Want a Cool House? Start with a Cool Roof

Posted by Nina Govedarica on Dec 4, 2018 11:46:23 AM

What to know what’s cool? Me. Want to know my secret? The roof above my head. Studies have shown that colour of your houses roof helps tremendously with passive heating and cooling of your home! 

The Low Carbon Living Cooperative Research Centres (LCLCRC) have released a report on cool roofs, and besides being really interesting for a sustainability nerd like me, it’s a great chance to recap the low down on cool roofs. Let’s dig in!

What makes a roof cool?

A question as old as time, what really makes something cool? Luckily for us, the answer for a roof is literal.

A cool roof is a roof covering that is highly solar reflective or has a low solar absorptance (ABS) – basically it is a roof with a light colour. ABS is a measurement of how much radiation (heat) a material absorbs / lets through. The measurement is on a scale from 0 to 1, 0 being no radiation at all and 1 being the full force of the sun coming through your roof. If that's still confusing, ABS refers to  how much heat from the sun is coming into our homes through our roof. Low ABS = little heat, high ABS = lotta heat.

The Building Codes of Australia (BCA) define a low (cool) ABS value of being between 0.0 and 0.4, medium sits between 0.41 and 0.6 and high ABS is 0.61 to 1.0 (note - if you live in New South Wales, your assessor will have to follow the BASIX protocols in which the range for light, medium and dark differ slightly).

white roof

 

So, what’s the ABS of my house?

If you have a Colorbond roof you’re in luck! Check out this link to Colorbonds table, match the closest to your home and the number in the left column is your ABS. If you have metal cladding on your external walls, the same ABS applies, however walls (generally) have less of an impact on your homes energy needs than the roof.

Have a zinc or galvanised roof with no Colorbond? 0.55 is our go-to here at SWA.

Have a tiled roof? Just as easy! Unfortunately, there is no handy website like Colorbond, but we do follow this general guide:

Slate (dark grey)

0.90

Red & green

0.75

Zinc aluminium (dull) & galvanized steel

0.55

Light grey

0.45

Off white

0.35

Light cream

0.30

 

The above table is of course used as a reference and is not definitive, we will always use our best judgement and so should you. For example, a roof that is light green will obviously have a lower ABS than a dark green roof.

So how does this impact me?

In very few words, a low ABS means your home won’t heat up as fast. But we’re not here to discuss things in few words. I’m a woman who likes to talk. And my boss has given me a blog to write (probably to stop me talking). Here are some energy assessor buzz words you’re going to learn in the next couple of paragraphs: low energy consumption, climate zone & urban heat island effect

Low Energy Consumption

The biggest pro for cool roofs. Logically, if your house is absorbing less heat you won’t need to cool it down as much in the warmer months. But if you delve further into our logical thought process, you will realise that your house will also not absorb as much heat in the colder months either. This is where climate zone matters (see our blog!). If you live in a warm-temperate climate zone (climate zones 1-5), you will end up saving energy & money as we do pump the aircon more in the summer than the heater in the winter.

Climate zones 6 and 7 are a bit trickier and has arguments for both sides. Yes, it gets hot, but it also gets very, very (and in climate zone 7 – very) cold. Cool roofs have other benefits beyond energy consumption which should be considered but talk to your assessor and see what’s the best roof colour for your situation.

In climate zone 8, the alpine region, there’s a whole new factor to consider. Snow! Theoretically, if your roof is covered by snow the radiation won’t be reaching the roof anyway, your roof can be fuchsia and it won’t make a difference (except the eyesore). Secondly, studies have shown that snow actually helps insulate your home. This study was conducted in the USA, so take it with a pinch of salt on how it will impact Australian alpine homes, but still something to consider.

Urban Heat Island Effect

Refers to the ‘phenomenon’ that densely populated cities are warmer by 4-5°C, sometimes even higher. This is due to the number of humans, our human activities and all the human buildings stopping the land from absorbing the solar rays*. But then this leaves the radiation with nowhere to go but bounce around us! Hence a heat island effect, as we live in a pool of hotness.  

A cool roof will reflect the radiation back in the atmosphere, not to us little humans. This is especially effective with large buildings, as discussed in our report of choice Driving Increased Utilisation of Cool Roofs on Large-Footprint Buildings Final Report.

As always, there is a downside to this. Researchers from California, USA have found that radiation (UV Rays) being bounced back into the big blue sky may be leading to increased levels of ozone in the atmosphere. Poo! But, in my humble opinion, it is the lesser of two evils, as our cooling methods produce more toxic gases then what can be reflected by a light roof.  If you’re that concerned, consider planting a tree to offset your carbon footprint.

*Fun fact, the Earth naturally has a high ABS which is why the soil doesn’t burn our feet. This is also why running across the beach is a nightmare in summer, because sand is white and has a low ABS.

Help my roof is black! What can I do?

There is absolutely no need to change your existing roof, for a number of reasons.

Firstly, your climate zone really does matters so if you live in a colder climate you’re probably going to benefit more from a dark roof anyway. Keeping your house cool normally isn’t as big of an issue.

Secondly, if you have Anticon (a form of roof insulation) the impact of solar radiation is significantly reduced and changing your roof is a lot of work for something that won’t make a big difference. This also applies if you’re in the design stage of your home and love the look of a dark roof. Anticon! Anticon!

Lastly, if you do think that the ABS of your home is giving you heat stroke and forcing you to crank the aircon 24/7, upgrade your ceiling insulation. It is a more cost effective and easier solution. Contact some local builders for quotes.

If you’re a DIY kinda guy (or girl!) you can pick some up from Bunnings (we recommend R4.1 or higher), watch some YouTube videos, find the man hole to your ceiling and go for it. Thinkinsulation is great for working out how much you’ll need to buy, or the packaging on your insulation should help. Energysaver has some great tips, although it is based in the USA. Full disclosure, I have never done it myself so please don’t call asking for advice, I can’t give you any. I can only reassure you on how cool you are.

Tell me more!

I’ve given you the basics, but if you’re as crazy about solar reflectance as I am you should read the CRCLCL report Driving Increased Utilisation of Cool Roofs on Large-Footprint Buildings Final Report (9th September 2018). The CRCLC is a government funded group focussed on reducing the impact the built environment has on the planet. The report does a fantastic, well researched overview that I recommend to everyone looking at building/designing a new home. Although it is based on large buildings (in this case shopping centres), all of the outcomes of the report are also relevant to residential houses, and their synopsis’ of other research on the topic is also well thought out and easy to understand.

 

Got any questions? Ask here

 

 Sources

http://www.lowcarbonlivingcrc.com.au/resources/crc-publications/crclcl-project-reports/driving-increased-utilisation-cool-roofs-large

http://www.pnas.org/content/114/34/8991

 

Images

https://www.realestate.com.au/blog/images/800x509-fit,progressive/2014/02/whiteroofs550.jpg

Topics: energy, energy efficient homes, energy efficient roofs, solar absorptance