Image Credit: Image #1: Green Energy Times
More Musings of an Energy Nerd
GBA Prime subscribers have access to many articles that aren’t accessible to non-subscribers, including Martin Holladay’s weekly blog series, “Musings of an Energy Nerd.” To whet the appetite of non-subscribers, we occasionally offer non-subscribers access to a “GBA Prime Sneak Peek” article like this one.
Everybody loves passive solar design. Back in the 1970s, “passive solar” was the essential first step for cold-climate builders. It was considered an approach with obvious advantages over complicated “active solar” schemes that required pumps, fans, and electronic controls.
While the definition of a “passive solar house” was well established by the 1980s, Wolfgang Feist muddied the waters in the 1990s when he decided to call his new superinsulation guidelines “the Passivhaus standard.” Ever since that fateful day, journalists and owner/builders have confused passive solar design principles with Feist’s superinsulation standard from Germany.
Rather than focusing on the confusion between passive solar design principles and the Passivhaus standard, however, I’d like to travel back in time to the 1970s, the heyday of the passive solar movement, to identify the original principles espoused by passive solar designers. Once these principles are identified, we’ll examine how many of them have stood the test of time.
Here are the five bedrock principles of passive solar design for a cold climate:
Solar vs. superinsulation
In October 2009, the Passive House Institute U.S. invited me to give a presentation at the Fourth Annual North American Passive House Conference in Urbana, Illinois. In that presentation, “The History of Superinsulation in North America,” I discussed the debate between “solar house” advocates and superinsulation advocates during the late 1970s and early 1980s. After Joe Lstiburek and John Straube saw my presentation online, I was invited to present it again at the 14th Annual Westford Symposium on Building Science (August 3, 2010).
Some of the information from that presentation was incorporated into a 2010 GBA article, Solar Versus Superinsulation: A 30-Year-Old Debate.
Here’s a quick summary of the relevant history: during the late 1970s and early 1980s, advocates of superinsulation raised questions about the validity of passive solar design principles. A debate ensued, and superinsulation won.
Although I’m quite familiar with the historic debate, and I side with the superinsulation crowd, certain aspects of the passive solar approach — an emphasis on careful solar orientation, a concern for proper roof overhangs on the south side of a house, and a preference for south-facing windows over north-facing windows — seem embedded in my DNA.
Lately, however, I’ve begun to wonder whether there is any technical justification for these recommendations. Do these design principles result in energy savings? Or am I just dragging around the stubborn legacy of my hippie past?
Forget the thermal mass
Some passive solar principles — especially the old belief in the near-magical effects of thermal mass — never made much sense to me. Thermal mass is expensive. Thermal mass complicates remodeling. Thermal mass makes a home unresponsive to sudden changes in the weather. By keeping a home cold when the occupants want to warm it up, or by keeping a home hot when the occupants want to cool it off, thermal mass is as likely to interfere with occupant comfort as it is is to contribute to energy savings.
For most cold-climate builders, the disadvantages of extra interior thermal mass outweigh any advantages. Even radiant floor designers, many of whom sang the praises of thermal mass in decades past, have mostly accepted the new consensus: low-mass floors are easier to control, and result in higher levels of occupant comfort, than high-mass floors. (For more on this issue, see All About Thermal Mass.)
How much south-facing glazing?
My faith in another passive solar principle — adding plenty of south-facing glazing — was first shaken by Gary Proskiw’s 2010 paper, “Identifying Affordable Net Zero Energy Housing Solutions.” (For a discussion of the report’s ramifications, see my GBA article, “Study Shows That Expensive Windows Yield Meager Energy Returns.”)
Briefly, here’s what Proskiw found:
- South-facing windows are so expensive that the value of the heat gathered by the windows is too low to justify the cost of the windows.
- Money that a builder might want to spend on extra south-facing windows would be better invested in other energy-saving measures.
- The area of south-facing glazing “should be limited to that necessary to meet the functional and aesthetic needs of the building.”
It turns out that every extra square foot of glazing beyond what is needed “to meet the functional and aesthetic needs of the building” is money down the drain.
In a way, this advice is liberating: it compels the designer, secure in the knowledge that no technical or functional issues are at play, to think about aesthetic issues — and that’s almost always a good thing.
Proper orientation
What about orientation? According to conventional wisdom, the wise designer studies a site carefully, looking for a knoll with good southern exposure, and tries to align the long dimension of the house in an east-west direction.
Lately, building scientist Joe Lstiburek has delighted in puncturing this balloon. “I don’t think orientation matters anymore,” Lstiburek told me on the phone. “I see passive houses that are overheating in summer as well as winter — in Chicago! These houses need to reject the heat, not collect the heat.”
So where did the passive solar design principles come from? What’s changed since the 1970s?
Today’s houses are better insulated and less leaky
For one thing, passive solar buildings never worked all that well. Even back in the 1970s, they were cold on winter mornings and hot on sunny afternoons. But most solar enthusiasts were so excited by the idea of “free heat” that we accepted uncomfortable conditions as a necessary part of the brave new solar future we were all busy creating.
Second, today’s houses are better insulated and a lot more airtight than they used to be. That’s good, because they require less energy to heat and cool than homes built in the 1970s. However, recent improvements in insulation and air-sealing standards make homes with lots of south-facing glazing more susceptible to overheating — so it’s more important than ever to avoid excessive glazing area.
It’s also essential that we make the right decision when choosing between high-solar-gain glazing and low-solar-gain glazing. That decision has gotten trickier lately, especially since Proskiw’s calculations have called into question the entire idea that south-facing windows are heat-collecting devices. Some designers (including Joe Lstiburek) have abandoned the idea of orientation-specific glazing specifications and now advise that all windows should have a low SHGC.
Lstiburek says, “Don’t do it”
In a 2014 article titled “Zeroing In,” Lstiburek addressed passive solar design principles with his usual bluntness.
“Don’t bother with the passive solar,” Lstiburek wrote. “Your house will overheat in the winter. Yes, you heard that right. Even in Chicago. … You should go with very, very low SHGCs, around 0.2, in your glazing. If this sounds familiar to those of you who are as old as me, it should. We were here in the late 1970s when ‘mass and glass’ took on ‘superinsulated.’ Superinsulated won. And superinsulated won with lousy windows compared to what we have today. What are you folks thinking? Today’s ‘ultra-efficient’ crushes the old ‘superinsulated,’ and you want to collect solar energy? Leave that to the PV.”
Why passive solar doesn’t work very well
Four salient facts undermine the old premises of passive solar design:
- In a well-designed house, the energy required for space heating represents a smaller percentage of a home’s energy budget than it did in the 1970s. In many low-energy homes, domestic hot water requires more energy than space heating. For more information on this concept, see It’s Not About Space Heating.
- While large expanses of south-facing glass help heat up a home on a sunny day, the solar heat gain doesn’t come when heat is needed. Most of the time, a passive solar home has either too much or too little solar heat gain, so much of the solar heat gain is wasted.
- At night and on cloudy days, large expanses of south-facing glass lose significantly more heat than an insulated wall.
- These days, investing in a photovoltaic array yields more useful energy than an investing in a south-facing window.
A new look at the old principles
So what kind of advice would I give a young designer contemplating the five passive solar principles listed at the beginning of this article?
The long axis of the house should be oriented in an east-west direction. I still have a sentimental attachment to this principle, even though I know it won’t save any energy. The reason I like to follow this principle — at least when the site allows it to be followed — is that it allows more rooms to get sun during the day. If you live in a cold climate, winter sun is cheerful. [P.S.: In Comment #6 below, Dana Dorsett makes an important point: While the passive solar principle favoring east-west orientation may be hard to defend from a space heating perspective, most new homes should, if possible, include a roof that is optimized for the installation of a PV array. An east-west orientation makes this possible.]
The rooms where people will spend most of their time should be located on the south side of the house, while utility rooms, bathrooms, closets, stairways, and hallways should be located on the north side of the house. It won’t save any energy, but this is still a good principle, for the same reasons that it makes sense to orient the long axis of a house in an east-west direction. However, if you live in a mixed climate or a hot climate where the sun is oppressive and shade is your friend, this principle can be ignored.
There should be lots of extra glazing area on the south side of the house, and little or no glazing on the north side of the house. This principle is overstated. If your site has a wonderful view to the north, of course you want to include north-facing windows — and you may want your living room or dining room to face north. Moreover, there is no reason to include extra glazing on the south — only what’s necessary (in Proskiw’s words) “to meet the functional and aesthetic needs of the building.”
That said, every house I have ever designed had more south glazing than north glazing — because sunshine is cheerful and I like sunny rooms. (Up to a point; watch out for glare. Many passive solar houses are so sunny on winter afternoons that the occupants all flee to the home’s dark northern corners.)
The roof overhang on the south side of the house should be designed to shade the south windows during the summer solstice, but allow the sun to shine through the south windows on the winter solstice. Although there’s nothing wrong with this idea, it’s worth pointing out that it has always been impossible to design an overhang that will keep out the sun when it is unwanted and admit the sun when it is wanted. At best, the designer can come up with an overhang that kind-of, sort-of, almost works, but not quite. The sun is tricky. It follows the same path through the sky in March, when sun may be welcome, as it does in September, when it may be unwelcome. Moreover, at 10:00 a.m. and 2:00 p.m., it sneaks in sideways, at an angle, and stubbornly undermines the intent of the designer’s overhang.
So it’s OK to shrug your shoulders and accept imperfection in this department — especially if you take Lstiburek’s advice and just jump on the low-SHGC bandwagon.
The house should include extra interior thermal mass to soak up some of the solar heat gain that comes through windows on a sunny day. I’m happy to throw this principle out the window. However, if you live in a hot climate with high air conditioning bills, you may want to build a house with a lot of interior thermal mass. Just remember that many of the benefits of thermal mass can be achieved at a lower cost by installing extra insulation.
GBA Prime subscribers can read a great many posted comments at the page where this article was originally published: Reassessing Passive Solar Design Principles. If you are a GBA Prime subscriber, that page remains the best place to post comments. Non-subscribers are invited to post comments below.
P.S.: Readers interested in reading a 1978 research paper on this topic may want to check out this report by Rob Dumont: Passive Solar Heating. Our thanks to Bronwyn Barry for sharing this document.
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27 Comments
Martin - Good article,thank
Martin - Good article,thank you. You refer to "see comment #6 below from Dana Dorsett" but that must have been in reference to another blog, as there are no comments here yet- I think I'm the first. I like a lot of south glass. I'm a sun lover and I love to lay down on a floor or a window seat and bask in it on a cold, sunny winter day,like a cat. While Proskiw talks about 6% south facing glass to floor area, I'd like more like 8-10%. Having a slab (cheap mass) then helps absorb the extra heat. Kind of a down-sized passive solar approach.
Important article but not sure about solar control glass
Great article, can't be said often enough, I often reference your original article.
However my colleagues and I have rejected solar control glass even for very compact blocks of flats that need no extra solar heat.
The samples and examples we have seen cut out light transmission and make a bright day look dull and grey so we haven't gone below about 48% g (and about 67% light transmission). Better (cheaper and nicer) to have less glass, especially below working surface height.
Unfortunately, in the UK, the window energy ratings favour higher g glazing and 60% is becoming the norm for triple but we aim for about 50% g.
Dense urban sites don't usually allow the luxury of every living space facing south so currently arguing for east west orientation (and modest glazing) so everyone sees some sun indoors in winter. Not for daylight or heating.
Cheers
Response to Kevin Zorski
Kevin,
You wrote, "You refer to 'see comment #6 below from Dana Dorsett,' but that must have been in reference to another blog, as there are no comments here yet - I think I'm the first."
See my italicized paragraph at the very end of the blog. This is a "sneak peak" version of a blog that is behind the paywall. ("Sneak peak" versions are a way for non-subscribers to see what they are missing.) Here is the link to the original article: Reassessing Passive Solar Design Principles.
The original blog has 15 comments (as of the morning of Oct. 10, 2015). If you are a GBA Prime subscriber, you can read all of those comments, and add your own comments to the growing thread of comments there.
low mass solar porch
I have recently built a "low mass solar porch". South facing using 2, 10 foot sliding glass doors. It works well in that the doors can be opened in summer and closed in winter and the single insulated door which accesses the house can be either closed during the night or open during the day to allow heat in. Because of the low mass, high insulation and high heat gain glass, very little heat is lost at night and all that one wishes can be allowed into the house. This solves a lot of typical solar passive high mass problems. Now, after one year, I can say it works well and doesn't seem to have any down side to it. The winter gain, especially in October, February, March and April is truly welcome and I frequently find people basking in 85 degree heat when the outside temperature is zero. Snow bounce is all the more sublime for countering the winter blues.
Passive Solar history
I would recommend to anyone interested in the history of passive (and active) solar to get the book The Solar House: Pioneering Sustainable Design by Tony Denzer. It is illuminating on how solar design became mainstream and then lost ground with cheap fuels, technical complexities, and comfort issues.
Having completed a passive solar Passive House it does take a bit of work explaining the difference, but free heat is free heat so with the energy modeling tools and windows available today to design without overheating or too much energy loss is not that difficult.
@ Andrew
I think passive solar has a romantic notion to it, who wouldn't like free heat, but it does cost you in many indirect ways, the sun is not constant, you have sunny and cloudy days, windows are expensive to buy/install, thermal mass costs a lot, windows lose heat easily, their R value is not excellent, you can't predict an accurate heat loss/gain over different weather patterns because of many factors, your thermal mass is nice until its exhausted and you can't store large amounts of heat for long, you can't recharge in summer then live from the heat all winter.
Solve all these problems and we all all be happy.
Martin - A, Joe - F
Nice post Martin. I'll be sending you a copy of a paper Rob Dumont wrote in 1978 called "Passive Solar Heating - Results" where he essentially confirms all your advice above, except for that which you attribute to Joe Lstiburek regarding using all-low-SHGC (or g-value) glass.
That's a dumb idea for the reason outlined by Nick Grant above, and because it does affect performance. Why install windows that lose more energy than they gain? Living in a dark box with dark windows may have been popular in the 70's, but countless studies have shown it's not healthy for humans. Needing additional electric lighting during the day because your glazing blocks out most of the available natural light is also not efficient. Nor beautiful (which Joe apparently finds more important than performance these days.)
Incidentally, my business partner, Allen Gilliland, delivered a paper at NAPHN15 entitled 'Mass Voodoo' that you may enjoy here: http://oneskyhomes.com/about/presentations/mass-voodoo-debunking-passive-solar-naphn15. It's based on measured performance from our projects in California, plus a few references to other studies debunking that tired 'thermal mass is good' meme.
My own recent presentation at the Wood Window and Door Conference in Wisconsin confirms Rob Dumont's findings where he recommends "one should limit the southfacing window area to less than 8% of the floor area of the dwelling." Although this is only a modeled study of a house in Saskatoon, it shows how the Passive House standard can be met in Saskatoon by not being greedy with glass and making the same 'Passive Solar' mistake unfortunately many are still making: http://oneskyhomes.com/about/presentations/high-performance-matters-keynote-wwdc2015.
Response to Ven Sonata (Comment #4)
Ven,
I'm glad that you enjoy your sunroom. Sunrooms that work well (like yours) require active homeowner involvement; in other words, the homeowner has to be present to open the doors between the sunroom and the house during certain weather conditions, and to close the doors when those conditions change. Fewer and fewer homeowners have schedules that allow this type of careful monitoring and adjustment.
For the lucky few -- retirees, people with home offices, and operators of Buddhist retreat centers -- these sunrooms are delightful. In all cases, however, the amount of heat collected by the sunroom is too small to justify the cost to build the sunroom.
Response to Andrew Michler (Comment #5)
Andrew,
Thanks for recommending the book by Anthony Denzer. Denzer has written for GBA, so anyone interested in sampling Denzer's writing can check out his blog: Historic Solar House Has Been Bulldozed.
I'm glad you are pleased with the performance of your passive solar house. Some passive solar homes are prone to overheating, while others -- those with limited south-facing glazing -- perform well. In most cases, the passive solar design features cost more than the features needed for a home without them, and are unlikely to reduce the energy required to heat and cool the house. That said, if you are happy with your house, then it must meet your needs -- so that's good.
Response to Bronwyn Barry (Comment #7)
GBA readers interested in reading the Rob Dumont paper that Bronwyn mentioned can find it here: "Passive Solar Heating -- Results From Two Saskatchewan Residences." I have added a P.S. at the end of my blog including the link.
Bronwyn,
Thanks for the report card, and thanks for sharing Rob Dumont's paper. Concerning high-SHGC glazing vs. low-SHGC glazing: as you probably know, I have long advised cold climate builders to choose orientation-specific glazing -- specifically, to choose high-SHGC glazing for the south orientation, and low-SHGC glazing for the west orientation. (See, for example, my 2009 article, High-Solar-Gain Glazing and my 2010 article, All About Glazing Options.)
Joe Lstiburek has noted that many cold-climate homes -- especially homes designed according to outdated passive solar principles -- are overheating. I've discussed this problem as well -- for example, in Study Shows That Expensive Windows Yield Meager Energy Returns. I think that you and I can agree that our goal is to design homes that don't overheat. There are two ways to solve the overheating problem: reduce the area of south-facing glazing, or switch to low-SHGC glazing. You seem to favor the first approach, which is the one I advocate as well. However, as long as the result is a house that doesn't overheat, either approach can work.
(Although I'm assuming that overheating problems arise from an excessive area of south-facing glazing, a badly designed house can also have so much east-facing glazing or west-facing glazing that the house overheats. These problems on the east and west elevations are so elementary that I hope that most passive solar designers have avoided them.)
Euro windows aren't that expensive
It's interesting to note that the study that shows triple pane windows aren't "worth it" is based on a fixed window being $45 per square foot. Our company is providing that same window at less than $20 per square foot.
I doubt that the R-44 SIPS wall outlined in the study has come down at all in price and has likely increased. I think it might be worth a recalculating with more updated values.
Response to Ben Freed
Ben,
I assume that you are talking about Gary Proskiw’s study -- the one I discussed in the article titled Study Shows That Expensive Windows Yield Meager Energy Returns.
As my article noted, Proskiw's calculations "ignore the costs associated with framing the rough opening, the cost of the header, and the cost of trimming the opening." This work isn't free, so it's fair to say that even windows that can be purchased for $20 per square foot cost significantly more than $20 per square foot once they are installed.
Balancing, rather than adding window area.
The simple dumb financial analysis is predicated on the marginal benefit value of ADDITIONAL window area relative to the marginal cost of the wall displaced, which is something of a red-herring or straw-man type of argument. There is only additional cost if there's additional total window area, but the case has yet to be made that there would necessarily be more window area than in an average house. I'd argue that in a well analyzed NZE design there would normally be LESS glazed area than typical code-min houses. (I suspect Proskiw might agree.)
In a well designed passively tempered house in a heating dominated climate there wouldn't be any "extra" glazed area, but there would be optimizing/balancing of the glazing sizes & orientation, in most cases with more glass on the southern exposure, and far less (or none) on the west, and north & east facing windows sized primarily for daylighting (and code-required egress area for bedrooms, etc.) Rather than looking a bit like a '70s era passive solar design, even with a higher fraction of south facing glass, it can usually be done with less than the typical ~15% of total window / floor area seen with the average house.
I'm not necessarily disagreeing with his general conclusions, only that the comparing marginal cost of additional window area analysis is junk, since it doesn't describe what really happens. With windows (any performance level of U0.35 or lower), less is generally more from a total energy use & comfort point of view, but the balance changes. Even though the optimal balance has more south facing glass than other orientations, many decent well daylighted NZE houses would have less total window area than a typical house not additional total window area to be applied on the south side.
Some generalities:
Even at code-min U-factors of 0.35-0.32 windows tend to be net energy gainers, even in winter, even on the north side of the house, but without sufficient thermal mass buffering capacity there's no advantage go going hog-wild on glazed area. Extra glazed area drives peak heating loads despite net-energy positive .North facing windows sized for daylighting are both a net passive energy gain, but also reduce the amount of power necessary for adequate lighting- it's not all about space heating/cooling energy cost.
West facing windows drive peak cooling loads, increasing both the size/cost of mechanical systems & total cooling energy use, and are usually best minimized (or even eliminated.) East facing windows have the same total energy gain as west facing glass, but it occurs during a much cooler part of the day, and can be a welcome gain for the majority of the year in cooler climates, and doesn't usually increase the peak cooling load, even if it increases the average cooling load somewhat. East facing window area trades welcome AM heating during the winter & shoulder seasons against summertime & shoulder season cooling energy use, but doesn't drive cooling system sizing very much- nothing like unshaded west facing or south facing glass.
Well designed passive solar can still be the best option
If you have the good fortune to have a site with good solar access, passive solar heating is still the cheapest best option for cold climate renewable energy collection.
Three of our net zero energy houses in our 9200 HDD climate, are getting 40 to 50% of their annual heating demand simply through well designed and optimized south facing windows. A combination of carefully designed overhangs (moveable in two cases- whatever happened to awnings), additional thermal mass ( 2 1/2" decorative concrete overlay on wood framing) has resulted in very high occupant comfort.
Overheating is not a big problem in from October to April in Edmonton ( or Chicago I suspect). On the rare occasion when it occurs, there is plenty of free cooling to be had by just opening a couple of windows.
This is not to disagree with that there are many badly designed uncomfortable passive solar houses. Just that after durable highly insulated, thermal bridge free, air tight envelopes, carefully optimized south glazing is still one of the best tools in our box.
Unfortunately, good solar access is pretty rare in the dense urban places where we should be building. It makes no sense to piss away the savings to be had from that perfect solar site by driving back and forth to work every day.
Response to Dana Dorsett (Comment #13)
Dana,
You criticism of Proskiw's analysis raises a fundamental question: What is our working definition of "passive solar design"?
I'll try to elucidate my point with an imaginary dialog between a client and a designer. We'll listen in on a meeting at which they discuss the size of the windows in each room of the house.
Client: I'd like to build a house, and I think it's best if there are no windows on the north side.
Designer: OK. That's possible. Are there any rooms that you're contemplating that don't need windows?
Client: Sure. The mechanical room. The broom closet. A few others.
Designer: OK, let's try to put all of those rooms on the north side of the house.
Client: Wait -- I just read a comment from Dana Dorsett. He says that north windows are OK, but it's best to have no windows on the west side.
Designer: No problem. I think we can come up with a design the puts the mechanical room, the broom closet, and all the similar windowless rooms on the west side. Will that work?
Client: Great! I feel better now. So how do we determine the size of the windows in the rooms facing north, east, and south?
Designer: Let's start with the Proskiw Principle: we'll size the windows “to meet the functional and aesthetic needs of the building.”
Client: Great! Are we done?
Designer: I think so.
* * * *
So, here's the question: Is that a passive solar house? I think not, Dana, but you might say it is. If so, I guess that our conclusions depend on our definitions.
Proskiw assumed that many passive solar designers deliberately add extra south-facing glazing -- more than necessary to merely “meet the functional and aesthetic needs of the building.” That assumption is the basis of his analysis.
If you are correct, Dana, and passive solar designers never do that -- then your criticism of Proskiw's analysis has some merit. Otherwise, your criticism stands on a shaky foundation.
Liberating
Especially in urban situations, there are usually more important concerns that should determine the orientation of buildings than passive solar - and usually does. Almost all of central Vancouver faces north to the mountain views, which define the city. In its generally overcast climate the diffuse light brought in through large expanses of north-facing glass is very appropriate.
Having an energy strategy that doesn't rely on a particular orientation and access to direct sunlight can open up the design possibilities to many other delights
Overheating and opening windows
It seems clear that some of the 70's era rules of thumb for passive solar need some revision. However, my experience (and intuition) leads me to believe that too much is being made of the overheating issue. There is a solution to the overheating issue in cold climates. That solution (also mentioned by Peter Amerongen) is to open a window. I live in a super-insulated, passive solar home in Vermont. I don't believe it has ever overheated in the 4 winters I've lived there. It very occasionally does so in the Fall, but that's easily solved by opening a window or two. Since the heat was all free to begin with, nothing is 'wasted'.
As for the South windows that are installed in a house in a cold climate, it seems to me that high SHGC glazing and a properly sized overhang are still good ideas. There is essentially no additional cost for these, free heat is still free heat, and any overheating can easily be handled as mentioned above.
response to Martin on solar porch cost etc.
Yes, it is a squeaker calculating the ROI...especially since we just collect firewood for exercise only and save the cost of Gyms! But I evaluate our heat costs 'as if' we had to provide it by propane. The porch, by calculation produces conservatively 20 mbtu through the heating season. I reduce that to about 13 mbtu to account for various losses. This is annually about $500 of propane in our strangely expensive Canadian propane market. The porch, since it was already roofed cost about $4500 to build and so 9 or 10 year payback. To get a payback competing against free firewood is impossible, but the other aesthetic factors and psychological factors make the economics worth it. And, as you say, one needs to actually be home during the day to appreciate it. Alas, for the working person, the home life is almost all in the dark.
Great article and comments as
Great article and comments as always. I recently posted a new blog on Passive Solar Design. It doesn't conflict too much with what Martin wrote but does recommend choosing high SHGC for a well shaded south and views passive solar design in a more favorable light.
I think Dr Joe's comments are an effort to simplify for the masses. The advice is great for production builders but for overall better performance, I think designers and custom builders should be paying attention to passive solar design and these discussions regarding the details.
I think passive solar design is best described by prioritizing south-facing windows. As Dana points out, this doesn't mean extra or excessive. Prioritizing does not mean replacing wall area with window area. If a home is given an average 15% total window area to floor area allowance, putting a majority on the south will result in more cost-effective free heat than a home that ignores orientation. The liberating part is that you needn't worry if you can't make it happen, especially if you are easily building to net-zero optimized levels.
Passive solar design also includes ways in which we block unwanted heat in the summer. Properly sized south-facing overhangs do not work perfectly but they work much better than "almost". Well placed covered decks or porches and landscaping has considerable influence on comfort and cooling loads which I crudely illustrate in that link. Designers can improve the energy performance of designs by paying attention to these simple, passive solar principles.
Another advantage for east to west orientation is that it tends to reduce the window areas on the east and west sides of the home thanks to reduced wall area. Both things help reduce overheating during the summer, especially with an airtight, well insulated, reflective roof.
I agree with Peter that good passive solar designs rarely overheat in the winter. It's the spring but mostly fall that causes discomfort and potential extra AC usage.
We've built a handful of passive solar homes in Asheville NC with south-facing window to floor areas of about 10%. I continually ask homeowners about overheating and none of them have mentioned it being a problem. Homeowners do seem to open the windows during warm fall days but are currently unanimous about preferring the way the design works with the sun overall. These homes feature local code to international code levels of insulation and would probably benefit from smaller glass ratios at super insulated levels. They all have better than international code levels of blower door verified airtightness which is obviously more important than passive solar.
It's amazing to work on a passive solar home in the wintertime. All trades appreciate walking into a warm house on a cold morning despite having no temporary heat sources. The free heating also helps dry the interior air quicker which helps with drywall finishing and interior wood acclimation.
Of course good design should prioritize a site's views but that doesn't mean ignoring the path of the sun. I think many urban sites without good views can increase performance by prioritizing south windows and designing a good landscape in that direction. Well shaded south-facing windows get more free heat (without necessarily overheating) by using high shgc glass.
The bigger description of what seems to be discussed here is south-facing window area to floor area. That's the metric that should suggest the SHGC value and any attempts to provide appropriate shading. I think the great info brought up and discussed here is important for those on a super performance path with needlessly high amounts of south-facing windows.
Home Power Magazine needs this blog
This just posted at Home Power Magazine: http://www.homepower.com/articles/solar-electricity/project-profiles/back-land
Response to Christopher Peck
Christopher,
I have subscribed to Home Power since issue #1, and fondly recognize the magazine's off-grid editors as kindred souls. The magazine is a source of excellent information on off-grid electrical systems.
In recent years, the editors have tried to expand their focus to include articles on energy-efficient construction methods, and they have unfortunately foundered, since many of their authors lack a good grounding in building science.
I saw the article you linked to when the issue showed up in my mailbox. The author of the article notes, "The home and its construction were not without challenges. The Clays will attest to the frustration of entrusting contractors with successfully implementing unfamiliar building methods, such as the three-layered glass wall and extensive venting and controls. In the absence of regular oversight, mistakes were common and sometimes costly to remedy. ... The main control board was eventually damaged from a power surge, and a combination of difficult-to-find replacements, in addition to the realization that no control is “smart” enough, led to a more involved approach for the Clays—manual operation of the fans and vents to suit their comfort levels." Problems like this are common on a house that tries to incorporate both passive solar and active solar components.
In spite of these trials, the homeowners are happy. That is also very common for owners of passive solar homes. These homes don't always work well. Even so-called passive solar homes often require active homeowners -- who are called upon to open windows, adjust dampers, close shutters, or unfurl awnings. But the type of homeowner who ends up doing these chores often has a smile on her face.
Home Power link
"On a few occasions during multiday winter power outages, the temperature inside the house never dipped below 55°F."
I suppose we'll never know to what extent this was due to the elaborate solar air management systems and basement thermal storage and how much simply to the high-R SIPs. The other conclusion to be drawn from this indoor air temperature number is that I'll never make a satisfactory Vermonter.
Do NOT forget the thermal mass
Dear Martin,
Thermal mass (tm) is not semi-magical, but real and if used right does work very well. Not on its own, and not in all climates, but many. And if you move from "traditional" tm to "smart-lightweight", ie Phase Change Materials (PCM) tm it works even better, because a PCMtm building behaves like a thermally lightweight AND very heavyweight. Such a building is quickly heated up to the PCM's switch point (maybe 23C), is very stable to 28C (5C comfort range) and any access heat beyond this point can be purged easily, as most of the heat is just hot air. Please have a look here http://ecobuildingboards.weebly.com/pcm-board1.html.
Regarding overheating in a cold climate, like maybe Scotland, please have a look at this graph https://www.flickr.com/photos/[email protected]/23473159816/in/datetaken/
And in the "Guide to California Climate Zones" in many zones tm is recommended as part of a efficient temperature management strategy like here http://www.pge.com/includes/docs/pdfs/about/edusafety/training/pec/toolbox/arch/climate/california_climate_zone_13.pdf
Just one more graph from a controlled lab test to show what a difference PCMtm can make https://www.flickr.com/photos/[email protected]/23417200921/in/dateposted-public/lightbox/
Regards,
Christian Nialki
Christian- since when is Scotland considered a cold climate?
By North American standard the climate of Scotland is quite temperate. Even the cooler eastern Scotland region only averages about 3000HDD-C (or ~5400 HDD-F), which is comparable to the cool-edge of US climate zone 4, warm edge of zone 5. That's half the heating degree days of an average US zone 7 location. Most of Scotland would be comparable to US zone 4C climate, which is considered by most N-Americans to be very temperate, neither hot nor cold (except for those who hail from much warmer climates.)
The graph linked to at the flickr site doesn't show up (maybe it does if you have an account and are logged in?)
The example in the link (California's Central Valley zone 13) is in US zone 3B, much warmer than the US average, with much larger diurnal temperature swings than seen in most US & Canadian locations.
Phase-change materials specifically designed for the human comfort zone has thermal mass characteristics very different from those of concrete or most standard construction materials. If the stuff ever becomes commercially available at a cost-rational price relative to the benefit I'm sure it'll get built into a lot of homes, but I'm not going to hold my breath while waiting. Phase change material wallboard & phase change insulation products have been "in development" in various forms for at least 20 years, maybe 30. Is EBB even available on the open market? If yes, at what price? If not, when will it be available?
Response to Christian Nialki
Christian,
Of course thermal mass affects indoor temperatures and shifts the time of day when heating or cooling is required. The question isn't whether thermal mass "works." The questions are:
1. Is the cost of installing additional interior thermal mass justified by energy savings or comfort improvements? and
2. Do high levels of interior thermal mass sometimes contribute to uncomfortable conditions?
For any thermal benefit from interior thermal mass, the occupants have to accept a wider interior temperature range than Americans have become accustomed to. You have to be willing to let the house get a little cold when the sun isn't shining, and then let the house get a little warm when the sun shines. You also need an HVAC system that is programmed not to try to correct these wide swings. This approach may be a good thing -- I'm not defending the thermal comfort expectations of Americans -- but the consequences need to be understood.
As Dana Dorsett pointed out, phase-change materials are (a) expensive and (b) generally unavailable to anyone but researchers. Moreover, they carry the same disadvantages of other types of thermal mass.
Home Power magazine
Home Power magazine is a wonderful publication.
I designed and built my own 2,000 sq ft passive solar home, In Bc Canada on Vancouver Island.
I used low E-glass coated to reflect the homes heat back into the building. The front of the house has a wall of low e glass 9ft by 40ft floor to ceiling. Roof is a green roof :)
The main floor has a 4 " concrete slab . At -4 in the winter the house will reach 25c, if I cover the concrete with a thin carpet the house over heats and I have to open the doors (@ -4c outside..). At night in -4c weather I drop 1 0r 2 degrees overnight on average9no other heat source). Thus proving the value of thermal mass. In the summer the house is shaded by large maple trees that drop they're leaves just as I need the extra solar in the fall. Using cost factors alone to validate passive solar is ignoring the value quality of life gained from living in a glass fronted home with a stunning vista.
Homes are more than bedrooms and cost factors, they are where we spend a large % of our life, no one will ever convince me a 2x6 insulated wall with basic windows is better than a passive solar wall.
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