I found this calculator http://www.susdesign.com/overhang/ From my input it seems impossible to have an overhang that blocks the sun during summer and still let's it during the winter... it seems that no matter what I try in term of combination, I get shading in winter or not enough shading in summer... any tought?

Posted By Alexis on 07 Oct 2012 09:14 PM
I found this calculator http://www.susdesign.com/overhang/ From my input it seems impossible to have an overhang that blocks the sun during summer and still let's it during the winter... it seems that no matter what I try in term of combination, I get shading in winter or not enough shading in summer... any tought?

That looks like a BETA version, so maybe it is not 100% accurate.

With that being said, from what I have read and learned, you are correct, it is almost impossible to have an overhang that blocks 100% of the sun during summer. Factor in spring and fall equinox and you got a whole other problem of peak sun angles when it's still hot. One cannot rely on overhangs alone and must utilize interior or exterior shades to avoid overheating. (the latter being more efficient but less practical)

I am in communication with someone who designed a passive house and is experiencing serious overheating issues during late spring, summer and early fall. This was designed by a "professional" and of course this company is refusing to take blame and is telling the homeowner to install shades to deal with it.

The east side and especially the west side of a home is next to impossible to provide enough overhang to block the sun. It's the south side that can be dealt with by overhangs, to a point.

Therein is the dilemma. Do you design a home from scratch that is completely dictated by solar gains/energy efficiency and aesthetics go out the window? Or do you design a home based on aesthetics alone and throw solar gains/efficiency out the window? I believe happiness is found in the middle and in the end a compromise must be met for each issue.

Another issue with overhangs is area wind codes. Some areas only allow for 24" overhangs. Remember, an overhang acts like a wind sail and catches the wind and the uplift forces want to peel away the roof via the overhang. I believe 24" is about average for an overhang, some go 36" but anything above that and it begins to look funky and engineering needs to be called in.

but anything above that and it begins to look funky and engineering needs to be called in

It only looks funky if you are trying to do something traditional and over stretching it to do something else. You may need to come up with a new concept to make it work. Engineering is nothing to be afraid of. For me, it has always been good money spent.

Posted By ICFHybrid on 08 Oct 2012 01:37 AM

but anything above that and it begins to look funky and engineering needs to be called in

It only looks funky if you are trying to do something traditional and over stretching it to do something else. You may need to come up with a new concept to make it work. Engineering is nothing to be afraid of. For me, it has always been good money spent.

I agree!


A 16" overhang can add almost 35% more uplift loads on a roof.  It's not only wind loads but gravity loads as well. The overhangs get hit with both positive and negative pressures. Uplift forces can be as high as 50 lbs per square foot. In high wind areas, I once heard a saying, "don't let the overhangs turn into Eaves of Destruction."   Don't forget that overhangs add to the possibility of ice-damming problems.
 

I believe in regards to SIP roofs, they have a 24" maximum overhang allowance.

Proper overhangs requires the architects involvement, the engineer and the local wind code to all work harmoniously.

As an alternative to very deep overhangs on the west and east sides, consider low-e glass, shades \ awnings, and deciduous (leaf shedding) trees.

In my next home, the west and east sides will have very large roof overhang on upper floor which will have a patio serving as overhang for the level below. Deciduous trees are already there. Will likely have exterior roller shades as well but mostly for efficiency and privacy, not for shading, since trees and overhang will take care of shading. South side will have only 2' overhang to maximize solar gain in winter.

Alexis-

You mention one of my favorite calculational tools, the overhang calculator at http://www.susdesign.com/overhang/.  You say that you cannot use it to obtain shading in the summer while still letting light in during the winter.  The model used in this software is just straightforward physics, and the calculations should work fine.  The calculated results, which I used to design my overhang on the south side, match well with what I observe in terms of window shading at various times of year.  

Let us establish some guidelines in the use of the program:

1.  Use the dates of the spring and fall equnoxes and the summer and winter soltices to represent the sun angle during spring, fall, summer and winter respectively.  So these dates are approximately March 21, September 21, June 21, and December 21, respectively.  Because of the thermal inertia of the earth, you might say summer should be represented by a later date than June 21, but the sun is highest in the sky on that date, so use that date.  Likewise, the coldest day of winter typically occurs after December 21, but that is the date when the sun is lowest in the sky, so use that date.  Start out doing the calculations for noon on the days of interest. 

2.  The calculations are easiest to picture for a house with sides that face north, south, east, and west.  If you rotate a house 45 degrees from this orientation, so that the "south" sides face southeast and southwest, then you cannot use overhangs as well to provide ideal shading.  Do not design a house or choose a house with this 45 degree offset orentation.  It makes it more difficult to use solar panels also.  

3.  You cannot use overhangs to provide ideal shading if you have floor-to-ceiling windows.  The physics does not allow it.  If you want to use passive solar heating with shading in the summer, do not include floor-to-ceiling windows in your design, or buy a house with that type of window already installed.  Likewise, you cannot use overhangs to provide ideal shading for a two-story house unless you offset the second story over the first story on the south side so that the offset becomes the overhang.

Please perform some sample calculations using these guidelines and share some specific results with us for these dates.  Specify the window size and overhang.  Also provide your latitude.

This leads me to believe that the only way to have real control on the south side of a passive building is to use a retractable awning like they use on RV. The problem is that I'm not convinced of the durability of those!

This leads me to believe that the only way to have real control on the south side of a passive building is to use a retractable awning like they use on RV

I am at 49N and I have good control without retractable awnings.

You cannot use overhangs to provide ideal shading if you have floor-to-ceiling windows

You are right, it is more difficult, but it also depends on where the floor and ceiling is. For example, I think my walls on the south are substantially higher than normal, but we still achieve control. I have some large french doors with glass nearly to the floor and some insolation does sneak in there during hot periods, but not for very long. What you get in the winter is far greater than the Summer burden.

Likewise, you cannot use overhangs to provide ideal shading for a two-story house

You can design a little roof that acts like an awning, but looks like a roof for the bottom windows. The structure also provides a great place to mount screens

I used the tool and my passive solar design works great. Yes, the sun waxes and wanes in the room as the seasons change, and occasionally I have too much radiant heat in spring and too little in fall. That's why you need thermal mass as well. The only discomfort I have experienced is sitting in direct sun; move three feet end of problem. As Lee says you can tune passive solar by lengthening overhangs, shortening windows or adjusting the space between them. My eaves are 30 inches -- 24" overhang plus 6" gutters. The trusses extend to the edge of the roof and I installed hurricane ties with four 2.5" screws.

BTW, if you model your house with google sketchup, you can use google earth to see exactly how the sun enters at any point in the year.

Here is an example of a roof that you can build onto the side of a two-story wall that can shade windows on the first floor.  An added benefit is that you can hang sunshades from the beam between the posts to completely shade the windows, or, in this case, windowed doors.  Very useful on the West side of buildings.

Posted By Alexis on 08 Oct 2012 02:11 PM
This leads me to believe that the only way to have real control on the south side of a passive building is to use a retractable awning like they use on RV. The problem is that I'm not convinced of the durability of those!

A cloth awning that is subjected to winds and rain will get destroyed pretty quickly. Steel or aluminum exterior shutters are the best but they are the most expensive and aesthetically it can be difficult to hide the assembly without looking like a prison or militia compound. I've seen some good details where they recess the roll-up shutters partially in the wall cavity but it requires some professional trades to do that detail properly.

In the end, the easiest energy efficient design is a box. While not the most appealing and it won't win any aesthetic awards, the square looking home is the easiest to design and to make energy efficient, especially with passive solar. One can hire a good architect and design an energy efficient home with passive solar that doesn't resemble a shoebox but it will take better architecture skills and more money. You will still have to compromise somewhere on either design or efficiency. Ideally both energy design & aesthetics will be allowed to express themselves in the homes design. If all you want is 100% passive solar, aesthetics inconsequential, all you have to do is draw a rectangle and poke a few holes in the south upper portion wall, and call it a day. If aesthetics matter and you want a design that reflects the area and your style, then it will take a lot more work and a good architect to complete the project.

I would encourage you to find some books on passive solar design, many of which have examples of house plans incorporating solar. You will see that it can be incorporated in just about any style, including soffit-free cape cods. This website will get you started: http://www.builditsolar.com/Projects/SolarHomes/guidesps.htm

My high mass house cost less than conventional construction. The foundation is slab on grade (no basement) and the concrete floor is finished with dark green ceramic tile. If control is your thing, and you don't want thermal mass, consider an attached sunroom/greenhouse. Put a blower on a thermostat to move heat into the house during the day; close the door at night.

Another tool to design overhangs is Google sketchup. It is a free download and you can plug in the location Lat/Long. Then the shading tool will show you any time and day exactly where to sun/shadow is.

There are some good heat loss analysis, passive solar design (including roof overhang design), hydronic radiant floor heating design, and integrated heating system performance calculators at this website:

http://www.borstengineeringconstruction.com/Calculators.html

There are also calculators for water work projects (e.g., cross flow turbine, water wheels, hydraulic ram pumps, ditch/pipe flow rate capacity, etc.)

We used the same program.Works like a charm during Winter and Summer. The issue comes to play at the mid point. March 21 is still cool but Sept 21 is still warm. The sun angle is the same both times. Our South facing windows are approx 8" below our 30" wide overhangs. As of recently, we are getting 100% of the sun. Our concrete floors hold the heat well. On sunny days with night time lows at 30 deg, heat is not needed. see attache photos; Patrick T

Patrick-

Sure, because of the thermal inertia of the earth, the fall equinox is warmer than the spring equinox. Window shades or blinds are required in the fall to reduce the solar gain if you want the solar gain during the spring.

Posted By Lee Dodge on 09 Nov 2012 07:14 PM
Patrick-

Sure, because of the thermal inertia of the earth, the fall equinox is warmer than the spring equinox. Window shades or blinds are required in the fall to reduce the solar gain if you want the solar gain during the spring.

Good points. That is why June is summer solstice but the highest temps are usually recorded in late July or early August. Winter solstice is in December but the coldest temps are usually recorded in late January.

It takes like 30 days or so for the earth to react to the suns angles. Sort of like concrete and thermal mass. It is slow to react to changes in heat and cold.

Lee I understand why the mid point is tricky. The original poster may have played with this program and found spring and Fall are a bit tricky. We chose to go with the heat gain in Spring and deal with the potential over heating in Fall. Our experience has been, Fall is a good time to have the windows open to cool it down in the event of a warm day or two. Also, AC is lower cost generally than heating. Sunny days to all Patrick T

Posted By PatrickT on 11 Nov 2012 07:35 PM
Lee I understand why the mid point is tricky. The original poster may have played with this program and found spring and Fall are a bit tricky. We chose to go with the heat gain in Spring and deal with the potential over heating in Fall. Our experience has been, Fall is a good time to have the windows open to cool it down in the event of a warm day or two. Also, AC is lower cost generally than heating. Sunny days to all Patrick T

There really is no way to prevent the overheating issues in spring and fall without installing exterior roll-up storm shades. Which most people will not do because of aesthetics and costs, except maybe those living in hurricane areas. Interior shades help but stopping the suns rays BEFORE they hit the window is the best way.

Like you mentioned, opening windows will help. Usually in the fall it will cool down after sunset and having the window open for a few hours will cool down the house. Even in the daytime it will help. When it 76F outside the other day, the glazing heated up the interior to 85F, I opened a window which slowly brought the interior temps down to 78F.

It can get bad if you have an unseasonable hot spring or fall, which might force you to turn on the A/C due to passive solar overheating. I've spoke to a few people who had passive house designs and they had to do this on a couple of occasions. It's not an exact science because spring and fall temps can be higher than average. When designing for passive solar, summer and winter equinox are the parameters and of course due south positioning.

What I've done is create a powerful solar chimney that can evacuate the hot air fairly quickly. I've got a thirty foot high atrium that is open to the passive solar area. It collects the overheated air. It has a motorized opener at the top and if the temperature rises above what the HRV can move out, the window opens and the hot air is flushed out. Here in W Washington, the overheating starts to occur at about 5 pm and its not too long before the evening air can bring some relief.