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Case Study > Kin Residence - Green Architecture

Passive Solar

Passive Solar Design

Straw Bale Construction

Passive Solar

Passive Solar is, as it probably pretty obvious, a system set up that does not use active mechanical methods to harness the sun. Passive Solar is a design philosophy that uses the natural characteristics of building materials and how they react to sunlight.

Passive Solar takes advantage of the way heat acts naturally, the most basic priniciple that heat moves from hot areas to cool areas. This is the fundamental underlying principle behind all passive solar designs. There are several other principles behind the way heat and passive solar design works:

Firstly, conduction is the basic principle outlined above. Heat is actually the vibration of molecules that are affected by some form of outside radiation. As one molecules vibrates, it will transfer its energy to the molecules surrounding it. As time goes on, the energy will spread out amongst the group of molecules. This is why placing a cool object on a hot object, let's say a pot of water on a stove, will make it warm up.

Secondly, convection is how heat circulates through fluid matter. As many who live and work in the higher stories of a building know, heat rises while cool air sinks. This is why a higher story on a building becomes much warmer in the summer than the lower stories, while the basement and ground floor will remain cool. Passive solar systems will use this to their advantage to distribute heat between rooms.

Thirdly, heat can move through the air itself through raditation. This also follows the law of conduction. Firstly, there is the raditation of the sun itself, which is the primary source of energy in the passive solar heating system. Then there is the infrared raditation emitted by objects that are heated up, which travels through the air and warms up other objects. This once again obeys the principle of conduction.

According to, there are five basic elements of passive solar design:

1) The Aperture/Collector
This is either the windows or other such area that will let light into the building. These typically face southward, as the winter sun hangs lower in the sky, and this allows this light to enter the building. It is also important that these windows are not obscured by any shade during the brighter part of the day during the winter.

2) Absorber
This is the part of the system that absorbs the sunlight and converts it into heat. Many different materials and systems can do this duty, and this can include water tanks or containers for usage in passive solar water heating.

3) Thermal Mass
These are the means by which the heat energy is contained and stored, allowing it to be released over time instead of spent all at once. This is typically underneath the absorber surface, and sucks in the thermal energy produced by the absorber material.

4) Distribution
This is how the heat is circulated throughout the house from the thermal mass storage to elsewhere. There are designs that are purely passive in nature, using the basic principles that heat follows; and there are other systems that will use mechanical methods such as blowers and ducts.

5) Control
Using long eaves and overhangs over your windows and other openings will shade them from the high summer sun, while letting the lower sun come in during the winter. Ergo, more sunlight and heat will be absorbed in the winter, but less in the summer. Other options exist, such as retractable awnings and canopies, blinds or vents.


Passive Solar Design

1) Direct Gain -

Direct gain is the simplest passive solar home design technique. Sunlight enters the house through the aperture (collector)—usually south-facing windows with a glazing material made of transparent or translucent glass. The sunlight then strikes masonry floors and/or walls, which absorb and store the solar heat. The surfaces of these masonry floors and walls are typically a dark color because dark colors usually absorb more heat than light colors. At night, as the room cools, the heat stored in the thermal mass convects and radiates into the room.

Some builders and homeowners have used water-filled containers located inside the living space to absorb and store solar heat. Water stores twice as much heat as masonry materials per cubic foot of volume. Unlike masonry, water doesn't support itself. Water thermal storage, therefore, requires carefully designed structural support. Also, water tanks require some minimal maintenance, including periodic (yearly) water treatment to prevent microbial growth.

The amount of passive solar (sometimes called the passive solar fraction) depends on the area of glazing and the amount of thermal mass. The glazing area determines how much solar heat can be collected. And the amount of thermal mass determines how much of that heat can be stored. It is possible to undersize the thermal mass, which results in the house overheating. There is a diminishing return on oversizing thermal mass, but excess mass will not hurt the performance. The ideal ratio of thermal mass to glazing varies by climate.

Another important thing to remember is that the thermal mass must be insulated from the outside temperature. If the thermal mass is not insulated, the collected solar heat can drain away rapidly. Loss of heat is especially likely when the thermal mass is directly connected to the ground or is in contact with outside air at a lower temperature than the desired temperature of the mass.

Even if you simply have a conventional home with south-facing windows without thermal mass, you probably still have some passive solar heating potential (this is often called solar-tempering). To use it to your best advantage, keep windows clean and install window treatments that enhance passive solar heating, reduce nighttime heat loss, and prevent summer overheating.


2) Indirect Gain -
These typically use something known as a 'Trombe Wall' between the rooms and the sun. The Trombe Wall acts as both thermal energy gatherer and distributor.

An indirect-gain passive solar home has its thermal storage between the south-facing windows and the living spaces.
Using a Trombe wall is the most common indirect-gain approach. The wall consists of an 8–16 inch-thick masonry wall on the south side of a house. A single or double layer of glass is mounted about 1 inch or less in front of the wall's surface. Solar heat is absorbed by the wall's dark-colored outside surface and stored in the wall's mass, where it radiates into the living space.

The Trombe wall distributes or releases heat into the home over a period of several hours. Solar heat migrates through the wall, reaching its rear surface in the late afternoon or early evening. When the indoor temperature falls below that of the wall's surface, heat begins to radiate and transfer into the room. For example, heat travels through a masonry wall at an average rate of 1 hour per inch. Therefore, the heat absorbed on the outside of an 8-inch-thick concrete wall at noon will enter the interior living space around 8 p.m."


3) Isolated Gain (Sunspaces)

The most common isolated-gain passive solar home design is a sunspace. A sunspace—also known as a solar room or solarium—can be built as part of a new home or as an addition to an existing one.

The simplest and most reliable sunspace design is to install vertical windows with no overhead glazing. Sunspaces may experience high heat gain and high heat loss through their abundance of glazing. The temperature variations caused by the heat losses and gains can be moderated by thermal mass and low-emissivity windows. For more information, see sunspace orientation and glazing angles.

The thermal masses that can be used include a masonry floor, a masonry wall bordering the house, or water containers. The distribution of heat to the house can be accomplished through ceiling and floor level vents, windows, doors, or fans. Most homeowners and builders also separate the sunspace from the home with doors and/or windows so that home comfort isn't overly affected by the sunspace's temperature variations. For more information, see sunspace heat distribution and control.

Sunspaces may often be called and look a lot like "greenhouses." However, a greenhouse is designed to grow plants while a sunspace is designed to provide heat and aesthetics to a home. Many elements of a greenhouse design that are optimized for growing plants, such as overhead and sloped glazing, are counterproductive to an efficient sunspace. Moisture-related mold and mildew, insects, and dust inherent to gardening in a greenhouse are not especially compatible with a comfortable and healthy living space. Also, it is difficult to shade sloped glass to avoid overheating, while vertical glass can be shaded by a properly sized overhang.

Straw Bale Construction

Straw bales were a fairly common building material in the United States between 1895 and 1940. Interest in straw-bale home construction began to re-emerge in the mid-1970s. But it wasn't until the mid- to late-1990s that building codes began to acknowledge it as a viable approach. The rising cost of conventional construction materials, techniques, and concern for our environment has fueled the growing popular enthusiasm for straw bale home construction.

There remains much we do not understand about appropriate ways to build with straw bales in different individual building assemblies, climate zones, and weather conditions. Two of the current straw bale construction methods include non-load-bearing or post-and-beam, which uses a structural framework with straw bale in-fill, and load-bearing or "Nebraska style," which uses the bearing capacity of the stacked bales to support roof loads.

The non-load-bearing construction method is the approach most regulatory authorities accept today.


Is a straw bale house more energy efficient?

Yes. A typical straw bale wall is roughly three times as efficient as conventional framing. Over the life of a typical thirty year mortgage, this superior insulation can reduce energy costs by up to 75%, saving money and vital natural resources.

Isn’t a straw bale home at greater risk for fire?

No. Canadian and U.S. materials laboratories have found that: “The straw bale/mortar structure wall has proven to be exceptionally resistant to fire.” In these tests, the flames took more than two hours to penetrate the plastered bale walls. Conventional framing built to commercial standards took only 30 minutes to one hour to burn. Due to their tight compaction, bales contain very little oxygen and thus resist combustion. It’s like trying to burn a phone book. Loose straw; however, is at risk for fire and should be cleaned from the job site daily. Walls should be plastered as early as possible to increase their fire resistance.
How does building with straw bales help our environment?

The use of straw bales can have a huge impact on our natural resources and air pollution. Each year, the U.S. alone burns or disposes of 200 million tons of ‘waste straw,’ producing massive amounts of carbon dioxide. The use of straw as insulation reduces the need for initial energy outputs in regards to manufacturing. There is less embodied energy in straw as it is available in almost every local market, thereby reducing transportation costs and efforts. Straw is a renewable resource that has a one year growth/harvest cycle. By using this local, agricultural by-product as a building material, we reduce energy expenditures, the amount of straw burned, and the use of fossil fuels needed for material transportation.