Q: I am a graduating senior from Antioch College in Yellow Springs, Ohio. For my senior project I am developing a plan to build ecologically sound affordable housing in my area. I fully intend on using passive solar heating as much as possible in the interest of maintaining the house's affordability, as well as environmental quality. I understand the need for thermal mass, but am confused on the kind/type of solar glazing that is needed for the windows. What kind of windows are best for passive solar heating? What kind of glazing do they require? I also am having confusion over how to reduce the amount of sun in the summer, yet still allow for full sun in winter. Is there a formula or method to figure the amount of sun a particular location will receive, the angle of sun, and then the amount of heat captured, and perhaps from there a way to calculate the needed strength of an HVAC system? I noticed several publications on your web site, but if you could recommend a good book in a student's price range that might outline some of the basics, that would be wonderful.

A: I could spend a lot of time trying to provide information on windows, but at the risk of sounding like I'm promoting my book, you should probably get a copy of my new book, The Solar House. It has a chapter on energy efficiency which contains a section on windows. This will help you understand what you need. Basically, you want double-paned low-E windows with a wooden frame and metal cladding on the outside for protection. The chapter will explain why this is one of your best options.

Chapters 1 and 3 in my book, The Solar House, cover this topic in detail. It shows the formula to calculate overhang and lists a web site that will run the calculation for you. You might also want to read chapter 7, which discusses software for analyzing the energy performance of solar homes.

Q: For the passive solar home we're building we would like to use double pane low e glass with a high shgc for the south side. Most low e glass (with a low u value) also comes with a low shgc. What values ( u value and shgc value) should we use? Who manufactures windows with these values?Our house is being built at 7600 ft in southern Utah.

A: I'd look for glass that has a U-value of at least 0.33 and a solar heat gain coefficient around 0.40 to 0.55.  Ask your local window suppliers. That's your best bet.

Q: I want to make a solar wall out of half gallon wine bottles creating bottle logs. Will this work?

A: (Kelly) It isn't clear to me whether this is to be used as a trombe wall, or more of a direct solar gain situation. If you intend to use the bottles in part of a trombe wall, then I would say that they might defeat the purpose to some extent, because the bottles will not act as thermal mass to hold and pass through the heat. In a direct gain capacity, they would serve you somewhat better, but would not be as ideal as ordinary flat glass, because they will not pass through as much of the light and heat that strike the wall. Bottles are best used in decorative ways in my opinion.

Q: I am a student performing an experiment on thermal absorptivity of different kinds of glass as a building material for different climatic condition. I will appreciate it, if you can give me any information on ways to carry out such an experiment.

A (Paul Shippee): The thermal absorptivity of glass is dependent on the iron content of the glass. This iron content can be assessed by looking at the edge of the glass. The greener the color, the more iron present and the more absorption occurs as sunlight tries to pass through the glass. Sunlight absorption in glass can be as high as 10% for high iron content, and as low as 0% for water white glass (as it is called).

However, usually the greater percent of sunlight is reflected off the glass.  For sunlight coming toward the glass this reflection is 4% for each air-glass interface. So, for single glass this reflection is 8%, and for double glass it is 16%. These losses occur in addition to what gets absorbed in the glass (in the iron) itself. For double glass it is not unusual to lose 20% of the sunlight; that leaves 80% of the sunlight that gets transmitted.

Now, to measure these losses, simply place a sunlight meter (a tiny photovoltaic cell can be used hooked to a DC amp meter) in front of the glass, then compare this reading to when the meter is placed behind the glass. This will give you the total loss --both reflection and absorption-- which is the valuable number relevant to solar gain in passive solar buildings.

Given all this, I think it is extra difficult to measure just the absorption of a piece of glass.

An interesting experiment is to put your face near a wood stove so you can feel the heat, then slide a piece of glass between your face and the stove.  You'll be surprised at what happens as the glass intercepts the long wave heat (infra red) radiation off the hot stove metal surface.

Q: I am planning to build a new home using passive solar design in Sturgeon Bay, WI. My question is in regards to windows and overhangs. Some window manufacturers now produce windows with blinds between glass. Would using these on the south side help reduce unwanted summer heat gain or would they inhibit full incoming winter sun? Also, is it true that the size of south overhangs are not an issue as WI summer sun is not intense enough?

A: I think I would stay away from windows with blinds between the glass. I haven't assessed their performance and don't know much about them, but it would seem that they would reduce solar gain in the winter. I have never heard the claim that the size of south overhangs is not an issue in the summer in Wisconsin. If summer temperatures are in the 80s and 90s during the summer, I think you would want to protect the south-facing glass (with appropriately-sized overhang) from additional solar gain.

Q: I am in an MBA program currently working on a project on energy conservation. Would like to ask your opinion on Electrochromatic windows and if you feel that this product would benefit in energy costs?

A: (Kelly) Electrochromatic windows are certainly an interesting new technological development. One estimate that I read is that they could potentially reduce peak electric loads by 20%-30% in many commercial buildings and increase daylighting benefits throughout the United States. This in its own right would be quite significant as a conservation measure.

For residential use, they could also save more energy than they require to operate. They are mainly useful in either allowing or limiting the amount of light and heat transmitted through them. As far as actually insulating a window space, they would not be as efficient as various sorts of thermal curtains or shutters that are manually operated. But, of course, manually operating such window curtains requires attention and time that many folks don't have.

I suspect that a well-designed passive solar home that was manually operated by its occupants would greatly out-perform one that relied entirely on this technology. Where such windows might really shine is in retro-fitting all of the windows that are inappropriately located and tend to either heat or cool the house too much.