How the two differ in kind
External shading is the most effective way to control solar gain, because it stops the energy before it passes the glass — once sunlight is inside, an internal blind can only convert it to heat in the room. The question is what form that external shading takes.
A FIXED OVERHANG shades by geometry, and the geometry it responds to is SUN ALTITUDE: how high the sun stands above the horizon. On an equator-facing facade that produces close to ideal behaviour, because altitude is high in summer, when the overhang's shadow reaches down the glass and excludes the sun, and low in winter, when the sun passes beneath it and the gain is welcome. It needs no maintenance, no controls, no power, and no user, which is why it is the first shading device to consider on that facade.
It also has a limitation people are consistently surprised by. On an EAST or WEST facade the sun is low whenever it is on that side of the building — rising and setting — so it arrives almost horizontally and comes in under any horizontal projection, however deep. Horizontal overhangs do not shade east and west facades. Those elevations need vertical fins, movable devices, or glazing selection instead, and the overhang that works beautifully on one wall is decorative on another.
The second limitation applies even where it works. Sun altitude at a given date is symmetric about the solstice, so an overhang sized to admit sun from the spring equinox onward admits exactly the same sun at the autumn equinox — and autumn is generally still warm while spring is generally still cool. A fixed device cannot distinguish two days with identical geometry and different weather, and it cannot respond to an overcast day, to a heatwave, or to a room that happens to be occupied.
ADJUSTABLE LOUVRES solve exactly that. Blades that rotate — manually, or automatically on a sensor and controller — can be closed against a hot afternoon and opened for daylight on a dull one, and they can work on any orientation, including the east and west facades a fixed overhang cannot. What they cost is maintenance, mechanism, control, and the risk that any of the three stops working.
The factors that actually differ
Show
| Fixed overhang | Adjustable louvres | |
|---|---|---|
| What it responds to | Sun altitude, through fixed geometry. | Whatever the control responds to — sun position, irradiance, temperature, occupancy, or a person. |
| Equator-facing facade | Ideal. High summer sun is excluded, low winter sun is admitted, automatically and forever. | Also works, and the added control buys relatively little on this orientation. |
| East and west facades | Ineffective. The sun is low whenever it is on that side and comes in under any projection. | This is where they earn their place, along with vertical fins. |
| Seasonal symmetry | Cannot distinguish spring from autumn — the same sun angle produces the same shading on a cool April day and a warm September one. | Responds to conditions rather than to date, which resolves it. |
| Maintenance | None beyond the building fabric it is part of. | Real: moving parts, actuators, sensors and a control system, all of which can fail. |
| Failure mode | None — it cannot stop working. | Stuck open, stuck closed, or a controller nobody understands. A failed system is frequently left in one position for years. |
| Daylight | Blocks a fixed portion of the sky permanently, including on an overcast day when the daylight is wanted. | Can open for daylight and close against sun, which is the main daylighting argument for them. |
| View | Unobstructed. The overhang is above the glass. | Blades across the view when closed, which occupants notice. |
| Cost | Low, and often part of the roof or a balcony that exists anyway. | High — the system, its controls, its commissioning and its maintenance contract. |
| Wind and snow | A structural projection, designed for uplift and for snow load where it applies. | Usually retracts or is designed to a wind limit, which the control has to honour — a system that fails to retract is a failed system. |
Which one, and when
Choose fixed overhang when…
- The facade faces the equator, where the geometry does the work by itself.
- There will be no maintenance regime, and a device that cannot fail is worth more than one that performs better when it works.
- There is already a roof overhang, a balcony or a walkway that can be sized to do the job.
- Simplicity and cost are the drivers and the orientation suits it.
Choose adjustable louvres when…
- The facade faces east or west, where a horizontal overhang does nothing.
- Daylight matters and a fixed device would block sky on the overcast days when it is wanted.
- The building is actively managed, with a maintenance regime and somebody responsible for the controls.
- Shading needs to respond to conditions rather than to date — a heatwave in spring, an occupied room, a cloudy afternoon.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Why doesn't a horizontal overhang work on a west facade?
- Because the sun is low in the sky whenever it is on that side of the building. A horizontal projection shades by intercepting sunlight arriving from above, so its effectiveness depends on the sun's ALTITUDE — and on a west-facing wall the sun is in view during the afternoon and evening as it descends, arriving at a shallow angle that passes under any overhang no matter how deep. The same applies to an east-facing wall in the morning. That afternoon western sun is also the hardest gain to deal with, because it arrives at the end of a day when the building has already warmed up. The devices that do work on those orientations are vertical fins, which intercept low sun from the side, movable louvres or blinds, and glazing chosen for a low solar heat gain coefficient on those elevations specifically.
- How deep should a fixed overhang be?
- Deep enough to shade the glass at the sun altitudes you want excluded, and no deeper than would block the ones you want admitted — which makes it a geometry calculation from your latitude, the window height and the dates you are designing to. The method is to establish the sun's altitude at the relevant dates and times for your location, then work out the projection that casts a shadow to the sill at the summer condition while allowing sun onto the glass at the winter one. Two things make the answer specific rather than general: latitude, which shifts every altitude, and the height of the window head below the overhang, since a projection shades a window immediately below it far more effectively than one further down. Deeper is not automatically better, because the same device blocks winter sun and permanent daylight.
- What is the spring-autumn problem?
- Sun altitude depends on the date's position relative to the solstice, and two dates equally spaced either side of it have the same geometry — so a fixed overhang shades identically in, say, April and September. The weather does not match: in most climates the spring date is still cool and solar gain is welcome, while the autumn one follows a summer of accumulated heat and the gain is not. A fixed device cannot tell them apart, because it responds only to where the sun is. The consequence is that any fixed overhang is a compromise between two seasons, and the sizing decision is effectively which of the two shoulder seasons to favour. Adjustable shading resolves it directly, and so does thermal mass with night ventilation, which handles the accumulated heat rather than the gain.
- Is external shading really better than internal blinds?
- Substantially, and the reason is where the energy ends up. External shading intercepts sunlight before it reaches the glass, so the absorbed energy is released to the outside air. An internal blind stops the light after it has already passed through the glass and entered the room — the blind absorbs it, warms up, and radiates and convects that heat into the space, which is now inside the thermal envelope with no easy route out. Internal blinds are excellent for glare control and privacy and are a weak solar control measure, typically rejecting a fraction of what an external device rejects. Mid-pane blinds within a sealed unit sit between the two in effectiveness. Where external shading is impossible, the effective alternative is the glazing itself, selected for a low solar heat gain coefficient on the exposed elevations.
- What maintenance do adjustable louvres need?
- Enough that the maintenance regime should be agreed before the system is specified, because an unmaintained automated shading system is the most common way this comparison resolves itself badly. Moving blades have bearings and linkages that wear and seize; actuators fail; sensors drift or get dirty; and control systems outlive the people who understood them. The characteristic outcome is a facade whose louvres are all in one position, having stopped years earlier, with nobody quite sure who is responsible. That is worse than a fixed overhang in every respect, since it costs more, blocks a fixed amount of light and provides no seasonal response. Where the building has facilities management and a maintenance contract, adjustable systems perform well; where it does not, the honest recommendation is the device that cannot fail.
- Do fixed overhangs help with daylight?
- They help with glare and they reduce overall daylight, which is a trade rather than a benefit. Excluding direct sun near the window head removes the harshest contrast and prevents the sharp patches of sunlight that make a space uncomfortable to work in — genuinely valuable. But the projection also blocks a portion of the sky permanently, including on overcast days when the sky is the entire light source and there is no sun to exclude. On a deep room that can push the daylight factor down and increase the use of electric lighting. The devices that address both are ones that separate the functions: a light shelf, which shades the lower window while bouncing light off its upper surface deeper into the room, or separating the facade into a view window with shading and a higher daylight window without.
- What about vertical fins?
- They are the correct answer for the orientations a horizontal overhang cannot serve, and they work on the perpendicular geometry. A vertical fin intercepts sunlight arriving from the side rather than from above, so it responds to the sun's AZIMUTH — its compass bearing — rather than its altitude, which makes it effective against the low morning and evening sun on east and west facades. Fins can be angled to favour a particular part of the day, which is why they frequently appear on a slant rather than perpendicular to the facade. Their costs are the view, which they obstruct more than an overhang does, and the daylight they block permanently. Many facades use both, with horizontal shading on the equator-facing elevation and fins where the sun comes in low, which is why shading strategies are usually orientation-specific rather than uniform.
- How does latitude change the answer?
- It changes every sun angle, which means an overhang depth taken from an example at another latitude is simply wrong. Nearer the equator, the summer sun is very high and a modest projection shades effectively, while the winter sun is also relatively high so admitting it is harder — and the seasonal swing that makes fixed shading elegant is smaller. At higher latitudes the summer sun is lower, so a deeper projection is needed to shade the same window, and the winter sun is very low, which makes it easy to admit and also means it penetrates deep into the room. High latitudes also have long summer days with the sun swinging well round to the north or south behind the building, which puts sun on elevations that never see it further from the pole. All of that is why the calculation takes latitude as an input rather than offering a rule of thumb.