Solar Greenhouses - Part 5

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Now that there has been some serious chill in the air for a few weeks, it is time to talk about heat. Just sit in your favorite chair in a sunny window one winter day and see how fast the sun warms you up. The sun creates heat.

All objects, including people, water, air, and soil warm up when they are exposed to the sun. However, some objects are better than others at storing the sun's heat. People aren't good at storing heat. When you move out of your sunny window, you'll feel cool pretty quickly, especially if the temperature in your house is about 55-60 degrees.

Other objects hold heat and cool off slowly, releasing heat into the surrounding air. Think about a cast iron pan. If you use one to stir-fry some veggies and turn off the heat, the pan will stay hot to the touch for quite a while. The hotter you get it, the longer it will take to cool off, and during the time it cools, it will radiate heat.

This ability of certain materials to store and release heat is one of the most important principles of solar greenhouse design. If enough heat-storing materials in a greenhouse are exposed to the sun, they will absorb the sun's heat by day, and release the heat in the evening, keeping the green house and its plants (people, too) warm.

Things that store heat well are generally those that weigh a lot for their size. Bricks, rocks, gallons of water, and cement are such objects, and they can also be used in construction. Yes, water can be used in construction. More about water next month.

Very heavy things such as steel and other metals, though capable of warming up quickly, also release their heat quickly. This makes them unsuitable for use in a greenhouse. What a good solar greenhouse needs is materials that gather heat slowly and release it slowly, keeping the greenhouse warm after the sun goes down.

Another important principle of greenhouse design is minimizing heat loss. Because the goal is to keep the greenhouse warm at night, not warm the great outdoors, the room must also be well insulated and weather stripped. Curtains, shutters, or movable insulation over the south facing glass will help slow heat loss through the windows at night.

A third design principle is ventilation, both between the house and the attached greenhouse and from the greenhouse to the outside. Ventilation into the home will allow surplus heat gathered during the day to be moved into the house where it can be used to regulate house temperature. The vents can also be used at night to move a small amount of heat from the house into the greenhouse to help keep it warm enough for sensitive plants. Ventilation to the outside is used in summer to keep the greenhouse from overheating.

Next month I'll cover the integration of these design features. Until then, one word of caution. If you're beginning to roam library and bookstore shelves searching for books on solar houses, rooms, and greenhouses, keep in mind that most of the designs you see will NOT work in the desert. Most are designed to gather more heat than what we need, and if you follow a "stock" plan, you may cook your plants right where they grow.

Don't plan to use glass or other glazings on the roof of your greenhouse. First, glass on the roof invariably leaks, and second, your greenhouse will be exposed to summer sun all day from May through September, just when you want the greenhouse to be cool. Third, don't slope the glass on the south side. Again, it will be more prone to leak and will gather too much summer sun. Think about well insulated ceilings and easy to build vertical walls. And begin gathering bricks and rocks.

Emilie Vardaman
November, 1993