Shading a window: what hot climates do to glass

01 Why external shading beats internal blinds

A blind inside the glass sits behind the point where sunlight has already entered the room. The shortwave radiation passes through the pane, strikes the blind, and is re-radiated as heat inside the thermal envelope. An external shutter, louvre or overhang intercepts the same radiation outdoors, so a large share never crosses the glazing line at all.

This is why Mediterranean building traditions put the shading device on the outside of the wall: shutters on the reveal, timber louvres, deep loggias, pergolas, and the calculated overhang above a south-facing opening. The principle is described in the same terms by Mediterranean house design writing on the relationship between facade, sun path and interior comfort, where the shading element is treated as part of the architecture rather than an accessory added later.

For an English reader the practical consequence is simple. If a room overheats, the first question is not which glass to buy but what sits between that glass and the sun.

02 Which shading habits travel well to England?

Not every hot-climate habit transfers. The English sun is lower for more of the day, the cooling season is shorter, and daylight is scarcer in winter. That changes the ranking.

Habits that travel well:

• External shutters or sliding louvre panels on east and west elevations, where low morning and evening sun is hardest to stop with an overhang alone.
• Adjustable external blinds or awnings, because they can be retracted in winter to let low sun and daylight in.
• Deep reveals and recessed openings, which shade the glass for part of the day at no running cost.
• Deciduous planting to the south and west, which leafs out in summer and drops in winter.

Habits that travel less well:

• Fixed horizontal overhangs sized for a high summer sun. In England the same overhang can cut useful winter daylight and still miss the low evening sun in June.
• Permanently closed shutters or heavy fixed screens on south-facing living rooms, which trade daylight for cooling in a climate where winter daylight is already limited.
• Very deep permanent loggias on the north side, where there is little solar gain to block.

03 Do overhangs and brise-soleil work at English latitudes?

They work, but the geometry has to be recalculated. An overhang shades an opening when the sun's altitude is above the angle subtended by the overhang from the head of the window. A device sized for a Mediterranean summer noon, where the sun can stand above 70 degrees, will be shorter than one sized for an English summer noon, where the maximum is closer to 60 degrees in the south of England and lower further north.

The useful check is the sun path for the specific latitude and the specific orientation, not a rule of thumb copied from a catalogue. Two numbers matter: the altitude of the sun at the hottest part of the afternoon, and the altitude at the times of day when the room is actually occupied. A west-facing bedroom and a west-facing kitchen need different answers.

Horizontal overhangs suit south-facing glazing, because the sun is high when it is due south. They do little for east and west glazing, where the sun is low and arrives almost horizontally. Vertical fins, or a combination of fins and an overhang, are the usual response on those elevations.

04 What does the building regulation actually require?

In England, Approved Document L of the Building Regulations deals with the energy performance of the fabric, including solar gain and the control of it. Approved Document O deals with overheating in new residential buildings and sets out how the risk is to be assessed and mitigated. Both are published by the government and are free to read.

Approved Document O does not mandate a particular shading product. It requires that overheating risk is assessed using a specified method and that reasonable mitigation is designed in. External shading is one of the recognised measures, alongside opening windows, cross ventilation, thermal mass and the orientation of glazing.

For replacement windows in an existing dwelling, the relevant trigger is usually Part L compliance and, where applicable, the requirement to notify through a competent person scheme. A FENSA certificate or an equivalent building control approval is the document a homeowner should keep. Shading added later, such as an awning or external blind, is generally not a notifiable alteration, but it can affect the stated performance of the window if it is included in a calculation.

05 How should shading be specified in practice?

Start with orientation. A south-facing window in England can be substantially managed with a correctly sized overhang plus an adjustable external blind for the shoulder months. East and west windows need vertical or adjustable shading, because no fixed horizontal device will catch a low sun.

Then decide what happens in winter. Any shading device that cannot be removed, retracted or opened will also block winter sun, and winter solar gain is useful in England. Adjustability is therefore worth more here than in a climate where cooling dominates all year.

Then check the glass. Solar control coatings reduce the amount of solar energy transmitted through the pane, and they are useful where external shading is impractical, for example on a listed facade or where a fixed screen would be refused. They are not a substitute for external shading, because they also reduce daylight and, in some specifications, visible light transmission.

Finally, record what was installed. Keep the window specification, the certificate or approval, and any shading calculation with the property documents. A future buyer, or a future building control query, is easier to answer with the paperwork to hand.

06 What is the short answer for an English homeowner?

Shade the outside of the glass before you change the glass. Prioritise east and west elevations, where low sun causes the most discomfort. Choose devices that can be adjusted or retracted so winter sun and daylight are not lost. Size overhangs to the actual sun path at the property's latitude rather than to a figure borrowed from a hotter country. Where external shading is not possible, solar control glazing and internal blinds are the fallback, in that order.

The hot-climate habit worth copying is not a particular product. It is the habit of treating shading as part of the design of the opening, decided at the same time as the window, rather than as something added after the room has already overheated.