How to reduce heat loss through windows

Glass is a poor insulator, which is why windows tend to be the main source of heat loss in any home. When temperatures are colder outside than inside, glass conducts the heat from inside the house to the outside where it quickly disperses. The air next to the glass inside, having lost its heat, becomes cold and dense and falls to the floor drawing in new warm air from above. This cycle continues until you lose all your warm air outside.

Ways to reduce the amount of heat loss through the windows include, lined curtains and blinds, sealing gaps, double glazing and its alternatives Let’s go into a bit more detail on these.

Seal any gaps

The very first thing to check is if you have any gaps around your windows that are letting cold air leak in; check both inside and outside. Gaps are easy to fix either by caulking them or applying weather seal tape where appropriate. This is available fairly cheaply from any hardware store and it’s easy to install; here’s a handy how-to video from Bunnings.

Double glazing

Double-glazing is proven to reduce heat loss through windows. It works by trapping a pocket of air – which is not a good conductor of heat - between the two panes of glass. You can even get triple-glazing nowadays! With quite incredible insulating performance, triple-glazing does come with some disadvantages such as needing an extra wide frame to accommodate the extra thick window panes, it doesn’t let in much solar heat so you would have to rely almost completely on a heat source within the home, light transmission can be quite low and for the most part, retrofitting cannot be done. To install double or triple glazed windows throughout your whole house would be an expensive (albeit worthwhile) task, so here’s a cheaper option:

Insulating window film

The cheapest option is to go the DIY route with a kit from the hardware store. 3M make a window insulating kit you can install yourself with nothing more than a tape measure, scissors and a hairdryer. Here’s a video to help you install it.

Low-E window film

This is a thin film with a micro-thin metal coating that sticks directly to existing glass to reflect radiant heat back into the home. It works best in homes that don’t experience condensation though. Find more information here.

Thick, properly lined curtains or blinds

Thermal lined curtains and blinds can significantly reduce heat loss through your windows. According to Energywise “Good curtains and blinds can reduce heat loss through windows by 60% for single glazed windows, and 40-50% for double glazing.”

A thick, closely woven fabric will offer the best heat loss reduction.

Ensure a snug fit on both sides of the window and at the top of the curtain or blind to stop warm air sneaking down behind it.

Install a pelmet above the curtain rail. This then creates that pocket of air, which helps to reduce heat loss. Or use curtain tracks that curve at the end and bring the curtain into direct contact with the wall to create a seal.

Make sure you keep curtains and blinds open during the day to let in any sun and close them at dusk to keep that warmth in and block the cold glass.

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