A hemp feature wall adds thermal mass for cooling benefits.

There are three simple principles for keeping a home cool in summer without inflating your carbon footprint, according to architect Steffen Welsch who will be one of the experts giving advice at the Alternative Technology Association’s Speed Date a Sustainable Expert event in Melbourne this weekend.

First, prevent hot air from entering; second, soak up any heat that does enter within the interior; and third, move air around inside to increase comfort. And most are achievable in an existing detached home or apartment, even if it’s a rental.

Keeping the heat out

Steffen Welsch

To prevent hot air entering is partly a matter of sealing the building envelope, but also preventing sun hitting the windows and other glazing, Welsch told The Fifth Estate. The simple solution is vertical blinds installed on the exterior of windows.

Conventional interior design dictates blinds hung inside to block sun, but Welsch says while the blinds might block the glare, by the time the sun enters through the glass they do not block the heat.

“Vertical blinds are also more efficient at shading glass than horizontal ones, and vertical blinds reduce the effect of radiant heat on the building,” he says.

Another easy cooling measure those in freestanding homes can implement is creating areas outside the house that soak up heat, such as garden beds and other vegetation, or a timber deck area that will absorb heat instead of paving, which radiates heat.

For an apartment, where many have only an outdoor terrace or balcony, plants in pots, timber decking placed over ceramic tiles or concrete, or even a large timber outdoor table can all help add shade and soak up the sun, reducing the amount of heat the outdoor space radiates.

Soak it up

Welsch says the principle of interior elements that will absorb heat is “underrated and often completely overlooked”. The basic principle involves having thermal mass inside, elements such as stone or concrete floors that will absorb heat from the air, or reverse brick veneer, where the brickwork is on the inside. Exposed solid plaster also works well, he says.

Timber floors generally perform better than carpeted floors for absorbing heat, and in homes where there are struts under a timber floor, it is possible to retrofit underfloor insulation.

Options for insulation include the new “phase change” materials that are only a few millimetres thick but deliver the insulation benefits of a 200mm brick wall. They work by absorbing heat, which changes the state of the material from solid to liquid when it is hot, and from liquid back to solid when it cools.

These are still quite expensive, however, due to the current small market share. Welsch does think the price will, however, go down.

Indoor breezes

The third principle of moving air about can be achieved through mechanical measures such as ceiling fans and through strategic window placement.

“When the air moves, then you feel more comfortable at higher temperatures,” Welsch says. Fans also assist with airing a house out when the cooler evening change hits.

“You also need to check the window openings are in the right place. For example, in Melbourne the cool change comes from the south west, so a low window should go there, and a high window in the north-east corner.”

The cool air entering shifts the hot air out the higher window, but window placement is another thing Welsch says is often not being properly considered. Generally, diagonally opposite windows will work better for cooling and ventilation than multiple windows on the same side of a room – a good thing to keep in mind when renovating, extending or planning to build.

Clerestory windows set up high under the eaves are also effective for ventilation and cooling. They rely on the natural stack effect of air at different temperatures. They can be manually operated, or it is possible to install automated window opening, which is particularly effective when combined with a temperature sensor control. These are not expensive technologies at the domestic scale – Welsch says an automated window opener can cost between $60 and $80, and a temperature sensor control for about $30.

More cool, green ideas

Even though sealing the building well is part of managing heat gain in summer and heat loss in winter, some form of ventilation is essential for occupant health. Having well-sealed homes also means it becomes more important to be careful in the choice of interior finishes and materials, including furnishings, to minimise the level of volatile organic compounds and other nasties.

Other things those looking to build or renovate would want to consider are double glazing, insulation and zoning controls for any ducted airconditioning system. These are, however, best avoided altogether, Welsch says.

Ideally, in terms of environmental impact, airconditioning should be viewed only as a back-up system, not the main solution. If there is no shading or other measures, a refrigerated ducted system is “questionable” in terms of carbon footprint.

Check the specs

Welsch says it’s best to do the simple things first, and with a new build, this means checking that what was promised is actually being delivered in energy terms.

“[In Victoria] even mass produced houses or dwellings need to have six star energy ratings. But people getting a new home built need to check the certification of the rating and check that all of the assumptions made in the certificate have been implemented.”

This includes checking doors and windows are sealed properly, and checking that where double-glazing has been specified, that’s actually what got put in throughout.

Commonsense is cooler too

“A lot of the solutions people can use are commonsense and good practice,” Welsch says.

That good practice extends to how people occupy their homes, so that when sustainability measures are installed, they are operated properly. Simple things such as closing bedroom doors and blinds before leaving for work while also leaving some strategic windows open for ventilation to let any hot air out.

“Appropriate use is very important. You have to be active about it.”

Growing green thinking needs to embrace new ideas

He says that over the past 10 to 15 years he has seen sustainability shift from being mainly architect-driven to something clients now increasingly want.

But while there is an increasing amount of knowledge, once the design starts to move into details clients can struggle to reconcile their green ambitions and old ways of thinking about design, construction and materials.

Heavy is cooler

Rammed earth wall (left) and hemp wall (right).

“A high performance building in our climate needs to be constructed with heavy materials,” Welsch says.

“The proposition with concrete [however] in terms of sustainability is the amount of CO2 it takes to produce. It is the ‘bad boy’ of building materials in terms of its embodied energy.”

Solutions for a lower footprint can include using concrete containing fly ash, or with recycled content. The real stars in sustainability terms for heavy materials though are rammed earth, also known as pise, which is very low on embodied energy, or the new hemp composite walls, which work extremely well as thermal mass inside a home.

Hemp walls also have the potential to be carbon neutral, Welsch says, as they are a store of carbon. They are not cost-competitive yet, and he would not recommend an entire house be constructed of them. Instead, a number of feature hemp walls could be incorporated, and the cost of this balanced out through downsizing airconditioning.

Ultimately, he says, the way to approach sustainability in a home is as it being something to be proud of.

“It is worth considering the social aspect, too, of people feeling comfortable in your beautiful, green home and enjoying spending time in it. When people want to be in your home, it makes you feel proud of it.”

6 replies on “The three principles of sustainable home cooling”

  1. One of the reasons our clients look to insulating coatings as part of the solution. Keeps aggressive heat out in summer, heat in in the winter. University of Melbourne report makes this clear.

  2. Great discussion topic however, as a general rule in Victoria we send more money in heating than cooling. These cooling principles would apply in warmer the states of Australia, the rest would need to incorporate “Thermal Mass” in the design.

  3. Operation of the home is significant when aiming to benefit from thermal mass and heat retention. (As well as passive solar design)

    Agreed Steffen, thermal mass can work very well in winter.

  4. Fortunately building with hemp is rapidly becoming more cost competitive in Australia. In the past 12 months more Australian farmers have planted hemp fibre crops, more processors have come on line and building materials are now manufactured and readily available in Australia avoiding the footprint of shipping and import costs.

    Hemp building costs will reduce even further as each state produces its own processed fibre, making for a win win in every direction. It’s good for farmers, good for consumers who want to live in energy efficient housing and great from the perspective of Australia growing more harvestable biomass that sequesters carbon, so we can start locking up substantial amounts of carbon in buildings.

  5. It is often overlooked that around 80% of the energy dedicated to keeping our homes comfortable is actually spent on heating. Do you think we overstate the importance of thermal mass? Yes, it works extremely well in summer, but can be a huge detriment in winter.
    There is the story of the 9+ Star home in Melbourne, where they purposely included an exposed concrete slab between floors for thermal mass. Just one winter in and they had to have a heater installed – it was just too cold!

    1. Thanks for your comments. I agree with you, more resources are spent on heating than cooling, although not 80% on average.

      Thermal mass works very well in winter if you apply passive solar design principles properly. Make sure sunlight (even diffused) warms up the wall or floor in the day so the heat can radiate back at night.

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