Does Secondary Glazing Help in Summer? Overheating and Heatwaves in London Homes
Dr Sarah Chen
Building Physics Consultant
A common worry is that secondary glazing will trap heat and make a home unbearable in summer. London flats, top-floor rooms and south-facing bays already get very hot during heatwaves. The truth is more useful: secondary glazing can help or hinder, depending on the glass you choose and how you use it.
How Heat Gets Into a Room in Summer
Most summer heat comes in as sunlight passing straight through the glass, then warming floors, walls and furniture. Ordinary clear glass, whether single or double, lets most of that solar energy through. Extra insulation slows heat moving through the window, but it does not stop sunlight on its own.
Where Secondary Glazing Helps
- Keeping the windows shut on noisy nights. Many Londoners cannot open windows at night because of traffic or nightlife. With good secondary glazing you can open it alongside the primary window for airflow, then close it for quiet when needed.
- Slowing heat from hot outside air. During the hottest part of the day, a closed, well-insulated window keeps hot air out and the cooler indoor air in, much as it holds warmth in winter.
- Solar control glass. Specifying solar control or low-e coated glass in the secondary pane reduces how much solar heat reaches the room, while keeping it looking clear.
Where It Will Not Help on Its Own
If a south or west-facing room is in direct sun for hours, clear glass alone will not stop it heating up. Blinds or shutters between the panes or inside the room, external shading, and solar control coatings do most of the work there. Rooflights and conservatory roofs are a separate problem; see our guides to skylights and conservatories.
A Simple Heatwave Routine
- In the early morning, open both windows to flush out warm air while it is cool outside.
- Before the sun reaches the window, close both panes and draw blinds or curtains.
- Keep them closed through the afternoon peak.
- Open again in the evening when the outside air is cooler than inside.
This is why opening styles matter. Vertical sliders and hinged units let you open the room fully, while fixed panels do not.
Frequently Asked Questions
Does secondary glazing make rooms hotter in summer?
Not if it opens and you use it sensibly. A fixed, clear pane on a sunny window with no shading can add to heat build-up, which is why we match glass and opening style to the room.
What is solar control glass?
Glass with a thin coating that reflects part of the sun's heat while letting most visible light through. It can be combined with acoustic laminate.
Is secondary glazing still worth it if summer is my main problem?
It can be, especially if noise stops you opening windows at night. For year-round savings, see how much secondary glazing saves on bills, or book a survey to discuss your rooms.
