TECHNICAL NOTE U-VALUES T: 01223 897100 www.VELFAC.co.uk
Introduction
Minimum requirements
In simple terms, a window’s U-value is the measurement given to the amount of heat allowed to escape through it (heat loss), or in other words, its ability to keep in heat.
Building Regulation Approved Document L Conservation of fuel & power, governs the energy efficiency of buildings and forms part of the Government’s overall strategy to reduce energy usage and CO2 emissions.
The measurement of heat loss is determined from the U-value (thermal transmittance), which is a measure of the rate at which heat is lost through the window. A lower U-value indicates a more energy efficient window, i.e. it allows less heat to travel through it. A U-value is measured in W/m²K (watts per square metre, per degree Kelvin). Although each component of a building (e.g. roof, wall, windows etc.) needs to achieve a minimum U-value, by improving certain components to achieve better U-values, you can improve the energy performance of the whole building.
The latest edition of Part L became effective on 6th April 2006, and stipulates the following maximum allowable U-values for windows, roof windows and roof lights (W/m²K).
Area weighted average
Individual element
Recommended weighted average
2.20
3.30
1.80
Important considerations for the performance of your windows We can consider the performance of windows both in terms of heat loss and heat gain. The sun's energy is short wave radiation, which passes through the window and is absorbed by your internal fixtures and furnishings as free heat. This energy is then transformed into long wave radiation similar to that generated by your heating appliances. Unfortunately long wave radiation flows from warm to cool, so your internal heat from the sun and heating tries to escape through the external envelope of the building – walls, roof, and windows – especially at night and during cooler times of the year.
The typical heat flow paths and thermal resistance considered for windows.
1 2 3 4 5
1
1. Heat gain and natural light from the sun through the glazed unit. 2. Heat loss through the glazed unit, commonly called the centre pane U-value. 3. Edge effect - some heat will be lost through the spacer bar at the edge of the glazed unit, and through the interaction of glazing and frame. 4. Air permeability of the whole system – the more airtight the seals and structure of the window, the better the performance. 5. Heat loss through the window frame – heat can transfer through the window framing material, and the amount of heat loss is subject to the materials used and the arrangement of any air gaps, or use of hollow and solid materials.
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TECHNICAL NOTE U-VALUES T: 01223 897100 www.VELFAC.co.uk
How to improve performance Heat will always find the path of least resistance, which effectively means that the improvement of the performance of windows must happen in all areas, not just the glazing for example. This diagram shows how heat flows across the whole window section including frame and glass. Outside - cold
Inside - warm
Low-e (low-emissivity) coatings Low-e coatings are thin films that are placed directly on one or more surfaces of the glazed unit. When applied inside a double-glazed unit, the coating is usually placed on the inner surface of the inner pane of glass, to reflect long wave energy (heat) back into the room, whilst permitting short wave light and solar heat through from the outside. It is possible to use 2 coatings within double or triple glazed units to improve performance, although the initial coating provides the greatest benefit. Outside
Inert gas within the glazing cavities Filling the gap between the glass panes with low conductivity gas such as argon or krypton at atmospheric pressure, improves the window performance by reducing conductive and convective heat transfer. This should always be used in conjunction with low-emissivity coatings. Spacer profiles Typically the spacer profiles that separate the 2/3 panes of glass around the perimeter are made from aluminium. A more energy efficient ‘Warmedge’ spacer can be used, which is made from a composite polycarbonate material.
Inside
Frame to glass ratio The easiest way to improve the performance of a window is to reduce the frame to glass ratio, or in other words, maximise the glass area. This is because modern glazed units perform better than the surrounding frame. This can be achieved by making windows larger, or by removing unnecessary mullions or transoms.
Examples U-values for different window configurations:
W1230mm x H1480mm
W1230mm x H1480mm
W1480mm x H1230mm
W1480mm x H1230mm
Standard V200: 1.55W/m²k
Standard V200: 1.72W/m²k
Standard V200: 1.82W/m²k
Standard V200: 1.91W/m²k
Optimum V200i: 0.92W/m²k
Optimum V200i: 1.09W/m²k
Optimum V200i: 1.19W/m²k
Optimum V200i: 1.28W/m²k
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