Are we getting the wrong advice about which way our homes and offices should face?
I have always taken it as given that the ridgeline of a home built in a northern shelf city like Ottawa should be east-west. When the ridgeline (i.e., the long axis of the structure—either a home or office or other type of building) runs east-west and the front faces north, your backyard/your deck/your primary windows face south. This is supposed to do several things: a) give you the most exposure to daylight, b) maximize the amount of solar irradiation so you can warm the structure with either active or passive solar and c) allow you and your friends to sit leisurely on your deck during warmer weather months for longer periods of time.
These are not inconsequential decisions that can—result in a reduction of heating costs as well as problems resulting from SAD, Seasonal Affective Disorder. But after examining the data and doing an informal survey, I am no longer sure this is right.
First, let’s look at the daylight assumption. Here we are measuring hours of exposure to daylight not the intensity of solar irradiation. So let’s examine the data set for Ottawa.
(You can download my data in .xls format from: http://www.ottawarealestatenews.ca/SolarIrradiation.xls. Or you can examine it in the postscript below.)
From first principles, we can determine that for a home with a ridgeline running north-south and the backyard facing due west (i.e., your deck or balcony faces west) and the front of the home facing east, the building will have sun coming in the east-facing or west-facing windows from sunup to sundown on both March 21st and September 21st each year. This is because the length of day at those times is about 12 hours and the sun rises directly in the east, moves in a ‘straight line’ directly overhead and sets in the west. At no time do any north-facing or south-facing windows get any direct sunlight. See my diagram below.
Now we have to examine two other conditions. What happens on June 21st (the longest day of the year) and December 21st (shortest)?
On those two days, the sun’s declination is either +23.44 degrees or -23.44 degrees in Ottawa. At 1 pm on June 21st (it’s 1 pm not noon since we use daylight savings time), the sun is directly overhead (its azimuth is zero) while on December 21st, this occurs (obviously) at noon.
Again (for your home with a north-south ridgeline), at sunrise on June 21st, light comes flooding in your east-facing windows and at sunset, your west-facing windows and deck are again bathed in sunlight. What happens in between is interesting: you still get more direct daylight into your home as the sun heads overhead until the azimuth reaches 45 degrees and now, finally, the north side of your building starts to get more direct sunlight than the east-facing or west-facing side. This starts at around 11:40 am and lasts until 2:30 pm (when the azimuth of the sun is -45 degrees) after which the west-facing side of your home starts to receive more direct sunlight. So when the length of the day in Ottawa is 15.73 hours, the north-facing side of your building receives more direct sunlight than the east and west walls for just two hours and fifty minutes. It’s no contest. And remember the south-facing side of your Ottawa home is in shadow during that time because of the positive declination of the sun at the summer solstice. If you want a sunny backyard in the summer, face your backyard north not south!
Now you can do the same analysis for December 21st and what you find out is that you do better—from 0841 to 1522, you are getting more direct sun on the south facing windows; that is, for 6 hours and 41 minutes out of the 8.78 hour day, you are getting more direct sunlight. This doesn’t mean that you aren’t getting direct sunlight in your east-facing and west-facing windows during that time, it’s just that the sun coming in those windows is coming in at a more oblique angle.
If you are more likely to be home before 8:41 am and after 3:22 pm, then again, you are going to want to orient your home’s ridgeline north-south so you can capture more daylight.
This analysis is based on the assumption that combating SAD (which can have significant mental and physical health impacts—daylight deprivation is no joke) is a top priority for you.
Offices where we tend to be present during the period from 8:41 am to 3:22 pm might be better oriented with their long axes pointed east-west, provided we only have offices on one side (the south side) of the building. If you have double-loaded corridors, the offices on the north side of the building will be horrible dungeons of deep, dark depression for three months of the year.
Now so far, we are only looking at day lighting, not solar power production—either passive or active. Again, the conventional wisdom is to orient your ridgeline east-west so you can capture the maximum amount of solar irradiation during those crucial winter months.
We know that solar irradiation at the earth’s surface is stronger when the sun is overhead. This is simple geometry. A one kilometre sunbeam hitting the earth’s surface from directly overhead heats up one kilometre of the surface; if the sun’s angle is, say, 30 degrees, then the area being irradiated is SQRT [(1/ tan (30))**2 + 1**2] or 2 km; i.e., half as much irradiation is occurring when the sun is lower in the sky. Other factors affecting solar irradiation are the amount of light reflected or refracted back into space by the atmosphere, dust, cloud cover, moisture content, length of day. These are not simple calculations which is, in part, why climatology is such a difficult field.
It seems clear that solar irradiation is gong to be a lot less when the sun is low on the horizon than when it is more directly overhead so that, if you are maximizing capture (for either your solar panels or simply your windows) during, say, the period of 0841 to 1522 on December 21st, then you should be better off with a south-facing backyard. But wait a second, in the case of windows, these aren’t lying conveniently on the ground or at an optimal angle for solar irradiation. They are standing vertically in your wall. For a kilometre wide sun beam falling on an imaginary (humongous) vertical wall facing south with a 30 degree angle of attack, the surface length is 1.1546 km. (2 x tan (30)). This isn’t too bad (you are only losing about 15% efficiency) so there is no need to build yourself a crooked house.
Obviously, solar cells or hot water heating systems in your roof can be made more efficient in the winter months by pitching the roof at an angle above 60 degrees so they can be 90 degrees to the angle of the sun. Now, no one (outside of a Swiss Chalet) would do this (it’s too steep) but a flat roof won’t do and less than 30 degrees is pretty bad too since your panels will be 30 degrees off normal.
But still one has to wonder to what extent those of us who live in northern climes want to go to maximize solar power at the expense of significantly less daylight in our homes year round. I suspect that aligning the ridgeline of your home in a north-south direction so that the backyard, balcony, deck, family room, kitchen and master bedroom (where you spend more evening hours) will face west while your exercise room and home office (where you spend more morning hours) face east will become more popular. Having light from sunrise and sunset flood your home at daybreak and day’s end with more daylight during most of the year is a plus, I believe. And I am quite sure that passive or active solar installations can work around the orientation of the ridgeline—hip roofs, for example, will work fine on end units or garages thus producing the desired orientation for these applications.
Prof Bruce
Postscript 1: An informal survey of knowledgeable people in the industry (Ottawa-based, residential REALTORS) found that 18 out of 26 or 69.2% wanted their decks or backyards to face west while 30.8% preferred a south orientation, that is, if they had a free choice in the matter. I was surprised that the margin was more than 2 to 1 in favor of west; when I asked them why, most of them simply said it’s nice to sit on your deck or balcony and watch the sun go down but I couldn’t help but think that they had intuitively grasped the fact that this orientation was producing more daylight inside their homes and those of their clients.
Postscript 2: Data Set Ottawa, Canada
Day and Month Declination Sunrise Sunset Length Length
of Day of Day
(degrees) (hours:mins.) (hours:mins.) (hours:mins.) (hours)
March 21st 0.61 700 1918 1218 12.3
June 21st 23.44 512 2056 1544 15.73333
September 21st 0.33 647 1903 1216 12.26667
December 21st -23.44 837 1724 847 8.783333
Time of Day 1000 1100 1140 1305 1430 1510 1610
Azimuth (June 21) 75.39 59.56 45.17 -0.29 -45.56 -59.85 -75.6
(degrees)
Time of Day 800 841 1201 1500 1522 1600 1700
Azimuth (December 21) 52.83 45.13 -0.02 -41 -45.35 -52.49 -62.98
(degrees)
Latitude and Longitude 45° 19′ N by 75° 40′ W 45.316667 75.66667
Storey Height 3 metres 2 storeys 3 average height of windows (m)
Source:
http://susdesign.com/sunangle/
Comments by Ralph Wiesbrock, Partner / Principal, KWC ARCHITECTS INC. and Mark Lucuik of Morrison Hershfield:
“Good afternoon Bruce,
We took a look at your post and have the following comments:
• As you implicitly suggest, the advice, wisdom, or otherwise, pertaining to building orientation depends on your goals and assumptions. Your analysis privileges quantity of daylight. It suggests some psychological benefits and the start of documentable consumer preferences but does not delve further into the finer grain of the qualitative aspects of daylight and orientation. Nevertheless, you have take into account factors like the stifling late afternoon heat and sun on the west facing deck at Ralph’s mother-in-law’s house in Thunder Bay; this would suggest that the issue is more nuanced than a simple matter of orientation.
• We can more readily control the sun on the south elevation. On the west and east we are prone to uncontrollable glare, which inevitably results in the use of sun shades (thereby eliminating light). Note this could be offset by specialty glass or similar systems….it could be better to use Solera (as an example) on a west elevation as compared to the south, as the lower sun on the west elevation could result in a stronger light and deeper penetration into the building. Having said this, we also need to recognize that that western daylighting is simply not there for a large part of the day.
• East and west sun tends to be lower in the sky. While this can result in a higher intensity of light, it also results in increased shading from adjacent trees, buildings, and such.
• Many buildings focus all efforts on north and south facades and sort of abandon the east and west. East and west can play a role for day lighting and a good design could bring in western or eastern light when northern light or southern light is not available.
• Your post seems to be predicated on a status quo tract housing model. One of the challenges wrt passive solar and daylighting from a green building performance perspective is that this development model is generally indifferent to questions of orientation. It is left to the consumer’s chance opportunity to select from available options on the site plan or MLS listing. That is why we see most of the innovative housing strategies such as those exemplified by CMHC’s Equilibrium competition winners focus on other measures for improving their environmental performance.
• We would suggest that the underlying issues concerning orientation deserve more consideration going forward. In the commercial / institutional sector there is considerable and growing recognition that each exposure and interior use of a building deserves a considered response. On a major new institutional building that is designed to meet the 2030 Challenge target of 60% energy reductions that we are involved, we treated the envelope design for each exposure and related interior activity differently.
• It would be great to see a new generation of development that offers a more nuanced approach to building design and site planning where solar and daylight orientation are better taken into account. Buildings have a significant role to play in our energy future and the leveraging of building orientation is and will be an important aspect.
So what we are saying is that your blog posting raises as many questions and points of further discussion as it answers, and that’s a good thing. Occupation of space (i.e. comfort – physical and psychological) are significant factors to be considered but should be separated from energy related performance which is the primary measure for most discussion of solar orientation. At that point, we need to do whole building analysis to examine the energy impacts of each potential design decision.
Now, just for the record, Ralph’s backyard (a small city lot) faces southwest but is blessed with a fine tree canopy that he relishes in summer for its fine cooling microclimate. Ralph has a wonderful red maple in the front yard and between these traditional passive solar control strategies we avoid the use of air conditioning.
Best wishes,
Ralph and Mark”