Follow these principles when building a sunroom addition and you can get the triple benefits of a cozy living space, free solar heat, and a super greenhouse for plants.
Think of a sunroom, sometimes called a sunspace, as a productive living area in your home with different benefits in different seasons. In spring and summer, a sunroom provides a relaxing living space. In winter, it’s also a great place to grow food plants. Even better, a sunroom can help heat your home! I estimate that a well-designed sunroom addition can pay for itself in less than five years through savings on food and home heating.
To realize the full benefits of your sunroom addition, you’ll need to incorporate the basic elements of passive solar design: orientation, glazing, thermal mass, insulation and ventilation. If you apply these principles, you shouldn’t need to heat or air-condition your sunroom — you will be able to keep this living space at a comfortable temperature by using natural systems.
Would a sunroom work for your house? If your solar exposure gives you at least four hours of sunlight around midday in midwinter, the answer is probably yes! The sunroom design strategies described in this article will work in almost all U.S. climate zones and southern Canada. However, you may want to research beyond the level of detail presented here to fine-tune your design for your specific climate, especially if you’re trying to optimize your heating potential.
Site Your Sunroom
You will want to orient your sunroom toward “true south” (which is usually a few degrees different than “magnetic” south) to best take advantage of the sun’s low angle in winter. There are several methods for finding true south. In general, orienting the solar window 20 degrees off true south reduces your solar gain by only 4 to 5 percent. On the other hand, if your glass is oriented 45 degrees off true south, solar gain will be 18 to 22 percent less.
Think About Glass
There are several types of glazing you can use for your sunroom, but I recommend glass because it traps more heat energy than plastic and because it’s durable. Plastics do not work as well as glass for trapping long wave heat energy, polycarbonates can scratch and yellow over time, and films are just too fragile for a house.
Dual glazing should be used in all but the most temperate climates, such as southern Florida. In colder climates, including much of the Northeast and northern Great Plains, triple and sometimes even quadruple layers of glass work best. I recommend clear, uncoated glass for sunrooms. Avoid reflective glazings and the newer low-e glass because they prevent much of the solar energy from entering through the glass, and they reduce the solar spectrum. If you want to grow plants in the back area of your sunroom, I also recommend skylights to bring in overhead sunlight.
For optimum winter collection of light and heat, sunroom glazing should be tilted so it’s perpendicular to the sun’s angle at winter solstice (Dec. 21). However, for several practical reasons, I’ve selected vertical glazing for my ideal sunroom design. Believe me, I’ve tried the tilted glazing on sunrooms, and it’s not the best choice for a living space.
Here are a few of the many advantages of choosing vertical glazing instead:
- Easier to shade in the summer and insulate in the winter
- Easier to vent windows
- Less prone to leakage
- More comfortable on winter nights because less radiation is lost through glass
- Less likely to be damaged by heavy snow or hail
So, how much glazing do you need to start producing heat for your home? Consider that you’re using vertical glazing with passive solar design principles for a house located between 30 to 40 degrees north latitude (roughly between Austin, Texas, and Denver). For a 1,200-square-foot house, 300 to 600 square feet of greenhouse glazing could provide 25 to 50 percent of the heat needed. Reflectivity off exterior surfaces, such as snow-covered ground, can add to the overall solar gain on a vertical surface.
Add Thermal Mass
The trick to keeping a sunroom comfortable is to provide thermal mass: dense materials that soak up and conduct heat when the sun is shining on them. I recommend using concrete, stone or tile floors — not carpet or wood. Without thermal mass (and ventilation), the temperature can swing from 40 to more than 100 degrees depending on the climate and weather. Generally, the thermal mass should be a darker color to absorb heat more readily, but it doesn’t have to be brown or black. Reds, blues and greens work well and create more playful sunrooms.
The wall between a sunroom and the inside of the house can also be a good heat sink, transferring heat by conduction and radiation into the house. Heat absorbed in the sunroom during the day will slowly conduct through the solid material to provide heat to the inside space at night. These walls can be mass walls or water walls. Mass walls can be rock, masonry, adobe, or concrete; water walls are vessels or tanks that contain water to store heat. A heat-sink masonry wall is usually about 8 to 16 inches thick, although the first 2 inches of thermal mass are the most effective in regard to daily heat shifts.
Water walls have different performance characteristics than masonry walls. Water vessels such as paint cans or 55-gallon drums are stacked between the sunroom and the adjacent interior rooms. You can also use large-diameter, vertical pipes or fiberglass tubes. The advantage of using a water wall for thermal mass is that the heat that strikes the surface of the wall is more readily absorbed. The heat energy is convected or mixed within the water vessels rather than slowly conducted through like the solid masonry. Use about a half cubic foot of water for each square foot of south-facing glass. A water wall 8 to 12 inches thick with a dark, exposed surface will have daily temperature fluctuations of 20 to 40 degrees.
Water walls hold more heat by volume than solid materials, and the water absorbs and mixes the heat more quickly. A masonry wall may allow a temperature fluctuation of 45 to 70 degrees within the sunroom if there’s no convection through an adjacent opening (keep reading for more information on ventilation). This may be too much fluctuation for many plants. Using additional mass such as containers of water will narrow the fluctuation to 20 to 40 degrees, which is more acceptable. It may also increase the sunroom’s humidity, which might be beneficial, but can cause condensation on the inside of the glass when the outside temperature is low. Generally speaking, a mass wall on the north side of a sunroom will effectively transfer 15 to 30 percent of the total heat collected through the glazing.
Don’t Hesitate to Insulate
I recommend a fully insulated roof for the sunroom, with an R-value of 20 to 50. I prefer to use structural insulated panels (SIPs) because they don’t allow moisture to penetrate. You can use fiberglass, granular, or foam insulation with conventional joist framing for the roof, but you need to prevent inside humidity from being absorbed into the ceiling. Typically, the roof joist space will need to be vented to prevent dry rot. You can use vapor-proof interior membranes or paints on the inside of the room, but I prefer SIPs to avoid any problems.
It is also important to insulate the end walls (R-value of 19 to 30), but don’t worry about the wall adjacent to the house. Rigid perimeter slab and wall insulation is necessary on the exterior foundation (R-value of 11 to 19) and, in colder climates, I recommend using rigid “closed cell” foam insulation below the concrete slab floor (R-value of 4 to 12). You can also use insulation or thermal drapes and radiant membranes (R-value of 2 to 8) on the glazed surfaces to prevent heat loss through the glass at night. This is a smart choice in especially cold climates.
Add Ventilation: Windows, Doors, and Skylights
In winter, ventilation is necessary to help the heated air go where it is useful. I recommend using natural convective ventilation. During the heating season, you’ll want to help warm your house. Vents high and low in the wall adjacent to the house’s interior allow the hot air to flow out the top, and cooled air to return low into the sunroom for another trip. Of course, doors can be opened to create a similar effect. I like a combination of thermal wall and sliding, or French doors, on the house wall.
In summer, or during the cooling season, it’s necessary to vent the sunroom to the outside so it doesn’t overheat. Vent skylights work well for this purpose, or you could use vents high in the end walls for the same effect. I also like to use low-awning windows or sliding glass doors on the solar facade to let the fresh outside air flow upward and out of the sunroom. These screened windows can be left open most of the cooling season. All vents should be airtight and even insulated when closed in winter.
One way to collect heat more effectively with a sunroom is to use a duct and fan system to pull heat from the top of the sunroom and direct it to remote areas of the house, such as north-side rooms. Slow-speed, in-duct fans, often called “whisper” fans, can be manually switched on when the sunroom has extra heat and the remote spaces are colder. Or they can be automatically controlled by a differential temperature thermostat, which senses the temperature in the sunroom versus the temperature in remote spaces. A general rule is to provide a fan capable of moving 50 cubic feet per minute with 6-inch ducting. Fans can be noisy, so it’s a good idea to insulate any ductwork.
David Wright is the author of The Passive Solar Primer.
Seasonal Variation
As the angle of the sun changes, so does solar heating potential. These figures show the heating potential of 300 square feet of glass at 40 degrees north latitude (about the latitude of Denver).
December: 494,000 Btu/day
January: 518,000 Btu/day
February: 519,000 Btu/day
March: 445,000 Btu/day
April: 307,000 Btu/day
Sunspace and Passive Solar Design Resources
Passive Solar Energy (second edition, 1994), by Malcolm Wells and Bruce Anderson
The Passive Solar Energy Book (1979) by Edward Mazria
Eco House (third edition, 2007) by Sue Roaf, Manuel Fuentes and Stephanie Thomas
The Survival Greenhouse (1978) by James B. DeKorne
The Complete Greenhouse Book (1978) by Peter Clegg and Derry Watkins
The Passive Solar Primer (2008) by David Wright
The Solar Greenhouse Book (1978) by James McCullagh
Online Resources
