Part IX: Tools, Calculators, and TestingChapter 15. Design Tools and Data
Modeling Projects in 3D (SketchUp/FreeCAD) for Shading
A lightweight modeling workflow for overhangs, trees, and the winter shadow of a neighbor.
- Difficulty
- Intermediate
- Cost
- Free
A three-dimensional model earns its keep when it shows where a shadow falls on glass or on a collector at a few hours you care about. SketchUp and FreeCAD can both place a sun. What you want from either one is a shadow you have checked against a tape measure and against true south.
Set the location and the date, then look at winter solstice and summer solstice, morning and noon and afternoon. Overhangs are the ratio guide. True south, which a compass will not give you, is the compass guide. A year's sun on the plane, as numbers rather than shadows, is the radiation guide. Examples in the links below are other people's models. This is the checklist.
Difficulty
- Level
- Intermediate
- Time
- An evening once the heights are measured
- Skills
- Setting north before you trust a shadow
Cost estimate
Free
Free versions of the modelers are enough. A tape, a way to estimate a tree height, and the patience to set true north are the actual cost. A beautiful render of the wrong north is free and useless.
Materials
| Item | Qty | Notes |
|---|---|---|
| Measured heights, not guessed ones | 1 set | Eave, neighbor wall, tree. A photo is not a dimension until you scale it from something you taped. |
| True south on the model | 1 | The software's north has to match the site. A default axis is a coin toss. |
| Solstice dates and a few hours | A handful | Winter noon, winter morning, summer noon. More hours if the view is blocked by something tall and close. |
A small model
Draw the glass or the collector, the overhang or the eave above it, and every mass that can cast a shadow: the neighbor's house, the chimney, the tree at the height it is now and a note about the height it will be. You do not need furniture, textures, or a sky background. They only make the shadows harder to read.
Set geographic north to true north. Magnetic declination is why a compass needle is the wrong axis. The compass guide has the correction. If the program wants a latitude, use yours, and remember this book's south-glass habit is a northern-hemisphere habit. South of the equator the winter sun is in the other part of the sky and the solstice that matters for heat swaps roles. Do not import a northern overhang into that climate and feel done.
Run the sun for the winter solstice around noon and again a couple of hours off noon. An overhang that shades the glass at noon in December is too deep for a heating climate. An overhang that leaves the glass bare at noon in June is too shallow for comfort. The ratio guide is the numerical version of that picture. The model is how you see a tree the ratio guide cannot see.
What the picture is allowed to decide
Use it to move a window, shorten an overhang, or give up on a collector location that is dark at the hour you needed. Do not use it to publish a kilowatt-hour. Shade the photovoltaic plan with the hours the model found, then let the production calculator apply a loss. A shadow that covers one corner of a string is the leaf problem, not a gentle percentage.
Check the model against the real sun once. On a clear day near a date you simulated, look at where the shadow of the eave falls and compare it to the picture. If it is off by a foot, the north axis or a height is wrong. Fix that before you believe a tree.
Composite: the missing eave
Invented model. The house is careful, the overhang is careful, and the neighbor's taller wall was never drawn because it was "just next door." In December the real shadow crosses the glass by mid-afternoon. The composite file looked sunny because the sun had an empty sky. Measure whatever blocks the sun, including what you do not own. The composite is not a real street. File examples are in the further-reading list.
Tape the heights that can cast shadows
Your eave, their wall, the tree. Scale a photo only from a measured length.
Set true north and the latitude
Compass north is the wrong axis until you correct it. Southern-hemisphere seasons are not this book's default.
Look at solstice mornings and noons
Winter heat and summer shade. Compare the noon picture with the overhang guide.
Check one real shadow
If the eave shadow disagrees with the model, fix the model before you move a wall.
Further reading
These links go to outside sources. This book cites them by link and does not copy their words or pictures.
- https://www.builditsolar.com/References/Calculators/
- https://www.builditsolar.com/References/Source/TMYWeather/TMYWeather.htm
- https://www.builditsolar.com/References/Source/RadOnColProgram/RadOnCollector.htm
- https://www.builditsolar.com/References/SketchUp/SketchUpEx.htm
- https://www.builditsolar.com/References/Suppliers/SolarHeating.htm
- https://www.builditsolar.com/References/HowMuch/HowMuch.htm
- https://www.builditsolar.com/References/BestRefsRS.htm
General education, not a permit or a stamped design. Follow the safety note in the guide, read the safety disclaimer, and hire a licensed trade when the work requires one.