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.
- (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.
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.
by Christopher Gronbeck. Seattle, Washington.
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.