North light, and why solar control is the wrong answer there

What north light actually is

A north facing surface in this country never receives direct sun in the middle of the day, at any time of year. What it receives is light from the sky itself: sunlight scattered by the atmosphere and arriving from every direction at once.

That produces light with two distinctive properties. It is remarkably steady, because it does not track across the room as the sun moves. And it is close to shadowless, because it arrives from the whole hemisphere of sky rather than from a point.

It is why north light is the traditional choice for a studio, and it is a genuinely different quality of light rather than simply a dimmer version of the south.

Why solar gain is not the problem

Solar control exists to reject direct beam energy. A north facing rooflight receives almost none of it.

There are exceptions and they are small: in high summer the sun rises well north of east and sets well north of west, so a north facing slope catches some direct sun very early and very late. Those are the coolest hours of the day and the sun is at a shallow angle to the glass, so most of it reflects.

The rest of the year, a north facing rooflight is collecting diffuse sky light and nothing else.

What solar control does to it

A solar control coating rejects part of the visible spectrum along with the infrared. It does that indiscriminately, to direct sun and diffuse sky light alike.

On a north facing rooflight the result is a straightforward loss: less daylight, for no reduction in a solar load that was never significant. The room reads darker and slightly cooler in colour, and nothing has been gained.

It also costs money, which makes it an unusual specification: paying more for a worse outcome.

What to optimise for instead

On a north facing rooflight, two things matter and neither is the g-value.

Light transmission. Take the highest you can get, because the light arriving is diffuse and there is no surplus to reject. Clear glass with a low emissivity coating and nothing else.

The U-value. This matters more on a north facing unit than a south facing one, because there is no solar gain offsetting the heat loss. A north facing rooflight is a net loser of heat in winter, all winter, so the insulating performance is doing real work.

The winter arithmetic

A south facing rooflight loses heat and gains it. Over a heating season the gain offsets a meaningful part of the loss.

A north facing one loses heat and gains almost none. Its net contribution to the heating demand is worse, unit for unit, which is the honest reason to spend on the U-value there rather than on solar control.

Where a budget has to go one way, on a north facing rooflight it goes into the insulating specification. On a south facing one it goes into solar control.

North facing does not mean dark

It is worth correcting the impression, because people sometimes avoid north facing rooflights on the assumption they will not deliver.

A rooflight faces the whole sky rather than a slice of it, and the sky is bright. A north facing rooflight in a room with a north facing window will deliver considerably more daylight than the window, because it is not shaded by anything and it sees far more of the sky.

Overhead glazing is efficient regardless of orientation. Orientation changes the character of the light, not whether there is any.

The rooms it suits

Steady, even, shadowless light is exactly right for several things.

• A studio or a workshop, which is the traditional case and still the best one.
• A home office with a screen, where changing direct sun across a desk is a nuisance all day.
• A kitchen worktop where somebody works with their back to a window, since shadowless light means no shadow cast over the work.
• Anywhere a room is used consistently through the day rather than at one hour.

In each of those, the absence of a moving bright patch is the feature rather than a compromise.

North on a lantern

A lantern has four slopes, so it never faces one direction, and that changes the argument.

With the ridge running east to west, the large slopes face north and south: a steady north slope plus a strong south one. That is the arrangement that gives the most even overall daylight and the strongest midday peak, and it means solar control on a lantern is rarely a clean north facing decision.

On a lantern, the glass is specified for the whole unit and the south slope drives the answer. Ridge orientation and what it does to the light has its own page.

Where north facing does need attention

Two situations, and both are about heat loss rather than gain.

A large north facing rooflight over a room that is difficult to heat is a substantial cold surface overhead. That is a case for the best U-value available, for triple glazing where the frame will take it, and for a properly insulated kerb or reveal.

And on a very cold clear night, a north facing unit is the one most likely to show external dew, because it has no residual warmth from the day. That is the unit working correctly rather than a fault, and the condensation pages explain why.

The condensation point

Because a north facing rooflight has the coldest inner pane of any orientation, it is also the one most likely to show condensation on the room side in winter.

That is a reason to specify the U-value well, and it is more a reason to look at the ventilation and the humidity in the room, which is what actually drives it.

Where one rooflight in a house mists and the others do not, a north facing slope is one of the usual explanations and it is not a defect in the unit.

North east and north west

Few roofs face true north, and the intermediate directions behave predictably.

North east catches early morning sun in summer, onto a room at its coolest, which is rarely a problem and is frequently pleasant. North west catches late evening sun in summer, onto a room that has been warming all day, which is the same difficulty west facing units have in miniature.

So a north west facing rooflight over a room used in the evening is worth a step of solar control where a north east one is not. It is the only intermediate case that changes the answer.

Skylight or window, and what north gives you

It is worth setting the comparison out, because it is the argument for a north facing rooflight rather than none.

A north facing wall window sees a slice of northern sky, and much of that slice is obstructed by whatever is outside: a fence, a hedge, a neighbouring building, the ground itself. In a terrace or a close plot, a north facing window may see very little actual sky at all.

A north facing rooflight sees the sky directly and is obstructed by nothing. It draws from a far larger area of bright sky, and the light arrives from above where it does most good. That is why a modest north facing rooflight transforms a room that a north facing window merely lights.

Two orientations in one room

Where a room can take more than one rooflight, mixing orientations is worth considering rather than matching them.

A north facing unit for steady working light and a south facing one for warmth and brightness gives a room both characters, and the two behave differently through the day in a way that is genuinely pleasant. They can carry different glass: clear on the north, solar control on the south.

Specifying two units to the same glass because they are on the same job is the easy thing to do and it is the wrong answer where they face different ways. It costs nothing to order them differently.

Getting the orientation right

It sounds obvious and it is worth checking on site rather than from a drawing.

Plans are not always oriented as people assume, extensions are not always square to the house, and what a client describes as the back of the house is not reliably north. A compass takes ten seconds and it decides a specification that cannot be changed afterwards.

Where a roof faces a genuinely intermediate direction, we say which way we are treating it and why, rather than rounding it to a cardinal point silently.

Rooms that are already too bright

There is one north facing case where reducing light transmission is legitimate, and it is worth naming so the rule above is not applied blindly.

A very large north facing rooflight over a small room can genuinely be too bright to be comfortable, particularly where the room contains a screen and the glazing is directly in the field of view. That is a glare problem rather than a heat one.

The answer is still not solar control glass, because a coating reduces light at every hour equally including the dull February afternoons when you wanted all of it. The answer is a blind, which reduces light when you choose and returns it when you do not.

Obstruction, and checking what the sky actually looks like

North facing performance depends on how much sky the rooflight can see, and on a constrained site that is worth checking rather than assuming.

A tall building, a mature evergreen or a rising bank immediately to the north of a rooflight removes a substantial part of the sky it would otherwise draw from, and unlike direct sun that obstruction affects it all day and all year. There is no hour when the sun moves round and improves things.

Standing on the roof, or leaning out of an upstairs window, and simply looking at how much open sky is visible in that direction takes a minute and tells you more than any calculation. Where the answer is very little, moving the unit a metre or turning to a different slope may be worth considerably more than any glass specification.

Self cleaning, which still applies

One coating that is worth having on a north facing unit is a self cleaning outer surface, subject to pitch.

It works by breaking down organic dirt in daylight and sheeting water so the loosened dirt washes off, and daylight rather than direct sun is enough for the first part. What it needs is run off, which is a question about pitch rather than orientation.

A north facing slope also tends to stay damp longer and grow more algae, which makes it arguably the orientation where a self cleaning coating earns its place most.

What we check on the glazing

We check the orientation with a compass rather than assuming, and on a north facing rooflight we specify clear glass with a low emissivity coating, the best U-value the product offers, and no solar control. We spend the difference on the insulating performance and on getting the kerb or reveal detail right, because that is where it does something.

Solar control on a north facing rooflight is paying to make a room darker. It is one of the few specifications that is simply wrong rather than a trade.