I was recently challenged by one of the students in the Small Home. Big Life. e-Cours(E) to talk more about passive solar opportunities in the small house/tiny house community. My initial response was that it was not in the course outline (for good reason, mind you) and that that was a huge challenge considering all of the layers of passive energy. After thinking about it some and even losing a little sleep over it I realized that the concept itself wasn’t all that hard and that some may want to know more about it or how to find out more about it.
A passive solar home means a comfortable home that gets at least part of its heating, cooling, and lighting energy from the sun. Passive solar homes collect heat as the sun shines through south-facing windows and retain it in materials that store heat.
Passive solar design takes advantage of a building’s site, climate, and materials to minimize energy use. A well-designed passive solar home first reduces heating and cooling loads through energy-efficiency strategies and then meets those reduced loads in whole or part with solar energy. Because of the small heating loads of modern homes it is very important to avoid oversizing south-facing glass and ensure that south-facing glass is properly shaded to prevent overheating and increased cooling loads in the spring and fall.
Before you add solar features to your tiny home design or even existing small house, remember that energy efficiency is the most cost-effective strategy for reducing heating and cooling bills. Because your house is small there is naturally less to heat and cool anyway so optimizing home energy is an easy DIY. If you’re remodeling an existing home, the first step is to have a home energy audit to prioritize the most cost-effective energy efficiency improvements. Some of the steps commonly employed by tiny housers already are:
- EnergyStar rated windows
- Spray On Cellulose Insulation
- Radiant Barrier
- Soffit Vents on the Eve of the House
- Alternative Heating and Cooling (not over using or under using)
- LED light bulbs
- 12 volt Power Systems
- On-Demand Hot Water Systems
This is the tricky part because there are few legal, available sites for tiny houses (especially tiny house trailers) to begin with. But if you have the luxury of space, land, etc., a portion of the south side of your house must have an unobstructed “view” of the sun. Consider possible future uses of the land to the south of your site—small trees become tall trees, and a future multi-story building can block your home’s access to the sun. In some areas, zoning or other land use regulations protect landowners’ solar access. If solar access isn’t protected in your region, look for a lot that is deep from north to south and place the house on the north end of the lot.
How It Actually Works
In simple terms, a passive solar home collects heat as the sun shines through south-facing windows during the day and then retains it in materials that store heat,. These materials are known as thermal mass. The share of the home’s heating load that the passive solar design can meet is called the passive solar fraction, and depends on the area of glazing and the amount of thermal mass. The ideal ratio of thermal mass to glazing varies by climate. Well-designed passive solar homes also provide daylight all year and comfort during the cooling season through the use of nighttime ventilation.
To be successful, a passive solar home design must include some basic elements that work together:
- Properly oriented windows. Typically, windows or other devices that collect solar energy should face within 30 degrees of true south and should not be shaded during the heating season by other buildings or trees from 9 a.m. to 3 p.m. each day. During the spring, fall, and cooling season, the windows should be shaded to avoid overheating.
- Thermal mass. Thermal mass in a passive solar home — commonly concrete, brick, stone, and tile — absorbs heat from sunlight during the heating season and absorbs heat from warm air in the house during the cooling season. Other thermal mass materials such as water and phase change products are more efficient at storing heat, but masonry has the advantage of doing double duty as a structural and/or finish material. In well-insulated homes in moderate climates, the thermal mass inherent in home furnishings and drywall may be sufficient, eliminating the need for additional thermal storage materials. Now this element can rule a lot of tiny housers out because most tiny houses do not use and cannot use brick, stone, tile, etc. due to weight restrictions. However, there are ways to incorporate elements and this could be essential if constructing or renovating a small “sticks and bricks” house.
- Distribution mechanisms. Solar heat is transferred from where it is collected and stored to different areas of the house by conduction, convection, and radiation. In some homes, small fans and blowers help distribute heat. Conduction occurs when heat moves between two objects that are in direct contact with each other, such as when a sun-heated floor warms your bare feet. Convection is heat transfer through a fluid such as air or water, and passive solar homes often use convection to move air from warmer areas — a sunspace, for example — into the rest of the house. Radiation is what you feel when you stand next to a wood stove or a sunny window and feel its warmth on your skin. Darker colors absorb more heat than lighter colors, and are a better choice for thermal mass in passive solar homes.
- Control strategies. Properly sized roof overhangs can provide shade to vertical south windows during summer months. Other control approaches include electronic sensing devices, such as a differential thermostat that signals a fan to turn on; operable vents and dampers that allow or restrict heat flow; low-emissivity blinds; operable insulating shutters; and awnings.
In The Details
Although conceptually simple, a successful passive solar home requires that a number of details and variables come into balance. An experienced designer can use a computer model to simulate the details of a passive solar home in different configurations until the design fits the site as well as the owner’s budget, aesthetic preferences, and performance requirements.
Editor’s Note: Some information provided by energy.gov
Image courtesy of http://solarenergyfactsblog.com/passive-solar-energy/