How the Right Windows Help Reduce Overheating in Homes

UK summers are getting hotter, and the well-insulated, airtight homes we now build to save energy can trap that heat just as effectively as they keep the cold out. Overheating has shifted from an occasional nuisance to a recognised health and comfort risk — so much so that it now has its own section in the Building Regulations. Windows sit right at the centre of the problem and the solution: glass is the single biggest route for solar heat to enter a building, but the right glazing, frames and opening configurations are also one of the most effective ways to keep a home cool. This guide explains how.

Why overheating happens through glass

Heat enters a home through windows in the form of solar gain — short-wave radiation from the sun passing through the glass and warming the surfaces inside. Those surfaces then re-radiate heat as long-wave energy, which the glass traps, much like a greenhouse. The more glazing a room has, and the more direct sun it receives, the faster it heats up.

This is compounded by modern construction. High levels of insulation and airtightness, introduced to cut heating demand under Part L, mean that once heat is inside, it struggles to escape. A south or west-facing room with large unshaded glazing can become uncomfortably warm even on a moderately sunny spring day.

The Part O context

Since June 2022, Approved Document O (Overheating) has applied to all new residential buildings in England. It tackles overheating in two ways: limiting unwanted solar gain getting in, and providing a means of removing excess heat once it builds up. Location, orientation, the proportion of glazing, shading and ventilation openings are all assessed.

Compliance follows one of two routes — a simplified method that caps glazing areas and sets minimum opening sizes based on orientation and risk, or dynamic thermal modelling for more complex or heavily glazed designs. It is worth noting that Part O applies to new builds (and conservatories on new builds); it does not apply to replacement windows, extensions or conservatories added to existing homes. Even so, the principles behind it are exactly what any homeowner should consider when specifying glazing to keep a property comfortable.

South-facing elevations: the biggest challenge

South and west-facing glazing is where overheating risk concentrates. The summer sun is high and strong on southern elevations through the middle of the day, while west-facing glass catches the low, intense late-afternoon sun when the building has already absorbed a day’s worth of heat. Large expanses of glass on these elevations — fully glazed rear extensions, bi-folds and picture windows facing a south-facing garden — look stunning but can turn a room into a heat trap.

The good news is that south-facing glazing is also the easiest to control, because the high summer sun can be blocked by relatively shallow shading while still letting in the lower, welcome winter sun. The measures below work together; the best results come from combining several rather than relying on any one.

1. Specify solar control glass

The most direct intervention is the glass itself. Solar control glazing carries a microscopically thin metallic coating that reflects a large proportion of the sun’s heat while still letting light through. Its performance is measured by the g-value (also called solar factor) — the fraction of solar heat that passes through the glass. A lower g-value means less heat gets in.

• Standard double glazing typically has a g-value of around 0.6 to 0.7.
• Solar control units can bring this down to roughly 0.3 to 0.4 on south and west elevations.
• Around 0.40 is generally regarded as the sensible lower limit for homes — going lower usually means heavily tinted or coloured coatings that affect appearance.

The key trade-off is light versus heat. A poorly chosen solar control glass can leave rooms feeling dim. The way around this is to specify a product with a high light transmittance (LT) but a low g-value — modern coatings increasingly let you decouple the two, keeping interiors bright while cutting solar heat. On north-facing elevations, where overheating is rarely an issue, standard high-performance glass is usually the better and more cost-effective choice, so glazing can be specified elevation by elevation.

2. Shade the glass from outside

External shading is far more effective than internal blinds, because it stops the sun’s heat before it ever reaches the glass. Once solar radiation has passed through the window, internal blinds can only do so much. Options that suit south-facing elevations particularly well include:

• Overhangs and projecting upper floors — a roof overhang, balcony or first floor that oversails the ground floor blocks the high summer sun while allowing the lower winter sun underneath.
• Brise-soleil — horizontal fins above the glazing, increasingly common on domestic projects, designed to cut out high-angle sun.
• External shutters, louvres and awnings — adjustable options that can be closed during the hottest part of the day.

3. Right-size and position the glazing

More glass means more potential solar gain. On heat-prone south and west elevations, moderating the glazed area is one of the simplest preventative measures — for example, balancing a large glazed opening with some solid wall, or distributing glazing so that the bulk of it faces north or east where gain is gentler. This does not mean small, dark rooms; it means putting the glass where it earns its keep. Where a big glazed opening onto a south-facing garden is the whole point of the room, the answer is to pair it with strong solar control glass and external shading rather than to shrink it.

4. Make sure heat can escape — ventilation and opening area

Reducing the heat coming in is only half the job; a home also needs to purge heat that has built up, ideally overnight when outside air is cooler. This is where the opening configuration of the window matters as much as the glass.

• Maximise free opening area. The usable open area of a window drives how quickly a room can vent heat. Side-hung casements and tilt-and-turn windows offer a large free area, which is why they are increasingly specified on overheating-sensitive projects, while fixed panes contribute nothing to purge ventilation.
• Enable cross-ventilation. Openings on opposite sides of a room or home let air flow through, clearing heat far faster than a single window. Designing for openings on more than one facade makes a measurable difference.
• Allow secure night purging. Tilt-and-turn windows and restrictor-fitted casements let a home ventilate overnight without compromising security — important, since ground-floor windows are otherwise assumed to be shut at night.
• Don’t rely on trickle vents for cooling. Trickle vents handle background fresh-air ventilation (Part F), but their tiny area does not count towards the rapid purge ventilation needed to remove excess heat.

Bringing it together

No single product solves overheating on its own. The most comfortable, resilient homes combine measures: solar control glass tuned to each elevation, external shading on south and west-facing glazing, sensible glazed proportions, and windows configured to give generous, secure opening area for cross and night ventilation. Treated this way, glazing stops being the cause of overheating and becomes a central part of keeping a home cool — without resorting to energy-hungry air conditioning.

Whether you are working on a new build that must satisfy Part O, or simply upgrading the windows on an existing south-facing room, specifying the right glass and frames from the outset is the difference between a home that bakes in summer and one that stays comfortable year round.

SevenDay supplies a full range of solar control glazing, sealed units and uPVC, aluminium and composite window systems across all 22 depots. Speak to your local depot for advice on the right specification for your project.

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Frequently asked questions – Overheating of homes & windows