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Types of Glazing for Passive Solar

Ken Haggard, formerly an architecture professor at California Polytechnic, is an architect and principal in the San Luis Sustainability Group. Since the late sixties, Ken has designed more than 200 solar buildings, from homes to large commercial and institutional buildings—as well as the first permitted straw bale building in California. An active member of the American and International Solar Energy Societies, he received the Passive Pioneer Award from ASES in 1999 and was made a fellow of ASES in 2000. His office and home—in Santa Margarita, California—are passive solar, off grid, and straw bale. With David Bainbridge, Ken wrote Passive Solar Architecture: Heating, Cooling, Ventilation, Daylighting and More Using Natural Flows, published by Chelsea Green in 2011.

Questions and Answers

Q: We are building an off grid passive solar rammed earth home in southern New Brunswick (Lat 45'). Trying to decide on adaquate glazing for the south face, which will be entirely windows, with the north and most of the east and west walls bermed. The south glaze front will be tilted to 68' to maximize our capture during the winter. From the Q&A site, I understand that aiming for a glazing with a U value .4 would be important - to add to the complexity, if we are also trying to grow food in the house from indoor grey water planters along the south face, what type of glass should we invest in to ensure plant life, solar gain and minimize heat loss?

A: Assuming New Brunswick has a high heating load, is the rammed earth the exterior walls? If so are they going to be insulated somehow? Rammed earth does provide good thermal mass but it doesn't provide much insulation. For a high heating load poorly insulated exterior walls would override all other considerations. My suggestions would be use straw bale walls for the exterior walls and rammed earth for the interior walls. This would optimize insulation and thermal mass and be easier to build as well. For your climate insulation of exterior walls should be at least R 35 and the roof R 55. If you still want to use rammed earth on the exterior the common fix is to insulate the outside of the wall with styrofoam insulation which then receives a weather skin, usually of stucco.

Sloping glass on the south such as you describe was done a lot in the 80s but it was found that this put more stress on maintaining the integrity of the multiple glazing system and really provided very little extra solar gain. This would particularly be the case here if the building is poorly insulated. Why maximize solar gain if you're just going to lose it through the exterior walls? For your climate, maximize insulation not solar gain. Once that's done then solar gain can become important but be careful not to over glaze because you need to minimize heat loss through the glass. I would suggest using vertical glass on the South in the amount of 10 to 25% of your floor area in south glass. Use the higher % if you can insulate the glass at night .

The type of glass is of course important. Fortunately in Canada you have a better choice of glazing than we do in The States where marketing concerns means most window companies won't carry high heat gain low e windows in many areas. You do want highly insulative glass (double pane low e at least) perhaps even triple pane low e. But you also want a high solar heat gain of at least .6 SHGC (solar heat gain coefficient). The common mistake in these parts is to use low e heat rejecting glass (. 30 SHGC). Under this circumstance a otherwise perfectly designed passive solar building won't heat much at all.

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: (Daniel Chiras) 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: (Daniel Chiras) 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: (Daniel Chiras) 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.

Q: There are commonly cited glass:floor area and glass:mass ratios for passive solar design. Some of these can be found in Dan Chiras' book. They quote a figure of 7-12% glass to floor area for a sun-tempered dwellings. Above this additional mass is recommended. Does this assume the sun-tempered dwelling essentially has no high mass components whatsoever? e.g. timber subfloor with timber wall framing and lightweight linings and claddings?

A:(Daniel Chiras) Yes...

Q: We plan to replace leaky windows on a south-facing wall. The low-E coatings limit sunlight coming in--yet all the companies advocate these and argon or krypton to prevent heat loss on winter nights. Should I just get well-built windows with clear glass to maximize SHG or use the full package they push. Pella and Peachtree said I can't increase SHG without losing insulating factor. SeriousWindows says I can, a little. Any ideas?

A: (Daniel Chiras) You may have to shop around a bit, but you should get a low-e window with an appropriate solar heat gain coefficient. I just ordered some low-e windows with a solar heat gain coefficient of 0.55 from Quaker in California. I think that Pela offers a low-e wind with a similar SHGC. You might also check out Canadian glass manufacturers. From what I've heard, they're offering windows that meet these specs.

Q: I am building a log sided home in Elliot Lake, northern Ontario, Canada. It is south facing on a small lake. It is a rectangle with a shed roof on the loft. It is 32'x43'. One of the 32' sides is facing the lake and this is the great room prow. It has not been blue printed yet so changes can still be made. Our temp is quite hot in the summer coming in the prow and obviously we get minus 30 degrees below centigrade in the winter. Can you advise me about windows? What type, best brand for my climate etc.?

A: (Daniel Chiras) If you intend to capture solar energy for heating, you will need to install low-e, high solar heat gain coefficient windows (SHGC of 0.6 or higher) on the south-facing edifice. You should install a window with a U-factor of 0.3 or lower, preferably 0.1 to 0.2.

You are in luck. There's an amazing window company in Canada called Thermotech which sells a product that will work for you.

Throughout the rest of the home, you will need to install low-e windows (with a U factor 0.1 and 0.2, if at all possible) for best performance. Solar heat gain coefficient for these windows is not as important as the south-facing windows. I'd shoot for a SHGC of around 0.35 for these windows.

Q: Is there an economical way to create double paned windows or do I have to purchase pre-made. How much does cost reflect quality?

A: (Kelly) The problem with homemade double pane windows is that it is nearly impossible to make them airtight, so then moisture will condense on the inside. My suggestion is to find seconds or windows that have not been picked up at glass shops that they usually sell for much less money.

Q: I plan to build a passive/active solar home in SE Pennsylvania. I've created a fairly involved spreadsheet that allows me to calculate heat load, heat loss, etc. My question is this: How is heat loss through windows handled, when the sun is shining through the window? Put another way, does heat loss through a window still occur when the window is in full sun?

A: (Kelly) Heat always wants to move toward the cooler space to equalize temperatures, so I doubt that you will get much loss during the time the window is in the sun.

Q: I plan to build a passive/active solar home in SE Pennsylvania. I've created a fairly involved spreadsheet that allows me to calculate heat load, heat loss, etc. My question is this: How is heat loss through windows handled, when the sun is shining through the window? Put another way, does heat loss through a window still occur when the window is in full sun?

A (Kelly) Heat always wants to move toward the cooler space to equalize temperatures, so I doubt that you will get much loss during the time the window is in the sun.

Q: I've imagined adding glass to the south-facing front room, plus some mass in the floor. We're also doing a lot of insulating, walls, attic, and underneath. My latest thought is that this room already has a big window and that punching more holes in the insulation would let out more heat than it would let in. Does that thinking make any sense to you? If it does, would it do any good to add mass to the floor?

A: (Kelly) Yes, with passive solar design there is a point where there is no advantage to adding more solar gain. There are some formulas for how much window area to floor space is optimal, and this varies with climate. In a moderate climate (like Olympia) they say between 0.11 and 0.25 square feet of window area for every 1 square foot of floor area is appropriate. This is assuming that you have a decent amount of thermal mass to absorb that heat...and this is best situated in the floor, but doesn't have to be. Dark tile is good, with a cement board backing. It may be that the size of the existing window fits this prescription, but it might be that adding more will optimize the performance.

Q: I've bought a beautiful hillside property in North Carolina. I am all about green building and read extensively about passive solar. Although I'd get my piece of land all over again for the amazing view of the Blue Ridge Mountains I am becoming increasingly aware its sloping wooded due west orientation is not conducive to passive solar ideal at all. Is my only option investing in active solar or is there a trick or two I could still use the passive solar approach?

A: There are a host of ways to work orientation conditions to have your cake and eat it too so to speak. In your case I think this could be done by separating the solar gain and view functions. Existing technological improvements in glazing could allow you to have your west views without also getting excess heating in the afternoon or excessive heat loss in the winter. Check out the solar gain coefficients of soft coat low e heat reducing glass (around >.3 ) and compare this to what you would want on south oriented glazing (around <.65) for heating. Where you should use hard coat low e glazing. This should allow you to get your passive gain without the common mistake of over glazing, The rule of thumb is you want only 15-20 % of the floor area served in high solar absorptive south glazing. This can be with south facing (plus or minus up to 20 degrees) walls or roof mounted dormers which might need to face different directions than the geometrical order of the rest of the house.

If you have separated this requirement from the view orientation you may get some complex geometry to the house which can be nice in my opinion if creatively worked with. I have found working quick sketch models to scale the easiest way to solve geometry problems. For sun angles manipulation with a sketch model you may need to use a sun peg device. (see page 54 of our book Passive Solar Architecture by Bainbridge and Haggard) I have found that very few sites are really passively impossible. Its all a mater of design.

 


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