Ask the Expert: How to achieve passive solar in a home

| by Teena Hammond
Ask the Expert: How to achieve passive solar in a home

Passive solar design is a way of making your home more energy efficient by reducing heating demand and creating a tighter building envelope. By siting a new home to take advantage of the sun's warmth in the winter, and avoid the sun's heat in the summer, it's possible to regulate temperatures to keep a home more comfortable year round.

In this Ask The Expert feature story, the ACE's at ProudGreenHome answered the question: How do you achieve passive solar in a home?

Sean Canning, LEED-register architect and founder of 10|70 architecture

Imagine if your home varied temperature by less than one degree Fahrenheit a day with no help from a heater or air conditioner. That is typical of a passive home design.

Passive design should not be confused with net zero design, which focuses on a zero degree plus/minus in terms of energy usage as a metric exclusively. Both would be considered extremely efficient/green well above building code standards. A passive home has three design criteria: reduce heating demand, reduce primary energy use, and a very tight envelope assembly.

A passive design requires the home to function in complete unison, from the solar orientation to the materials selected plus the way they are installed. Passive design is not an attachment or supplement to architectural design, but a design process that is integrated with architectural design. Although a combination of various sustainable techniques are needed to achieve passive standards, there are a couple consistencies that we find in these types of homes.

Passive solar is a technique that is consistent with most passive design. To maintain a consistent comfortable indoor air temperature, a thermal mass is installed. This is a component that will absorb solar radiation (heat from direct sunlight) during the day plus release that heat at night when the sun is down plus outdoor temperatures drop. The thermal mass can be as simple as a ceramic tile floor but is generally much more prominent in the design like a concrete wall up two-feet thick in some cases. The key to this component is it requires direct sunlight and the thicker/denser the mass, the more heat/energy that will be stored. In some cases the Earth itself can be used as a thermal mass.

Also, direct sunlight (which will heat up a space) should be limited in warmer seasons. In the summer months, the sun comes from a higher point in the sky. Exterior window shading devices, trellis structures and deciduous trees are common solutions. Deciduous trees will actually allow the solar heat gain in the colder parts of the year when they drop their leaves and provide shade in the warmer seasons.

The point of the solar heat gain and thermal mass combination is to regulate the indoor air temperature and reduce the need to spend energy to do this. Even so, generally a supplemental mechanical system is installed for extreme conditions and ventilation.

Ventilation can also be achieved passively through techniques utilizing stack effect (when heat rises and cold air settles) and naturally with operable windows. Ventilation should be considered from the very onset of the project.

Super insulation is the second component to passive design. Once we have the solar orientation correct and have determined which windows will allow solar heat gain, it is important to tightly seal the rest of the building envelope. This is generally achieved with a tight air barrier and an extra thick layer of thermal insulation in the exterior walls. This prevents temperature transition and air infiltration through the envelope (spray polyurethane foam insulation is ideal for this b/c it has an R-value of about 6 per 1-inch thick and doubles as a moisture barrier.) The importance and cost of super insulation will increase with harsher climates.

Once you have a comfortable indoor air quality, the super insulation will maintain that environment. It is important to note that when installing a tight building envelope the home should be checked for healthy indoor air quality. A minor mechanical vent leak from a water heater that could have went unnoticed in standard construction conditions will be exponentially dangerous in a tightly sealed home.

In order to achieve a passive home it is important to establish a qualified team of professionals starting with an architect and an energy consultant and engineer. The team (including the client) needs to establish project goals and expectations for a completely integrated design phase. Passive design is very challenging but the rewards are reduced energy dependence, reduced utility bills, and a comfortable home.

Ted Clifton, principal of Zero-Energy Plans LLC, and CVH Inc.

While sunshine may "just happen" and it happens better in some places than in others, effective use of passive solar energy needs to be very carefully designed into the home from the very beginning. Many homes will allow lots of nice warm sunshine in; that is not so much the problem. The problem is keeping it, and using it productively, rather than letting the home get overheated at some times, and remain cold at others.

The key to success is the balance between solar exposure and thermal mass storage. You need to know how much radiation you will receive from the sun, on average, during your heating season. Then you need to have the storage capacity in your thermal mass to hold that energy, without so much temperature rise that the room becomes uncomfortable.

In our area, in the Pacific Northwest, we will get around 50 Btu per square foot per average hour of exposure during the coldest two months of the winter. The reason this number is so small is that much of the time it is very cloudy here in the winter. When the sun is actually shining, we get up to five times that much direct radiation. Because of our relatively high latitude, we will only get direct solar radiation for about six hours each day, even though the sun will be up for over nine hours at the minimum. The rest of the time the sun will just be too low in the sky to get over the neighbor's house, or the trees across the street, even if you have planned for a clear area in front of your house for such a purpose. A well-oriented 2,000-square-foot home may have 200 square feet of south-facing window, and if the angles have been calculated correctly, most of that area will result in energy being transferred into the home.

Good windows, however, will have a Solar Heat Gain Coefficient (SHGC) of around .5, which means that only 50 percent of that energy will actually get through the glass. Windows with a lower SHGC will let even less heat through. If you multiply the window area (200 s/f) times the Btu/sf (50),then by the SHGC (.5), and finally by the number of hours per day (6), you will get the number of Btu you will receive on a winter day (200x50x.5x6=30,000). This would provide enough heat for one of our net zero energy homes for about 2-1/2 hours on the design degree day, and over four hours on a more average winter day. Since it will take six hours to absorb four hours' worth of energy, it is obvious that cloudy days in the winter are not the time when passive solar heating will require tempering with thermal mass.

Consider now the sunny winter days, when we receive around 240 Btu per s/f. Using the same formula, we would receive about 144,000 Btu per day, which would be enough to heat the same home on the design degree day for about 12 hours, or an average winter day for about 18 hours. Because the air inside the home can only hold about .018 Btu per degree F in temperature rise, and there are just under 22,000 cubic feet of air inside this particular vaulted-ceiling house, this would result in a rise in temperature of about 60 degrees per hour, if not for other mitigating factors! The first significant factor is that all houses have some thermal mass, unfortunately not enough to solve the entire problem. The second significant factor is that as the inside temperature rises, the difference in temperature between the inside and the outside increases, therefore the conductive heat loss increases as well. For every five degrees of temperature rise, there is about a ten-percent increase in heat loss per hour! If we can avoid any significant temperature rise, while still holding on to the energy gained, we can solve the passive solar heating problem.

This is where added interior thermal mass comes in. Concrete, brick, stone, or water, all have a significant ability to store heat, with a much lower temperature rise. They can then give that heat off more slowly, over a longer period of time. A pound of water, by definition, will rise in temperature by 1 degree per Btu. Since a cubic foot of water weighs about 62.5 pounds at room temperature, this means that water can store over 3,400 times as much heat as the same volume of air! Concrete, brick, and stone each store slightly less, but still a very significant improvement over air, or wood, or even dry wall. This means that when the same solar energy is directed into a concrete slab, a very small temperature rise will result, but the heat will still be there to radiate back out over the next several hours or days.

Not all of the solar radiation received is absorbed by the thermal-mass floor. Some if it is reflected off, bouncing around the room, heating the air and other objects. This typically is between 10 percent and 15 percent of the total received, not particularly significant when compared to the amount absorbed by the floor.

Now consider a spring or fall day, when the outside average day night temperature is a more modest 50 degrees, as compared to the design-degree day of 17 degrees. We are receiving as much as 275 Btu per square feet per hour when the sun shines, but we are receiving it for as much as eight or nine hours per day, and even on the average day (still often cloudy) we are receiving 120 Btu per hour per square feet. Because of the higher outside temperature, our heat loss is only 4,800 Btu per hour, yet we are receiving as much as 247,500 Btu per day on a sunny day, after the reduction of our .5 SHGC windows! This represents more than two days' worth of energy requirement, and even on the cloudy days we are receiving (and storing) almost enough to heat the home.

Each climate is different, and the calculations must be done specifically for your own conditions. Especially important is the shading of glass in the summer-time to prevent the sun from heating up the house when it is already warm enough. Proper roof overhangs, and the use of carefully placed deciduous trees and shrubs will all be a part of your passive cooling strategy.

When it is all added up, in the Pacific Northwest we can provide up to 15 percent of our heating needs during the coldest and cloudiest winter weather, and 100 percent of our heating needs for about eight or nine months of the year. What is the cost? Nothing, just good planning!

Heather Ferrier Laminack, marketing manager for Ferrier Custom Homes

Basically, designing your home to incorporate passive solar is all about paying attention to what's going on in the natural environment that surrounds your home. Passive solar design is a technique that has dated back as far as we have had dwellings. It requires no special technology, just your attention.

Things to pay attention to include where the sun rises and sits in relation to your home — You will want to take notice to this feature, because your climate will dictate whether you will want to block out or absorb the morning and evening sun. For example, I am in a hot climate, so we are always looking to orient and design the home so that the least amount of sun reaches inside the home when the sun rises and sets, which would require our air conditioning to kick in and compensate. This also means that we minimize the amount of glazing (i.e. windows, doors) on the east and west sides of our homes, and when present we incorporate overhangs, porches, shutters or landscaping strategies to control the sun's infiltration into the home.

Paying attention to the seasons — even in our hot climate in Texas we want the sun to reach inside the home during the winter months, which will assist in heating the home. We therefore invest attention into strategically placing windows on the south side of our homes with the proper overhangs. In the winter the sun sets lower in the sky than it does in the summer, so we design overhangs that block the summer sun out, but allow the winter sun in. Planting deciduous plants and trees also helps reinforce this method, because as they lose their leaves in the winter they allow sun to reach the home.

Noticing the natural habitat — we once had a homeowner who would drive out to his site and watch how the wildlife would nest on his land. He noticed that in the hottest months of the year the deer would congregate and sleep in one particular shaded area that stayed cool, and in the winter months they would return to this same location as it was at the bottom of a bluff that blocked the harsh northern winter wind. This is where he ultimately located his home on his land, as the natural habitat provided the most protection and aid. Fortunately he was building on 100+ acres, so the deer had plenty of other locations to settle into.

Lastly, we have strong southerly winds in our region. As a result, we position operable windows on the north and south of our homes so that we capture these breezes, which aid in cooling the home and ultimately delay the use of air conditioning.

When incorporating passive solar strategies into your project, you are essentially aiming to work with the natural environment as much as possible, instead of working against it. When this approach is taken, it will save you operating costs and result in a much more comfortable, harmonious dwelling.

Read more about building a green home.


Topics: Building Green, Windows



Teena Hammond
Teena Hammond has published more than 2,000 articles in People and W magazines, Women's Wear Daily, and in dozens of newspapers and books. She also wrote a home improvement, remodeling and decor column that ran in Gannett newspapers nationwide. She's interested in all things green and would love to hear from you with your story ideas.

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