Brise soleil and green walls in a daylight model

A brise soleil is part of the building, so it belongs in the daylight model as geometry rather than as a note on the elevation. Brise soleil daylight modelling goes wrong in two directions: shading is left out of the model entirely, so the reported figures describe a building that will never be built, or it is approximated as a transmittance reduction, which loses the directional behaviour that is the whole point of the device. Green walls raise a related problem, because they change over the life of the building while the model captures one moment.

Why shading devices have to be modelled as geometry

Start with the geometric tests, because shading affects them and the effect is often overlooked.

Vertical sky component is measured at the centre of a window from a vertical plane, and any projection above that window reduces the amount of sky visible from it. A horizontal brise soleil, a deep reveal, a balcony above or a projecting fin all cut into that measurement. Where the device sits on the proposed building and the assessment concerns a neighbour, the projection also increases the obstruction the neighbour sees, so it has to appear in the massing model used for their assessment as well as in the model of your own rooms.

For internal illuminance, whether through the average daylight factor approach or through climate based modelling, the device changes both how much light arrives and where it arrives from. That directional character is why a proxy is unsatisfactory. A louvre array that blocks high angle light and admits low angle light is not the same as a sheet of glass with reduced transmittance, even where both produce the same annual average.

The practical rule is simple. If a device would appear on a section drawing, it belongs in the model. If it is a product with published dimensions, use them.

Fixed louvres, fins and their real dimensions

Modelling a louvre array properly requires the things architects sometimes have not decided yet: blade depth, blade thickness, vertical or horizontal spacing, blade angle, profile shape, and the finish.

Four of those matter more than the rest. Blade depth and spacing set the cut-off angle, which determines what is excluded. Blade angle decides whether the device is symmetrical in its behaviour or biased towards one part of the sky. And finish reflectance decides how much of the intercepted light is redirected inward rather than absorbed; a pale, matt underside on a horizontal blade returns useful light into the room, while a dark anodised finish absorbs it.

Where the design is not fixed, model the worst reasonable case and say so. A model run on a preliminary louvre specification that is then value engineered to deeper blades at closer centres will produce results the finished building cannot achieve, and the discrepancy usually appears at technical design when nobody is looking at the daylight report any more.

Vertical fins behave differently again. They have little effect on a south facade and a substantial effect on east and west elevations, and on a north facing room they mainly remove diffuse light without any solar benefit, which is worth resisting. The starting position for a north facing room is already constrained, as daylight to a north facing habitable room sets out, and adding fins to a north elevation for visual consistency is a trade worth questioning.

Seasonal behaviour against annual metrics

Fixed shading is designed around the sun's altitude, and the metrics used to assess daylight are annual. The two do not line up neatly.

A horizontal device sized to exclude high summer sun will admit low winter sun, which is usually the intention. Annual daylight metrics in the European standard, expressed as a target illuminance achieved over a proportion of daylight hours, average across the year and can understate both the summer penalty and the winter benefit. Sunlight metrics, measured as probable sunlight hours including the winter proportion, capture the seasonal story more directly, and reporting both gives an officer a fuller picture.

Where the assessment concerns amenity in a historic setting, the argument is often about the device's appearance rather than its performance, and the framing in daylight assessment in a conservation area context is the relevant one.

One honest uncertainty. There is no settled convention on how much weight to give a seasonal profile when the headline metric is annual, and practice varies between consultants. Presenting the annual figure, the seasonal breakdown and the assumptions is better than choosing whichever framing flatters the scheme.

Green walls, growth and the maintained condition

A green wall is not a static surface, and a model has to choose a moment in its life.

Model the maintained mature condition rather than the day of installation. A living wall system reaches a designed coverage within a few growing seasons, and the difference between bare substrate panels and full coverage is substantial. Foliage reflectance is low, typically well below the values used for render or light masonry, so a green wall on an obstruction opposite reduces the light reflected into a room, and a green wall on your own facade around a window opening reduces the effective aperture as growth encroaches on the reveal.

Three details are worth specifying before modelling:

1. The designed coverage and the species mix, since evergreen and deciduous systems behave differently through the year.
2. The maintenance regime, because a trimmed edge line around openings is what keeps the aperture clear, and it has to be funded.
3. The depth of the system, including substrate and irrigation, which reduces the reveal depth available and can turn a generous reveal into a shallow one.

Free standing planting is treated differently. Guidance on trees and daylight generally declines to treat deciduous trees as permanent obstructions, on the basis that they admit light in winter, and that logic does not extend to a building mounted evergreen system. Where a scheme relies on planting for screening and on daylight figures calculated without it, say which condition each figure describes.

The solar gain case pulling the other way

Shading exists mainly to control solar gain and overheating, and the two assessments want opposite things.

The cooling hierarchy expects passive measures, including external shading, to be demonstrated before mechanical cooling is proposed. So the Overheating Risk Assessment has an interest in deeper devices, and the daylight assessment has an interest in shallower ones. The resolution is to design them together and to report the pair: the daylight results with the shading as specified, and the overheating results with the same geometry.

A scheme where the overheating model assumes external shading that the daylight model omits, or where the daylight model assumes clear glazing that the thermal model treats as solar control, is internally inconsistent. Officers and consultees now compare the two documents, and the inconsistency reads as either carelessness or selective reporting.

Where shading has to be deep, look for compensations in the daylight case: a larger opening, a higher head height, a pale soffit and reveal, and a shallower room. Those moves recover more than tinkering with blade angles.

Devices added after the assessment

The commonest failure in this area is chronological. The daylight and sunlight assessment is prepared, the scheme is approved, and shading is then added during technical design to satisfy the overheating strategy or the facade contractor's preference. Nobody re-runs the daylight model.

Two consequences follow. The approved drawings may no longer describe the building, which raises an amendment question. And the internal daylight the occupants receive is lower than the figures on which the permission was granted, which matters if a purchaser or a future officer looks.

Build the check into the process. Where shading is likely to be added, model it at application stage as a sensitivity case, and say in the assessment that the results hold for shading up to a stated depth and spacing. That gives the design team headroom and gives the authority a baseline. Where a condition requires shading details to be approved, ask for wording that requires the daylight position to be confirmed alongside them.

Documenting the modelled geometry

The assessment should state, for every shading element and green wall:

Its dimensions, spacing, angle and profile, taken from the specification or from a named product. Its surface reflectance, and whether the finish is matt or specular. Whether it was modelled explicitly or as a proxy, and if a proxy was used, what it was and why. Which rooms it affects. And a sensitivity run without the device, so the effect of the shading is visible as a number rather than as a claim.

Add a drawing. A section at each typical facade condition, showing the device against the window head, reveal depth and room depth, lets a reviewer check the geometry in the model against the geometry in the design, which is the check that actually finds errors.

A Daylight and Sunlight Assessment and, where neighbour amenity is in issue, an Overshadowing Study should both carry the same geometry. PlanScope drafts these documents structured to the tests councils apply, with modelling inputs listed as fields to complete rather than assumed, so the specialist supplies the model and the results. Councils normally expect a suitably qualified author for this work. Reports should be reviewed before submission, and PlanScope does not guarantee planning approval.

This is general guidance, not legal advice. Check the current BRE and European standard guidance, and your authority's policies, before relying on it.