Image Credit: Image #1: Robert Opaluch
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This article explores the viability of passive solar and solar-tempered space heating in northern U.S. regions and metro areas. I will quantify solar heating potential by looking at climate data for 22 large cities across the northern U.S. The winter climates in these cities differ not just in temperature ranges, but also in the amount of winter sunlight. Winter temperatures and cloudiness are major determinants of the potential and the cost-effectiveness of exploiting solar heat gain for space heating.
This article also explores the difference in solar gain between south-facing windows and windows facing other directions. Most consumers do not realize the extent to which window orientation affects the amount of light and solar heat gain.
Passive solar and solar-tempered buildings
Thirty-five years ago, I designed and built a classic passive solar home in Colorado. The Climate Zone 5 site has 5,600 heating degree days.
My goal was to build an energy-efficient home using materials that cost no more in total than typical home construction. Passive solar heat gain through large south-facing windows provided most of the winter space heating energy. The design was intended to reduce supplementary space heating substantially and minimize utility bills. The upstairs solar-tempered rooms only needed supplementary heating on the coldest nights (about one-third of mid-winter nights). Passive solar heat gain almost eliminated the need for space heating on the main floor. Unlike the upstairs, the first floor has a tiled, concrete floor (with large amounts of thermal mass) to absorb heat to radiate later, stabilizing interior temperature fluctuations.
This raises the question: Could a passive solar home be built successfully and cost-effectively in other cold, northern climates — climates more challenging than the sunny but cold climate of the Rocky Mountain Front Range? For a classic direct-gain passive solar design with a south-facing window wall paired with thermal mass, the answer seems to be “no” or “not easily.” Five cloudy days in a row is not unusual for New England, for example. Long stretches of overcast days produce large heat losses through large windows, with minimal solar heat gains to compensate.
Even if it is not possible to build a classic passive solar home in other climates, solar gains can be used for substantial solar-tempered wintertime home heating.
Winter conditions and solar heat gains in northern U.S. metro areas
Table 1 lists 22 northern U.S. cities. These cities are geographically dispersed, with more cities located in the northeast and Great Lakes regions. Much of the population of northern U.S. live in or near these metropolitan areas.
In Table 1, each city is listed with wintertime climate statistics
Annual heating degree days (HDD), an index of the amount of heating needed each winter season. The warmer HDD numbers are highlighted in tan, and colder are highlighted in ice blue.
Average January temperature (°F), the coldest month of the year.
Winter design temperature (°F), or “coldest expected temperature.”
North latitude location (degrees north of the Equator).
Average percent of sunlight shining in January (opposite of cloudiness). The highest percentages are highlighted in green, and the lowest red and pink.
Average daily solar heat gain per square foot of south-facing window glass in January.
Solar gain compared to Denver, the city with the highest solar heat gain in the list. Cities in the table are listed in descending order of their mid-winter solar heat gain.
Table 1 shows wide variation in heating degree days, winter design temperatures, and January’s average percentage of available sunlight. The lowest numbers are about double the highest. For January solar heat gain from south-facing windows, the highest numbers are triple the lowest.
An interesting result: It turns out that when these cities are ordered by average solar heat gain, they happen to be ordered somewhat by geographic areas:
- Denver, with the highest average solar heat gain in January.
- Kansas City (east of Denver), second highest.
- Next seven cities are all located in the northeast, along the Atlantic coast (Providence; Hartford; Boston; Portland, Maine; New York City; Philadelphia; and Concord, N.H.).
- The following four cities are along the north central and western U.S. (Minneapolis; Salt Lake City; Bismarck, N.D.; and Billings, Montana) .
- Next, six cities in the Midwest and Great Lakes area (Chicago; Indianapolis; Detroit; Burlington, Vermont; Pittsburgh, Pennsylvania; and Buffalo, N.Y.).
- Two cities along the Pacific northwest coast (Portland, Oregon; and Seattle).
- Finally, Anchorage, Alaska, along the far northern Pacific coast, with by far the lowest solar heat gain.
This geographic ordering is illustrated in the map in Figure 1.
Figure 1 depicts the grouping of northern U.S. cities by solar heating potential, as listed in Table 1. The specific microclimate of any building location should be used in building design, rather than the rough approximation of solar potential shown in this map or Table 1. The resources cited in the Appendix or other internet resources may provide climate data for your location or a similar climate nearby.
From this ordering of cities by average mid-winter solar heat gain potential, we may begin to conclude:
- Denver is the best location for wintertime daylighting, for using solar heat gain for space heating, and for PV.
- Kansas City and cities along the northeast coast also look promising for wintertime daylighting, space heating, and PV.
- Alaska, the Pacific Northwest, and Midwestern cities would not be good candidates to rely on wintertime solar heating.
- Overcast winter conditions are more likely for cities west of large bodies of water.
Next, we need to explore solar heat gains and losses in more detail for each metro area.
Solar heat gain computations
When designing a building, the solar heating potential can be computed. The amount of solar heat gain from windows varies tremendously. If windows get direct sun in mid-winter, solar heat gain might provide the majority of needed space heating energy for a well-insulated, airtight building. Some important factors for the amount of solar heat gain are the:
- Size of the window glass;
- Window’s orientation or direction (e.g., facing south, east, west or north);
- Solar Heat Gain Coefficient (SHGC), the solar gain potential, usually 0.35 to 0.7, or 35% to 70% for efficient new windows; and
- U-factor (inverse of R-value), which measures the rate of heat losses through that window to the cold outdoors.
Table 2 shows the amount of solar heat gain per square foot of window glass for the cities listed previously.
Table 2 lists solar gain data for the 22 northern U.S. cities.
Columns 1-3 lists the same city data, in the same order, as Table 1.
Columns 3-6 lists the daily amount of solar heat gain in January, per square foot of glass, for windows facing south, east, west, and north (and the total of all four), respectively. Note that east- and west-facing glazing provide an equal amount of solar heat gain, but east gains mostly in the morning, and west mostly in the afternoon.
Column 7 shows the percentage of the total amount of solar gain that comes from the south-facing glazing. Note that the majority of the total solar gain comes from the south-facing windows for all cities in January. The further north, the greater the percentage of gain from the south-facing windows vs. other directions during mid-winter.
Column 8 shows (with red negative numbers) the average January daily heat loss per square foot of glass (assuming glazing is rated R-5 or U=0.2, with window coverings adding some insulation during nighttime hours to attain U=0.15). A new, affordable but well-insulated window (with cellular blinds used at night) would attain values similar to those listed in the table.
Column 9 shows the net heat gain per day for south-facing windows (which is the solar heat gain minus the heat loss).
The last column (#10) shows the percent of net heat gain compared to the heat gain for window glass. This shows that a good percentage of the gains from south windows are retained, despite losses, except in Anchorage, Alaska.
Note (in the last two columns, #9 and #10) that Denver has better net solar heat gains than anywhere else in the list. Kansas City and the seven northeastern seaboard cities do almost as well with average net solar heat gain in January. At the bottom, Anchorage is the only city that has average net heat losses through south-facing windows. There is so little solar heat gain through all windows in Anchorage in January (and such large heat losses), that heat losses far exceed the gains, even for south-facing windows. For all other areas, solar heat gain through south-facing windows exceeds the heat lost through the glass.
In column 7, “South Percentage of Total Gains,” note that south-facing windows provide the majority of the total solar gains for every city in the list. In January, south-facing windows always account for more solar gain than east + west + north combined. South-facing windows provide between 59% and 77% of the total solar gains, despite making up only 25% of the glazing of the four windows of equal size. To maximize wintertime interior daylighting and solar heat gains, south windows should be larger in size or in number than windows facing other directions.
Note that south-facing windows typically have solar heat gains (column 3) at least double the heat losses (column 8). In contrast, the east, west, and north-facing windows lose more heat (column 8) than they gain in January (columns 4 or 5), except in Denver. (For Denver, east- and west-facing windows do have a net solar heat gain of 25%, which is positive, but only one-third as much as south-facing window’s 75% net gain.)
Buffalo, Pittsburgh, and cities in the Pacific Northwest do not do as well as most other cities. Their net solar heat gain is only about a quarter of Denver’s. Other cities in the list across the Midwest and northern central U.S. get about half the net solar heat gain of Denver.
Assumptions and exploitation of solar gain data
To model anything, you have to make some reasonable assumptions. The windows specifications used here might be typical for a Pretty Good House design, being relatively high-performance windows but in the affordable range.
Only 22 cities are examined, and only in the northern U.S. Comparable data was not readily available for Canada or Europe, and limits on the length of the article and readable table size limited the cities listed.
The window orientations were limited to true south, east, west, and north. Similar analysis could be performed by using web resources to compute solar gains for windows in other orientations.
Heating Degree Days vary year-to-year, so other HDD numbers may be found. Cost of electricity varies over time as well. But overall, if values are changed to reflect other information sources, a similar overall pattern will emerge, unlikely changing any of the main conclusions of this modeling effort. Window specifications and climate data for your specific construction project are more important than the generalizations made here.
Exploiting knowledge of window orientation in building design can magnify the effects noted in these tables
Designers and builders can use higher solar heat gain windows on south-facing windows and higher R-value (lower U-factor) windows on north, west, and east-facing windows to further increase solar gains and reduce heat losses overall. In passive solar and solar-tempered homes, typically there are more or larger windows facing south, and fewer or smaller windows facing other directions.
Varying window sizes and numbers can increase or decrease solar gains and heat losses to affect overall energy performance.
The ease and value of using solar heat gains
It is easier to maximize wintertime solar heating in some climates than others.
The cost-effectiveness of solar heating varies among regions as well. The cost of electricity, natural gas, or other heating fuels can impact the use of passive solar heat gains for wintertime space heating. Passive solar heat gains are more valuable in areas with higher costs for electricity or other heating fuels, or with occupants who are less affluent, or architects, owners, or builders more concerned with sustainable building.
Well-insulated and airtight energy-efficient homes are likely to use electric minisplit heat pumps, resistance electric heat, or radiant electric heat as backup or alternatives for passive solar or solar tempered space heating. The smaller amount of heating needed for space heating in high-performance homes makes more costly or elaborate heating systems no longer cost-effective or necessary. So the cost of electricity is used to compute the value of solar heat gains in the following analysis.
Table 3 shows the net solar gain and value of solar gains in northern U.S. cities.
The first six columns list the same 22 U.S. cities and other previously displayed data, which are helpful in determining the value and ease of using solar heat gains for space heating.
Column 7 computes an index of how easily solar heat gains from window glazing can be used to heat a building for each of the cities. This Solar Gain Index uses south window glazing net BTUs of heat per day per square foot of glazing, and divides by the Heating Degree Days for that location. (The result also is multiplied by 100 so that numbers are transformed into a simple single digit range.)
The larger the net solar gains, and the smaller the winter heating needed, the better the score on the index.
Locations that lack significant net solar heat gain, or that require a lot of wintertime heating, score lower on this index.
This simple computation provides a metric to rate locations for ease of using passive solar or solar tempered winter space heating.
Column 8 computes an index of how cost-effective or how valuable solar heat gains can be for a location. This Solar Value Index uses south window glazing net BTUs of heat per day per square foot of glazing, and multiplies by the Heating Degree Days and by the price of electricity. (The result also is divided by 10 million so that numbers are transformed into the single digit range.)
The larger the net solar gains, the larger the amount of winter heating needed, and the higher the price of electricity, the better the score on the index.
Locations needing a lot of heating, that have high electricity prices, and that have good net solar gains, get higher scores.
Locations get lower scores if they can’t generate much net solar gains, need less winter heating, or have cheap electric rates for minisplit heat pumps, electric radiant heating, or resistance electrical heating. High-performance homes likely use these space heating appliances, since less space heating energy is required in well-insulated, airtight homes.
This simple computation provides a metric to rate locations for the value or cost-effectiveness of passive solar or solar tempered winter space heating.
Many more cities in the U.S., Canada, Europe, and elsewhere could be added to this list to compute comparative solar heating ease and value. Only their (1) net south-facing glazing heat gains; (2) winter heating degree days; and (3) cost of electricity, would be needed to create a more comprehensive list.
The list of 22 northern U.S. cities can be re-ordered by the two indices for the ease of heating by solar gains and the value of solar heating. Tables 4 and 5 show the re-ordered lists. Note that northeastern coastal cities now rate highly along with Denver and Kansas City, for the value of solar heat gain (due to high electricity rates in the Northeast). Again, the northeastern coastal cities look promising for using solar heat gains for wintertime space heating. Otherwise, the ordering remains similar to the previous ordering by amount of solar heat gain from south windows (Tables 1 through 3).
Seasonal variation in solar heat gain
Locations in the northern U.S. are winter-heating-dominated (meaning that more energy is needed for winter heating and much less for summer cooling). So the solar heat gain analysis has focused on the coldest month (January) for the 22 northern U.S. locations. For the months of December and January, the sun is low on the horizon during midday. Consequently, south-facing windows capture far more light, and thereby produce far more solar heat gain, than windows facing other directions.
However, many northern U.S. locations can experience uncomfortably hot summers as well as cold winters. Heat gains from windows can contribute to overheating. There is more total sunlight shining in summer (June to August in the northern hemisphere) than mid-winter (December to February). Before choosing window locations, sizes and performance characteristics, we need to examine solar heat gains throughout the year, not just January.
Table 6 lists the BTUs per square foot of window glass per day for Providence, R.I. (the northeastern coastal city that looks most promising for solar heating).
The first five columns list each month of the year, and the daily solar heat gain per square foot of window glazing for windows facing south, north, and east, or west , and the average heat gain over the four window directions.
The rightmost column 6 of the table shows the percentage of gains for one south-facing window compared to the combined total of three windows facing north, east, and west.
At the bottom of the table, the yearly gains are totaled for south-, north-, east-, and west-facing windows.
The data in the table indicate:
- South-facing windows produce the most natural daylighting and solar heat gain from the month of September through the month of April. Daylighting and heat gain are desirable especially in cold winter-heating-dominated climates of the northern U.S.
- Incredibly, south-facing windows gain about fifteen times as much light and solar heat gain as north-facing windows in December and in January.
- South-facing windows gain about 3.5 times as much light and solar heat gain per square foot than either east- or west-facing windows in December and in January.
Note from the cells highlighted in yellow: During the winter, from November through February, one south-facing window would produce more light and solar heat gain than the total gain of three windows facing north, east, and west, respectively.
East- or west-facing windows produce the most heat gain from May through August. Heat gain during the summer months is usually undesirable, especially in cooling-dominated climates of the southeastern U.S.
North-facing windows produce the least natural daylighting and solar heat gain for every month of the year. North windows would be most desirable in cooling-dominated climates, or during hot summer periods anywhere in the U.S. — but not during the cold and darker winters of northern U.S. locations.
Providence, R.I., is not unique among northern U.S. cities. All of the 22 northern U.S. locations examined previously exhibit the same pattern of superior winter energy performance for south-facing windows.
In January, one south-facing window produces more solar heat gain than the solar gains from three windows facing north, east, and west for all 22 cities. The south-facing window advantage ranges from at least 140% (Indianapolis) and 144% (Denver), to the most extreme 179% (Seattle) and 338% (far-north Anchorage, Alaska).
Taking account of heat losses from the windows produces much more extreme results favoring south-facing glazing. In Table 2, it was noted that north-facing windows are always a net BTU loss, since heat losses exceed the meager solar heat gains for all 22 cities. Even for east- and west-facing windows, losses exceeded solar heat gains except for Denver. Table 4 only looks at solar heat gains without considering losses. The advantages of south-facing windows become even greater when losses are incorporated.
Positioning and sizing windows to improve home energy performance
Since heat gain is desirable during January for homes in northern latitudes, locating windows on the south side is far more beneficial than other orientations, assuming that that south window is not blocked from getting sunlight from obstructions during midday.
In the hotter summer season, solar heat gain through windows is usually undesirable. Even though south-facing windows have much higher solar gain during December and January than other orientations, the situation in summer has changed significantly due to changes in the position of the sun at midday. During the summer months, the path of the sun has changed. In June, the sun is closer to overhead at noon, so south windows have far less solar heat gain. Meanwhile, the solar heat gain through east- and west-facing windows is more intense due to the greater amount of solar radiation around June (for the northern hemisphere). East- and west- facing windows gain far more heat than south-facing windows during the summer months, until late August. West-facing windows gain that heat in the afternoon, usually during the hottest time of the day, making west-facing windows particularly undesirable unless well shaded.
Note that the values in Table 6 highlighted in light blue show that south-facing windows gain less heat in June and July compared to east- or west-facing windows.
The data on solar heat gains is summarized in Figure 2, below. This information in an aid in choosing the best direction and size of windows facing different directions.
Conclusions
- (1) To exploit solar heat gains from windows in mid-winter (and minimize heat gain in summer), we can build on a building lot that has good south-facing solar access during mid-winter, when the sun appears lower on the horizon mid-day. Modify the landscape of the property to optimize wintertime solar heat gain (and reduce summertime solar heat gain from the west).
- (2) To exploit solar heat gains from windows in mid-winter (and minimize heat gain in summer), we can, as much as practical, locate more and larger windows facing within 15 degrees of south, and try to reduce the glazing on walls facing north and west (and perhaps east).
- (3) Overall, unobstructed south-facing windows gain more heat than they lose during mid-winter in almost all U.S. climates.
- (4) South-facing windows can provide about fifteen times as much light and solar heat gain in winter as north-facing windows.
- (5) During mid-winter, south-facing windows can provide more than triple the amount of light and solar heat gain in winter than east- and west-facing windows.
- (6) South-facing windows have less undesirable solar gain during summer than west- or east-facing windows.
- (7) East-facing windows provide sunlight early in the morning when a house interior is the coolest, so they are more useful than west-facing windows. East-facing windows can be more useful on winter mornings to warm up a cold house, but are quickly overtaken by warming from south-facing windows.
- (8) Unobstructed west-facing windows produce heat gains mostly in the afternoon. Even during winter afternoons, additional space heating may not be needed by the afternoon. In the summer, that afternoon heat gain typically is undesirable. Try to design smaller and fewer west-facing windows.
- (9) Design for the local climate, considering winter temperatures (heating degree days and winter design temperatures), cloudiness (or average percent of available sunlight in January), and costs of electricity (or fuel for heating). Climate zone temperatures are important, as are available solar heat gains and utility prices.
- (10) Calculate solar heat gains when designing, and compare to heat losses.
- (11) Make adjustments to window locations, sizes, and glazing options (SHGC and U-factor) to optimize natural lighting and solar heat gain in winter and summer seasons.
- (12) Consider different glazing for windows facing different directions. South-facing window glazing may optimize heat gains with higher SHGC, and west-facing windows with lower SHGC glazing. Glazing with lower U-factors (higher R-values) for north- and west-facing windows tend to have lower SHGC.
- (13) Design roof overhangs, patio or deck coverings, and landscaping to preserve unobstructed sunshine in winter, and shade west- and east-facing windows during summer months. South-facing windows also have increased solar heat gain in late August to consider.
- (14) Exploit sustainable solar heating to lower the building’s heat load, utility bills, and supplementary space heating systems.
- (15) Consider less costly minisplit heat pumps or even simpler resistance or radiant heating in energy-efficient high performance homes, which have a much lower heating load.
- (16) Reduce the amount spent on larger and more complex supplementary space heating systems to allocate funds for more insulation, air-sealing measures, and more energy-efficient windows and doors. Re-allocating costs would reduce the cost and complexity of the space heating system, and can enable solar heating to provide a greater portion of space heating.
- (17) Cities and towns can promote energy efficiency and public health by incorporating zoning laws and incentives that provide building lots with south-facing solar access, and protect solar access for existing buildings.
Appendix: References
The following web resources were useful for gathering climate and solar gain data. Other internet data sources may include somewhat different data, but the patterns of data would lead to similar conclusions.
Sustainable By Design by Christopher Gronbeck. Seattle, Washington.
Appendix D, Degree Day and Design Temperatures.
_________________________________________________________________________
-Bob Opaluch designed and built a passive solar home in Colorado, renovated two homes in Massachusetts, and has many years of renovation, maintenance, repair, and furniture-building experience. He led a course in Sustainable Architecture for Lifelong Learning Collaborative, an adult ed organization in Providence, R.I. Bob has degrees in applied mathematics and in philosophy from Brown University, and psychology from UCLA. He was a psychology professor for five years, and a software and web site usability and design engineer for 20 years.
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25 Comments
Excellent Resource!
Thank you Bob for sharing this very thorough article!
I would also like to hear more about your/GBA community's thoughts on thermal mass and its relationship with solar heat gain. I understand that some passivhaus builders pour 8 inch thick concrete slabs for heat distribution from solar gains. Is this really necessary? To what extent does this help? Should homes with floor joist/subfloors opt for tile over wood?
One quick note: It appears that you are assuming a SHGC of 0.5 for the heat gain calculations. Is this correct? (My apologies if these assumptions are printed and I overlooked them.)
Thermal mass
Rick,
Thanks for your comments. Yes the analysis assumed SHGC=0.5 for all windows, which wasn't stated earlier in the article. Higher SGHC=0.7 could be used on south-facing glazing to increase solar gains, but still get decent heat loss performance (e.g., Cardinal LoE-180 glass with SHGC=.69, U=26). Most insulating glass units in windows sold in USA are closer to SGHC=0.3 for avoiding excess solar gains in summer. Those won't work very well for passive solar homes.
Modeling thermal mass performance seems difficult or approximate. From what I've read, slab depth beyond 4" thick doesn't get you much additional heat storage or dampening of temperature fluctuations in direct gain passive solar designs. Passivhaus is known for having lots of foam insulation below slab floors (8" to 10" thickness might be common) to keep within their limited heat loss budget. Its important to insulate thermal mass well, but Passivhaus insulation levels might be overkill in a passive solar home.
Tile floors over wood doesn't provide much thermal mass (about 1/2" vs. 4" slab), but more than wood floors. So you could have a solar-tempered interior with tile or wood floors over wood frame construction, but not fully passive solar, unless you also have other significant thermal mass elsewhere.
[For those unfamiliar with the terminology…
-- Buildings are considered "solar tempered" if they provide enough wintertime solar heat gain to keep the building's interior warm during sunny days.
-- "Passive solar" requires sunlight to shine on thermal mass (e.g., concrete slab floors, masonry walls, large containers of water) so that most of the solar heat gain is stored in the thermal mass, to avoid overheating the interior air during the day. The thermal mass also will radiate stored heat when the sun isn't shining to keep interior temperatures from getting too cool.) So thermal mass dampens daily temperature swings from solar gains, keeping interiors within about a ten degree Fahrenheit range (5 degrees C), if designed reasonably well.
-- "Active solar" involves capturing solar heat gains, and moving the warmed air or liquid into some storage location via pumps or fans, to be used for heating when needed.]
I plan to say more about designing passive solar homes effectively in a future blog.
Robert
This is a very basic question I confess not knowing the answer to.
When architects (yes I've done it for clients) draw diagrams showing the solar exposure of south-facing windows, they often decide on the depth of overhangs for shading based on the angle of the sun at noon in both summer and winter. But this angle represents a brief moment and most of the arch of the sun's path, while still influencing the solar gain, is much lower during the periods of time on both sides of noon.
Is it possible to determine an appropriate period of shading for summertime sun that also allows peak wintertime solar gain? Is it, say, the angle of the sun at 11:00am and 1:00 pm? And during this period, should the window be completely shaded, or just shaded at noon and partially on each side of that time? How do you optimize this shading in a more so sophisticated way that the common arc diagram?
Thank you so much for sharing
Thank you so much for sharing this with us.
Very helpful
Thank you for sharing your thoughts, Bob. Very helpful. I look forward to reading your future articles.
Intro to roof overhangs and shading
Here's some information not included in the article that provides background to those unfamiliar with the topic raised by Malcolm.
-- The highest sun angle (for the northern hemisphere) is around June 21st, when the sun is nearly overhead at noon, just a little toward the southern horizon. The sun rises in the east, goes overhead, then sets in the west, with little sunlight directly striking a south-facing wall or window. That's good for south-facing windows, because during the hot summer, you don't want the sun beating in your windows, causing solar heat gain in your home's interior.
-- The lowest sun angle at noon is around Dec 21st,, when the sun is low on the southern horizon, even at noon. The sun is even lower before and after noon. Therefore, the sun is shining on your south-facing walls and windows during wintertime. That's good for south-facing windows, because in the cold winter, you want the sun beating in your windows, warming your home's interior (and providing daylighting too).
-- Seasonally, the coldest temps of winter, and the hottest temps of summer, lag the sun's seasonal changes in path and intensity. Unfortunately, by late August, it is still hot summer weather, but the sun's path has changed to be lower on the horizon than June 21st. You want to shade your south-facing windows from the sun to avoid solar heat gain. One shading method is to have roof overhangs stick out above the south-facing walls enough that they cast shade on south-facing windows in late August. Unfortunately, whatever shading is caused by these roof overhangs also causes just as much shading on April 21st (two months before June 21st) as two months after (August 21st). So overhangs tend to keep the house cooler in the Spring as well as late summer. But hey nothing is perfect, so architects and builders could design roof overhangs to reduce summertime overheating.
Overhangs, shading and sizing of west-facing and east-facing windows is most important to prevent summertime overheating! The intense summer sun on the east side of the building in the morning, and the west in the hot afternoon, is greater than the sunlight striking the south side. See Table 6 for details.
Designing south-facing roof overhangs
Malcolm,
First, thanks for sharing your many helpful comments and ideas on GBA articles on Q&A. Many of us benefit significantly from each other's experiences and insights.
I think that the key design criteria for passive solar involves locating and sizing windows, thermal mass, and doing sufficient insulating and air-sealing so that only solar heat gain can provide most of the home's wintertime space heating, yet avoid summertime overheating. For south-facing windows, designing overhangs to maximize mid-winter solar gains yet reduce summertime solar gains is useful, but more of a detail than a major contribution to passive solar heat gain management. I know some others disagree with this perspective.
Shading analysis of the building site's landscaping and adjacent buildings is critically important. A passive solar building's south-facing windows need good solar access from 9 or 10AM through 2 or 3PM midwinter, when the sun is low on the horizon. That's where your observation about winter sun angles before and after solar noon is most critical.
Personally, I think we should focus on better window shading methods than roof overhangs. Roof overhangs can shade the top of a window yet allow solar gain at the bottom. Adjustable awnings, curtains, window shades, exterior shutters, or even deciduous trees, or other shading methods, can provide more flexible and more complete shading than a fixed roof overhang. They can work well in late summer hot weather, yet not block the sun on cooler days in the Spring. So I'd recommend designing overhangs for minimal shading mid-winter, and not worry so much about summer (for winter-dominated northern climates).
As you point out, it's a complex problem to design a good south-facing roof overhang for the windows below. Ideally, the goal is to not block wintertime sunlight on south-facing windows. Designing roof overhangs to avoid shading windows at noon on Dec 21st results in some shading at the top of the window at noon in January, February and November, when you still want that sunlight (though at 10AM or 2PM you won't be shading the top of the window since the sun is lower on the horizon at that time of day). And if you design for best wintertime performance, you will get undesirable solar heat gain in the late summer.
If you were to draw a Dec 21st shading line for the top of the window, then draw a shading line for June 21st from the bottom of the window, the point of intersection could be used to set the lowest, farthest out point of the roof overhang. That would provide ideal sun shading for those dates at solar noon. But it would not shade the window as much as you'd like by late August, or earlier or later during the day on June 21st, as you point out. And it would shade some of the top of the window during January, February and November, when you'd prefer full sun. But it might be a good compromise.
The larger the roof overhang, the more important it is to do shading analysis, as it makes a big difference in the size of the shadow. So for a traditional Cape style home, the small overhang doesn't shade the windows much in any season. Extending them would have an effect on the top of the window but not extend very far down the window. On a traditional southern plantation style home with porches and monster roof overhangs, you could design a south-facing overhang that would completely shade south-facing windows in June, yet not shade much of the top of the window during winter. Its no accident that Cape style homes appeared in winter-dominated climate up north; and southern plantation style homes with big windows, big overhangs and wide porches in the cooling dominated climate of the southern US. They fit the climate.
Elevates mood in winter
Thanks for all the great details and comments, and your work on this type of efficient home design.
Having lots of sunlight entering the home in the winter is a way to elevate mood when many folks find it hard to be outside like when it's warm. Kids in school and adults at work have been shown to have better moods and do complex work better with sunlight entering the room.
We have a passive solar house we designed and built ourselves in SE Michigan in the late 80's. Simple 2x6 with exterior 1" aluminium skinned polyiso foam, south facing rear with larger windows and doors, few windows on other walls. We have about 2' overhangs on all south facing windows and doors which work well in this latitude. Still, don't have AC. We find the coldest days are very clear and let in a lot of light and heat contrary to what some say about passive solar in this area. These cold sunny days the furnace rarely comes on all day long. Kinda like today, 11 degrees, very sunny and pleasant indoors, with a high-pressure system in control.
We pull tight insulating shades over the windows in the evening to reduce heat loss.
Our energy use for the area is almost off the chart lower than other homes mainly due to passive solar design at not much/or no higher cost than conventional.
Will be adding solar panels on our large south facing roof when our city, state and utility stop punishing those with a solar tax who have. Would seem 'Damn the torpedoes, full steam ahead' to catastrophic Global Warming.
Dark days of winter affects mood
Vince,
Totally agree. Your home and the Colorado passive solar house I designed and built seem quite similar in construction. I enjoyed the bright sunlight and warm interior temps mid-winter too. (Plus the low utility bills and resilience in case of power outages.). Being in a solar home definitely cures the winter blues and Seasonal Affective Disorder (SAD). My great aunt used to live with family in Oregon, but had to live elsewhere during the overcast winters there.
Optimizing heat gain
To optimize the heat gain through windows heavily insulated shutters will change all the numbers. I built and installed R20-30 closing shutters both internally and externally more than 5 years ago in Western Canada at 51 latitude north. This is a huge house over 10,000 sq ft with 1300 sq ft of double glaze windows. Those panels closed during the 15 hour nights of winter make far more difference than thermal mass or solar gain. It was an experiment, and I can now, after 5 years, say it exceeded my expectations.
As well, 3 years ago, having been so pleased with the shutter effect, I built a "low mass, super insulated south facing solar porch. The window area is 400 sq ft and the secret is low mass. The solar gain is not stored near the windows to be lost at night but is captured through an open door into the main building. Just that insulated door is shut at night and the porch is then isolated from the main heated building. This is also hugely effective. We want to keep our solar heat, not lose it back through the same window that brought it in.
Robert
Thanks for the kind words - and thanks for the explanation. I found it both interesting and very useful. Worth remembering that those early-'80's houses are one of the main reasons we are having such fruitful discussions today!
Solar Pathfinder
Great Article!
For those trying to figure out directions and overhang sizes and angles, the best tool is the Solar Pathfinder and many new apps like that which show all angles and azimuths of the sun for any location throughout every day of the year. Used by solar PV installers for the most bang for the buck.
sketchup
I used Google Sketchup, the free version, to roughly model shadows on my soon to be built house. You build a simple house, and then you can set your latitude, and then choose the date/hour. It is neat because you can pick a date, then move the hour from am to pm and watch the shadows dance across the house.
My house is orientated 45* off south, with a large wall (38') with 4 large-ish windows orientated south-west, and 2 largish windows orientated south-east. 36" overhangs, 9' walls. December 21st there is no shading from overhangs. June 21st, only sees direct sun in the early hours of the morning, and the window son the north-west side of the house in the late evening. southernwalls see about 6" of sun exposure at the bottom of each window. September 1st/March 1st modelling revealed modest exposure as the sun has dropped enough to negate most of the overhang benefit. However, a few strategically placed deciduous trees and i have all but eliminated my summertime window solar exposure. Our property has some young-ish maples that are already 30' tall and are perfect for transplanting into the exact spot that we need to maximise winter gains and reduce summer gains.
Motions of the Sun Simulator
I used this website to analyze my site. It's free and doesn't require a download. NAAP Labs Motions of the Sun Simulator
What about glare?
Getting free heat and sunlight to raise mood is great.
But, how to deal with glare? Is there anything else other than minding where you put your furniture?
For example, If my living/dining/kitchen open space is taking up the whole south facing side of the house, where do I put the TV? Not that I watch it much (or at all) during daytime), but still :)
Also for example, if I am to have brunch/lunch during daytime, where do I put dining table? We designed it to be in east west direction, and on the south side we have a large 2.5m wide doors. Will there be sun in the eyes of people sitting on the north side of the table during winter?
Response to Davor Radman
Davor,
You bring up an important point. I'm old enough to remember the passive solar homes of the 1970s and 1980s. Many had problems with glare. Too much south-facing glazing can make a home very uncomfortable and unpleasant in February and March.
Note that the photo at the top of the page shows a passive solar house from 1982. Unless I'm mistaken, the house has interior shades or curtains, and these are in use for many of the south-facing windows. I saw that a lot: first, the designer included too much south-facing glazing; then the homeowners installed interior shades to cut down on the glare.
Solar Gain Overheating Interiors
Note that this article doesn't promote classic passive solar design, even though a well-designed passive solar home works great in dry cold winter climates (but not in climates with overcast winters). I'll address passive solar and other home design issues in the next article.
This article notes the importance of using south-facing windows to capture warmth (and natural lighting) in winter, yet not suffer as much heat gain in summer (vs. west and east-facing windows). It quantifies how some US climates (and not others) can use solar heat gain for space heating, or reduce utility bills cost-effectively, by designing to manage solar heat gain. Those principles work in all types of buildings, and unfortunately can be ignored in subdivision design, building design, and building orientation. I would hope that we learn to manage solar gain better, to reduce our dependence on auxiliary heating and air-conditioning, on fossil fuel usage, and to reduce utility costs for those who can't afford it. Not just to design passive solar or solar-tempered buildings.
Martin I agree some passive solar homes did create window walls of south-facing glass, likely without quantifying solar heat gain beforehand. Lack of sufficient thermal mass was likely a common problem too (resulting in large interior temperature fluctuations in winter, i.e., overheating). I bet under-insulating was also a common problem, especially for that thermal mass that serves as an overnight heating system. We would agree heat loss and solar gain calculations should be done to optimize any home design. I see lots of examples where people try things without "doing the math" beforehand. Overglazing, underinsulating, oversized HVAC and using stock home plans without site analysis, seem commonplace. IMHO the typical USA under-insulated home with oversized fossil fuel heating and AC is the failed design in need of our greatest attention. I would hope this article would lead people to consider quantifying solar gains for any type of home, not just solar-oriented designs.
Glare
Glare can be an issue for south-facing window walls around noontime during the winter, for west-facing windows late in the day (especially during summer), and for east-facing windows in summer. It can't be a problem for north-facing windows, which receive almost no direct sunlight any time of year. Like most things, you can have too much of a good thing. Daylighting is good, and solar heat gain is good in winter and not good in the hot summer. Avoiding south-facing windows because in rare cases people created window walls without considering the solar gain or glare impact would be a mistake. West-facing windows are a bigger concern, needing shading of some sort during hot late afternoons. There are lots of ways to provide that shading, or using low SHGC windows for west and east-facing windows to avoid interior overheating during the summer.
For optimum absorption of solar heat in passive solar homes using a direct gain slab (like in the photo shown), many solar home designers recommended slab floors be a dark color. They also recommended ceilings be white, to reflect light back to the floor. Engineering-wise, this makes sense, but not for human perception. I think these recommendations are a mistake, as that high contrast of dark floor and white ceiling and walls adds to the perception of glare. The home show in the photo used ceramic floor tile which varied in color but overall was a coffee color. (If interested, see interior photos in this GBA article:
https://www.greenbuildingadvisor.com/homes/passive-solar-home-1980s
I changed the color of walls from off-white to the same coffee color (from advice in an interior design class). Although glare hadn't been much of a problem, that uniform interior color reduced color contrast, and did seem to reduce the perception of glare. Its likely that if the floors were a dark color, the house would have performed slightly better, but glare likely would been a much greater problem.
Not sure when I took the photo, other than it shows AM shadows, and not during winter, as the deciduous plants had their leaves. Glare would not be the reason shades were drawn at that hour for south-facing windows in any home, unless someone needs a dark interior, like trying to sleep late. I don't ever remember using shades to reduce glare or overheating during the winter. This home had sufficient, well-insulated thermal mass tiled slab floor, engineered quantitatively to offset wintertime heat losses with solar heat gains. However, I did have problems with too much solar heat gain from west-facing windows during hot summer days.
Shades, overhangs, or many other alternatives can be used to cut down on solar heat gain during hot days in any type of home, to reduce AC or uncomfortable interior temps. Using window shades is not a defect of passive solar or other types of homes.
If glare is an issue, you could manage lighting levels with sheer curtains or partially drawn "buttom up, top down" cellular shades. However, if shades are needed to reduce overheating mid-winter, likely there's a design error of over-glazing with west or south-facing windows. I did heat loss, air infiltration, solar gain, and hourly interior temperature calculations for the home pictured (using pencil and paper, before the days when home computers were available). Currently, there's plenty of software and consultants to perform calculations, or do it yourself with spreadsheets. For any home design, heat loss, solar gain and other analysis needs to be done for best results, especially considering the high cost of building a home. "Do the math!"
Positioning furniture
Davor,
I'm less familiar with these lighting issues, so hope someone else can chime in.
Light through windows isn't all direct sunlight. North windows are a great example, since with the exception of dawn and sunset around June, there's no direct light through north-facing windows. That indirect light comes in all directions, including from south-facing windows, though the direct sunlight is much stronger.
In a room, its likely you have light coming from multiple windows on different sides of the home. Even if there's only one south-facing window in the room at mid-day in winter, light will bounce off the window sill and floor and diffuse the light, especially at the other end of the room.
At solar noon on Dec 21st, light from a south-facing window at typical height would cast direct sunlight about 13' into the home's interior. Hours earlier and later, the light could extend further but would be less intense. Again, it would reflect off the floor and other surfaces to diffuse the light. Similar for west or east-facing windows, except later or earlier in the day, respectively. In Table 6, note that light from west or east facing windows in summer, is about the same as south-facing windows mid-winter.
I think glare would be a bigger problem if you were sitting in direct sunlight, not at the other end of the room. So I don't think glare around the dining table on the opposite side of the room would be much of a problem.
If your TV screen is in direct sunlight, it would be difficult to see the screen. If the TV were on the same wall, next to a south window around noontime mid-winter, the light from the window would be likely be much brighter than your TV or computer screen. You wouldn't be able to see the TV well, and would experience glare from the window. (Of course you could use a light filtering curtain or shade to fix this.). Same for east or west-facing windows, early or late in the day, respectively. North windows get almost no direct sunlight, so would be less of a problem.
If the TV itself is not in or adjacent to direct sunlight, and at a 90 degree angle to the window receiving direct sunlight, it should be okay. Contrast between the direct sunlight and the TV screen needs to be avoided. Reflections of the window or objects in direct sunlight (that are behind you) could be visible on your TV or computer screen as reflections on the screen. The glass on the TV or computer screen can also be highly reflective smooth glass for best visual screen performance, or can be less reflective glass for less interference with reflections on the screen.
In the early days, computer screens had hoods that blocked light from above and the sides. Those screens weren't very bright. But if you really don't want to move your TV and have glare issues, besides curtains or shades, you could use other ways to block light from reflecting off your TV screen or creating too much glare competing with the TV in your viewing direction. You also may be able to increase the brightness of your screen, which some computer screens do automatically (depending upon ambient light levels).
Hope this helps!
Glare
We don't seem to have an issue with glare. We do have medium dark wood floors, linoleum and tile. We have white walls and flat white ceilings in the main living spaces. We don't have extensive open space, placing furniture near windows and glass doors. This may help break up the glare.
My wife has a small office at work with lots of south facing glass and pulls the shades on sunny days. At home she is the first to pull them up on a sunny morning, maybe because we have more room at home. Generally she, me and the cat sit in the sun if we can.
I'm not so sure about small windows of a dark passive house that is currently expensive to build, even if you can find someone who can do the work right, but the very low energy use is great. Maybe we should not let the perfect be the enemy of the good.
Response to David Martin
David,
Rather than looking for a window with a high U-factor, you should be looking for a window with low U-factor. The lower the U-factor, the better the performance. [I notice that after I wrote this response, you edited your comment, changing "high U-value" to "low U-value." That makes more sense.]
A U-factor of U-0.24 isn't bad, by the way. A SHGC of 0.39 would be considered low-solar-gain glazing, not high-solar-gain glazing. So if you have your heart set on high-solar-gain glazing, you should keep looking.
For more information, see All About Glazing Options.
glazing
I have 35 year old home-made fixed glass windows on the south side of my house that need to be replaced. I have not been able to find glass at local Vermont suppliers with both low U values and high SHGC. Should I be satisfied with a U of 0.24 and SHGC of 0.39? If not, how can I find a better alternative at a reasonable price?
High SHGC glazing
Unfortunately in the US, most windows sold are low SHGC. Canadian companies might be your best bet (e.g., Thermotech, Accurate Dorwin, Inline).
You might find a double or triple pane sealed insulated glass unit (IGU) with no "heat mirror" low-e treatment on any of the panes, or rare low-e treatments that don't reduce solar gain as much. Argon filled would be affordable and add about R-1.
Cardinal glass makes LoE-180 for passive solar applications. However I don't believe they sell sealed glass units retail, just to window manufacturers.
http://www.cardinalcorp.com/products/coated-glass/loe-180-glass/
PPG Industries manufactures two low-e/high SHGC products: Sungate 500 and Sungate 100 window glass with SHGC of about 0.7. Like Cardinal, PPG only manufactures window glass, and I don't think sell glass units retail, just to window manufacturers.
http://www.vitrowindowglass.com/lowe_glass/sungate_500.aspx
You might have to order windows, rather than just the sealed double or triple-pane IGUs.
Since I built that passive solar house back in the 1980's, there were no low-e windows available. I used double-pane windows, and four large double-pane IGUs (for site built fixed windows). U-factor = 0.5, or R-2, very poor by today's standards. I used pleated or cellular shades at night, and sometimes insulating shutters to reduce heat loss during snowy days. You might be able to find or make untreated triple pane glass units, or even try to use three layers of glass for your own site-built fixed window if you are looking for the ultimate cheap solution. In Vermont you'd probably have difficulty with fogging due to moisture between glass layers. Maybe some desiccant packets could help reduce fogging?
Whatever IGUs you might find for sale retail, please share with us.
high SHGC options
In colorado I found Milgard and Pella had high SHGC, low E options.
Great article! One of the best I've read here.
Awesome article..
Ive been under the impression that south facing super insulated walls are more efficient in the winter, than the solar gain potential if replaced by windows. But maybe that's because of my partly cloudy location of zone 5 Iowa City Iowa?
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