Efficiency
Keeping a house cool is mostly about stopping heat getting in
Once sunlight has passed through glass its energy is inside the building, so external shading beats every internal measure and beats them cheaply.
By Arjun Nair4 min read
Solar gain is the dominant summer load
A house overheats for three reasons: heat conducted in through the fabric, heat generated inside by people and appliances, and sunlight arriving through glazing. In most dwellings during a hot spell, the third is by a wide margin the largest, and it is also the one most amenable to being stopped.
The quantity involved is easy to underestimate. Bright sunshine delivers on the order of a kilowatt per square metre to a surface facing it, so a modest area of unshaded glass receiving direct sun is admitting heat at a rate comparable to several electric heaters running in the room.
That framing tends to settle the argument about priorities. No amount of insulation compensates for a wall of glass in full sun, and no fan removes heat arriving at that rate.
Outside the glass, not inside it
Once solar radiation has passed through a window, most of its energy is inside the building. Internal blinds and curtains absorb it and then re-radiate and convect a good share of that heat into the room, so they reduce glare effectively and reduce heat gain only partially.
Shading fitted outside the glass intercepts the radiation before it enters, and what it absorbs it releases to the outside air. External shutters, awnings, brise-soleil and simple roller blinds mounted outside are all dramatically more effective than their internal equivalents for the same coverage.
This is one of the clearest examples of a measure that is common in hot regions and rare in cool ones, purely by habit. Buildings in traditionally warm climates have shutters for exactly this reason, and buildings in temperate climates increasingly need them as summer heat becomes a design problem rather than an occasional inconvenience.
Geometry does some of the work for free
The sun is high at midday in summer and low in winter, and a fixed horizontal overhang above an equator-facing window exploits that difference. Sized correctly for the latitude, it shades the glass through the hottest part of a summer day while allowing the low winter sun underneath to warm the room.
The sizing genuinely depends on latitude, and an overhang designed for one place performs poorly a few thousand kilometres north or south of it. This is a case where the calculation is simple trigonometry and where copying a detail from a photograph isn’t enough.
Overhangs work far less well on east and west elevations, because the morning and afternoon sun is low and comes almost straight at the glass. There, vertical shading, external blinds or planting are the answers. A deciduous tree in the right place is a shading system that appears in summer and disappears in winter without anyone touching it, which is a quietly elegant solution where there’s room for it.
Night ventilation works in some climates and not others
Where nights are appreciably cooler than days, opening a house up after dark flushes out accumulated heat and cools the structure itself, which then absorbs heat slowly through the following day. Heavy masonry buildings do this well because they have thermal mass to charge; lightweight ones do it poorly because they have little.
The strategy fails in two situations. In humid tropical climates the night air brings moisture as well as a modest temperature drop, and the comfort gain is small. In hot regions where nights stay warm, there’s nothing cool to bring in at all, and the correct approach is to keep the building closed and shaded through the whole cycle.
Security and insects are the practical obstacles rather than the physics, and secure ventilation openings or shuttered windows that lock in a partly open position are what make the approach usable in an ordinary house.
Fans, and when cooling equipment is the right answer
A fan doesn’t cool a room. It moves air, which increases evaporation and convection from skin, and that makes a person feel cooler at the same air temperature. The motor actually adds a small amount of heat to the room, which is why running a fan in an unoccupied room is pure waste.
Within their limits fans are excellent, because the comfort they buy costs a small fraction of what mechanical cooling costs. They stop working when the air itself is hotter than skin, at which point moving more of it over a person stops helping.
Air conditioning is a heat pump running in the direction the household needs, and its efficiency obeys the same rule as any heat pump: the smaller the temperature difference it works across, the better it performs. So a setpoint a few degrees higher costs considerably less than one set aggressively low. There is one genuinely happy coincidence here, unlike with heating: cooling demand peaks when the sun is strongest, which is exactly when a rooftop array is producing most, so solar and cooling line up in time in a way solar and heating never do.
Common questions
Does painting a roof a light colour help?
In a hot, sunny climate it measurably reduces the heat absorbed by the roof and the temperature of the space beneath it, and reflective roof coatings are standard practice in some regions for that reason. In a cool climate the summer benefit is offset by a small winter penalty, and the case is much weaker.
Is loft insulation useful for keeping a house cool?
Yes, and it is often overlooked in that role. Insulation slows heat transfer in both directions, so it keeps the heat accumulating in a hot loft from reaching the rooms below. Combining it with adequate loft ventilation is what makes the difference, since a ventilated loft sheds heat rather than storing it.
Are portable air conditioners a reasonable option?
The single-hose type is a compromise, because it exhausts warm air outside through a duct and draws its replacement air from the house, which pulls warm outside air in through every gap. They do work, they work less well than their rating suggests, and a twin-hose or fixed split unit is substantially more effective for the same electricity.
Staff writer, Power Your Roof
Arjun has been reporting on solar basics, batteries, bills & tariffs since long before it was fashionable and thinks most subjects are more interesting once you know how they work.
