Passive Solar Heating and Cooling – Part 1

Heating and cooling consume a massive amount of energy–over 40% of the energy residential homes expend. (1) For all our advancements, we shade ourselves from a giant heat-producing ball of fire in the sky above and rest on top of a cool earth beneath, so we can spend massive amounts of energy heating and cooling a home which then leaks much of that out of the windows and walls. Passive design is a few simple principles that take into account the wealth of natural energy potential around us, and incorporate it into the design of the house, without needing to use moving parts. There isn’t a big difference in cost between conventional and passive designs, and part or all of a household’s heating and cooling needs can be taken care of by them. (2) Methods range from natural to cutting edge, simple fixes to primary design factors.

The basic principles are to face the long end of a house towards the sun, which in the Northern Hemisphere means towards the South. Then have the majority of the windows on that side. Strategic shading lets more sun in during the winter and less during the summer. The heat from the sun then gets absorbed by a heavy material called thermal mass. This is like a heat battery, because it takes a while to heat, and a while to let the heat go. The thermal mass soaks in heat during the day and releases it during the night, balancing out the temperatures. The house is carefully insulated and sealed, so the temperatures are more controlled.

In part one I want to look at methods to capture the sunshine and collect it in the thermal mass. Before we begin to look at this in more depth, as a disclaimer, it is possible I am not right about everything, so do your due diligence and verify before implementing.


First you want the long side of the house facing the sun. You should have most of your windows on this side.

Window size to percentage of floor space:

South: 7-12%

North: </= 4%

East: </= 4%

West: </= 2%

These are general recommendations that seem to work well in many climates. (3)
If you have this ratio of windows to floors, you shouldn't need any thermal mass. If you have more, you should put in thermal mass to soak up the excess heat. You can calculate that amount here.

The type of windows you choose is also important. You want to allow heat to come in on the South side, but also don’t want to let too much out, especially on the other sides.
Windows are classified in a couple of ways.
The u-value is the amount of non-solar heat the window lets through.

Well insulated = .20
Poorly insulated = 1.20

The Solar Heat Gain Coefficient, or SHGC, is the same idea, but specifically for the sun’s heat.

Well insulated = 0
Poorly insulated = 1

People generally look for well insulated windows, around .25 or less, as windows are a major cause of unwanted heat loss or gain. However, if you want to bring heat in on the South side, you should choose windows with a higher SHGC. While you can play with the exact number to get the amount of sun you want, an example would be:

South: >.65
N/E/W: <.35

U-value and SHGC are ways to quantify the performance of low-e windows. Low-e, or low emission windows, have a thin film of metal sprayed on that reflects heat but not light. Depending on which side the coating is on, it can reflect the heat out or keep it in.

"Windows manufactured with Low-E coatings typically cost about 10 percent to 15 percent more than regular windows, but they reduce energy loss by as much as 30 percent to 50 percent." (4)

Double paned windows use two layers of glass with a sealed of section of air or gas in between. Heat passes through glass much more easily than the stagnant air, so this is a much more efficient style of window. It has a higher upfront cost, but if insulation is a factor, it will probably be worth it.

Because some of the solar heat is reflected off or absorbed into the glass, losing 20% of the sun's heat is not uncommon. (5)

If for some reason you can't get your windows to face south, another method is to construct panels with air or water circulating, covered with glass, and place that in direct sunlight. Then pipe the heat from there to the house. You can see four examples here:

In cold environments, where you do not have access to sunshine, you want to minimize your windows which let heat escape. You want to have less exterior wall space, so the house is more compact and "bundled up." If possible, have more levels and put more windows on the sunny side in more of the rooms.

In the warmer months, you will want to limit the amount of sunlight you get. There are several clever ways to do this. One is to use an awning or an overhang over the windows. This works because the sun is higher in the sky during the summer, meaning it is effectively blocked. However, the exact angle of the sun depends on your latitude. You can calculate the angle of the sun with these useful tools:

A second technique is to place gardens or deciduous trees in front of the windows. These will provide shade during the summer and shed their leaves in the winter to allow the sun in, besides offering beauty and nourishment.

Shade can be an extremely effective way to keep the house cooler. It can be used over whole sides if necessary. Get a good set of curtains or shutters on windows.

The color of the walls and roof can absorb more heat if dark, or reflect it if light. Lighter roof shingles, metal roofing, or Spanish tiles can serve to make the house cooler. (6)

Radiant barriers that are designed to reflect heat waves like Mylar can be placed where needed, either to keep heat out or in. Just note that these need an air gap to work. They block radiant heat. Something hot that is in direct contact can still pass heat through. I have had very good success using $0.60 emergency blankets in front of windows to block the heat. I could easily apply them with a spray bottle of water and a squeegee. A warning on this, if you use it on double paned windows, the difference in heat between the panes could compromise the seal.

Thermal Mass

Thermal mass is something heavy that can absorb heat and release it back slowly. Because heat moves through it at a slow rate, the heat from the sun collected during the day can slowly release during the night. Heat moves through masonry at a rate of 1 inch per hour. Note, thermal mass can both heat and cool. Because it absorbs heat, it makes hot days feel cooler. When the outside is colder than the thermal mass, the temperature will want to reach equilibrium, and bleed back out. Thermal mass stabilizes temperature. So it is important to get the right amount of mass for the heat that enters the house. Overheating may mean you need less window and more mass, and under heating may mean you need more window and less mass.

As we have seen in the previous section, walls and floors already have some mass and perform this function, so if you have the right window to floor area ratio, you should already be good. Again, to calculate how much more thermal mass you need, you can visit:,0

A recommended book on the topic is Ed Mazria's The Passive Solar Energy Book: A Complete Guide to Passive Solar Home, Greenhouse and Building Design.

Mass should be put where it will get direct sunlight, but it should also be distributed for more comfort. (3)

Common types of mass are stone, concrete, brick, and tile. Earth is also a thermal mass, and Adobe, rammed earth, or other earth based products could function as thermal mass. Water stores twice as much heat as masonry per cubic foot, but is more difficult to store. (7) A series of products called phase change are also more efficient. (8)

Thermal mass does not need to be deeper that 4 inches. (3)

It should be insulated from the ground or outside so the heat does not leach out in the wrong direction. However, I have read that ancient peoples who did not have the luxury of windows built earth walls thick enough to absorb enough heat that there was enough for the inside despite what was lost to the outside. Some systems are also designed to tap into the earth’s constant temperature to create a year-round equilibrium, such as the PAHS (passive annual heat storage). This is better for colder climates. Earthships attempt to do this. (9)

One way to control the heat levels in thermal mass is to have running water contact it. The water can bleed the heat out, effectively creating more thermal mass. This can cool the space down, but it can also allow the mass to absorb even more heat, increasing the size of the battery as needed.

What I have described so far in this section is called direct gain, because the thermal mass is directly gaining heat from the sun. There is also indirect gain, where the greenhouse effect is created in one place, and the heat is piped into another.

The solar heaters described before would be an example.

Another popular one is the trombe wall. This is a masonry wall, with a glass pane in front of the sun and an air gap in between. As the sunlight heats the air in the chamber, it begins to rise. Vents placed at the top and bottom of the masonry wall and glass case pull in cold air from the bottom, heat it in the chamber, and send it up through one of the vents on the top. Depending on whether you wanted to heat or cool, you could open the top of the trombe wall to direct the hot air into the house, or open the top of the glass case to create an air current that also pulls the cooler air from a low vent into the house. The masonry wall acts as a thermal mass as well. This method uses 30-45% of the sun’s energy striking the glass, as compared to 60-75% in the case of direct gain. (10)

In Part 2 we will look at insulation and ventilation.












Passive Solar Design

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