Passive Solar Heating and Cooling — Part 2


A passive design without good insulation is like a boat with holes in the bottom. On a cold day, in order to benefit from the sun’s heat, it needs to be kept from pouring out. On a hot day, you need to be protected from the heat on the outside.

There are two kinds of Insulation, bulk and reflective. Bulk insulation works by slowing the path of heat with a series of air gaps. It slows both heat in contact with it, and radiant heat. The reflective kind blocks radiant heat the way mirrors block light, but does not block heat which comes in contact with it. Bulk insulation is rated with resistance value, or R-value. The higher the value, the better the insulation. Passive homes should have very high R values, for example: ceiling insulation of R50 or better and wall insulation of R30. (1)
There are many types of insulation. Factors to consider when making a selection include cost, toxicity, whether it can burn, attract insects, develop mold or mildew, and its longevity. You can see a list of insulation types and there R values here. (2)

It is also very important to make sure the insulation is installed properly, as the effectiveness of some are dependent on correct
building techniques.

Areas of weakness in the insulation include thermal bridges, doors and windows, and air gaps.

A thermal bridge is something that extends through the insulation that does not have as high an R value. This could be a bolt, a wooden stud, or even a concrete slab extended to make an exterior deck. Thermal bridges are avoided where possible in strict passive designs. Often, designs will have an insulative envelope around the exterior walls. One product, called a structural insulated panel, or SIP, is like a sandwich of siding and insulation. Insulated concrete forms or ICF use rigid foam insulation on the inside and outside of reinforced concrete, and work well for exterior walls. (1) For masonry or cement walls, two to four inches of foam board can be attached to the outside using glue and tie wires. (3)

Doors and windows are the low hanging fruit of improving energy efficiency. Inexpensive and readily available products like weather stripping can make a big difference. Some ideas to insulate windows are:

    Reflective barriers, such as heat blocking window tint or emergency blankets. As already discussed in part one, reflective barriers on the windows can block a substantial amount of heat. I have used emergency blankets by themselves during summer months and it was enough to keep the rooms cool. (See my warning about double paned windows.)
    Window plastic.
    Room darkening curtains. Complete products are commonly sold, and room darkening fabrics can be found in fabric stores.
    DIY fabric curtains with quilt batting. You can see a tutorial here, or this tutorial that features three designs. These curtains can be sealed against the window frame with velcro to block drafts. A vapor barrier helps address condensation issues.
    Comforters. Warm blankets are a simple way to block or retain heat.
    Insulated shutters. There are both commercially available products and do it yourself options. Here is a tutorial for the DIYers.
    Sealed pelmets. These can block the air flow at the top of the window.

To check for air tightness, you can have a blower door test conducted. An expert will use a fan to pull air out of the house and check for drafts where air is going into the house to find gaps to be sealed. You can also try a DIY version described here.

A less complicated housing design is better for air tightness. (4) Other simple fixes include weather strip on doors, high quality caulk on windows, expandable foam in gaps, special foam gasketing for outlets, louvers on the outside of your stove vent, and rubber sweeps under exterior doors. (5)



Air flow is important for health and comfort. It can be taken into consideration starting with landscaping and the building’s geometry. If you are in a warm climate, you want to direct the wind towards you, and block it if you are not. Hills, fir trees, and fencing can direct the wind.

In warmer climates, a breeze can be encouraged above, through, and beneath a house (if it is elevated). Have large openings to the outside. Wing walls, or perpendicular structures placed on the side of a window, can draw breezes in, even in cases of poor cross ventilation. Casement windows that swing open can have this effect. (6) A single-room depth building is best for cross ventilation, as well as an open floor plan, high ceilings, and strategically placed doorways. Openings in rooms should be placed so that air circulates throughout the room, rather than just a corner.

Louvers are a series of slats at an angle. They can direct the air in a specific direction that may have otherwise been neglected. A canopy over the window draws the air upward, while a gap between the canopy and the window will conversely push it down. A sun shade canopy will push the air current even farther down. (8)

As heat rises, cooler air can be drawn in from lower down as the hot air is vented out from above. Having a fan in the attic pulling air out is a simple and effective method to amplify this process.

The ancient Persians used windcatchers, which were towers designed to catch and draw in the wind creating a sense of cool. (7) Some of these towers are still in use in Iran today. A modern take on the same technology is this natural ventilator put out by Monodraught.

Creating spaces of increased heat can create an upward draft that then pulls the air behind it, drawing in the cooler air from below and actually cooling the house. When air is heated in the building, exits out the top, and cold air is drawn in from the bottom, this is called the stack effect. It can be used intentionally to passively ventilate a home. Smaller spaces can be specially designed for this effect using solar heat. Trombe walls, wind towers, and solar chimneys can all do this.

In dry and warm climates, heat can be purged from the house at night, and the cool night air can be kept in the house during the day.

Evaporative cooling is when the air is cooled down as it comes into contact with water. Water absorbs heat as it evaporates. (9) I read once that British soldiers living in India would place a curtain made of natural fibers like linen or cotton in front of an open window, and put the bottom of the curtain in a bucket of water. The natural fibers would wick up the water moistening the curtain, and as the air blew through it would automatically become cooler. There are many DIY variations of this. The windcatchers would also sometimes use evaporative cooling by bringing the air current along an underground aqueduct, using both the water and cool earth to cool the air. Pools or ponds by houses may have this effect. Commercially available swamp coolers work off of this principle as well. Note that evaporative cooling works primarily in dry climates.

In colder climates, you want to block the wind to the house, have lower ceilings, and control airflow between floors, being sure to seal the attic. (10)

Other less passive ways to heat or cool air temperature is with an energy recovery ventilator which captures the temperature of the outgoing air and transfers some of it to the incoming fresh air (11), and geothermal heating, which draws heat from under the earth. Geothermal heating has a higher upfront cost, but is energy efficient once installed. You can look at some of the pros and cons here.

These are only some of the creative ways simple designs can harness the forces of nature to our benefit. Progress, technology, and wealth are often just ways to say we have found a way to reach the goal without expending as much effort. We need not burden ourselves with wasteful complexity when the solutions are literally raining down around us.


Disclaimer: I am not a building expert. Please do your due diligence and verify any advice before you implement it.

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