The language used when discussing windows’ thermal performance-U-values, thermal breaks, low-E glass, and warm-edge spacers-sounds more scientific than it has to. All of these concepts are fairly easy to understand once explained properly and will definitely affect the quality of decisions made when making comparisons. Another option is to leave everything up to the supplier and just assume whatever the supplier says. This approach might work if one believes the supplier uses like-for-like comparisons and the correct data to quote on compliance. At Citi Home Windows, we think that the customer should understand what he buys. Sowe prepared this guide.
U-value: definition and what number to request
The U-value represents the heat transfer coefficient of the building component concerned and is measured in watts per square meter per degree Kelvin (W/m2K). The lower the number, the more effective it will be at retaining heat. According to the Building Regulations Part L of England, the mandatory minimum whole-window U-Value for replacement windows in dwellings shall not fall below 1.4 W/m²K.
There are two separate figures for each and every window system, both different. One, the centre pane U-value, measures the amount of heat loss that occurs through just the glazing part of the system, specifically in the middle of the pane where the insulating cavities work most efficiently. The other, the whole-window U-value, measures the heat loss from the total window system, including the frame, the glass, and the edge spacers, and is always a bigger figure, since these components do not perform as efficiently as the central part of the glass pane. It is the whole-window value that matters as far as meeting regulations, and which gives the true picture of system performance, making it alone worthy of comparison between systems. Some manufacturers give the centre pane figure in their literature, simply because it is smaller.
Why are thermal breaks important, and how to recognize them?
Aluminum metal conducts heat about a thousand times better than wood, and about several hundred times better than glass. If the aluminium window frame remained unaltered, it would serve as a continuous thermal bridge from the hot indoors to the cold outdoors, making the building fail the Building Regulations significantly and causing severe condensation of the frame when cold.
The thermal break is a strip of polyamide, an insulating material, which is attached to both parts of the aluminium frame to separate them from each other. This is precisely what makes aluminium windows viable as a choice for domestic use and a very important characteristic that defines the performance of aluminium window systems. A deep thermal break works better than a shallow one, while the more consistent the strip is, the better. Shallow thermal breaks, which do not meet the required specifications and are only minimally functional, can be found in budget aluminium window systems; in these cases, they provide worse results than a quality system in the same conditions.
Gasfill: argon krypton, on the indication of standard air fill
The space between the glass panels in a double or triple-glazed window is not filled with air but with an inert gas to reduce heat exchange through conduction and convection processes. The most commonly used gas is argon, as it is easily obtainable and relatively cheap compared to the total cost of the product, yet while offering an improvement of around 30% over an air-filled cavity with equal depth. Krypton is superior to argon in performance but much more costly; its major use is in high-performance triple glazing where space is limited.
Standard air fill in a quoted specification is a sign of a basic or budget product. The cost of argon fill to the manufacturer is not high, and its absence is a reasonable indicator that the specification has not been put together carefully. If a quotation does not specify the gas fill, it is worth asking directly before accepting it as a comparable basis for evaluation.
Low-eglasscoating-what does it do, and one thing that you need to know about it
Low-emissivity glass coating is simply a very thin coating of metal that is applied to one side of the glass. The purpose of this coating is to prevent long-wavelength infrared rays-heat rays-from passing through the glass out to the cold environment. This coating is invisible when applied and does not increase the cost of the window by much, a nd yet its effect on improving the U-value is great: it reduces it from 1.6 to 2 W/m²K to 1.0 to 1.1 W/m²K.
The point here to consider is that while the coating serves to decrease the thermal heat loss, at the same time, it will also prevent the solar gains from the sun’s radiation, which will decrease the heating requirement for the elevation facing southwards. Here, the g value or the solar factor of the glazed unit comes into play. If the g value is higher, it means that the glazed unit will transmit more solar gains. On the contrary, a lower g value will mean a reduced amount of solar gain. A specification made with only concern for minimizing U values could prevent useful solar gains.
Warm edge spacer bars: a specification to take note of
Spacer bars are the elements in the glass that keep two pieces of glass apart and ensure the gas inside the gap remains sealed between the edges of the glass panes. Spacer bars made from aluminum are very good thermal conductors. They act as cold bridges in the perimeter zone of the glass – the junction of glass and frames – and thus lower the U value of the window and form condensation lines in windows on cold days.
Warm-edge spacer bars are produced using polymer composite materials. This addresses both these issues because they minimize the amount of heat that escapes at ththe edgesnd iiimprove the overall window U-value by about 0.1-0.2 W/m²K when compared to aluminum spacer bars. The updated guidance on UK Building Regulation onns published n026,6 means that aluminum spacer bars are no longer used in window installations. In this respect, if the quotation mentions aluminum spacer bars, it might be worth questioning this issue.
Important questions to be answered before making a commitment
The key questions to ask a window provider aren’t technical, but the answers will tell you much about what they’re offering. Find out whether the U-value quoted applies to the entire window or only to the center pane. Ask how thick the thermal break is on the aluminum used in the window system. Check whether warm-edge spacer bars come as standard equipment or as an additional paid option. Find out whether the window has been filled with argon or air. Request both the U-value and the g-value of the glass used and ask why the provider has chosen to recommend such a combination. The provider who can answer these questions is a serious competitor in the market. The provider unable to provide consistent answers should be approached with caution.
