Are White Roofs Really That Cool?
Tags: Sustainable Development, Climate Action, Climate Adaptation, Sustainable Living

A near-white roof can save you around 7% off your annual energy bill.

A local planning authority has mandated that new house roofs must be a light colour rather than dark, as part of their ‘cooler city’ strategy. The local development sector is up in arms and objecting to the change, claiming that it’s ‘unviable’.

This particular part of the city is already seeing peak temperatures of 48C (118F) in summer. 50C (122F) is just around the corner. I’m not sure how 50C is ‘viable’ for the people and animals living there but rather than get emotional about it, let’s look at the facts — are light coloured roofs actually cooler, and if so in what ways?

Photo by Le Toan on Unsplash

Firstly, the basics.

Solar Reflectance Index

The Sun’s energy lands on a surface, and the nature of that surface — it’s colour, material, density and so forth, will determine how much of that energy makes its way to the other side (for example inside the roof space or house) and how much of that energy is rejected back to atmosphere.

We can measure this using the Solar Reflectance Index or SRI, which is one of the less complex ways to measure how much of the Sun’s heat is rejected by the surface. Solar Absorptance is another way and more commonly adopted for building codes, but at least for now I’m choosing SRI because it’s a quick way to understand the mechanics of a cool roof.

An SRI of 0 is fully absorptive, i.e., it absorbs ALL of the Sun’s energy. Asphalt or bitumen when freshly laid has an SRI of close to 0, and at the other end of the scale an SRI of 100 is 100% reflective, turning away all of the Sun’s incoming energy. This might for example be a mirror.

On a black tile roof, you could fry an egg, and on a cool roof you could walk on it in your bare feet. That’s the magnitude of the difference. (Chris Jensen from the University of Melbourne)

A dark grey roof has an SRI of around 20

A white or near-white roof has an SRI of around 85–89

There are many ways that we can make our roof cooler, many of which are relatively inexpensive and have a high adaptive value. The white roof may be the highest payoff, but there are additional things we can do to drive the heat and energy costs down even further.

Is the White Roof Actually Cooler?

What does this actually mean for us when we’re living under these roofs? How much difference does a light-coloured roof make over a dark-grey roof? Here are some facts.

On a typical summer afternoon, a clean white roof with an SRI of 80–85 will stay about 31°C (55°F) cooler than a grey roof with an SRI of only 20 (Calculated under the standard summer afternoon weather specified in ASTM Standard E1980–11, using the medium wind speed condition.)

A clean white roof reflects around 85% of the sunlight that hits it — and heats to just a few degrees warmer than the outside air temperature. A black roof, by contrast, can heat to more than 80C (176F)

A clean white roof can translate to the indoor temperature being lower by 2C to 5C compared to darker roofs. This depends on what type of insulation is inside the roof space

Increasing a roof’s SRI from 10–20 to around SRI 60 can cut a building’s cooling costs by more than 20%

If you’re building new, the additional cost of a white roof over a dark grey roof is zero

A white roof can also improve the performance of solar panels which further contribute to your annual energy savings.

The conclusion is that a light-coloured roof does make a difference and can save you money every year for no additional outlay (if you’re building new). I’d call this ‘viable’ for the occupants, for the neighbourhood and for the local flora and fauna. Developers just need to lift their game and make white roofs sexy.