Home-owners in the UK have been trusting double glazing to keep their homes warmer and their energy bills lower for decades. When triple glazing was introduced in recent years, the question had often been raised if that additional piece of glass can really make such a big difference. Even though triple glazing had been proven to be far superior to double glazing in Scandinavian countries, there are still some doubts as to the impact this can have on both cost of running a home and the comfort level of its inhabitants. Let’s look at the different components of double glazing and triple glazing in order to make a direct comparison between the two.
Double glazing is made up of two panes of 4mm glass. The glass has a gap of 20mm between them, and that gap is typically filled with Argon [Ar] gas. The total thickness of the unit comes to 28mm. A triple glazing window also makes use of 4mm glass, but the 3 panes are separated by 2 gaps of 16mm each. These gaps are filled with Krypton [Kr] gas. The total thickness of a triple glazed window therefore is 44mm.
Double glazing typically has a weight of 20kg/sqm, where triple glazing has an average weight of 30kg/sqm. Both window types may require specialist installation equipment or scaffolding depending on the size, weight and position of the window.
There is little difference in the frame materials of double and triple glazing. Both window types are available in wood, aluminium, uPVC and aluminium-clad options. Where windows are extremely big however, certain restrictions may apply to the choice of frame material for triple glazing. This is because of the weight associated with the extra pane of glass and increase thickness of the frame.
While both Krypton and Argon have thermal barrier properties, Argon has thermal conductivity 67% that of air, while Krypton’s thermal conductivity is 40% when compared with air. Krypton is therefore much more effective as a thermal barrier, and because triple glazing has 2 of these gas-filled chambers, it creates an even higher barrier. While Krypton is more expensive than Argon, Krypton is more efficient in a smaller area, allowing less gas to be used to fill the 16mm gap in triple glazing than the 20mm gap in double glazing.
Both double and triple glazing have increased safety over single glazing. The additional layers of glass make the window unit harder to break, so for that reason alone triple glazing is 50% safer than double glazing. To prevent thermal stress cracking, the central pane in triple glazing is also tempered, increasing its strength even further.
Double glazing manufacturers offer various options to homeowners to improve the efficiency of their double glazing. Triple glazing is manufactured with low-e glass that reflects long-wave infrared energy, or heat. This means that interior heat is being reflected back into the room instead of escaping to the outside. The glass typically used for triple glazed windows are also low-iron, which means that they are extra clear to allow even more light and solar heat gain into the home.
Due to the additional materials needed for its construction, triple glazing is between 20% and 40% more expensive than double glazing. The cost of having double glazing installed in an average home with 3 bedrooms and 15 windows will be around £5,000. The expected reduction in heating costs after this upgrade, will be £65 per year, which means double glazing will take 77 years to show a return on investment when saving alone are taken into account.
Triple glazing on the other hand, will cost around £7,275 to install – an increase in cost of 31%. With an annual saving of £162.50, it will take 45 years to get your money back when energy savings along are considered.
What does however have to be taken into consideration as well, is the level of comfort of the home, as well the added value of the property.
The U-value of a window is the rate at which heat is lost through the window. It is calculated on the rate at which heat transfers through 1 square meter of a structure or material, and where the temperature difference between the inner and outer face of this material or structure is 1 degree Celsius. The lower the U-value, the slower the heat loss through that material or structure.
U-Value (w/m²K) = the measure of structural heat loss per unit of surface area
Because the U values take the entire window unit into consideration, all of its elements will have an influence on the rating: the frame, glass used, quantity of panes, the gas in the cavities and the spacers between the glass.
Double glazing typically has a U-value of 1.4 and triple glazing has a U-value of less than 0.7, meaning the third layer of glass and additional Krypton gas-filled chamber significantly reduces the rate of heat lost to the outside.
The resistance of heat transference from a warmer (or interior) space to a cooler (or exterior) space through a structure or material, is measured as an R-value. The greater the resistance to heat transferred, the higher the R-value. Double glazing has an R value of 3, where triple glazing has an R-value of 5. This proves that the additional glass and gas chambers offer a 66% increase in efficiency when it comes to heat lost to the outside.
Because of temperature differences between interior and exterior window surfaces, condensation tends to form on window glass. While external condensation is completely acceptable and indeed expected, interior condensation can cause damage to window frames and surrounding paint work.
A thin film of water forms on glass when the surface temperature of that glass falls below dew point. Dew point in the UK varies between -1 and 5 degrees Celsius during the colder months. When the interior glass surface then falls below that temperature, condensation will form on the inside panel.
With an interior temperature of 22 degrees Celsius, double glazing can maintain a stable interior surface temperature of 16 degrees, while triple glazing maintains a temperature of 18 degrees. Both of these are well above dew point and therefore interior condensation shouldn’t occur. Where there is a draught however, or if the gas chamber is leaking, then interior condensation may be the result. Because of the additional glass and gas chamber of triple glazing, this is highly unlikely to happen.
The British Fenestration Rating Council, or BFRC, put in place a standardized system for rating the energy efficiency of windows and glass doors in 2004. Since then, both Certass and the British Standards Institution (BSI) have also set up their own similar rating systems. These ratings allow us to compare windows from different manufacturers on equal footing.
All products from a manufacturer are independently evaluated for their total heat transmittance, its U-value, its solar gain and the R-value or resistance to heat transference, and the amount of air that is allowed to flow through the window seals.
Products are rated on a scale from E, being the least energy efficient, to A++, being the most energy efficient. Triple glazed windows have the highest WER of A++, while double glazing windows have a maximum rating of A or A+.
While double glazing will no doubt be beneficial to your home, they are less energy efficient than triple glazed windows, show a slower return on investment, and add less value to your home.
Published on : 5th March 2017
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