Return to the Renewable Energy Forum | Post a Follow-Up

 o
Passive solar and roof overhangs

Posted by windchime (My Page) on
Mon, Mar 22, 04 at 22:23

I live in Northern NJ, and have a ranch house with a hip roof. The roof overhangs about 2 feet all around. It's a great design for passive solar. With the sun so low in the sky in the winter, it just pours in. And, in the summer, the roof overhang shades the windows nicely.

The only problem is that the house is too small, and we are definely planning on adding a second floor. I have been working on the plan myself, and have given much thought to the roof overhangs. I am leaning towards a home style that would have a large gable on the front and back of the house (east and west) with the roof sloping down on the sides (south and north.) The roof overhang on the south would provide shade on the first floor, and I would build dormers on the south side of the roof (presumably with overhangs as well) to provide headroom, windows, and ventilation to the second floor. I also have a mature maple tree close to the house on the south side, which provides a tremedous amount of shade.

Here's my problem: how am I going to prevent overheating of the first floor on the east and west? On the west, which is the back yard, I've planned a large arbor which I think will do a fine job when covered with vines during the warm months. But, how about the east? Am I doomed? Please don't say "less glass" because I really enjoy the views out the front, and my lot affords me plenty of privacy. I'm beginning to think that I need the large gables on the south and north instead, since I have the large shade tree on the south. I'm going for a "cottage" look. Something like a cape-cod type, with the dormers, yet, I need a lot of space upstairs. The only thing that I think would solve this delema is the prairie style, with hip roofs around both the first and second floors. Yuck! That style would NEVER fit in this historic neighborhood. I need ideas...


Follow-Up Postings:

 o
RE: Passive solar and roof overhangs

Hi windchime,

Can you spell, "awning"?

Maybe either the add-on kind, or design some into your building plan as you upgrade?

joyful guy


 o
RE: Passive solar and roof overhangs

Oh yeah, Ed. I forgot to mention, I don't want to block my views. I had thought of awnings, and decided that I didn't like that option because of my views. I'm beginning to think that the best thing is to reorient the roof so that I can have the overhangs in the East and West, since I have my tree in the South already. I had already drawn out some elevations, though, and I really liked the large gable in the front, but I guess I should start over, because I need it to function as well as it looks...


 o
RE: Passive solar and roof overhangs

Are new windows for the first floor a possibility? A friend of mine recently got 'low-e' windows on a west-facing wall and LOVES them. I don't know if they block so much, though, that you lose solar gain when you want it in the wintertime.


 o
RE: Passive solar and roof overhangs

Oh yes, new windows are definitely planned for the first floor. And I was planning on energy efficient windows. I was wondering about losing solar gain in the winter too, though. Anyway, it's not a problem on the west, because the sun never reaches the west during the winter here. I don't know if it's the latitude (NJ) alone, or if it also has to do with the fact that my house is built on a wooded hill/small mountain, and the sun is behind the hill before it ever reaches "west." And, also, I'm planning a large arbor on the west side, which is the back yard, to reduce/eliminate gain in the summer.

The south is not a big problem, because I have a mature maple tree growing fairly close to the house (10 feet.) Although the overhangs in place right now do contribute to the shading. How do homes that are designed to be passive solar get around all the gain from the south in the summer? I know that my tree is growing a lot closer to the house than is normally recommended, and planting trees farther away on the south really doesn't help because the sun is so high in the south sky during the summer. Is there something else I can be doing on the south to reduce gain in the summer?

My big problem is the front, which is east. I wanted a large gable in the front, but I have no way of blocking the sun in the summer. Currently, the overhangs do a nice job of shading out the sun in the summer without blocking my views. I guess low-e windows will help, but I don't want to lose the heat gain that I currently enjoy all winter long. I am beginning to think that a re-oriented roof is the answer. And I have time before choosing windows, but I'm beginning to worry about finding windows that will do what I want!


 o
RE: Passive solar and roof overhangs

windchime,

How about adding a flat construction protruding horizontally above the windows, far enough above them and protruding just far enough that it will allow the slanting rays in winter to enter the entire area of each window, but cause the more highly angled rays in summer to hit the wall below the window?

Each protruding construction would need to be somewhat wider than the window to allow for it to be effective throughout many hours of the day, as the rays of the sun arrived at different angles horizontally.

Viewed from directly in front of the window, it might need to be elliptical in shape to be most effective, to allow for the sun's rising and falling position during its daily traverse of the sky.

Viewed from a position parallel to the wall, it might need to have an arced shape, as well, to achieve the same result of allowing for the differing positions of the sun during its daily traverse of the sky, and its varying positions north to south through the seasons.

Those arcing constructions might impede your view from the window somewhat, but not seriously, I think.

I'm no engineer, but it seems to me that such a construction would achieve the results that you seek.

Good wishes for avoiding any appearance of claustrophobia.

joyful


 o
RE: Passive solar and roof overhangs

Read up on low E glass as it will explain a lot about reflective in BOTH winter and summer. Here is a web site for a reflective panel that may help. inflectorpanel.com Also check Accuratedorwin.com They are windows we plan to use and give a good report on heat loss/gain. Gooooood luck! PS A friend in the Il Solar Association uses inside insulated padded blinds.He varies the open/close positions as needed.


 o
RE: Passive solar and roof overhangs

There is a lot of incorrect information floating around about low-e glass and how it performs under various conditions. The biggest misleading thing stated is that low-e will keep your house cooler in the summer. This is only true if you have only reradiated (long wavelength) heat hitting your windows. Sunlight heat (short wavelength) will pass through just as it does in the winter. What I did finally manage to find out after conversing with low-e experts at the University of Minnesota and at Berkely is that you can reflect solar heat back out a low-e window with quite high efficiency and no damage to the glass. It reflects at appoximately the same wavelength that it came in as, so the low-e glass will let it through. A highly reflective bronze or silver mylar window shade works remarkably well at doing the reflecting. Plain curtains or fabric shades absorb the heat and reradiate it at a longer wavelength, so the low-e glass will stop the heat from leaving the building. I have tested this at home and it does really reduce the heat gain.

Your other option would be to use louvered awnings that you can see out of through the horizontal openings, but they still block almost all of the direct sunlight.


 o
RE: Passive solar and roof overhangs

My house is designed to take advantage of passive solar principles.

The second roof has a large overhang that blocks the 2nd floor windows in the summer but not the winter.

On the first floor, trees are planted in front of the windows.


 o
RE: Passive solar and roof overhangs

Have you looked into tintable windows at all they dont block your veiw.

Here is a link that might be useful: Tintable windows


 o
RE: Passive solar and roof overhangs

Ed's idea for the protrusions is actually a very good idea, I think, for a contemporary house. But, since I live in a 1950's house in a historic district, I was planning on giving my house a more historic look with the remodel.

So, I guess, landmarker, the only option is to plant trees in front of my house to block solar gain in the summer from the east. I already have a couple of really nice trees there, including a full moon maple and a red horsechestnut. I really don't want to plant any more in the east, and even if I did, because of the steep slope of my lot, they would be planted downhill from the house, and would take years to grow tall enough to shade the house. I might not ever see it in my lifetime! The trees that I have are old, and tall, and not nearly shading the house, except really early in the morning, when the sun is low in the sky.

I have actually considered putting a porch across the entire front of the house, but, I don't want to lose all of the solar gain that I get in the winter. So, I am strongly considering changing the roof lines in order to get the slope in the front. This is the best compromise that I can think of.

Concerning the low-e glass then...Is it really better at keeping the heat in in the winter than it is at keeping the heat out in the summer? I keep my windows open almost all summer long anyway. So, the heat should be able to escape on it's own, right? We're also planning on installing a whole house fan for the evenings.

The tintable windows are an option that I'm not really happy with, again, because of the loss of winter heat gain. Also, although they don't necessarily block the view, the tinting does affect the outdoor scene in a way that is unappealing to me.


 o
RE: Passive solar and roof overhangs

In general the low-e glass is much better at keeping in heat in the winter than it is in keeping heat out in the summer. Summer or winter, any heat from direct sunlight that gets in, will stay in, unless it is reflected back out. Insulated curtains are generally considered a bad option for a low-e installation as they absorb the solar heat in the summer and trap it, as the reradiated heat off the curtains will not pass through the low-e glass. If the curtains are tight to the window, you can overheat the glass. If they are not tight to the window, all the heat goes into the room, so they do no good. Not as big a problem in the winter, but the very large difference between the inside and outside glass temp can cause thermopane seal failures. You can buy your low-e glass in low and very low sunlight transmitting versions, to keep you cooler in the summer, but then you lose the heat gain you would like in the winter. Putting film on the windows can also overheat the glass, unless it is a reflecting, not absorbing style.

In my opinion, the best setup is a high sunlight transmitting low-e window (like an R7 Heat Mirror) with a rollup mylar reflecting shade. You get the best of winter and summer performance that way. Of course, an outside roll up blind or awning is even better, but not as easy to do.


 o
RE: Passive solar and roof overhangs

Windchime, I just visited a friend in Phoenix. She has exterior sun shades that are controlled electronically (manually or can be set on a timer). They are on tracks running up/down alongside the window frame. I'm sure they are expensive, though, since she lives in a VERY upscale development. The shades do obscure the view somewhat, but let a breeze in. If they were set on a timer, you could have them lowered just when the sun is at it's worst, then raised the rest of the time for the view. Just one more thing to consider....

I found that when I got my roof insulated (from R2+7 to R-27+7) that my house stayed cooler in the summer if I closed the windows all day (while at work) and opened them in the evening/night. The daytime 'breezes' were warmer than the inside air. With the addition of a second floor, the first floor -if you can keep solar gain to a minimum- should be much cooler and may benefit from modifying your normal way of doing things.


 o Post a Follow-Up

Please Note: Only registered members are able to post messages to this forum.

    If you are a member, please log in.

    If you aren't yet a member, join now!


Return to the Renewable Energy Forum

Information about Posting

  • You must be logged in to post a message. Once you are logged in, a posting window will appear at the bottom of the messages. If you are not a member, please register for an account.
  • Posting is a two-step process. Once you have composed your message, you will be taken to the preview page. You will then have a chance to review your post, make changes and upload photos.
  • After posting your message, you may need to refresh the forum page in order to see it.
  • Before posting copyrighted material, please read about Copyright and Fair Use.
  • We have a strict no-advertising policy!
  • If you would like to practice posting or uploading photos, please visit our Test forum.
  • If you need assistance, please Contact Us and we will be happy to help.


Learn more about in-text links on this page here