Roofs in black and white

The current issue of  the Australian on-line magazine The Fifth Estate features a Special report: the case for white roofs by Cameron Jewell, and the byline:

With externalities including an increase in the urban heat island effect, peak electricity demand and climate change, is it time to ban black roofs?

The best thing about the article is that it draws together in one place most of the conflicting considerations in choice of roof colour, and even to some extent, roof material. The worst thing is that it is missing perhaps the most important variable: the construction of the roof.

The issue with roof colour is the absorption of solar radiation. In any climate where summer overheating is an issue, the solar radiation absorbed rather than reflected has the same effect as raising the outside temperature, and therefore the temperature difference between outside and inside with which any insulation under the roof then has to deal.  The equivalent rise in temperature is known as the SolAir fraction, and can be very significant in the total cooling loads.

But this is only true if the roof is mainly relying on reducing the heat flow by conduction. It becomes less important if the roof system has low emissivity cavities which are effectively ventilated to the outside.  In an ideal version of such a roof, the ‘inside’ face of the cavity would be barely warmer than the temperature of the ventilating air, and therefore the effect of the solar radiation would be effectively neutralised, regardless of the roof colour.

In some other countries, such ventilated roofs are commonplace, even usual. It is important to understand that we are not talking about a ventilated attic of the kind to be found on a typical Australian pitched roofed house. Rather, the ventilated cavity is directly under the roofing material, with the low emissivity achieved by a downward facing foil faced membrane. The cavity is then formed by a rigid sheet lining, supporting the second of the ‘double sarking’ layers, this time with upward facing foil. The secret to the arrangement is to leave an unobstructed air path that will turn into an efficient buoyancy driven air stream, inducing intake at the low point and exhausting hot air at the ridge. This is achieved by the primary battens supporting the roofing running up the slope of the roof, rather than across the slope.  And of course it works best with roofs that are simple gable shape.

Anyone who has followed the description so far will realise that the system, ironically, works best if the roof sheeting is dark, because that will provide the necessary heat to drive the ventilation.
So what about winter, when it would be good to benefit from the extra heat absorbed by the dark roof? An additional elaboration of the system switches over to capturing the warm air in the cavity, and pulls it into the house interior. Admittedly, this may require a fan, but the fan power is small compared to the heating obtained.

OM Solar system Japan

In case anyone thinks the description is fancifully theoretical, it is in fact essentially the type of roof construction employed in Japan, parts of Europe, and of North America. Only the last bit, the winter heat capture is a relatively recent development, as exemplified by the Japanese OM Solar system, which gathers so much heat from a typically small Japanese house roof, that it also puts a heat exchanger in the loop to provide the prodigious amounts of hot water needed to support traditional Japanese bathing habits.

Summer daytime operation

So why don’t we build such roofs in Oz? Because we traditionally used to think it’s for those wankers overseas, while we can save heaps doing away with the plywood sheeting, and even sarking under the tiles. Well, the time has come to look again at what those wankers are doing, why they are doing it, and to spend a little more on building energy efficient smaller houses, rather than the world’s biggest poorly built homes.

Oh yes. And then you can have your black roof and feel good about it.