The Center for Ecological Design - Passive solarize your space

Passive solarize your space

Most heating and cooling systems used today are active: they use some resource like gas or electricity to produce an outcome that will heat or cool your house. However, it is possible to use some basic principles of thermodynamics in addition to climactic knowledge to cool and heat your home without no external inputs- or in a passive way. While passive systems are usually not enough to provide all of the heating and cooling needs of a home, they can go a long way to lowering your bills.

Great news! You don't have to build a custom house to incorporate passive solar design into your life. In this post, methods are adapted from passive solar design principles to apply to your already-existing apartment or house, using affordable materials.

Step 1. Aperture (windows) and Control.

Identify your north, south, east, and west facing windows. In a traditional passive solar design, most of the windows are placed on the south side. Adapting this principle, identify the windows in your house that give you the most sun, and the times of day they receive it. For me, this is my east facing windows- most of the windows in my house are on the east side. Because these windows face east, the morning sun shines aggressively into my house. Once you have these identified, you will have two modes to adopt your passive solar system: summer and winter.

Winter

In the winter, you want to capture as much sunlight- and warmth- as you can. Make sure that your sunniest windows are not blinded during the hours you receive direct sunlight from them. Close these blinds (more on recommended thermal blinds below) when the sun is not shining in to help increase your insulation.

Summer

In the summer, the opposite is true- you want to reflect away as much sunlight as you can when it is shining directly into your house. Fully close your blinds or curtains when the sun is shining into these windows. You can open them again when the sun is not shining directly in.

Step 2. Absorber.

The second principle of passive solar design is to use an absorber to help maximize the warmth generated by the sunlight coming into your space in the winter. This principle is based off of the concept of albedo- which is the amount of heat absorbed or reflected by different colors. Think about how hot you get wearing a black tee shirt on a summer day compared to a white one- that's because white has a high albedo or reflectivity. To engage this principle, to the extent possible try to use dark colored furnishings and finishes on your sunny side to absorb warmth in the winter.

Step 3. Thermal mass.

Ideally, a passive solar design would include a thermal mass underneath your absorber to help store the heat generated by the absorber. This unfortunately is quite difficult to achieve by other means in an already-constructed home.

Step 4. Distribution.

The distribution principle is all about how you move hot or cold air around your space. If you were building a new house, you would identify prevailing winds and face your windows towards them to achieve cross-ventilation for cooling in the summer. We can still use this general principle no matter where your windows are. The first step is to identify where your breeze typically blows and when. At my house, there is an evening breeze that comes down the canyon, blowing east-west. Each summer evening I open the windows on the east and west sides to let this breeze roll through. The results are incredible, in part helped the large diurnal temperature range in my area. If you don't have a breeze, you can use fans!

... more on Control

There are three other tricks you can try out to help with controlling the heat that enters your space.

Thermal Blinds

One of the most important things you can do to passive solarize your house is to buy thermal blinds. You will make back their cost on your heating and cooling bill, so select a good pair. Ideally, they will be white on the outside (facing window) to reflect solar and thick enough to be insulative.

Awnings

If you don't want to cover up your view, an awning is a good option. They are less expensive than you think, and if you're on the ground level they can also bring you the added bonus of a shaded space to sit outside. For some further reading on overhang sizing, see this checklist.

Trees and plants

If it is possible, plant a deciduous tree on the south side of your home. This will provide shading in the summer, but not the winter since the tree will lose its leaves. If you are in a shorter-term situation, using a fast growing non-evergreen vine can also work (I recommend hops for this purpose).

Further Reading:

Checklist: A primer for passive solar Checklist: A primer for passive solar. Maggie Leslie.

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