Michael Green

Writer and producer

Solar thermal heating and cooling

In Architecture and building on

HEATING and cooling churns through nearly 40 per cent of Australian household energy use. Energy prices are set to hit ever-higher levels, so the more efficient a temperature control system is, the more attractive it looks – almost day-by-day.

There’s a growing industry selling units that use the sun’s free energy to ventilate, warm and even cool your home. They can also improve indoor air quality by reducing humidity, moisture build up and mould. Broadly, the systems fall under the banner of ‘solar thermal air heating and cooling’.

So what’s commercially available and how well do they work?


Roof space systems

These units work by moving the air in our roof cavities. When the sun strikes the roof, it warms the air below – just as cars get stuffy on clear days even if it’s chilly outside. For heating in wintry weather, roof space systems pump that warmer air inside the home. To help with cooling in summer, some units can extract the hot air from the roof during the day, and also, blow cooler outside air into the house overnight – useful if you’re worried about security or the noise of open windows.

There are two types of roof space solar collectors. One kind, sold here by businesses such as HRV, Solectair and Smart Roof, uses the whole roof cavity as a collector. The intake is near the top of the space, where the warm air rises. A fan sucks the air through a filter and ducts it into the home.

The second kind of technology, offered by Smart Roof in its Smart Breeze system, is suitable for metal roofs. It captures heat in the gap between the roof sheet and a layer of insulating foil or sarking underneath. The warm air rises to the apex of the roof cavity, where it’s nabbed and whisked into the home (using a solar-powered fan). “The ribs act as ducts and we seal the ridge so the air can’t escape,” says Smart Roof CEO Robert Semmel.

He says that the system can significantly reduce bills, but won’t do away with other sources of temperature control. “Our aim is to reduce heating and cooling by up to 40 per cent, not to replace it. If you’ve got a miserable, wet day there’s no radiant heat and on really hot nights there won’t be any cool air to bring in. But those are extremes.

“If the sun is out on a 15 degree day, we get temperatures up to 40 degrees [in the roof]. When cloud cover comes over, we get 30 degrees. There’s a lot of wasted energy we can use,” Semmel says.

According to energy efficiency expert, Adjunct Professor Alan Pears from RMIT University, the catch is that our roofs aren’t very good solar heat collectors. “A roof space can be described as an unglazed, very leaky solar collector, with a substantial proportion of its area facing the wrong direction,” he says. “So in that sense it’s not very efficient.”

But all is not lost. “On the other hand, it’s a very large area, so you can collect quite a lot of useful heat, especially in mild seasons,” Pears says. Light-coloured roofs with insulation directly underneath will collect much less warmth than a dark roof without insulation. Similarly, the roofing material will influence the way the systems function. Metal roofs heat up (and cool down) quickly, whereas the thermal mass in tile roofs means they take longer to warm, but stay hotter for longer in the evening.

If you’ve ever crawled into your roof space, the idea of pumping that air into your home might make you splutter. But Pears argues the air near the top of the roof space is likely to be no worse than other sources of ventilation in our homes. “The air coming through windows or under doors is unfiltered and often highly polluted,” he says. “In any case, most roof space systems offer a filter. The filtration systems available are very impressive, as long as you buy a good one and maintain it properly.”

Solar air collectors

A solar air collector consists of a clear plastic or glass-fronted panel that uses solar radiation to heat air, like the way a solar hot water unit heats water. The panel is mounted outside the house, either on the roof or a wall. A fan (usually solar-powered) blows the warmed air into the house. For cooling at night during summer, it may also blow cooler outside air into the home.

In Australia, a variety of solar air collectors are available, including products from SolaMate, Solar Breeze, Sun Lizard and Solar Venti.

Arne Hachmann’s business, Global Eco and Environmental Solutions, sells Solar Venti units. If you want to ventilate your home, he advises that a 3 square-metre panel will suffice for an energy-efficient 250 square-metre home. If heat gain is your priority, he recommends one unit of that size per 100 square metres of floor space.

He says the Solar Venti warms air to about 35 degrees above the outside ambient temperature. “If you have a sunny 15-degree day, you would expect to get 50-degree air ducted into your house for free, at a rate of about 200 cubic metres per hour.”

Pears has a SolaMate prototype solar air collector installed on his Melbourne house. The panel is three square metres and feeds into his hallway and living room. “In reasonably sunny winter weather, it’s enough to make a significant difference to the temperature of my fairly small and well-insulated home,” he says. “It makes a bigger difference in spring and autumn when the house cools down a lot overnight but the weather outside is pleasant.”

He warns that householders should buy big units if they want to collect a lot of heat. “Most systems are not very large, so they may not collect a lot of energy, especially on cloudy days. Also, if they’re single glazed, their efficiency may not be very good in colder weather.”

He says that solar air heaters work better in winter if they’re steeply sloped to face the low sun. “It may also be preferable to angle them slightly east of north, so they warm up the house more in the morning when air temperatures are lower and the building interior is colder.”

Cost and benefit

According to the Solar Thermal Air Heating and Cooling Association, both kinds of systems start from about $2,500 installed, with an average price of $4,500 installed. It estimates that the return on investment will vary from two to ten years, depending on the house and the habits of the householders.

Dr Bob Fuller, a low-temperature solar thermal researcher from Deakin University, is not yet convinced of the effectiveness of the systems. When it comes to heating, “there’s a big mismatch – the resource is low in winter when the demand is high,” he says.

Another problem is that there is no Australian Standard for this technology. “There needs to be some independent modelling and testing,” Fuller says. He maintains that as a first step, householders are best served by improving energy efficiency in other ways. “I would spend the money on conservation – on better insulation, curtains, and shading in summer.”

Certainly, these products should not be considered as a replacement for appropriate solar passive design, either when building from scratch or retrofitting. Semmel, from Smart Roof, says that if sun is pouring through a bank of west-facing windows, a ventilation system can only do so much. “In that situation, stopping heat getting in from the ceiling is not your major concern.”

For heating and cooling, the systems are most useful in a well-designed and insulated building. Where a home has thermal mass, the heavy materials will store the extra warmth and ‘coolth’ provided by the units. In a well-sealed house, the systems not only provide fresh air securely and noiselessly, but also, less of the airflow will escape through gaps.

Both Fuller and Pears note that there is a trade-off between the temperature and the volume of the air coming into the home. That is, the systems take time to heat the air, but if the fan runs too slowly, then not enough will enter the home to make any difference. “In some cases, because of other heat losses or gains, to maintain a temperature you might need enough volume for up to ten air changes per hour,” Fuller says.

Pears says it’s vital that solar air collectors include a high-capacity, variable-speed fan that can adjust flow rates according to the heat of the air in the panel. Likewise, the ducting on both kinds of systems should be good quality, and as short and straight as possible, so as not to reduce airflow.

Some systems, complete with clever thermostats, can adjust and make the tricky decisions for you. Set to function automatically, they will sense the temperatures in the collector, as well as inside and outside the home, and heat or ventilate as required.

But even the best designed systems, says Fuller, will not always be able to provide enough warmth and airflow to maintain the temperature inside the home. So it’s important not to expect miracles. These are supplementary systems – they won’t do the job all by themselves. 

Published in Sanctuary Magazine

Small houses

In Greener Homes on

Compact homes take less and give more.

WHEN Edward Vinas and his family began planning their new home in East Brighton, they thought they’d go big. “It was probably because everyone else was building big places,” he says. “In our area, there are so many new houses and they’re built almost fence to fence.”

They visited volume builders, but it wasn’t long before they realised a large home would mean wasted space. “We’re a small family – my wife, myself and our son. When we thought out what we needed and why we needed it, a small house suited us better,” he says.

Zen Architects designed the family a two-bedroom home, with a convertible third bedroom and a study. It measures 140 square metres, including the outdoor decking. “It’s the perfect size for us. It feels spacious and we’ve got enough room to entertain both inside and outside,” Mr Vinas says.

The cost savings due to the modest size allowed for design and sustainability features they couldn’t otherwise have afforded, such as a reed-bed and sand filtration system that treats all greywater onsite.

“We don’t feel as though we’ve lost anything,” he says. “We feel like we’ve gained spare time. We didn’t want to be slaves to the home for its cleaning and maintenance. We’ve got better things to do.”

If house size is a guide, Australia is becoming a nation of constant cleaners. Research by the Australian Bureau of Statistics last year showed that our new houses are the largest in the world. The average floor area of new free-standing dwellings is 245 square metres. Meanwhile, through the decades, our households are comprised of fewer and fewer people.

Sustainability consultant Malcolm Wilkie says house size isn’t just a matter of lifestyle, but also ethics. Larger homes require more materials, more appliances and more stuff to fill the space, as well as more energy for heating and cooling. Inevitably, extra bathrooms mean higher water consumption.

“To sustain human life on the planet we have to conserve our resources and use only what we need,” he says. “We’re beyond the point where we can just say, ‘I earned it, so I can spend it.’ There’s a moral obligation to the next generations.”

Mr Wilkie argues that choosing a humble abode doesn’t mean sacrificing quality of life. With thoughtful, elegant design – and consideration of what really makes a house a home – a compact residence can improve your day-to-day existence. “A good way to think about it is, ‘What makes the building sing?’ I think a small house, with the right-sized spaces in it, is a nicer house to live in. If it’s too big, you lose the heart.”

In the US, the ‘small house movement’ is setting out to counterbalance oversized dwellings. The trend is founded on affordability, sustainability and simplicity, and a belief that larger lives are lived in littler spaces. The Small House Society, founded in 2002, acknowledges that ‘small’ is relative to occupancy and needs. Its supporters own homes ranging from just a dozen square metres, up to hundreds. Jay Shafer, author of The Small House Book, lived in only eight square metres.

“That’s going to the extreme to demonstrate what’s possible,” Mr Wilkie says. “They can do a lot with that living area – people start to get really clever when they’re tight for space.”

Contact Malcolm Wilkie.

Small and simple

In Blog on

I have worked on the little building with Michael Kelly for half a day each week. Mostly, we’ve been outside, in his narrow, paved courtyard. Courtyards fascinate him.

In the window of the shop, next to the flowerpots, a framed A4 printout is headed PRISONER GETS “LIFE”. It begins: “At a time when many young people are beginning a career or university, Michael Kelly was doing hard time for armed robbery. Life and death stood before him. He chose to make good use of his time in study, physical training and art.”

After prison, he was accepted directly into post-graduate study at the Sydney College of the Arts. The story continues: “In the years since, Michael has applied his art in a unique, hand-made building style.”

In jail, one of the benign things he discovered was that when one courtyard was uninhabitable because of brutal sun, another, on the shady side, might be too cool for comfort. “Courtyards can be their own little worlds,” he said one afternoon.

With a sideways, impish smile, he told me his incarceration might also explain why he had become so intrigued by small spaces. Planned and fitted out with care, they can enlarge even the most confined of lives.

At that time, I was reading The One-Straw Revolution, by Masanobu Fukuoka. The book, first published in 1978, is the Japanese farmer’s manifesto on growing and eating food, and on the limits of human knowledge.

As Michael spoke, I recalled Fukuoka’s observation: “…if one fathoms deeply one’s own neighbourhood and the everyday world in which he lives, the greatest of worlds will be revealed.”

In the book, Fukuoka recounts his quest for simplicity in ‘natural farming’. “‘How about not doing this? How about not doing that?’—that was my way of thinking.”

One chapter explains the cycles in his rice fields and outlines his practices. “There is probably no easier, simpler method for growing grain,” he concludes. “It involves little more than broadcasting seed and spreading straw, but it has taken me thirty years to reach this simplicity.”

Michael’s design for the courtyard studio is the result of steady simplification, stripping out anything unnecessary in the structure. Each of the four wall frames is separate. The roof frame rests above, on a rectangular timber plate.

After the first day, in which we built the frame, we have worked on the cladding for the roof and walls. Using the thin strips of Oregon lath (reclaimed from demolished lath-and-plaster walls), we have built lightweight panels. We overlapped the lath, like pixie weatherboards. The whole building can be easily dismantled and moved.

That kind of elegance takes thought. On his chalkboard one day, Michael wrote, “There’s no wisdom on a silver platter”.

Courtyard

Wall and floor insulation

In Greener Homes on

Insulation works all around the house.

CEILING insulation is taking a lot of political heat this year. But while the federal government admits its rebate scheme was flawed, the insulation itself shouldn’t be left in the cold.

Caitlin McGee, from the Institute for Sustainable Futures at the University of Technology, Sydney, says good insulation is always a crucial part of construction. “It has many benefits: greenhouse gas reduction, better comfort and lower energy bills.”

And the ceiling isn’t the only spot for it. “The roof cavity is the most important place, but if you want to insulate well, you need to think about the walls and floors, and the building shell as a whole,” she says.

If you’re building your house, get the wall insulation right first time – it can be difficult and expensive to add later. According to Ms McGee, in existing homes, the best opportunity to retrofit is “when you’re renovating or pulling apart your walls for some other reason.” In all cases, it’s wise to consult a building sustainability assessor for detailed advice.

Because heat transfers in different ways, a combination of both reflective (foil) and bulk insulation (such as batts) works best. When you’re choosing a product, consider its green credentials, such as recycled content, as well as its performance, measured by the R-value.

In Melbourne, the building code requires that walls rate a minimum of R2.2. “Generally, the more extreme the climate, the more insulation you should have,” Ms McGee says. But she warns DIYers not to buy overly fat batts that must be squashed to fit. Bulk insulation works by trapping air; it’s less effective when compressed.

Another retrofitting alternative is to add the insulation outside. On her home, Ms McGee affixed polystyrene and cement panels to the external walls. “The material I used as my cladding is also part of my insulation strategy,” she says. “It’s worthwhile thinking about less conventional materials that have good insulating properties.”

Underfloor insulation is more straightforward, so long as there’s enough access space. Maurice Beinat, from home retrofitting business ecoMaster, says you need about 400 millimetres to work in.

Although floors cause less heat loss than ceilings and walls, insulating them can make a big difference to winter comfort. “The special thing about floors, particularly polished timber, is the contact your feet make with them,” he says. “Floors don’t need to be very cold to make you uncomfortable.”

He suggests that well-insulated floors should reach R2.5. (Uninsulated timber floors rate R0.7, and with good-quality carpet and underlay, they rate R1.)

Mr Beinat says there are two requirements for floor insulation: that it doesn’t hold moisture and won’t become a rat nest. For those reasons, he recommends polyester insulation (manufactured in a roll, rather than a batt, for convenience). Before stapling it in place, seal any gaps in the flooring.

In homes that have a lot of underfloor airflow, such as weatherboards, ecoMaster also fastens a layer of reflective insulation to the joists, making the sub-floor nearly air tight. They charge between $28 and $35 per metre, installed.

Mr Beinat says it’s a job well suited to DIYers, but with one serious warning: “The main danger is electrocution by stapling through wires. People do die underfloor.” He advises purchasing a double-insulated, electric stapler, rather than cheaper handheld models that won’t protect against electrocution.

Knowing how hard to push

In Blog on

ON the first morning I worked with Michael Kelly, I arrived at eight o’clock, as agreed. “Let’s see how much we can get done in a day,” he’d said. He planned for us to complete the framing for a small timber studio, even if we had to work until dusk.

His shop-home is a model in the artful management of space. It has a narrow paved courtyard, only eight feet deep and fifteen wide. Yet Michael and Nadeen sort and store stacks of salvaged timber out there, and inside too, in neat overhead racks and shelves Michael has crafted. Chairs hang from hooks high on the walls until they’re needed.

We began. The studio was to be seven feet long and high, and four feet wide, plus a steep pitched roof. Michael assembled the simple tools: hand saw, tape measure, pen, set square, hammer and nails. He let me do the sawing.

“I’ve got a simple tip for sawing a straight cut,” he told me. “As you saw, lean the blade lightly against the inside top corner of the timber.” It worked like a charm – I guess it stops the blade wobbling while your elbow pumps back and forth. Next, I learnt to place the groove precisely, by steadying the blade with my other thumb as I began.

In Michael’s hands, the saw flowed over the timber like a stream over a stone. In mine, it stuttered and stopped. I didn’t know how hard to push.

Last year, I was caught up in melancholy and a longish writing project – perhaps the two go together – and didn’t often go outside. I grew unaccustomed to using my body. When I re-emerged in the summer, I wasn’t so sure how my limbs would respond to orders. One day, I watched some new friends practicing acrobalance, a kind of circus balancing act in pairs. I hung back because I had no idea how strong, or weak, my muscles were. I didn’t know what my legs would do if I tried to use them. Likewise, when I danced, I wasn’t quite sure how my booty would shake (this may be congenital).

That hesitance lingers. For people unused to making and doing, it takes practice to gain confidence in your hands. When I pushed on the saw, swung the hammer or lifted timber, the physics didn’t click. My reactions seemed less than equal-and-opposite.

We cut the first length from a long piece of timber and used it as a template for the others. When they were all done, we began nailing. The initial, unfinished frame was beautiful; so satisfyingly square and logical.

We worked diligently throughout the morning, stopped for a coffee, then worked through lunch. Michael regularly tidied our offcuts and swept up the sawdust gathering in the courtyard. Small spaces demand order.

The pitched roof required angled cuts. Again, we created a template – this time a triangle – and cut matching pieces and nailed them in place. By three o’clock, well ahead of schedule, we set solid the quivering ribs of our roof and manoeuvred the frames together: the studio’s skeleton assembled in but one short day.

Later, I walked home grinning stupidly, closed my bedroom door, turned up the music and danced my joy away, limbs flailing.

The frame