Environmental Analysis with Ladybug

Every Ladybug study starts with an EPW file - an EnergyPlus Weather file, a plain-text record of one representative year for a specific location: 8,760 hourly rows (365 days times 24 hours) of dry-bulb temperature, relative humidity, wind speed and direction, sky cover, and the direct and diffuse solar radiation that lets Ladybug reconstruct exactly where the sun was. You download the one nearest your site (Ladybug's own interactive map, or the DOE / climate.onebuilding.org archives), drop the path into an EPW file path panel, and feed it to the Import EPW component. Out of that one wire come dozens of hourly data streams you can plot, filter and map.

The mental shift is that weather stops being an abstraction ('it's hot here') and becomes data on your model. A Delhi EPW and a London EPW produce visibly different sun paths and radiation maps, and the same facade will light up differently under each: the sun sits far higher in the tropics, so the shading logic that works in northern Europe can be actively wrong nearer the equator. Because it is just data, you can also slice it with Ladybug's Analysis Period and boolean masks: analyse only the cooling season, only occupied hours, only the afternoons, only the hours above 28 C - the specific questions a designer actually asks rather than a vague yearly average.

Two honest caveats up front. First, a typical-meteorological-year EPW is a statistical composite assembled from many past years, not a forecast and not last Tuesday - it is the right tool for design decisions and for comparing options, and the wrong tool for promising a client an exact temperature on an exact date. Second, the file describes a weather station, which may sit at an airport with a different microclimate to your dense-city or hillside site; pick the closest station in a genuinely similar climate, and treat the numbers as well-grounded guidance rather than gospel.

The SunPath component turns the EPW's solar data into the familiar dome of arcs - the sun's track across the sky for every hour of the year, positioned correctly for your latitude and orientation. On the Grasshopper canvas it is not a printed chart but live geometry: sun vectors and sun points you can pipe straight into shadow, shading and view studies. You can even colour the sun points by any EPW value - temperature, for instance - to see the sun positions that coincide with your overheated hours, which is precisely the sun you most need to block.

With those vectors you can do the thing environmental textbooks only describe. Take the actual sun position on 21 June at 3 pm, cast your building's shadow, and see whether the courtyard is shaded when it matters. Sweep a shading fin's depth and watch the summer sun get blocked while the low winter sun still slips underneath to warm the room - the logic behind every good brise-soleil, now measured rather than guessed. Because the sun path is wired to your geometry, you are testing shading on the real form, at real times, for the real site, and the moment you change the fin the shadows update.

The design payoff is direction, literally. A south facade (in the northern hemisphere) faces the high midday summer sun and responds well to horizontal shading - overhangs and light shelves; east and west facades face a low, blinding morning and evening sun that skims in almost horizontally, which horizontal fins barely touch and which usually calls for vertical fins, deep reveals or a perforated screen. North facades (again, northern hemisphere) get little direct sun and can often be glazed generously for even, glare-free light. You do not have to remember these rules abstractly - Ladybug shows them to you, on your building, so the shading device you draw next is an informed response to this site rather than a transplanted habit from another climate.

The signature Ladybug study is incident radiation. The IncidentRadiation (radiation analysis) component takes your geometry as a mesh, the EPW's sky (via a Cumulative Sky Matrix), and a time period, and colours every face by how much solar energy - in kWh/m2 - falls on it over that period. The result is a heat map painted onto the building itself: hot reds where the sun pours in, cool blues where it rarely reaches.

Here is a worked reading. Suppose the study reports a peak of, say, 720 kWh/m2 on the upper south-west glazing over the cooling season and only 180 kWh/m2 on the north wall. You do not treat those as absolute truths to two decimal places - they are a relative map. The message is that the south-west upper glass takes roughly four times the solar load of the north, so that is where a shading device, a lower glazing ratio, or a solar-control glass earns its keep, while the north can be glazed generously for daylight without a heat penalty. One number in isolation means little; the pattern across the surface is the design brief.

Read correctly, that map is a brief. High radiation on glazing in a hot climate is a cooling-load and glare problem to shade or reduce; the same intensity on an opaque wall might be an opportunity for solar panels. In a cold climate you may want that winter gain and design to invite it. The map does not decide for you - it tells you where the stakes are, so your shading, glazing ratio and material choices land where they matter instead of being spread evenly out of caution.

The deeper move is optimization-in-the-loop. Because the radiation number is a live output, you can wire it to a slider - louvre depth, panel rotation, an attractor that opens and closes a screen - and watch the total incident radiation drop as you tune. Later modules push this to automatic search (Galapagos, Wallacei); here the value is simply that the analysis and the geometry are the same live model, so improving one improves the other in real time.

Climate is not only sun. From the same EPW, Ladybug builds a wind rose - a radial plot where each petal's length shows how often wind blows from that direction and its colour shows the speed band. Read it and you know which way to open windows for cross-ventilation, which cold direction to buffer with a solid wall or planting, and where an outdoor terrace will actually be pleasant rather than perpetually gusty. You can filter it, too: a summer-only wind rose might reveal a reliable evening sea breeze worth capturing, while the annual rose is dominated by cold winter winds you would rather exclude - two different design briefs hiding in one dataset.

Ladybug also draws the psychrometric chart, plotting your climate's hours against temperature and humidity and overlaying comfort zones and passive-strategy polygons. It answers a genuinely useful question: for this climate, which single move - shading, thermal mass with night flushing, natural ventilation, evaporative cooling, passive solar gain, humidification - would drag the most hours into comfort? In a hot-dry climate the chart might point emphatically at mass and night ventilation; in a warm-humid one at shading and air movement. That is passive-design reasoning made visual, and it sets the hierarchy of what your architecture should do before any mechanical system is even considered.

Now the boundary, stated plainly. A wind rose is regional weather at a station, not airflow through your building - real cross-ventilation, stack effect and pedestrian wind at the base of a tower need CFD (computational fluid dynamics), and detailed thermal behaviour needs the whole-building simulation you will meet in the next lesson. Ladybug is superb at situating a design in its climate and steering the big early decisions - orientation, massing, where to open and where to close - but it is not a substitute for engineering analysis or code compliance. Use it to make better choices sooner, and let the specialists, and ultimately the certifying engineers, do the rigorous confirmation later.