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Sizing overhangs for passive solar  RSS feed

 
Ben de Leiris
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Location: Hinesburg, Vermont
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We all know the basic idea is to size overhangs so that they let the sun in during the winter and shade the windows in the summer. I'm curious about the details though. That ubiquitous solar angle diagram usually implies that on December 21, the sun hits the whole window, and on June 21 the whole window is shaded. Is this the best way to match the solar gain with the varying seasonal heating loads? Is the 7-12% glazing area assuming overhangs are designed this way? Or do you ever design so that the window is in full sun for, say, 3 months of the winter, in full shade for 3 months of the summer, and in transition in the intervening seasons?

I know everyone's favorite response is "it depends." So here's a little more detail on the design I'm working on. A small house in western Colorado, latitude 39 N and elevation 5700, which has hot summers, and cold but still very sunny winters. Not arctic Minnesota winters, but down in the 10's and 20's. From late November to early March, we would welcome all the solar gain we could get. And from June through mid September, we'd be happy with full shade. So I'm thinking about having the full-sun/full-shade dates, instead of falling right on the solstices, bridge them by a month on either end. Obviously it's always a little fudged since the solar altitude is out of phase with the actual heating demand. I'm just wondering if there is any merit to this design, if I should just stick to December 21 and June 21, or what else should be considered.

Thanks.
 
Casie Becker
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Location: Just northwest of Austin, TX
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I think that's inspired reasoning.
 
Terry Ruth
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Ben,

We don't design overhangs primarily for passive solar these days, old design rules no longer apply. We design them primarily to shed water away from the foundation/wall depending on wind direction, orientation, etc. 18-24" is a good rule of thumb, I use 24 w/a energy heel. Of course we don't want to solar shade either, unless intentionally. There is simulation software like SAM to do this if you want, even site phone apps, we use it to design solar panels systems.

Smart window technology like thermochromic and low e-coating's replaced other inaccurate methods and control the amount of short/long waves per season automatically. The manufactures have different names depending on series so be careful, some manufacture their own. Cardinal glass the industry glass pro will offer it this year called "click". For now, Anderson has a solar passive smart tech glass with different SHGC's and U-values. Raven is a top producer. Marvin is another. PV/DC windows already hitting the commercial markets in time will have a much higher cost of performance than solar mass.

Further, on average the solar gain depending on mass, HVAC load loss/gains ~10%. A desiccant mass systems would be higher. A comparable solar panel systems investment ~ 30%+. It is next to impossible to determine HVAC loads that consider passive solar gains/losses per zone without the proper design tools and knowledge. Manual J has limited capability. Same for a solar panel comparison.

Good luck!
 
Ben de Leiris
Posts: 61
Location: Hinesburg, Vermont
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Terry, I'm confused as to why you would disparage the use of roof overhangs as part of a passive solar design. I think it is an incredibly elegant solution which is low tech, easily understood and implemented, and usually adds nothing to the cost of a design. As you say, overhangs are important anyway for protecting exterior walls from the elements. My working design began with a planned overhang of 24"-30", and by moving them up or down, in or out by a few inches, I can make it so the house will be warm and sunny in the winter, cool and shady in the summer. What's not to love? (I've found SketchUp's shading tool to be very helpful, as well as plain old high school trig!)

I agree that glass selection is important as well. I'll certainly be looking for low-E, low U-value windows, with high SHGC on the south. I understand the basic idea of thermochromic coatings but don't hear much about them... are these widely available? Do you know what kind of cost you can expect over more standard high-performance windows?
 
Rebecca Norman
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I think your idea of doing shallow overhangs, not as deep as the June 21 shadow sounds good.

I've been living in houses with only solar heat for the past twenty years, in high desert with cold sunny winters. Very low tech, no special coated glass, nothing too airtight, and no overhangs on most parts of the buildings. My residential rooms are exactly solar south facing; while our school building with classrooms and offices is 15 degrees east of south. Our windows are not flush with the exterior wall, because structurally, the window frame is embedded somewhere near the middle of the adobe wall, or about 6 inches in from the outer surface. We're at 34N, and for most of the summer in the perfectly south-facing rooms, the sun does come in, but just barely. I mean, plants on the windowsill don't get any direct sun, or just on the pot, not on the plant. And in December, the sun streams deep into the room. It works really well, in my experience. We use insulated curtains at night in the winter, and personally, I use a thin white curtain for privacy in the summer which might provide a little shade. We also add attached greenhouses from October to April on most of the buildings, and that's a huge source of winter heat.

During the ice-skating season here, I'll admit our rooms get a bit too chilly at night, like below 14C (57F) for evenings and nights for a week or two, which is really when I start wishing for backup heat. But as a matter of principle and example we don't use backup heat, and so okay, I wear a hat indoors for a few weeks of Jan, and use a hot water bottle under my feet as my backup heat.

 
Terry Ruth
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Ben, I use alot of old school materials and high tech solar passive and active systems that may not apply here. I invite you to quantify in BTU/HR, what gains and/or losses you expect to see before you build and post them. Many have had poor results, as in overheating, cold rooms, from guessing. BEOPT or WUFI is the best sources. Many have had success, you may be one without the data.

You environment is entirely different in Vermont than Rebeccas that reminds me of CA where they don't have overhangs, or some do some don't. You know you need them.

I get discounts as a builder here, the cost of drywall is double on the east cost then here. Anderson has a passive window with a thermochromic called low e "Passive Sun" with higher SHGC for south/west depending on HVAC zone loads. Smartsun everywhere else. Get your dealer to compare cost, here there is little difference. I see a huge difference in triple pain for little 20% increase U. That is going to change soon, unlike the past, now all the big players are offering triple in all climate zones since alot of smart builders were getting them from Europe at the cost of double here with higher U-values and higher VT. There is a whole lot more to windows. They like PV are a science project in themselves. I'm a production net zero/positive designer, not a one-off, so I have to keep up with good technology, it's sink or swim

http://www.permies.com/t/53957/passive-solar/Sizing-overhangs-passive-solar#443210
 
Irene Kightley
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I was reminded of this part of the forum by the Permies daily e-mail and started looking down the threads and this one caught my eye. It's an old thread and there may be something newer about this in the forum but I'll add some details to it anyway.

We use our south facing overhangs to warm thermal mass from the autumn equinox until the vernal equinox in March when the sun is prevented from entering the house. In fact, we didn't think of equinoxes when we were building the house, we just put chalk marks on the walls where the shadows fell when it started to feel chilly and used them to size the barrier. When I checked the dates for the yearly "opposite" to know when the overhangs would start to keep out the sun, it just happened to be the day of the autumn equinox.

This barrier in this photo isn't a roof "overhang" because we built a terrace on the south side covered with clear polycarbonate sheets but the photo shows clearly how the wooden part of the roof works in cold and warm seasons. We built our south facing upstairs windows, measured inside the walls; and added overhangs using the same dimensions.





 
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