Need Help With Passive Solar Design: Roof Overhang Over Clerestories

Hello Permies!

My husband and I are getting ready to build our home this year but I need help with one last design detail.

We're doing a passive solar design modified for our hot humid climate (Central Texas). I.E. More glazing on the north and roof coverings over the southern glazing.

We have 5 clerestory windows on the southern side that need shading from the roof overhang and I was wondering if someone could help me calculate how much overhang is needed.

I already calculated it myself but I'm an amateur at passive solar design so I was hoping someone else would do it as well so I can double check my number.

There are six windows pictured here but we're really doing 5.


Some numbers you may need to know:

Latitude 29.8

Windows: 5' wide 2' tall

Home is facing pretty much directly south, perhaps 3 degrees eastward.

I unfortunately don't know how far the windows are from the roof line but I used 6" for my measurement. I just asked the architect for more clear elevations here.

We would like 100% shading for September

This community is always so helpful! I greatly appreciate it :)

My roof overhang is two feet.

Here is a diagram that might help:



Source

This might help:

Overhang sizing rules:

   Draw the wall to be shaded to scale.
   Draw the summer sun angle upward from the bottom of the glazing.
   Draw the overhang until it intersects the summer sun angle line.
   Draw the line at the winter sun angle from the bottom edge of the overhang to the wall.
   Use a solid wall above the line where the winter sun hits. The portion of the wall below that line should be glazed.
   Design rooms to match the passive-solar design. Place rooms that have minimal heating and lighting requirements (such as garages and storage rooms) on the north side of the home. The kitchen is also a great choice for a room on the northern side because it produces its own heat. Keep in mind that furniture, rugs and tapestries will affect the thermal mass performance. Daylighting is an added benefit of passive solar design. Generally, a ratio of 5-percent glazing to floor area provides enough light for the room. Skylights admit light — but can offer unwanted heat in the summer. Solar tubes may be a good alternative.

Overhang Size Rules

Anne Miller wrote:My roof overhang is two feet.

Two feet? I was getting like 12-15” for mine!

How far is the window from the eaves and how tall are the windows?

The Clerestories windows are about 1.5 feet tall.

What I like about the larger overhang is that gives the house added shade.

Plus when I walk the dog it will shelter us from some rain.

Our windows open vertically towards the outside so this overhang allows us to leave the windows open when it rains. I monitor them though if the wind is blowing hard I close them.

I have lived for the last 40 years in a house that is equivalent to the back half of your house design.  Big angle single plane roof with a 3' 6" overhang off the front with clerestory.  My latitude is 44.48 and the total system is set up so the first tiny sliver of light comes in the 18th of Aug.  

First I will say you need way more information than you have provided to calculate it.  Window height from over hang, angle of back roof, how the glazing is built etc.

Second I will question your September?  Have you looked at when you quit needing heat in the spring too?  This house starts winter heating about 6 weeks early and is too warm for that part of the season but in the spring the sun goes out too soon and make up heat is needed for roughly the same 6 weeks because the sun has gone back out before the weather has warmed up enough.  So is your end of September properly splitting when you need heat in the spring vs heat in the fall?  If I were shuffling this one I would be adding remote control insulated shutters to block light and heat in the fall a bit so I could have the sun for longer in the spring.

Now there are a few things about how the glazing is built that would cause me to change.  My glass is centered in the wall and is vertical.  There are (4) standard 28" X 80" patio glasses laid on their sides and (3) 2'x2' awing style crank to open anderson windows that make up the clerestory row here.  I would be sure you had some high ventilation windows in that to let heat out. I have a roughly 10 foot pole here that I reach up to run the cranks on the windows with.  In my dream world I would have those automated too.

Another change would be the top edge of the exterior window frame.  It currently comes out square.  The problem is that it extends about 3 inches beyond the glass so even on the shortest day of the year it shades the top 1 1/2" inches of the glass at noon.  Since glass is 28" and I lose about an inch for mounting leaving 27" meaning I am losing 5% of my solar gain while it is still costing me that lost all night.  And when the sun is at a 45 angle I am losing 10% of my gain at noon.  So I would angle the top frame up to let more direct sun go thru the glass.  

Another change is I would angle the glass back slightly at the top and out slightly at the bottom to improve slightly the number of hours of the the sun makes it in the window..

I calculated just over 2 feet based on the solar elevation at 12 noon on September 30th being around 57°. I guessed at your roof pitch being ~18° (4:12). See sketch attached

C. Letellier wrote:
First I will say you need way more information than you have provided to calculate it.  Window height from over hang, angle of back roof, how the glazing is built etc.

Roof has a 4/12 slope.

C. Letellier wrote:
Second I will question your September?  Have you looked at when you quit needing heat in the spring too?  This house starts winter heating about 6 weeks early and is too warm for that part of the season but in the spring the sun goes out too soon and make up heat is needed for roughly the same 6 weeks because the sun has gone back out before the weather has warmed up enough.  So is your end of September properly splitting when you need heat in the spring vs heat in the fall?

I don't care about heat gain in the Spring because our average highs for September are in the 90s and I don't want to burn ourselves alive in our house We had an 80 degree Christmas this year to give you an idea.

C. Letellier wrote:
I would be sure you had some high ventilation windows in that to let heat out.

This is really the main reason for the clerestory windows in addition to natural light.

Aaron Yarbrough wrote:I calculated just over 2 feet based on the solar elevation at 12 noon on September 30th being around 57°. I guessed at your roof pitch being ~18° (4:12). See sketch attached

This is SO helpful! Thank you.

It appears I was over simplifying the mathematics of it by only being concerned for the lateral distance of the roof... but that doesn't necessarily work since the angle of the sun means the light will not just stop at the lateral distance of the roof but extend beyond. I knew I was doing something wrong but did not know what!

I'm going to go back in and play around with the numbers myself now that I realize this :)

You are also missing the effect of wall thickness.  If you work from this you have I think all the pieces.