Concept

When people think sun and windows, they often think of a greenhouse. Certainly a greenhouse goes well with a solar home, making it even more green.

A greenhouse can help in greening a house in two ways: it can be a heat source or it can be a source of vegetables. Typically a greenhouse can’t do both functions well, at the same time.

A sunroom to provide heat is used during sunny days to collect heat and transfer it into the house. At night the connection between the house and the sunroom is closed and it gets quite cold in the sunroom – too cold except for the hardiest of plants. One idea is to have "hot boxes" inside the sun room that can be closed at night to keep the plants healthy. This can work but on a limited basis.

If a structure’s primary purpose is to grow food, then it must function differently. Sun comes in and is used directly by plants and absorbed by thermal mass – yes more thermal mass. The heat is stored both in the earth and the other mass – often concrete or water. At night the stored heat is given up to keep the greenhouse warmer. There is another factor when growing plants and that is humidity. A greenhouse is quite humid – typically more humid than you’d want in your home. Most greenhouses that operate through the winter require supplemental heat – usually from fossil fuel.

Application

We are building a small greenhouse attached to our basement. It will be located on the west end of the house, with its south wall in line with the house’s south wall, to maximize solar gain.

The floor of the green house is about 3.5’ above the basement floor; the south wall will be 3.5’ high, about 1 foot above grade. The overall profile will be like a lean-to, with the north wall full height and made of concrete, painted black to maximize sun heat absorption.

The main windows are recycled from our last house – they were skylights that we replaced with coated glass for a low SHGC (solar heat gain coefficient) to reduce solar gain in the summer. The old glass is perfect for a greenhouse in that it is insulated double glass, without any low-e coatings.

We hope to be able to grow salad greens all winter, but at the least we will be able to greatly extend the growing season without using any fossil fuels.

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