Thursday, May 12, 2005

Shading through the south-side overhang

Yesterday when I shared the 3D Model slide show with my colleague Jim, he inquired whether the soffit overhang on the south side will provide enough shading in the summer.

Well it had been about 1.5 years since I last researched "sun altitude" and "sun azimuth". In the meantime the geometry of the roof and south wall had changed quite a bit, so it was time to confirm the sizing of the overhang. I found this site from the U.S. Naval observatory to get sun (or moon) altitude and azimuth tables for a given date at a given location.

Seasonal Sun Penetration - click to see full sizeIn the picture on the right you can see the different penetration levels at 12 PM noon on June 21, March & September 21 and December 21. As you can see, the overhang completely shades the midday sun in the summer (most left beam) whereas in the winter, even at high noon, the sun will penetrate as far as the back wall of the living room. (Click on the picture to open a full-size version in a separate window.)

The other study/simulation that I did, is to simulate the actual penetration during the middle of the summer. Two values determine the suns position: the sun altitude (i.e., the angle of the sun up from the horizon; 0° being on the horizon and 90° being directly above) and the sun azimuth (i.e., the angle along the horizon, with 90° being east and 180° being south). So the actual penetration through the south windows is not only a factor of the sun's altitude and the overhang geometry and window height, but also of the sun's azimuth and the windows' widths. The image below shows the penetration at 9:45 AM (yellow) and 14:35 PM (orange). As you can see only a fraction of the possible direct sunlight can enter the room during these times,Seasonal Sun Penetration - click to see full sizesince the sun is pretty much still in the ESE (112.5°) or WSW (202.5°). The suprising thing for me was that the overhang pretty much prevents even that early sun to penetrate the room - the deepest penetration is about 3 feet (at an azimuth of 109° with an even lower percentage of what makes it in). and the penetration rapidly declines to zero by around 11:10 AM and does not penetrate at all until 13:10.

As some of you might recall, on the east side of the dining room, we do have the same large windows as on the south side, but with no overhang (at least only in the gables), so the sun will penetrate the living room from that side for the most part of the morning. This is actually by design, because in the spring time, it tends to stay pretty cold (yesterday the high temperature was 42°F), especially in March or April. At our place the sun rises pretty much in the east, but due to some trees we probably won't see much of it until it is about 10° high, at around 7 AM at 10° south of east. For the summer, when the sun reaches 10° altitude at 5:40 AM in the ENE (67.5°) we definitely will have to employ shades on these windows. Fortunately, and also by design, this is not so much of a problem on the west side, where the windows are much smaller, due to the sitting area. But shades would probably also help here as well.

Sun Penetration Graph - click to see full sizeThe last simulation I ran with the data is to plot the penetration depth for each of the four dates. The graph on the right shows the penetration level at the various times of the day for the four dates during the year. The nice thing that you can see is that the penetration during the winter time can reach the backwall unhinderedly when the sun's azimuth is between ESE and WSW.

In summary, the theory looks good for the sizing of our overhang on the south side. The 80" (6' 8" or 203 cm) windows now allow unhindered penetration into the room during the winter months which should help with the heating on sunny days. The 475 sq. ft., four inch thick cement slab definitely will have enough capacity to store any heat coming through the windows, and with 168 sq. ft. of window space for 1,500 sq. ft. of main and 2nd floor living space (i.e., 11%), we might be a little bit over the recommended percentage for south-facing glazing, but we'd rather shade windows in the summer time then having to freeze. And our large thermal mass (including the masonry stove) should help mitigate overheating. We can also always open the door to the basement - to add another 1,100 sq.ft. of living space...(or the garage to add 485 sq. ft.).

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