Daylight factor in a workspace

Daylight factor in one paragraph

The measurement is deliberately taken under a standard overcast sky rather than sunshine, because an overcast sky is roughly predictable and sun is not. That makes daylight factor a worst-realistic-case figure: if a space works under cloud, it works. It also means the number is independent of orientation and season, which is why it survives as a design tool despite being crude.

It is measured at working plane height, conventionally 850mm above the floor for a desk and sometimes 700mm for a bench. Quoted as an average across the space, or as a minimum at the worst point, and the two tell you very different things.

The numbers people actually work to

The 2% figure is the workhorse. Below it, people switch the lights on and leave them on regardless of the weather, which defeats the point of the glazing. Above 5%, you are usually solving a different problem than the one you started with.

Why roof glazing beats a window of the same area

A rooflight sees the whole sky dome. A vertical window sees roughly half of it, and the half it sees is the lower, dimmer part, with the ground and whatever is opposite taking up a good share of the view.

The working figure is that a horizontal rooflight delivers around two to three times the daylight of a vertical window of the same glazed area, and the gap widens as you move away from the wall. In a deep-plan space the difference is not two or three times, it is the difference between light and no light, because wall glazing stops contributing at roughly two and a half times the head height into the room and roof glazing does not care about depth at all.

Working out roughly how much glazing a space needs

For a first pass on a flat or shallow roof, the glazed area needed for a given average daylight factor is approximately the floor area multiplied by the target daylight factor, divided by the visible light transmittance of the glass, then multiplied by a correction for the room surfaces. In round numbers, a rooflight area of about 10% of the floor gets you into the 2% to 3% band with ordinary double glazing, and about 20% takes you toward 5%.

Those are sighting figures, not a design. They ignore obstruction, they assume the glazing is spread rather than clustered, and they assume light-coloured internal surfaces. But they are close enough to tell you whether a scheme is plausible before anybody pays for modelling.

Uniformity, and why one big rooflight is usually wrong

Two schemes with the same glazed area and the same average daylight factor can perform completely differently. Four units spread across a roof give a fairly even wash. One unit of the same total area gives a bright pool underneath and dim corners.

Uniformity is expressed as the minimum daylight factor divided by the average, and something above 0.4 reads as evenly lit. Below about 0.25 the space feels patchy and people notice the contrast more than the light. On a wide-span roof, the practical rule is that rooflight spacing should be no greater than the height from the working plane to the glazing, and closer than that where the ceiling is low.

Visible light transmittance and what the glass takes away

Every layer takes a share. A clear single pane passes around 90% of visible light. Ordinary double glazing with a low emissivity coating passes around 70% to 80%. Triple glazing drops toward 60% to 70%. Solar control glass with a low g-value can sit anywhere from 70% down to below 40% depending on the coating.

That range matters more than people expect. Specifying a strong solar control glass to deal with heat, then wondering why the space is dim, is a common sequence. The number to ask a supplier for is the light transmittance and the g-value together, and the ratio between them, sometimes called the selectivity, tells you how much daylight you are keeping per unit of heat you are rejecting.

Diffusing against clear glazing over a work surface

Clear glazing gives a view of the sky, sharp shadows, and moving pools of sun across the floor. Diffusing glazing, whether an obscure interlayer, an opal polycarbonate or a translucent insulated panel, gives an even wash with no shadows and no view.

For a workshop, an assembly bench or a sports hall, diffusing is usually the better answer: the light is even, the contrast is low, and nothing casts a hard shadow onto the work. For an office or a break space, people want the view and the variation, and clear glazing with shading is worth the extra design effort. Mixing the two across a single roof is legitimate and often the right answer where the space has both functions.

Glare on screens, and where it comes from

Screen glare from rooflights arrives two ways. Direct glare is the bright source in the field of view, which happens when a rooflight sits low in the ceiling and within about 30 degrees above the horizontal from where somebody sits. Reflected glare is the image of the bright rooflight in the screen itself, which depends on the geometry between the glazing, the screen and the eye.

The design responses are ordinary. Keep bright sources out of the direct view by placing rooflights away from the wall people face. Use a splayed reveal so the transition from ceiling to glazing is gradual and the contrast at the edge is lower. Diffuse the glazing where screens dominate. Where a deep reveal is possible, it does more for comfort than any coating.

BS EN 17037 and the shift away from an average

The European daylight standard moved the conversation from average daylight factor to the proportion of the space that achieves a target illuminance for a proportion of the daylit hours. The recommendation is expressed as target lux on a fraction of the reference plane for at least half the daylight hours, with minimum, medium and high levels.

In practice that requires climate-based modelling rather than a hand calculation, and it produces answers that are more useful and harder to sanity-check. For most rooflight decisions the older daylight factor arithmetic gets you to the right specification, and the standard becomes relevant when a scheme is being formally assessed or when a credit is being claimed under a sustainability rating.

Daylight and the lighting bill

Daylight only saves energy if the electric lighting responds to it. A space at 3% daylight factor with lights on a single switch uses exactly as much as the same space at 0.5%, because somebody turns them on in the morning and nobody turns them off.

Photocell dimming on the rows nearest the glazing, or at minimum a separately switched perimeter zone, is what converts daylight into money. It is a lighting contractor’s work rather than ours, but it belongs in the same conversation, because the payback case for rooflights on a commercial roof frequently rests on it.

The overheating trade-off

The same aperture that brings daylight brings solar heat, and on a horizontal or shallow surface the summer sun is close to normal to the glass, which is the worst case. A space glazed to 20% of its floor area with clear glass will overheat in July unless something is done about it.

The available moves, in rough order of effectiveness: a lower g-value glass, a smaller and better distributed glazed area, opening units at high level to let the heat out, external shading where the roof form allows it, and internal blinds as the weakest option since the heat is already inside by then. Getting the glazed area right is nearly always better than glazing generously and then fighting it.

Part L, glazed area limits and U-values

Building Regulations set a limiting U-value for rooflights and a limiting proportion of roof area that can be glazed in a new or materially altered building. The rooflight U-value is measured in the vertical for the unit and adjusted for the pitch it is installed at, which trips people up when they compare a datasheet figure to a target.

Two consequences for a daylight scheme. Beyond a certain glazed proportion the fabric calculation stops balancing and something else has to compensate. And a large glazed area of good units still loses more heat than the roof it replaces, so the daylight benefit has to be worth the fabric cost. On a working space with long occupied hours it usually is.

Spacing rooflights across a wide-span roof

On an industrial roof the glazing is normally in-plane sheets in the covering, laid in bands. The spacing decision is driven by the purlin layout as much as by the daylight, because the sheets have to land on supports.

The pattern that works is bands of glazing running across the slope at regular intervals rather than a large block at one end, sized so that the gap between bands is no more than about one and a half times the height from the floor to the roof. On a 6m high unit that means bands roughly every 9m. Anything wider and the dark stripes between them become obvious from the floor.

Measuring what a space already has

Before adding glazing it is worth knowing what the current position is, and that takes an hour with two light meters on an overcast day. One outside on an unobstructed horizontal surface, one moved around the working plane inside on a grid. Divide the inside readings by the outside reading, multiply by a hundred, and you have a daylight factor map of the space.

It is crude and it works. It also frequently changes the brief, because the dark zone people complain about turns out to be one corner rather than the whole room, and the fix is two units in the right place rather than a roof full of them.

Where daylight modelling earns its cost

Simulation is worth paying for when the space is tall and complex, when an obstruction outside is significant, when a formal standard is being demonstrated, or when the glazing budget is large enough that a 20% error in the area is real money.

It is not worth paying for on a workshop where the answer is obviously four units instead of two. Being told that plainly is more useful than being sold a study. Where we think a scheme genuinely needs modelling before an order, we will say so and you can appoint somebody for it.

Turning a daylight target into a rooflight schedule

The output of all of this should be a short list: the number of units, the aperture size of each, the spacing, the glass specification with its light transmittance and g-value, and which of them open. That is what gets priced and installed, and everything above exists to produce it.

Send us the floor area, the ceiling or roof height, the roof construction, what the space is used for and how many people are in it, and whether screens dominate. We will come back with a range by email and a suggested layout, then a fixed price after a survey. Call 01865 704245 or email info@heritageskylightsoxford.co.uk.