Passive Cooling

Passive cooling starts with proper window shading so that excess heat gain is avoided, and of course a high level of insulation to keep heat out.  Depending on climate, you may also want to use a cool roof (light colored and reflective) or a green roof to reduce heat gain thru the roof.  Although a cool roof will reduce roof surface temperature significantly, roof temperature still tends to be higher and air temperature, hence the heat gain thru the roof will be higher than is indicated by air temperature.  Reducing internal heat gain (largely electric usage, but would include anything that generates heat inside, including human bodies which product 3-400BTU/hr) also reduces cooling load.  Energy efficient lighting, electronics and refrigerators are in this class--every bit of heat that generate (which turns out be most of the energy they use), ends up as heat that has to be removed.

Additional cooling is then provided by one of three methods: night ventilation, evaporative cooling or radiant cooling.  Night ventilation only works if the night air is cooler than room temperature, evaporative cooling only works in fairly dry air, and radiant cooling only works if the night sky is colder than air temperature, which generally also means fairly dry air.  If neither is true, you will need an air conditioner to provide cool.

Cooling by ventilation: the two techniques here are cross ventilation to capture any prevailing breezes, and convection (stack) ventilation, which vents warm air out of the upper part of the building.  In both cases, either you must manually open the windows are night, or the opening could be automated, although automating it adds complexity and significant cost, and hence is more appropriate for large buildings (a notable example being the Congregation Beth David synagogue in San Luis Obispo, CA).  Cross ventilation is just a mater of providing opening windows on opposite sides of the building, preferably facing the prevailing winds.  For stack ventilation, ideally there is an opening that leads straight up (ie thru the roof)--either an opening north facing skylight or a controllable roof vent, or a solar chimney which has a vent up high on the wall

Cooling by evaporation: in this method, evaporating water absorbs heat from the air, and this cooled air then drops into the building. In the US, this is mostly done with a roof mounted device called a swamp cooler, but can also be done with a chimney located inside the building.

Cooling by radiation: this method requires the roof insulation to be movable, which adds enough complexity that this method has not been frequently used.  The idea is that when the night sky is clear and dry (and especially at higher elevations), it is much colder than air temperature (maybe 10-40° colder) so the roof will radiate quite a bit of heat to the sky.  By removing the insulation, this cooled roof surface will cool the air below it.  If in addition, the roof is massive (concrete or water, as in a roof pond), it will store quite a bit of cool--essentially its an like an indirect gain system, but in reverse.

Theoretically it would be possible to use a solar-thermal heat pump for cooling.  This would be use a heat driven compressor, as is done with propane refrigerators, but driven by a solar collector instead of burning a fuel, which means that it would only work when the sun was bright enough to generate enough heat in the collector. This wouldn't likely be considered a passive technique, but it would likely use very little external energy--at most some electric to run a pump. As far as I know, it has not been done commercially.1

Ceiling fans are also a cooling method that's not passive, but is lower energy than air conditioning.  A fan will not actually cool the room, but moving air will help increase the evaporative cooling from the body.  If will also increase the convective heat transfer from the skin to air, and psychologically moving air also tends to make people "feel" cooler.  The catch is that as air temperature rises to near body temperature the convective effect disappears (and as air temperature raises above 100, moving air will cause the body to gain heat instead of lose it.  As long as the humidity is low, evaporative cooling (ie sweating), is a powerful heat transfer mechanism, but once humidity gets high, this mechanism is drastically reduced.

As with passive heating, the point of passive cooling is to reduce cooling loads with the understanding that sometimes, particularly in some climates you still might want air conditioning.

Passive solar heating as a cooling strategy

Window placement and overhangs: All the design principles that keep a passive solar house from overheating apply to any house: minimize windows on the east & west, and shade them with very wide overhangs (like 8 feet or more, eg a porch)--and potentially use a very low SHGC glass if the sun can't completely be avoided2.  Shade the south windows so that they have no sun on Jun 21st (how much more around that date depends on climate).  And, of course, don't put any more glass in than necessary.

It is truly unfortunate how many new homes, especially in the "modern" style with floor to ceiling glass (but by no means limited to them!), violate the passive solar principles and end up having blinds drawn for much of the summer.  Likewise houses that for some reason face west (view, that the way the street was etc), are often exposed to much summer overheating.

Thermal mass: can also help by absorbing some heat from the day (although this absorption is all by conduction, so it will be quite slow), helping to keep the temperature down.  Of course, it will release that heat at night, so there isn't any reduction in the heat inside the building, just a stabilization of temperature from day to night.  Alas, if thermal mass is allowed to get too warm, the building will become very difficult to cool. Thermal mass, as a cooling strategy, works best when the periods of high heat are short, or the night air is cool enough to cool the mass back down.

Cool Roofs: Another passive strategy is the "cool roof" approach, where the roof is designed to absorb less of the sun's heat and therefore stay at a cooler temperature (for more detail on radiant energy transfer, see the heat loss section).  This approach will always benefit in hot climates, but will also benefit in any climate where summer heat gain on the roof outweighs winter heat loss.

The simplest approach to creating a cool roof is to use light colored roofing, but beware that light colored composition (asphalt) roofing isn't generally as reflective as light colored metal roofing.4 An even more effective approach in hot climates would be to use a roofing that is coated with a material that has high emissivity and low absorption (high reflectance)--essentially building a solar collector in reverse.   High emissivity coatings will increase winter heat loss, so usually don't make sense in cold climates.

There are other factors that can affect a roof's performance in both hot and cold weather.  Snow cover eliminates radiant losses from the roof, so if the roof is snow covered most of the winter, a cool roof would have no winter penalty.  Vented roofs stay slightly cooler than unvented ones, but how big an effect this is depends on the material.  A vented tile roof, for example, is actually double vented because the arc of the tile minimizes contact with the roof sheathing.Tile roofs also have thermal mass, which, by delaying the peak roof temperature toward evening, keep the roof sheathing cool until nearer the time the air temperature starts dropping,6 hopefully allowing other passive strategies to keep the building at a comfortable temperature (or otherwise reducing the peak air-conditioning load).

Energy-Star labels roofing products with the Energy-Star label as long as their reflectance is high enough (which depends on whether the roofing is for sloped or flat roofs).  Most of the listed products are metal roofs, but there is also white asphalt shingles and tiles.3  While Energy-Star focuses on reflectivity, the EEBA hot-dry builders guide claims material is more important than color, saying for example, a red tile roof will perform better than a white composition roofing roof.

In climates with hot summers and cold winters, determining whether a cool roof makes sense can be a challenge.  For a better guess it what the tradeoff is in your climate, try one of the "cool roof calculators" (from  and from the DOE).  Essentially you are determining if reducing the radiant gain (and hence lowering the roof temperature) in the summer saves more cooling energy than the additional heat loss due to losing the solar radiant gain (and hence keeping the roof colder) in the winter.

Ventilation Cooling Strategies

In climates where it regularly cools of during the night, an especially when there is at least a gentle breeze, the house temperature can be lowered significantly overnight (assuming of course that you don't have a lot of thermal mass that was heated during the day).  Taller buildings (ie two stories or more) can use the stack effect, eg by opening a vent high in the building the hot air in the house will rise up and out.  The cooler it is out, and the taller the vent, the stronger the air movement.  If there is not enough stack effect pressure, an attic fan can supply the same pressure.

By designing the house with good cross ventilation, any nighttime breeze will both provide cooler night air in a horizontal direction, but help force air out any ceiling vents.

Since the ground temperature in most areas stays relatively cool (typically below 60 degrees), we can use the ground as a thermal mass also by using a pipe under the house to draw cold air out of the ground.  Transferring too much heat will cause heating of the ground around the pipe, rendering it as ineffective as any other thermal mass, so the usual cautions apply.

Evaporative coolers

When the relative humidity is low, passive cooling can be increased by evaporating water (such as the body's perspiration system, or the cooling towers used in the middle eastern countries), which takes advantage of the additional heat absorbed by the evaporating water.   Depending on the demand, an evaporative cooler can easily use 30-50 gallons of water a day.  Since they work best in dry climates, and water tends to be scarce in dry climates, they're use it limited to areas where there is plentiful source of water.  Their main advantage is that they use less energy, there disadvantage is the increase in water usage, and sometimes increasing the room humidity too much.

The typical evaporative cooler used in the US is the "swamp cooler" which usually sits on the roof, or in a wall and has a fan that blows air over a wet, sponge like material.  Water evaporates into the air, cooling the air, but also increasing its humidity, typically above 70% relative humidity.


As most of the focus of solar energy has been on heat gain, cooling is often not thoroughly covered.  The references on the main solar page all have some mention of passive cooling.

Wikipedia has a number of decent articles.

In addition to the energy-star and DOE cool roof websites, the California Energy Commission has one also.


1: I've seen it mentioned as a possibility, but never seen a product using the strategy.

2: note that a low SHGC is .3, meaning that 30% of the solar gain still gets in.  Using an overhang that blocks the sun eliminates 100% of the solar gain.  Of course, heat will get in by conduction or radiant transfer from any hot objects.

3: as long as the reflectance is greater than 25% for a sloped roof, the product qualifies, but I couldn't find what category of roof generally qualified, just long lists of products that did.

4: the light coating on composition roofing isn't as complete as light paint on metal, but smooth surfaces are more reflective than rough ones.  Also note that metal roofs have almost no thermal mass, while comp roofing has some; uncoated metal has a very low emissivity while composition has a reasonably high emissivity.

5. obviously the more common concrete/clay S-tile has a larger contact area than the more traditional (and much more expensive) traditional clay tile.  How much a factor this is, isn't clear.  My reference is the EEBA builders guide to hot-dry climates, and alas I haven't had an opportunity to field check anything about cool roofs.

6: total heat transfer does change, it just moves when it happens.  If the night is also hot and the tiles don't cool off, there would seem to be no advantage to the mass, other than to shift cooling load to nighttime.