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Are External French Doors Energy Efficient?

French Doors are an extremely popular means of bridging a home and a garden, and have been so for centuries. They allow light to filter into the home, contribute enormously to the sense of space, and look fantastic. But, with energy efficiency being such a growing concern for modern-day homeowners, does this classic design measure up to the standards of today?

If you’re trying to boost the energy efficiency of your home, it’s crucial to bear in mind that some materials conduct heat more willingly than others. As such, the glass in your windows represents a more pressing concern than the brickwork that surrounds them. Boosting the energy efficiency of windows has been an obsession for homeowners for decades – as typified by the relentless pursuit of ‘double-glazing’ technology. The same is true of glazed doors, whose efficiency has come along in leaps and bounds in recent years. But exactly how do we compare the energy efficiency of one set of doors to that of another?

How is Door Efficiency Measured?

If you’re considering a set of doors, then you’ll want a means of comparing its heat-retaining ability to other sets. The efficacy with which materials conduct or retain heat is most often gauged using one of two measures: the R-value and the U-value. The former measures the resistance to heat-transmission. It’s not terribly useful for describing doors, however, because it applies only to given materials like glass and timber, rather than to an entire set of French doors. The latter measures the ability of a door to conduct heat. The lower this number, the better: a hypothetical door with a U-value of zero would be perfectly energy efficient, and precisely zero heat would be able to pass from one side to the other.

U values are calculated according to the energy a given square metre over time. The average quality set of doors will have a U value of around 1.8W/m2K. Again, it’s easy to be misled by these figures, as the values quoted might refer to an individual component of the door in isolation (like the glass), rather than accounting for the entire thing. Moreover, the U-value of a given sheet of glass might vary from one side of the door to the other.  In the real world, doors are not uniform – and so these measures, while they’re technically accurate, don’t give a realistic picture of the door’s performance.

A more reliable indicator of the door’s efficiency comes in the form of a rating from the British Fenestration Rating Council (BFRC). This body’s job is to translate the more scientific (though less intuitive) measures into a helpful colour-coded system of categories which you can use to make a better-informed decision about which door to buy. The best sorts of energy-efficient external doors and windows are given a purple “A++” rating, while the “E” rating is the worst you can get while still satisfying building regulations. While you might expect to pay a little more for a more efficient door, this investment might be worthwhile in terms of both the direct energy savings you’ll make, and the contribution a superior door will make to the value of the property.

Are French Doors Energy Efficient?

External French Doors are characterised by their large glass panels, which really help to enhance the sense of space in the home. But don’t these large sheets of glass represent something of a liability for heat-containment?

If you’re considering installing a set of new patio doors into the rear of your home, then you might understandably be concerned about the potential impact on heat-efficiency. After all, aren’t French doors just larger sorts of window that you can walk through?

It’s important to bear in mind how technology has come along over the years. A pair of energy efficient external doors from the 1990s would be considered bog standard today. This is down to incremental improvements in glazing technology, like the practice of trapping an inert gas (like argon) between the double-glazing, or using a microscopically thin layer of metallic particles to reflect the heat from the interior back inside.

We should also consider where in your home you’re going to be installing your French doors.  In the northern hemisphere, doors installed into a south-facing wall will receive a great deal more sunlight than those installed into a north-facing one. This makes innovations like triple-glazing more trouble than they’re worth, as they’ll be repelling much of the energy which would otherwise heat your interior.

How to Increase Door Efficiency

If you’ve already got a set of doors installed and don’t want to tear them out and replace them, then there are several options available. The most obvious of these is to place another obstacle behind the door, thus restricting the flow of heat. This obstacle typically comes in the form of a set of heavy curtains. Blackout curtains are great from a security perspective as well as an energy-efficiency one. They’ll not only help to contain the heat of your home; they’ll also prevent would-be intruders from seeing inside and taking stock of all the high-value items they might like to steal.

As wooden doors age, they’re prone to warping in response to heat, light and moisture. This is because the fibres will expand and contract, unevenly changing shape. This phenomenon is much reduced in modern engineered doors, which bring together many different wooden panels into the same structure. But in older ones it can be a serious flaw – and one that can be combatted only through regular maintenance, cleaning and refinishing.

To combat any gaps around the edges of your door, you’ll want to replace the rubber (or foam) seals around the edges, and place a draught-excluder around the foot of the door. These devices are effectively cushions which serve to obstruct airflow through the bottom of the door. They’re particularly handy during winter, when freezing breezes can really put a dampener on spirits!

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