Passive Solar Design

Kelly Hart is your host here at greenhomebuilding.com, and has been involved with green building concepts for much of his life. Kelly spent many years as a professional remodeler, during which time he became acquainted with many of the pitfalls of conventional construction. He has also worked in various fields of communication media, including still photography, cinematography, animation (he has a patent for a process for making animated films), video production and now website development. One of the more recent video programs that he produced is A Sampler of Alternative Homes: Approaching Sustainable Architecture, which explores a whole range of building concepts that are earth friendly. Kelly is knowledgeable about both simple design concepts and more complex technological aspects of home building that enhance sustainable living. He has even designed and built a solar-electric car that he drives around his neighborhood. Kelly, and his wife Rosana, live in the earthbag/papercrete home that is profiled on the earthbag page. He is available, at a modest fee, for consulting about sustainable building design, either for remodeling existing structures to more fully embrace these concepts, or for new architectural designs.

Questions and Answers

Q: Can you please advise me on the relationship between thermal mass and insulation, as I don't fully understand it. For example in a cob building in a cold temperate climate, it seems important to insulate the home well, but insulation as I understand it resists heat transfer, so if one has to insulate a cob home on the inside, does that not inhibit the passive solar qualities of the building? ie the insulation although preventing heat loss, would also resist heat coming in from the walls (heat absorbed from the sun). In the same way, if one insulates a rammed earth building on the outside, how does that affect the ability of the thermal mass absorbing radiation from the sun?

A: There are two basic approaches to passive solar heating. One is the concept that you are referring to here, where you expect a thermal mass material (like adobe or cob) to transfer the heat of the day into the house to warm it at night. This only works in certain fairly moderate climates, where nights tend to be cool and days are warm. In harsher climates that are hot or cold most of the time, or have seasonal extremes, then this strategy doesn't work very well, because unwanted heat or cold is transferred directly into the house, and there is no way to control this.

The other approach to passive solar heating provides greater control and comfort in all climates. This is where the shell of the house is insulated and the heat of the sun is allowed to enter the building through glass. Ideally, the sun strikes thermal mass materials directly that are arranged in the floor, walls, or built-in features. In this scenario, the heat from the sun is collected in the thermal mass, which then releases this heat slowly over time...but this heat is not allowed to escape easily through the walls (or the glass, if it is insulated at night). This tends to provide greater comfort, because of the inherent stability of the temperature over time.

Also how does thermal mass behave in cold conditions? Does a large thermal mass absorb much heat when the sun is shining, but the air temperature is low, say around zero celsius?

Thermal mass materials will absorb the cold as well as heat, but with the sun, it is the radiant heat (infra-red) that is more readily absorbed by the mass material. This means that at zero degrees with the sun shining on thermal mass materials, they will warm up relative to the ambient air temperature...but when the sun goes down this heat gain will be dispersed in all direction.

I wonder also how one handles a building with a high thermal mass, when there are longer periods of low temperatures and no sun?

This is one of the main problems with the first approach to passive solar heating, since such buildings become cold and remain so. It is only with a great deal of supplementary heat that one can warm such a space, and much of this heat is being transferred outside and wasted. This is why the old stone castles were so notoriously uncomfortable places to live.

Q: I want to build a home by digging a large hole and cementing the floor and all walls, then making a slanted roof for water runoff, with windows on the south side. How will I keep a steady temperature of 68 degrees with no moisture?

A: You need to insulate and place a moisture barrier on all of the walls and floors that are buried. You need to balance the amount of sunlight that is allowed to enter the house and be stored in the thermal mass with the particular climate where you are.

Q: The reason why it is appearing that sustainable living will be a bit more of a challenge for me in this region is that the climate is one of extremes, The summers are frequently Very Hot and humid. Summer daytime temps reach over 100F fairly often, and winter temps have been as low as -30 in recent memory. I am uncertain if it is cloudier and more overcast then other regions but it does seem that we get a lot of storms. Provided my region isn't at a solar disadvantage I am still left wondering which extreme will cause me the most difficulty with any given idea I try to implement.

My current idea's are hovering around a heavily insulated partially recessed/totally recessed underground home of approx 1300-2000 sq ft in size. It would be designed for extreme efficiency and to come as close as possible to being able to sustain optimal comfort temperatures without requiring additional heating or cooling (I know this is a lofty goal but I like to do things in the extreme :) I am currently leaning towards a air-tight wood stove/fireplace backup for the extreme winter conditions (I like woods, so I plan to live in/near one.)

I am EXTREMELY drawn to the sustainable route but I am leery of biting off more then I can chew, especially for my first owner built home. So I have been trying to weight the consequences of either system carefully. But I haven't come to any conclusions as of yet.

A: With the kind of extremes that you mention, you have all the more reason to build with careful attention to what I would call sustainable features, especially good insulation, digging into the ground, and passive solar, compact design. We live at about 8,000 feet in Colorado and it gets as cold as Indiana. We probably have more sunshine, and slightly cooler summers, but these factors shouldn't inhibit employing these principles almost anywhere. When the sun shines, you take advantage of it; otherwise, the added insulation will only make the house more efficient in both cooling and heating. I suggest focusing on the best design relative to the specific site that you find, and then choosing materials that serve the design in an ecological way.

Q: I'm a student at university in Tanzania, East Africa and am searching for a PhD thesis topic. My interest is the use of clay/mud/soil to achieve thermal performance in walls. Its almost hot throughout the year here. Could you please advise?

A: I am sure that you could write a reasonable thesis on this topic that would help people understand the value of earthen materials in architecture and their vernacular use throughout the world. There is much science and practical experience to draw on.

Q: Could you please specify the areas that I could explore in such a study?

A: There are various areas to focus your attention:
1) Experiments that show the thermal effects of various thicknesses of solid earthen walls.
2) Experiments that evaluate the use of an earthen plaster of various compositions and thicknesses on walls made with other insulating materials, such as compacted straw, or earthbags filled with rice hulls.

Q: I have 12 Acres of Land in South Carolina. I intend to move back in the next 2 years. I had hoped to build a "Green Home" and escape the rat race. I am finding it VERY difficult to get information that can be used in that climate. Almost all the information is directed towards a cold mountainous environment. South Carolina in at sea level and hot and humid in the summer and rather cold in the winter.

A: A well-designed passive solar home generally also performs well in hot and humid locations because of the excellent insulation and the use of thermal mass materials on the inside. Often, the only part of the design that differs is perhaps less southern window exposure to reduce the solar gain somewhat. I especially like the idea of berming and earth-sheltering in these situations.

Q: I'm very interested in these forms of buildings and homes. All the examples I found were located in California or Arizona. Maybe because of the hot climate it suits the earth and the East coast rigid cold weather (which isn't that rigid anymore) may cause the moisture in the soil to freeze and create a freeze thaw effect. Regardless of that, I still would like to know if there is a solution for people in the east coast who would like to own a home like the ones featured on your site.

A: Most well-designed passive solar homes would do well on the East Coast. Some designs that utilize uninsulated earthen walls (adobe, cob, rammed earth) would not be a good choice, but cordwood, strawbale, and some earthbag buildings would do just fine.

Q: I have heard all sorts of variations on the basic passive solar design - trombe walls, double-envelope houses, earth sheltering, passive annual heating storage, annualized geothermal heat storage etc. These sound either expensive or large engineering challenges. How much better are these alternatives to a basic above-ground direct-gain configuration.

A: That's a great question, and one that is difficult to answer quantitatively, since there are so many variables. Certainly all of these strategies do work, at least to some extent. I lived for many years in a mostly above-ground passive solar home in a very cold climate, and I can attest to the efficacy of this arrangement. It was dug into a south-facing hillside, so it did have that advantage. However, during lengthy periods of gray days, I would have to rely on my backup heat source for comfort.

Most passive solar designs would perform best when also accompanied by at least some degree of earth-berming or earth-sheltering...the more the better.

The concepts that arrange for annualized heat storage are more likely to provide heat over longer periods of lack of solar input, but then they also can take years to get up to speed thermally, and their performance often depends on the site-specific considerations and design, so they are somewhat controversial.

The double envelope house design I think is a waste of materials and embodied energy and not worthy of the attention that they have gotten.

Q: What is the difference between thermal mass and insulation?

A: Thermal mass is heavy, dense material that will hold its temperature for a long time and passes cold and heat through fairly easily. Insulation is light, airy material that tends to block the transfer of heat and cold, but does not retain a given temperature.

Q: And if dirt or sand piled on top of each other in earth bags is good insulation for a home why not poured inside the holes of the concrete block?

A: Dirt or sand does not yield good insulation, whether in bags or in concrete blocks. Depending on how thick the walls are, they will act like they are insulating because they slow the passage of outside temperatures to the inside, but it is just a matter time before this happens. In some fairly temperate climates, this can work out OK, especially if it is cool at night, so that the cool arrived to the inside just as the day begins to heat up...but this is special situation and won't work out most localities.

Q: Do you recommend passive or active means for building green homes? Why?

A: I recommend both of these approaches, since they both help contribute to a healthy environment, but I have a special fondness for passive concepts because they are simpler to implement and less likely to require maintenance over time.


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I specifically disclaim any warranty, either expressed or implied, concerning the information on these pages. Neither I nor any of the advisor/consultants associated with this site will have liability for loss, damage, or injury, resulting from the use of any information found on this, or any other page at this site. Kelly Hart, Hartworks LLC.