the difference between needing an air conditioner or not.  To keep a house cooler than the outside temperature on a hot day without using any energy, several strategies need to be used in conjunction – including well insulated walls (especially those exposed to direct solar radiation), correct window orientation to prevent solar access, and adequate natural air movement (preferably from a passive stack system that doesn't rely solely on wind velocity).  When combined with surrounding vegetation and ground water (ponds or pools), a house may be several degrees cooler than the outside temperature on a hot day.  This may not seem a dramatic difference, but when it's 30°C outside and only 23°C inside, a house will feel incredibly cool.


Cellulose is probably the most environmentally friendly insulation material of all, especially when made predominantly of recycled newspaper.  However, it is usually treated with borax and sometimes other chemicals as fire and mould retardants.  Cellulose can also be made of hemp, straw, cotton, or sawdust.  Recycled newspaper is either loose filled or ‘wet’ sprayed into place.   Post-settlement of loose-filled cellulose is easily prevented by adequate compression during installation.  It is not suitable in any situation where it could get wet or where compressive strength is required.

Rock wool is made by melting a raw material such as stone or iron-ore slag at high temperature, spinning it to produce thin fibres, coating those fibres with a binder, and forming it into semi-rigid or rigid slabs 50 to 150 mm thick.  Some binders emit low levels of formaldehyde, and the fibres are respiratory irritants, but these are considered irrelevant once sealed inside a structure, and there are manufacturers whose products are formaldehyde free.  Rock wool can absorb water but will have added water repellents when made specifically as cavity-fill insulation.  Glass wool has similar properties but is more water repellent.   Mineral wool’s greatest advantages are that it is not damaged by water, is non-capillary, mould proof, and highly fire resistant.

Expanded polystyrene (EPS) is a naturally-white, synthetic, closed-cell insulation.  It is made in a variety of densities.  It’s economical, widely available, easily transported, handled, and cut to size (available to order in sheets up to 900 mm thick), lightweight (being 98% air), inert, durable, recyclable, rigid, water resistant but breathable (so avoids the problems of trapped moisture and condensation that can arise with non-breathable materials) and allows no capillary action.  It contains no CFC or HCFC blowing agents or formaldehyde.  It can have reasonably good compressive strength (from 40 to 345 kPa @ 10% deformation) and will not lose thermal resistance over time.  However it can be vulnerable to fire as it tends to melt and flow when exposed to high temperatures.  Best used as under-slab or foundation insulation with moderate compressive loads (most domestic buildings).

Extruded polystyrene (XPS) is very similar to EPS, but manufactured with a different process.  It can have a slightly better insulation value than EPS, can be made with a higher compressive strength (up to 700 kPa), is more water resistant (less permeable), which is an advantage if used underground in constantly wet soil, and has better fire resistance than EPS.  However, XPS is more expensive than EPS, contains colour dyes, loses some of its thermal resistance over time as it off-gasses, and is generally only available in sheet thicknesses up to 240mm.  XPS is best used as a medium to high load-bearing under-slab or foundation insulation.

Polyisocyanurate (PIR) and Polyurethane (PUR) foams are thermoset plastics which means they do not melt and flow when exposed to high temperatures.  So although they are flammable, they are more fire resistant than EPS and XPS.  Some products are a mixture of rigid polyurethane and polyisocyanurate.  They are often faced on both sides with low emissivity foil, have high water vapour resistance, and can achieve a thermal conductivity as low as 0.02 W/mK.  However, they usually have a compressive strength only up to 140 kPa (insulation directly supporting a timber floor should have a minimum compressive strength of at least 150 kP), and are generally only available in sheet thickness up to 200mm.  Polyurethane is often used as spray foam, which is useful to air/water-seal complex structures, but has some off-gassing issues.  However, as a board it is chemically inert.  Best used where minimum wall or roof thickness is vital and a vapour barrier is required (but not breathability), and where compressive strength is not critical.

Cellular Glass is a soda-lime silicate glass closed-cell inorganic material with no fibres or binders, with high moisture resistance, no capillarity, high compressive strength (620 kPa), and a reasonable lambda value of 0.04 W/mK.  Cellular glass is also very stable and non-combustible.  However it is generally very expensive and not as widely available as other materials.  Often used in commercial under-slab and foundation situations requiring high compressive strength and high fire resistance.


The list below is of commonly used materials in order of decreasing insulating effectiveness (with the best lambda value claimed by manufacturers in brackets) keeping in mind that the performance and properties of all materials vary considerably with different sources and manufacturers.

Polyurethane (PU) foil-faced boards  (0.020 W/mK)
Polyisocyanurate (PIR) foil-faced boards  (0.020)
Extruded polystyrene (XPS)  (0.025)
Expanded polystyrene (EPS)  (0.030)
Glass wool  (0.030)
Rock wool  (0.034)
Sheep’s  wool  (0.034)
Cellulose  (0.035)
Polyester fibre  (0.035)
Hemp fibre  (0.039)
Wood fibre  (0.039)
Vermiculite  (0.039)
Cellular glass  (0.040)
Straw bale  (0.047)
Perlite  (0.051)
Hemp lime  (0.067)
Straw board  (0.081)
Lightweight aerated concrete block  (0.110)
Softwood  (0.125)
Hardwood  (0.138)
Plywood  (0.138)
Particleboard  (0.144)
Gypsum plasterboard  (0.170)
High strength aerated concrete block  (0.190)
Hardboard  (0.216)
Gypsum plaster  (0.370)
Loose soil  (0.375)
Sand-cement render  (0.533)
Fibre-cement sheet  (0.580)
Brick  (0.800)
Glass  (1.053)
Concrete block (hollow)  (1.100)
Sandstone  (1.298)
Concrete  (1.440)
Rammed earth  (1.510)
Granite  (2.927)
Steel  (48.000)
Aluminium  (211.000)


What is Passivhaus?

Not to be confused with ‘passive-solar design’ (see below) the Passivhaus Institute was established in Germany in 1991 to help design and build healthy, affordable, low-energy buildings.  Since then, thousands of Passivhaus-certified buildings have been designed and built throughout the world, but mostly in northern Europe.  And people often assume that it's only relevant to houses in a northern European climate, but in reality, the Passivhaus concept is applicable to buildings in any climate.  The concept is not a building style, or even a design system. The institute simply set an energy-usage target (which is considerably more demanding than any other building standard in the world), developed some ways to achieve that target (specifically, using super insulation, airtight and thermal-bridge-free construction, triple glazing, preheated fresh air for ventilation [usually with a mechanical ventilation and heat exchange unit], plus appropriate window orientation and shading), then developed a software package that allows a designer to enter all the elements of a building – including materials, areas, volume, orientation, shading, services, appliances, and local climate data – and calculate the predicted energy consumption.  There are various quite arduous certification requirements, which include airtightness pressure-testing and regular inspections during construction.  And the Passivhaus level of energy efficiency and certification is virtually impossible to achieve using standard design and construction detailing.

So what then is 'passive-solar design'?

According to passive-solar design principles, a maximum amount of glazing should face directly towards the equator (due south in the northern hemisphere and due north in the southern hemisphere), and the width of eaves over this glazing should be calculated so that sunlight is allowed to heat high thermal mass floors and walls in winter months, when the heat is wanted, but prevented during summer, when it could cause overheating. This is possible because the midday sun is always at a lower angle to the horizon in winter than in summer.  In fact we can calculate the exact angle and direction for each hour of the day, every day of the year, for every location in the world, and design accordingly.   However, while the importance of correct orientation for maximum solar gain in winter shouldn't be ignored (why wouldn't you want direct sunlight in a room on a cold day), and if it saves you having to turn on a heater or light a fire on many cold but sunny winter days it's certainly of benefit.  But it's not always possible and the advantages of this as an overall heating strategy are generally overstated.  Unlike the inclination and direction of the sun, daily temperatures and cloud cover are less regular or predictable.  Even more importantly, five or six hours of direct sunlight (which is about the most you can expect on a clear winter’s day) will penetrate less than 50 mm into a dense concrete or masonry surface, while the heat stored will dissipate in less than 24 hours.  So if you get several days of cold, overcast, conditions (which of course is very likely to happen in winter in most climates) with absolutely no sunshine, even the best high-performance triple-glazed windows will actually lose far more heat than they can possibly help to gain.  And increasing the thickness/thermal mass of the floor and walls isn't going to help.  Energy modelling programs generally gloss over this unfortunate reality by using climatic averages; where the total heat-gain averaged over a month might equal the total heat loss.  While in reality, there will be days when the building is overheated and other days when it will require a significant amount of heating.  In summary, if a building has a good energy-efficient heating and ventilation system that keeps the internal temperature around 20°C and is powered by solar-charged batteries, it doesn’t particularly need or want a random and uncontrolled input of extra heat from direct solar radiation.

What is 'thermal mass' and how does it help a building be sustainable?

It's generally believed that all energy efficient buildings should have a good amount of internal thermal mass to help stabilize the inside temperature.  The theory is that concrete and masonry walls and floors take a relatively long time to heat up or cool down, so if the internal air temperature changes, the high-thermal-mass walls and floors remain the same temperature for many hours.  But in reality, if the internal air temperature of a building is maintained at a constant level with a well-designed, regulated, ventilation system, internal thermal mass becomes almost irrelevant.  So, despite what all the sustainable building experts tell us, it is possible to build a very energy-efficient building with virtually no internal thermal mass.  Greens Beach SOLARHOME is proof of this.

How important is glazing in an energy-efficient building?

The amount of glazing in a house is probably the single most important factor (other than airtightness) in regards to energy consumption, which is why one of the guiding principles of an ultra-low-energy house design is to have no more than a 25% glazing to floor area ratio.  Even the very best high-tech triple-glazed windows are only about 15% as effective at preventing heat loss as a well-insulated wall; to the extent that if you have huge areas of glazing and skylights it's almost pointless having any insulation.  And glazing that allows the penetration of direct sunlight (even in a cold climate) will have more to do with overheating the house than any other factor.  Everyone likes a room with plenty of natural light, but you don't need a huge area of glass to achieve this.  Rooms less than six metres wide with well-positioned windows on two opposite walls and a ratio of about 25% glazing to floor area will have plenty of natural light without compromising energy efficiency.  And you don’t need direct solar access for natural light.  People often confuse the terms ‘sunlight’ and ‘daylight’.  Direct sunlight will usually cause glare and overheating.  Daylight is just natural light – more even and softer, but still bright and enticing, especially if there’s a pleasant outlook or spectacular view.  In very cold climates, Passivhaus-certified, triple-glazed, airtight windows should be used, while double glazing is usually adequate in milder climates.

What about 'airtight construction', surely it will cause a building to be stuffy and isn't required in a warm climate?

People often question the need for airtightness in a warm climate.  But with an airtight building, when it’s hot outside the windows are opened, like in any other building, so airtightness becomes irrelevant – unless of course the building is air-conditioned, in which case it will be far more effectively cooled. In fact airtightness saves a lot of energy in all climates. In winter it prevents heat escaping, and in summer it prevents hot air entering a building and cooler air escaping. However, in a naturally ventilated building, the importance of airtightness in a warm climate has as much to do with air quality as saving energy. Tests have shown that air quality in a ‘leaky’ building is often very poor. Contaminated air may be leaking into a house through damp, insect-infested wall cavities, damp and/or chemically-treated (usually for termites) underfloor spaces, and dusty ceiling spaces with loose fibre insulation. Toxic chemicals, materials off-gassing, mould spores, insect droppings, and dust particles all contribute to unhealthy indoor air.   A fresh air supply in a modern home is even more relevant than in the past because many modern materials, finishes, electrical appliances, furniture, carpets, rugs, curtains, cleaning products, insect repellents, air fresheners, and even tap water contain chemicals that add toxins to the air. Plus gases such as carbon dioxide (from occupants), carbon monoxide (a by-product of incomplete combustion of fossil fuels used for cooking and heating), radon (a radioactive gas released by rocks), sulphur, volatile organic compounds (in many building materials), combined with particulates, microbial contaminants (mould, fungi, and bacteria), and dust mites, along with high humidity, can all contribute to an unhealthy indoor environment. So it’s essential to continually replace stale, contaminated air. This is not adequately designed for in most houses.   A well-designed airtight house provides better indoor air quality than a normal house by utilizing natural cross-flow ventilation combined with a passive-stack ventilation system and/or continuous mechanical extraction. Cross-flow ventilation is not enough on its own to ventilate a house because it’s totally dependent on wind speed: if there’s no wind, there’s no ventilation – so even a house with well-designed cross-flow and all its windows wide open can still have minimal air movement and surprisingly poor air quality. Passive-stack ventilation uses the energy of rising hot air (generated by internal heat gains from occupants, electrical equipment, bathrooms, and cooking) to continuously draw fresh air into each room, even when there’s no wind.

Are detached houses sustainable, don't we need high density housing to save energy?

Although there is a general trend towards apartment-living, in larger, denser cities, there still is, and probably always will be, plenty of people throughout the world who prefer – and can only afford – to live away from city centres in detached houses on their own small plot of land, with a garden and backyard for their children to play in.  And like it or not, many countries such as Australia, New Zealand, and parts of North America actually have minimum subdivision sizes around 500 square metres in many suburbs; much larger in semi-rural areas. That’s not generally regarded as an ideal sort of urban development by most environmentalists.  However, if these people build small ultra-low energy homes, establish indigenous gardens, grow some of their own food, collect rainwater, and treat and recycle their own waste-water, this type of low density urban development may not be as environmentally unsustainable as often depicted – especially if electric vehicles with solar-charged batteries become an affordable reality in the future. In fact, urban development of this kind could be seen as an integration with – rather than destruction of – natural landscapes and productive rural land.  And a move towards increased self-sufficiency in this way may help create a more robust and sustainable society than is possible with high-density apartment living. Complex, over-dependent, centralised systems often breed inefficiency and waste, and tend to be dangerously fragile when things go wrong, which they invariably do from time to time.  The worst scenario is probably the recent trend in many modern cities – small plot sizes with over-sized detached and semi-detached McMansions tightly clustered together; without the room for gardens, or any form of self-sufficiency, and without the population density to support the services which discourage a dependence on private transport – surely the worst of both worlds.

What are the advantages of smaller homes?

Many homeowners in countries like Australia, New Zealand, the USA, and Canada have been seduced in the past by the “supersize-me” marketing strategies of builders and real estate agents that has resulted in whole suburbs of McMansions. It might seem better value to build a larger house (with a bigger mortgage made possible by historically low interest rates) because it costs less per square metre. And “optimizing the land potential” may sound like a clever investment strategy (it certainly is for banks and the housing industry), but it is still costing more money overall for something not really needed.  An aging population has a higher proportion of older couples whose children have left home, plus younger couples deciding to have fewer or no children. And many are realising that a smaller house is easier to clean and maintain, takes up less land (allowing more garden space), has less embodied energy, consumes less energy to operate, and of course is more affordable to build. Unfortunately those who do want to downsize but would still prefer to live in a similar quality detached house, often end up having to purchase an apartment because that's the only option. So smaller, high quality detached houses are in short supply in many areas. They may in fact turn out to be a much better investment than a larger house. It certainly will be for our environment.

What about prefab houses, aren't they more sustainable?

A genuinely prefabricated house, which has been completely built and assembled in a factory before delivery to site, does have some real advantages; especially in regards to quality control, in locations where local labour simply isn’t available, and where weather conditions make site work extremely difficult.  But a house of this type is limited in size to a completed structure that can be transported on roads, which means it can be no more than about 3 metres wide by 15 metres long, at the very most.  So in reality, the majority of “prefab” houses are assembled on site from prefab components, which is something quite different.  In fact these often end up having the worst of both worlds – requiring plenty of site work from specially trained labour, with a complex high-tech construction system.   The claims that prefab homes are by default “sustainable” and “eco-friendly” simply don’t stack up; they may well be, but not necessarily.  And the quick construction time is often just an illusion.  It might have been assembled on site in only a couple of weeks, but it took many weeks in a factory beforehand to actually build.  Most prefab houses still need site works of clearing, levelling, and trenches dug for services and concrete footings anyway; which is where wet weather delays usually occur, when the site needs to dry out before heavy machinery can gain access.  Additionally, components of a prefab house must be engineered to be transported.  Once in place, they are over-engineered structures with special, often complicated joining systems; having more strength and material than is actually required.  They are, by nature, usually high tech and highly processed, requiring special transport (often on oversized semi-trailers), huge (and very expensive) cranes to lift into place, special equipment, and specialized skills to assemble (usually by non-local labour).  And finally, prefab houses are rarely cheaper than traditional houses of a similar size and quality anyway.  The highly processed, over-engineered materials and components, factory overheads, oversized transport, crane hire, and special assembly all add to the cost.  There are also considerable sales and marketing overheads involved with most prefab housing companies.

"When we try to make something true and certain that is not then we delude ourselves and others and bind ourselves to dogmas and practices that may not serve us at all well."  (Dr Craig Hassed)