Not only are Chelsea Green authors experts in their fields, from organic farming to green building, they’re incredibly nice folks! Just last week we got an email from James Kachadorian including this article full of tips to help fight the extreme heat a lot of the nation is feeling.
So here they are, some passive solar strategies for keeping your home cool without frying the planet.
by James Kachadorian, Author: The Passive Solar House
With temperatures soaring across our country and heat records breaking on a daily basis, let’s consider the summer cooling aspects of a properly designed solar home. On the cover of my book, The Passive Solar House, one will find the words The Complete Guide to Heating and Cooling Your Home.
Is this an oxymoron? Most people only associate a passive solar home with heating. However a properly designed passive solar home can and will be cooler in the heat of the summer than a conventional home.
First let’s start outside, with proper siting and the strategic planting and placement of trees. The following two pictures illustrate what can be done with deciduous trees. When the leaves have dropped, the winter sun is allowed to penetrate through the bare branches to heat the home. When the trees have leafed out, the summer sun is blocked. We all know it’s a lot more comfortable to sit under a leafed out maple tree in summer than to sit in the open sun.
The trees in these pictures were all planted when the home was built.
Next, by siting the home to face true south, the summer south sun that does get to the home is so high in the sky that there is little to no penetration via the south windows and patio doors. The next picture shows the amount of sun penetration at solar noon on the June summer solstice. Not only does the sun penetrate the home just a few inches; but the high angle of incidence “bounces” most the sun’s heat away from the patio doors and windows.
However, this is not the case for east and west facing windows. These windows are important heat collectors in spring and fall but can be detrimental in summer. The reason being that the sun’s east-south-west sweep around the home is greater in summer. The next diagram shows how the “slice of the pie” gets greater from December to June. The south aperture goes from 106 degrees in December to 234.6 degrees in June at north latitude 40 degrees.
Window treatments are effective in controlling east and west heat gain in summer. Keep in mind that the sun’s rays are almost perpendicular to east and west windows in summer making the heat gain far greater than what happens with the south faced windows. The following diagram and photograph show how east and west windows can be shuttered to keep the morning and/or afternoon sun out of the home. Of course, these same Thermo-Shutters are used in winter to reduce heat loss at night.
So far we have seen how we can control the macro environment by proper siting and the use of deciduous trees. We have also seen how we can keep the morning and afternoon sun out of the home by utilizing window treatments.
Now let’s go inside the home and see what we can do to keep the home cool. The most obvious is proper insulation. Almost everyone understands the importance of roof and wall insulation in controlling heat. So let’s move on to discuss the use of thermal mass and simple ventilation techniques.
Thermal mass is used in a solar home to absorb heat both winter and summer. In winter the sun’s heat is stored in the thermal mass during the day and then released to the home at night when the sun is no longer shining. If the thermal mass is sized properly, the home stays at a comfortable temperature day and night. My term for this is that the home is in Thermal Balance. Or, the home has thermal inertia. Too much thermal mass and you have a cave. Too little thermal mass and you end up with a hothouse.
In summer, if the thermal mass is sized correctly, by the time that the house heats up, the heat of the day is over. Because of this, a thermal lag has been built into the home. In summer, the technique is to ventilate the home at night and let the night’s cooler air displace the heat retained in the thermal mass.
The formally patented Solar Slab described in The Passive Solar House book effectively accomplishes these heat exchanges because air is allowed to pass through the mass. The following diagram illustrates the air flow.
Note the ventilator placed in the attic. By running this fan at night the cooler night air drawn into the home and reduces the temperature of the Solar Slab. The home is then ready to take on the heat of the next day.
There are areas of the country where all the techniques discussed will not be sufficient to control heat and humidity; and mechanical air conditioning is needed. In these areas the return air to the central air conditioner (heat pump) is passed through the Solar Slab. This will pre-cool the air and constantly assist the central air conditioner. Experience has shown that size of the air conditioner can be reduced because of the pre-cooling of the returning house air. This is analogous to present day hybrid cars where gravity is used in tandem with the mechanical engine. Whereby gravity charges the battery when the hybrid decelerates. In the solar home described, the engine is the central air conditioner unit and “gravity” is the Solar Slab thermal mass.
Hopefully this discussion has demystified and illustrated how a solar home can be cooler in summer than a conventional home.