You can use roof overhang to govern how much sun gets into your home throughout the year. This will greatly affect how much you have to spend on heating and cooling. In the absence of scale drawings, use a little trig to calculate how big your overhangs should be.
One of the keys to building an energy efficient structure is to understand how the sun hits the building. In the northern hemisphere, north of the Tropic of Cancer, the sunniest side of any structure is always the southern side. However, in Hawaii for example, which is a tropical island that lies south of the Tropic of Cancer, most of the time the south side is the sunniest side, but for a short time every year the northern faces of buildings get some direct sun. Moving further south, buildings that lie directly on the equator get an equal amount of direct sun on their southern and northern faces each year. Here in Mount Juliet, Tennessee, just east of Nashville, we sit at 36.17° north latitude, therefore our direct sunlight always comes from the south.
Let’s say you want to build an energy efficient home, one that takes advantage of passive solar heating in the winter and yet keeps the sun out during the hottest summer months. The key is to concentrate on your south-facing windows since that is where the sun’s rays are going to enter your home during the heat of the day. You can always hang blinds and shades over those windows, but if you want to take advantage of the natural light while still reducing the solar heat coming in, there are two main things you can do: buy windows with a small solar heat gain coefficient (SHGC) and design the roof overhang (or awnings) of your home to work with the seasons. I will cover window SHGC in a different post, but here’s the skinny: A SHGC of zero lets in no solar heat, while an SHGC of 1 lets in all the solar heat.
On to the overhangs. This will be a multi-step process:
Find the maximum and minimum solar angles at your home. This will give you a baseline to work with. This is easy. This step is optional, but I think it’s kind of fun.
Mine the climate data to find the average maximum high temperatures during the summer. This is also easy.
Find the solar elevation angle when you want the sun to stay out of your windows. Easy peasy.
Calculate your overhang at the south-facing windows. You guessed it: easy.
Step 1, Find Your Solar Angle Max and Min (Optional)
The summer solstice, around June 21, is when the sun is directly over the Tropic of Cancer (23.5° North) at about noon. That’s when the solar elevation is the closest it will get to 90° in the northern hemisphere, and when places like Alaska, Norway, Canada, and Russia experience the midnight sun. It’s the longest day of the year. Conversely, the winter solstice, around December 21, is when the sun is directly above the Tropic of Capricorn (23.5° South), thus at its lowest arc in the sky for those of us in the north. This is when Prudhoe Bay, Alaska is dark around the clock. You need to know two pieces of information to calculate your maximum and minimum solar elevations: your home’s latitude and 23.5°. Where I live, at about 36.2°N, the maximum solar angle is 90 - (36.2 - 23.5) = 77.3°. It’s important to take your latitude and subtract 23.5 first, then subtract that answer from 90. So in my case, from a line straight horizontally from my house foundation, the sun will rise to a maximum of 77.3° above my head at the end of June. To find the minimum, the tiny little equation is almost the same, except you add 23.5, not subtract: 90 - (36.2 + 23.5) = 30.3°. [Note for the mathematically picky: we’re actually subtracting a negative 23.5, since we’re dealing with the Tropic of Capricorn, which is at -23.5°.] In the end, my maximum and minimum sun angles (solar elevations) in Mount Juliet, TN are 77.3° and 30.3°.
Fun side note, and we’re honestly getting a little far into the weeds here: at 66.3° N, the maximum sun angle during the day is 90 – (66.3 – 23.5) = 42.8°. But if you use a solar calculator like the one in the next step, you’ll find that on the same day, in the middle of the night, the sun angle is still above zero. 66.3° is the Arctic Circle, the dividing line between the Land of the Midnight Sun and the rest of us. At the Arctic Circle, there is one 24-hour period each year where the sun does not set. The farther north you go from there, the more days like that there are.
Step 2, Mine the Climate Data
What you want to do here is look at the climate data for your location and find out when the average daily temperature (low plus high, all divided by 2) is around 72°. I browse usclimatedata.com for this. At the Nashville airport, the first time each season the average hits 72° is on about June 1, and the last time the average stays above 72° is about September 17. On each of those days, the low temp is about 61° and the high is about 83°, giving an average of 72°. Now that you have those dates, (June 1 and Sept. 17 for me), you need to find the sun angle (a.k.a., solar elevation) on those days.
Step 3, Find the Target Solar Elevation
There are lots of solar elevation calculators available on the Internet; I’ve used the one published by the National Oceanic and Atmospheric Administration, located here: https://gml.noaa.gov/grad/solcalc/. To use it, you enter your latitude and longitude, the time and day you want to calculate the solar elevation for, then watch the solar elevation box. Here is a screen shot for this particular tool:
As you can see, the maximum sun angle on September 17 is 55.81°. On June 1, the sun angle is around 76°. If we want to design a roof overhang that will provide shade throughout our date range, it will have to shade the lower angle, 55.8°.
Step 4, Calculate Your Overhang
Remember in 10th grade trigonometry when someone asked, “When will we EVER need this in our lives?!?” Today is that day, my friend. But it’s pretty easy. Let’s set up the problem. Say you’ve got a home with 9’ high walls, 12” thick, with a 5’ window set 2’ off the floor:
You’ve got two sunbeams you’re working with, the September beam coming in at an angle (to the ground) of 55.8°, and the other one, the June 1 beam, coming in hot at 76°-ish:
Clearly, we can ignore the June 1 beam since the September overhang will also block that beam. What we need to do is solve for the top side of this yellow triangle:
We know the vertical side of the triangle is 7’, and we know the beam’s angle to the ground is 55.8°. That’s all the information we need, but we have to do two simple calculations to get our answer. Now, as you can see here, the bottom exterior angle of that triangle is 55.8°. To get the interior angle, we need to subtract that from 90°: 90° – 55.8° = 34.2°
Once we know that, you’ll recall the words of your own Mr. Paulsen from 10th grade, “The tangent of the angle is equal to the opposite side over the adjacent side.” Admittedly, if you are drawing to scale and you can accurately draw your angles, you can simply measure how long the overhang should be. I kinda like the math though, to be honest:
As you can see in this diagram, once we solve this thing, the top of our triangle is 4.75’, or 4’ 9”. Since our wall in this example is 12” thick, the overhang is 3.75’, or 3’ 9”.
Is that overhang too big? Will it make your house look like a mushroom? That’s up to your personal tastes. If you think it will, you can install the windows higher up off the floor or use an awning directly above the window. The closer the awning to the window, the shallower it can be.
By the way, if we build this 3’ 9” overhang that protects us from the September 17 sun at the end of summer, when will it start protecting us in the Spring? Well, September 17 is about three months from June 20, so if we go back about three months from June 20 and check with the solar calculator, we discover this overhang will start keeping the noon sun out of our room on about March 25.
And what if you do not have an overhang on the south side of your house? Or any side, for that matter, and you don’t want awnings? There are always blinds or shutters. And in these days of widely available home automation products, you can program your blinds to lower and raise with the sun during the heat of the day. Your blinds may be fully closed at noon, but open again at 5:00 in the evening, with no intervention from you.
Building a home that benefits from passive solar heating in the colder months yet keeps out the sun in the warmer months is not difficult, it merely takes a little planning. An architect is a very handy resource to have, someone who works for you to design the home you want, that functions the way you want, and fits well into its surroundings. We at Terra Southeast are happy to make recommendations, and we will look forward to building your home with you and your architect. Please give us a call at (615) 898-9115.