Reconciling daylight with solar gain in glazing design

Two assessments now sit on every residential scheme and they pull in opposite directions. Daylight assessment wants more glass. Overheating assessment, under Approved Document O and the dynamic modelling route behind it, wants less solar gain. Run them sequentially and the second one undoes the first, usually at technical design stage when the elevations are already fixed.

The resolution is not a compromise on glazing area. It is a different set of levers: glass selectivity, external shading geometry, room depth and window head height. Those four settle the conflict in most flats, and they have to be set at concept stage.

Why the two objectives pull apart

Daylight inside a dwelling is assessed against illuminance-based criteria drawn from the European daylight standard and reflected in the current edition of the BRE guidance, which replaced the older average daylight factor approach. The measure rewards light reaching the back of the room across a representative part of the year.

Overheating is assessed under Approved Document O by one of two routes: a simplified method that caps glazing area by orientation and requires a minimum free area for removing excess heat, or dynamic thermal modelling following the relevant CIBSE methodology for homes. Both penalise solar gain, and the modelling route penalises it hour by hour through a summer season.

The conflict is real but it is narrower than it first appears. Daylight cares about diffuse skylight and about how deep into the room it reaches. Overheating cares about direct beam solar radiation at particular times of day on particular orientations. Those are separable, which is why shading geometry works.

Glazing area and orientation

Glazing area is the blunt lever and the one most schemes reach for first. It is also the one that produces the worst outcome, because reducing area cuts diffuse daylight everywhere while only partly addressing gain at the hours that matter.

Orientation does more work. A south-facing window receives high-angle sun in the middle of summer days, which a horizontal overhang can intercept almost entirely while leaving the sky view below it intact. East and west-facing windows receive low-angle sun morning and evening, which no horizontal element will block, and which drives most overheating failures in flats with single aspect east or west living rooms.

North-facing rooms have the opposite problem: no gain to control and not much light to work with, which is its own design exercise as set out under daylight to a north facing habitable room.

So the orientation audit comes first. List every habitable room by orientation and aspect, identify the single aspect east and west rooms, and treat those as the constraint that sets the approach for the block.

Solar control glass and what it costs in daylight

Glass is specified by two numbers that matter here. The g-value is the total solar energy transmittance, the proportion of incident solar energy that ends up inside. The light transmittance is the proportion of visible light that passes through.

Ordinary clear double glazing passes most of both. Solar control coatings cut the g-value substantially, and they also cut light transmittance, which is the trade-off. The figure to compare between products is the ratio of light transmittance to g-value, sometimes called selectivity. A higher ratio delivers more daylight per unit of heat admitted, and the difference between a mid-range and a high selectivity product is often enough to close the gap on its own.

Three cautions when specifying.

Coated glass shifts colour rendering and external appearance, and a tint visible from the street becomes a planning matter in a sensitive context. Ask for samples viewed in situ rather than deciding from a datasheet.

The g-value in the overheating model must be the one for the glazing as installed, including any frame factor and any secondary element, not the centre-pane figure from a brochure.

And solar control glass works all year. It reduces useful winter gain and daylight in December as well as in July, which is why it belongs alongside shading rather than instead of it.

External shading that keeps the light

External shading intercepts solar radiation before it enters, which is more effective than any internal blind, and the geometry can be tuned so it blocks the sun without blocking the sky.

Balconies deserve particular attention because they are frequently the accidental shading device on a scheme, and their effect on the flat below is rarely modelled deliberately. A continuous balcony above a window can remove more daylight than any glass specification decision, and the comparison between daylight inside new flats and daylight to neighbours usually shows the internal problem is the larger one.

Design the overhang from the solar geometry rather than from a rule of thumb. Set the projection so it shades the glass at the summer solar altitude for the site latitude and leaves the window exposed to the lower spring and autumn sun.

Room depth and head height as the better lever

The most efficient move available is usually not on the elevation at all.

Daylight penetration into a room scales with the height of the window head above the floor, not with the width or area of the glass. Raising the head by 300 millimetres pushes usable daylight measurably further into the room, and it does so without adding glazed area at low level where it contributes to gain and contributes little to the back of the room.

Room depth is the other half. A deep single aspect living and dining space is hard to daylight at any glazing area, and the answer is to reduce the depth, borrow light from a second aspect, or accept the depth and place the kitchen at the back where the criteria are less demanding.

Both moves cost almost nothing at plan stage and are impossible later. Which is the argument for running the two assessments before the plan is fixed.

Sequencing the two assessments

Commission a combined early study rather than two reports at different stages. The study should test three or four glazing and shading options against both criteria at once, on the worst-case rooms identified by the orientation audit.

Choose the Part O route deliberately. The simplified method is quicker and it fixes maximum glazing areas by orientation, which can be irreconcilable with the daylight target in a deep room. Dynamic modelling costs more and gives the design room to move, because it accounts for shading geometry, thermal mass and ventilation strategy rather than applying a cap.

Bring the acoustic and air quality position into the same conversation. Approved Document O's reliance on openable windows for removing excess heat runs into trouble where night-time noise or air quality means windows cannot be opened, and that combination is common on urban sites. A scheme that solves overheating with an openable window and solves noise with a sealed one has solved neither.

Presenting one coherent case

Most authorities assess overheating at planning where local policy requires it, and in London the cooling hierarchy in the London Plan means the sequence of measures has to be demonstrated rather than the outcome alone.

Produce a single statement that carries both sets of results, or make sure the Daylight and Sunlight Assessment and the Overheating Risk Assessment use identical geometry, identical room labels and identical glazing specification. Reviewers compare them, and a g-value that differs between the two documents undermines both.

State the specification explicitly: g-value and light transmittance as installed, frame factor, shading projection and its height above the window head, room depth, window head height, and the ventilation strategy assumed. Then show the options tested and why the chosen one was preferred.

One honest uncertainty. Overheating models are sensitive to assumed occupancy patterns and to whether occupants open windows at night, and two competent modellers can reach different conclusions from the same geometry. Say which assumptions you used rather than presenting the result as settled.

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This is general guidance, not 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.