Keeping heat out

Everyone knows how important it is that buildings can keep heat in. But unless they can keep heat out, they’ll be uninhabitable in heat waves, especially in muggy, still-air conditions. The most important heat to keep out is sunlight. Hot air is hot, but convection from sun-warmed surfaces makes it hotter – especially indoors. For both windows and outdoor sitting places, this makes shading important. Deep roof overhangs, awnings and solar panels can ensure summer noon shade for south-facing windows. The need to shade from sunlight is obvious, but internal blinds transmit heat indoors, whereas external blinds transmit it outwards. Sunlight is also reflected from paving, which green ground cover can eliminate. Walls need to be shaded. Vine shade on walls can reduce summer heat gain by up to 95%.

Overheating is a summer problem; and in summer, the sun is high, so shines down on to roofs. Conse­quently, tree shade can be worth 100 air-condition­er-unit hours/day. Vine shade is quicker to establish and carpeting roofs with Virginia Creeper can reduce temperatures by over 11^oC (23^oF) and cooling costs by 73%. Green roofs both insulate against heat gain and transpire and evapo­rate water in summer, so keep roofs an average of 15°C (27^oF) cooler. In winter, they keep roofs around 4.5°C (8^oF) warmer and can halve heat loss due to wind.

A passive method of cooling the house interior is through natural air movement, by cross- or stack- ventilation. which is most effective at night-time when the air is cooler. As cool air is heavier than warm air, high-level openings, open at night, but closed in daytime, can let night air form cool ‘air ponds’ indoors. This cool-air reservoir can last all day, cooling a building’s thermal mass.