Build a Greenhouse: The Amazing, Low-cost, Multipurpose, Solar-heated Greenhouse/Guesthouse

Image Gallery This greenhouse was designed to be used as a guesthouse or studio. KENTON KNOWLES

The idea behind my greenhouse/guesthouse concept is to build a greenhouse that has everything you need to live, and not a centimeter to waste. But you don’t have to use it as a greenhouse. It can be a guesthouse or an energy-efficient small home. The design is flexible, so you can incorporate whatever recycled materials you can find. Even if you buy some of the materials and hire a bit of help, you can build a greenhouse for about $3,800.

If you plan to use the building as a greenhouse, use insulated, wood-framed windows that aren’t coated with low-e or solar film, because the coating limits heat gain. Adequate thermal mass and a well-insulated floor and walls are necessary to store enough heat for a couple of days without sun. Thermal mass, such as concrete, brick or stone, helps moderate heat, tempering the thermal swings throughout the day and night.

To make a guesthouse, include a loft large enough for a double bed upstairs — the warmest part of the building — for cozy winter nights. (See photo in the Image Gallery.) In summer, the soffit design encourages airflow in the loft to keep it cool. To live totally off-grid and harvest water, install a metal roof at an angle appropriate for optimal solar performance for your area. To find this information, go to the Astronomical Applications Department of the U.S. Naval Observatory. (Click “Data Services,” then select “Altitude and Azimuth of the Sun or Moon During One Day.”)

Because the greenhouse/guesthouse is so small, you shouldn’t need a building permit in most locations, but check with local authorities before you build your own greenhouse. The maximum size of a building allowed in my county without a permit is 10-by-12 feet, so the greenhouse/guesthouse is designed to those dimensions.

The Benefits of the Greenhouse/Guesthouse

• Salvaged materials keep costs down.
• Lots of glass on the south wall provides light and heat for a winter greenhouse and/or solar heat for living space.
• Wide overhangs shade the south side in summer, and soffit vents provide natural cooling.
• Inexpensive DIY solar collectors on the roof transfer heat to the radiant floor via an automotive heating core.
• Small footprint may make this structure exempt from building codes.

How to Build Your Own Greenhouse

To build your own greenhouse, start with a level building site, but make sure the site slopes enough to drain water from the building. Then cover the area where you’ll build the greenhouse with a weed barrier and gravel.