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Designing a Livable Passive Solar Home

‘Temperate Climate’ Articles at Permaculture Reflections

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The following is a writeup of a workshop given at the fall 2010 conference of The Permaculture Project – GTA.

This guide is aimed at helping you to design an energy efficient, passive solar home in a temperate environment. However, there is more to consider when designing a home than just energy efficiency. The interior of the home must also be functional and inviting. Fortunately, this can be done without compromising on the gains derived from passive solar design.

What is Passive Solar?

As opposed to active solar, which is used to generate an electrical current, passive solar is a means of utilizing heat from the sun.

At 45° latitude (north or south) there are 957 Watt hours per square metre available for heating on a winter day. We can use this energy to assist in heating rather than relying totally on external sources of heat.

Taking advantage of free solar heat will not only save money, it will also help in making a building sustainable over its lifetime.

Passive Solar Design

Firstly, good solar access is necessary for passive solar design. A suitable site will need good access to the winter sun from at least 10 AM to 3 PM to be effective. While deciduous trees minimize the amount of shadows compared to evergreens, they still reduce the amount of solar gain and really interfere with passive solar design.

Slopes that face the sun (southern slopes in Canada) are good candidates, though flat sites will works as well. North-facing slopes are poor sites for passive solar unless the slope is very gentle.

The building itself will be oriented perpendicular to the sun at midday so as to maximize the exposure to the sun. If the building is slightly off, the loss is negligible.

Solar energy is let into the building through its sun-facing windows (south windows in Canada). However, having too few windows will will lead to insufficient heating, whereas having to many will lead to overheating in the day and heal loss at night. The simple rule of thumb is to match the percentage of window coverage on the sun-side wall with the latitude. So, a home at 45o north latitude would have about 45% of it’s south wall covered in windows.

The energy gained from the direct sunlight will be lost after the sun goes down if it is not stored. We store it by means of a thermal mass. A thermal mass is simply a dense object such as rock or concrete that will store the sun’s energy.

The easiest was to do this is by making the building’s foundation a monolithic slab on grade. In other words, the foundation is an insulated cement pad that the building rests on. The good news in that a slab on grade foundation is simple to build and avoids the problems of a leaky basement by eliminating the basement altogether.

The slab is insulated to retain the heat within the building. It is also a good idea to put “frost protection,” or “frost wings” around the perimeter of the foundation to help reduce heat loss.

Here, “frost wings” have been added in addition to the perimeter insulation.

Additional thermal mass can come from structures such as a brick wall inside the house or a masonry heater.

This house in Melbourne, Australia interior brick walls in the kitchen to store solar heat.

A masonry heater such as this one can provide a backup heat source while also being a thermal mass to capture solar heat.

Additional gain can be achieved by using reflectors to direct sunlight into the house that would otherwise be lost. This approach is somewhat limited as the sun sits very low in the sky in Canada during the winter and the pitch of roofs is generally too steep.

Two additional approaches to capturing solar heat are the trombe wall and the thermal siphon. From the outside, a trombe wall looks like a window built over a wall, but it is a very effective passive solar technique. A thermal mass wall is built then covered with glass to trap the heat it stores.

Trombe wall

The thermal siphon is simply a window over an object that heats up (such as a piece of steel painted black) with vents to allow the heated air to escape into the house.

Thermal siphon.

Something to remember is that not everyday is sunny, and that a backup heat source is necessary. Masonry heaters (mentioned above) make excellent backup heat sources. Unfortunately, building codes in Canada require a mechanical backup heat source, meaning that, unless you are building a cottage, you are required to install a conventional heating system. Though electric baseboards are the most expensive way to heat, they are the cheapest to install and will not be needed in a well designed passive solar home anyway. Code will also force you to have a better insulated home, if you are using electric heat, and that never hurts.

Room Layout

The building will ideally be a long, thin structure with the long edge perpendicular to the sun. To maximize solar gain, keep the sun-facing edge of the building a flat wall. Angled or rounded shaped decrease solar gain.

Generally, you want to position the rooms so that their usage throughout the day follows the sun. So, the kitchen will be in the east, receiving the first sun of the day. Most people do not mind sleeping in cool rooms, so locating them on the shade side of the building (the north side in Canada) frees up sun space for other rooms used during the daytime. That said, an east-facing bedroom (perhaps on the northeast corner) is a glorious thing.

Setting the Roof Overhang

The maximum and minimum sun angles during the summer and winter solstices respectively are found with by the following formulas.

Winter sun angle “A”:

A = 90° – (your latitude + 23.5º)

Summer sun angle “A”:

A = 90° – (Your latitude – 23.5º)

Knowing this, you can use trigonometry to set your roof overhang so that you let sun in during the winter and keep it out during the summer.

There is good news if you don’t want to muck about with trigonometry, though. You can have your rafters cut at a suitable length by nailing two pieces of wood together at the angle of the winter sun. Then place this guide flush against the wall at the top of your window and mark the point on the rafter that the guide intersects it.

A guide set to the same angle as the winter sun can be used to determine where to cut the rafters to set the overhang.

With Canadian latitudes, this will mean that the eaves of the house are set rather large – on the order of two feet or even more. While this means more material cost in the roof, it also means the building will be better protected from the elements.

Making It Functional and Beautiful

While the techniques covered so far can help you to build an energy efficient, passive solar home, they won’t guarantee that it will be a home that you would want to spend any time in. There are architectural patterns that will assist in designing a livable home, however.

First, recognize that your home is both a place to meet people and a sanctuary to get away from the world – and it can be both simultaneously. Design your home with spheres of privacy from the public, where you can greet people stopping by, to the semi-private, where you can invite guests to sit down, to the private, where only the family goes.

Next, a good home has a flow to it. Movement through the home takes place through rooms rather than through hallways or passageways. This makes the rooms social and inviting.

There should be a central common area that the major flows of traffic through the house intersect.

That common area will be improved if it has its own semi-private areas where people can sit off to the side but still be apart of the common area itself.

A room will be much more pleasant to live in if it has natural light coming in from two sides.

Crenelate the edge of a building. This will create usable and inviting spaces both inside and outside the building, increase airflow in the summer, and help to create rooms with natural light coming from two sides.

Having different ceiling heights in different rooms creates varied atmospheres in a home ranging from lively to more intimate. Such effects are easy to achieve in a 1.5 story house, which also happens to be the most efficient home to build in terms of building material.

Finally, a large, sheltering roof that the home is built into is both functional and aesthetic. Make at least one slope of the roof visible from the ground; and, if possible, low enough to the ground in places to touch. If this is done on the north side (in Canada) it will help the building to deflect cold northern winds in the winter.

Next steps: You might be interested in our upcoming online course on Temperate Climate Permaculture.

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Retrofitting For Passive Solar

‘Temperate Climate’ Articles at Permaculture Reflections

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The following is a write up of a seminar given at Noah’s Café in Tokyo. The focus of the seminar was suggesting potential retrofits to the café to reduce the heating costs. More will be written on passive solar design in the future, but for the purposes of this article, we will just look at enough to understand how to retrofit existing buildings.

In cold climates, 20% of the economy can be dedicated in one way or another to keeping warm. For areas below the Arctic Circle, this need not be the case. On a sunny winter’s day at 45°N (or south) latitude, there is around 957 Wh/m2 available to us for heating… if we use it. This is almost equivalent to the energy used by running a 1000W hairdryer for one hour falling on every square metre of ground.

There are 3 types of heating available to us: convective heating (heating using hot air), conductive heating (an object coming into contact with a hotter object), and radiant heating (heat radiating off a hot object such as the sun). While conductive heating is the most efficient, it is not practical for heating living spaces. Convective heating is the most common, but is also the least efficient. Radiant heating is the most efficient means of heating that is available to us. Luckily, we can get the sun to help us to use radiant heat.

To capture the sun’s heat, we use a technique called passive solar heating. We allow visible sunlight to enter a building through a window. When the sunlight hits objects in a room, some of that light is transformed into longer wave infrared light or heat. The windows then prevent most of the infrared light from escaping the building. Massive structures (concrete, brick or even water tanks) called thermal mass are used to store the heat generated and slowly re-radiate it into the building.

Although it seems obvious, it should be stated that passive solar systems need to be oriented towards the sun. It is worth mentioning because people have made the mistake of not doing this in the past. It is best that buildings are oriented so that they are within 10° of the polar axis (i.e. due south if you are in the northern hemisphere).

passive solar

Because the change in the sun angle is so great in temperate areas where passive solar heating is needed, the system automatically adjusts solar gain throughout the season. In the summer when heating is not needed, the sun is high in the sky meaning that the eaves of the roof cut solar gain. In winter, the sun is low admitting more light and allowing more solar gain.

passive solar

Existing buildings can often be retrofit to increase solar gain from windows that are actually on the shady side of the build. This is done be placing reflectors outside the windows on the shady side of the building. Designer Derek Wrigley has built these and reports that the 4.3m2 reflector system on his home in Canberra “is the equivalent of having a 1kW electric radiator on in the room for 9.72 hours each sunny day.”

passive solar reflector

With a little work, an electric motor can be attached to a reflector to make it rotate incrementally throughout the day allowing a steady beam of light to enter a given window. This technique could be used to target a specific thermal mass body. [Derek Wrigley has also installed this system in his home.]

Heliostat

A technique sometimes used on new homes is the trombe wall. From the outside, a trombe wall looks like a window built over a wall, but it is a very effective passive solar technique. A thermal mass wall is built then covered with glass to trap the heat it stores.

Trombe wall

A cheap, portable version can be made of water-filled bottles and placed next to a window to make use of that window’s solar gain. As sunlight strikes the wall, heat is stored in the water to be slowly radiated outward.

Trombe wall

Thermal siphons are another retrofitting option; and finished, ready to use units are sold in Australia under the name Sun Lizard. To save money, you can make your own out of easily found materials. To do so, you would construct an insulated box with a black piece of corrugated steel inside and a glass top. Solar powered fans can be used to increase the air flowing through a thermal siphon.

Thermal siphon
Thermal siphon

Windows will lose heat at night, so covering them up can save a lot of energy. The best method would be an internal shutter that completely covers the window. If that is not possible, heavy curtains can be used to help stop heat loss. The drapes need to seal off the window, so attaching the sides of the curtains to the wall with Velcro is a good idea. From there, the top is sealed off with a foam block so that air cannot enter from the top of the curtain rod. The bottom is sealed at the floor with cloth rolls on the inside and outside of the curtain.

Insulation

As an example, we took the building housing Noah’s Café in Nakano Ward in Tokyo. The orientation of the building was not bad (close to due south), but neighbouring buildings blocked a lot of sun reaching the first floor. The second floor and roof, however, have good solar access that could be tapped.

Some possibilities include a bottle wall in the second story window and thermal siphons on both the wall and the roof of the building as shown.

Passive solar retrofit

To keep in the heat generated, curtains could be fitted as described above. Additionally, blown-in cellulose insulation could be used to insulate the walls and attic of the building, which are currently uninsulated.