Reflectance, transmittance and maintenance assumptions

Three input assumptions decide more daylight results than the massing does: internal surface reflectance, glazing transmittance and the maintenance factor. All three sit in an appendix, none of them appears in the summary table, and a modest shift in each can turn a failing room into a compliant one without a single line moving on the drawings.

A reviewer reading someone else's daylight report should go to the inputs before the results. If the inputs are not stated, that is itself a finding.

Which inputs change the result most

Daylight methods fall into two families. The geometric measures, vertical sky component and the no-sky line, depend almost entirely on obstruction geometry and barely at all on surface properties. The illuminance based measures introduced through BS EN 17037 and carried into the 2022 edition of BRE 209 depend heavily on reflectance, transmittance and maintenance.

That split matters, because a report can show a room failing a geometric test while passing an illuminance test, and the difference may sit entirely in the modelling assumptions rather than in the daylight reaching the window.

Sensitivity, roughly in order:

• Glazing transmittance, because it scales the admitted light directly
• Internal surface reflectance, through the interreflection term, which matters most in deep rooms
• Maintenance factor, a straight multiplier commonly worth around 10 per cent
• Frame factor, the proportion of the window opening that is actually glass
• External ground and facade reflectance, which matters most on lower floors in narrow streets

Change all five optimistically and a room can gain a third of its modelled illuminance. Nothing in the report would look unusual.

Internal surface reflectance and standard values

The conventional set used in UK practice is 0.70 for ceilings, 0.50 for walls and 0.20 for floors. Those figures describe a light painted room with a mid tone floor finish, and they are the values a reviewer should expect to see unless there is a stated reason to depart from them.

Departures to watch for.

A wall reflectance of 0.60 or 0.70, which assumes near white walls across the whole room including any joinery, radiators and furniture. Possible on a show flat, optimistic for a dwelling.

A floor reflectance of 0.40, which assumes a pale timber or light tile throughout. A dark floor can be as low as 0.10, and residents choose their own flooring.

A single reflectance applied to a room with an internal partition, a kitchen run or a wardrobe wall, where the real average is lower.

Deep rooms are where this bites hardest. In a room 8 m deep lit from one end, the rear is lit largely by interreflected light, so the modelled result at the back of the room is substantially a function of the reflectance assumption. Ask for a sensitivity run at standard values before accepting a compliant result in a deep plan.

Glazing transmittance for different build-ups

Transmittance should match the specified glazing, and the value used should be the diffuse or hemispherical figure rather than the normal incidence figure, because daylight arrives from the whole sky.

The common finding is a model run at clear double glazing while the energy strategy specifies low emissivity coated units with a solar control layer to answer overheating. Those two documents frequently contradict each other, and the daylight report is usually the optimistic one. Cross check the glazing specification in the energy and overheating submissions against the daylight appendix every time.

Obscured, patterned or fritted glass to bathrooms and to overlooking sensitive elevations has lower transmittance again, and fritting applied as a bird safe measure or a solar control measure is often omitted from the daylight model entirely.

Maintenance and dirt factors

The maintenance factor accounts for dirt on the glass over time. For vertical glazing in a clean, non industrial setting a figure of around 0.9 is conventional, with lower values for a dirty urban or industrial location and lower again for sloping or horizontal glazing, which soils faster and drains less.

Two failings recur. The factor is omitted altogether, which is worth roughly a 10 per cent overstatement. Or it is applied at 0.9 to rooflights and atrium glazing, where the appropriate figure is materially lower.

Ask one practical question of any scheme with high level or inaccessible glazing: how will it be cleaned? Where there is no safe access and no cradle, the real maintenance factor over the life of the building is worse than any standard value, and that is a legitimate point to put in a review.

External ground and facade reflectance

Reflected light from the ground and from facing buildings matters most to lower floor rooms in narrow streets, which are usually the rooms under scrutiny.

Ground reflectance is conventionally taken at around 0.20 for a typical mixed surface. A model using 0.30 or 0.35 is assuming light paving across the whole street. Facing facade reflectance is also conventionally around 0.20, and a model using 0.40 is assuming light render or glazing on every obstruction.

Check two things. Whether the assumed external reflectances are stated at all, and whether they are consistent with the photographs in the site appraisal. A red brick terrace opposite is not a 0.40 surface.

Where the baseline itself is poor, the choice of inputs can decide whether the existing condition is modelled as acceptable, which changes the comparison entirely. We look at that problem in a daylight assessment with a poor existing baseline.

Documenting inputs in an appendix

A defensible report states, in one table:

1. The method and the standard applied, with the edition and year.
2. Weather file and location used for climate based modelling.
3. Reflectance for ceiling, walls, floor, external ground and each obstruction surface.
4. Glazing transmittance by window type, with the build-up it corresponds to and the source of the figure.
5. Frame factor, or confirmation that the glazed area is modelled explicitly.
6. Maintenance factor by glazing orientation.
7. Room use assumed for each space assessed, since the target illuminance differs between bedrooms, living rooms and kitchens.
8. Grid height, grid spacing and the proportion of the room area assessed.
9. Any obstruction modelled as a block rather than from survey, and the source of its geometry.

Where geometry comes from a measured survey rather than from assumed massing, say so, because reveal depth and balcony geometry change results more than most inputs. We cover that in daylight model accuracy, survey, massing and reveals, and the balcony problem in balconies and daylight failure in new flats.

Spotting an optimistic model in someone else's report

A short checklist for a reviewer with an afternoon and a PDF.

Look for the input table first. If there is no input table, ask for one before engaging with the results, and say in your response that the assessment cannot be verified as submitted.

Compare the glazing transmittance against the energy statement and the overheating assessment. Inconsistency between the three is the most common single finding.

Check whether a maintenance factor has been applied, and to what.

Look at reflectances against the room layouts. A 0.5 wall reflectance in a room with a full height wardrobe wall and a dark kitchen run is generous.

Ask whether balconies, external walkways, deep reveals and any projecting fins are in the model. Each of them reduces results, and each is occasionally absent.

Check the room use assumed against the plan, because a space labelled as a bedroom carries a lower target illuminance than the same space used as a living room.

Request one sensitivity run at standard values. A scheme whose compliance depends on non standard inputs will show it immediately, and a scheme that is compliant on standard inputs loses nothing by demonstrating it.

The Daylight and Sunlight Assessment page sets out what the report should contain. Councils expect a suitably qualified author and professional review is strongly advised, because a daylight result that cannot be reproduced from the stated inputs is of little use to a decision maker.

Enter your postcode on PlanScope to see which amenity documents a scheme of this kind is likely to need.

This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.