How to increase the u value of a window

Aluminum windows thermal insulation properties improvement - what else is possible?

Aluminum windows have gone through a huge progress in their development in recent decades - from the thermal bridge, they have evolved into a highly efficient component that is even suitable for passive houses. Of course, there is still much room for the improvement of the properties.

To begin with, there are still a lot of ways of improving aluminum composite windows. A new approach can always be found with a strong will and desire. As the writer Franz Kafka said:

Paths are made by walking them.

Tip 1: Improving U-values with wide polyamide insulating strips

Windows made of metals led to high energy losses due to the high thermal conductivity of the material in the past. In order to minimize the heat transmission through the frame of a modern aluminum window, the profiles are thermally separated today. This task was accomplished by insulating strips made of polyamide, since the material has a very low thermal conductivity. Also, the ABS insulating bars, which consist of the acrylonitrile-butadiene-styrene copolymer are being used. In order to achieve a further improvement of the U-values of the window frame, a larger insulating zone could be utilized, for example, by wider distances between the aluminum frame shells of the window profile. With appropriately fitted polyamide insulating bars with reduced lambda value, energy efficiency can also be increased. However, as the stability and usability of the window must be guaranteed, it is important to limit the broadening of the profiles and the increase in the distance between the aluminum shells.

Tip 2: Optimizing the insulation values by foaming the hollow chambers of the profile

Due to their design, aluminum window frames have some empty space inside. These can be used to optimize the U-value of the frame and thus the insulating effect of the window. For this purpose, some insulating foams are introduced into the interstices of hollow chambers insulating profile. For example, polyurethane foam insulation is commonly used for his cause. These measures allow to achieve efficient thermal separation.

Tip 3: Increasing the efficiency with the insulating glazing

Depending on the type of building, the glass surfaces of windows can make up a large proportion of the exterior wall and therefore they should have corresponding insulation values. Because of that, Insulating glazing, consisting of two or more panes is frequently used. The cavities between the panes get hermetically sealed and filled with gases as argon or krypton to minimize heat flow through the glass. Triple-glazed aluminum windows and polyamide insulating bars achieved sufficient enough U-values so that now they can be even used in the passive house area.

Tip 4: Lowering the heat loss with reflective foil or low-E foil

With highly reflective low-E foils, also known as reflection foils, the thermal properties of the frame and thus of the entire aluminum window can be significantly improved. The low-E foil is applied to the bar or the glazing across the direction of the heat transmission, reducing the energy losses caused by the heat radiation emerging outside. This allows the further optimization of their insulating effect and efficiency of a window without changing its structure. And of course, the highly reflective foils for glass surfaces are transparent, allowing the light into the room without major losses.

Tip 5: Energy consumption optimization through software

Improved insulation zones with insulating bars made of polyamide and foam-filled hollow chambers turns an aluminum composite window into highly energy-saving one and therefore environmentally friendly. Therefore, the profiles of such aluminum window frames are considered as "High Sustainability Profiles", i.e. as frame profiles that are most suited to ecological sustainability in all respects. Taking into account complex design and a large number of materials used in production, the level of U-value of an aluminum window can no longer be precisely calculated without the aid of software. The mechanical load capacity must be also included in the development of windows in addition to thermal protection issues. Software that visually displays heat flow helps identify profile vulnerabilities and optimize energy efficiency.

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You can try that for free with the Thermevo Flixo Cloud.

Adding insulation to walls in new construction or upgrading insulation in existing residential buildings has gradually become common sense in Australia.
It has been seen as a relatively cost-effective way of improving the star energy efficiency rating of new builds and renovations. However, we also know that holes and thermal bridges in the building fabric compromise thermal performance and a major source of these are the windows.
Windows are traditionally the weakest link in residential buildings when it comes to energy efficiency but here, we love our windows! So, can a window be as thermally efficient as a wall and help improve the energy star rating of our homes?
We normally measure the thermal performance of walls with R-values, the measure of resistance of a building fabric to heat flow, where the higher the better. On the other hand, the thermal performance of windows is usually expressed in U-values, the measure of heat loss through the window system, the lower the better.
For the purpose of comparison, using the U-value measure, an uninsulated cavity wall might have a U-value of approximately 1.6 W/m2K, while a solid wall approximately 2.0 W/m2K. Adding insulation will improve the U-values even further.

The Building Code of Australia suggests that for Melbourne, the minimum total R-value for walls should be 2.8, which is equivalent to a U-value of less than 0.4 W/m2K.
But window systems currently offered in Australia commonly have U-values of well over 4.0 W/m2K. In fact, of the WERS rated residential window products offered in Victoria for example, almost half have U-values at or higher than 4.0 W/m2K. Less than three per cent of products offered have U values under 2.0 W/m2K.
A 2010 report prepared for the government investigating the pathway to 2020 for low energy, low carbon buildings found that there are diminishing returns to adding or upgrading insulation in walls and roofs. Once insulation has been optimised, windows become the next most important element in residential energy efficiency.
Good building design optimises solar passive design through window distribution, glazing ratios as well as orientation. Good window design means considering the glazing, sealing, frame material and installation of the window system.

Glazing

Glazing options are growing in Australia as supply of insulated glass units (IGUs) and low emissivity coatings improves. Single glazed clear glass should be a thing of the past, as it is in American residential architecture. The standard in the US for some time has been an insulated double glazed low emissivity window.
Better performance is achievable with hermetically sealed IGUs with at the very least a 12-millimetre gap between panes. Less than a 12-millimetre gap and the insulation effect starts to drop exponentially. Increasing the gap to 16 millimetres can reduce U-values closer to, or below two where low conductivity frames are used.
To complicate things, it is not necessarily the same glazing system for every window in a building the choice of glazing depends on the building's design, the window's design and orientation as well as climate. Visible light transmission and solar heat gain coefficient are necessary considerations.

Seals

Higher performing windows have continuous, built-in double sealing systems and multiple locking points to provide a tight seal between the sash frame and the house frame reducing air infiltration. It is also worth looking at whether there are pressure seals on sliding glazed doors and windows to reduce the progressive wear and tear that otherwise occurs with regular seals.
It has been reported that a fairly common practice in Australia is to rely on the architrave to provide a barrier to the wall cavity instead of sealing windows between the window frame and house frame.

Frame material

The material used in frames plays a significant role, not just for structural strength but also for thermal performance of the window system. Frames should overall be poor thermal conductors. Aluminium, however, is the fourth most conductive material (after silver, copper and gold) and about 1,000 times more conductive than uPVC. It therefore needs thermal breaks to improve performance.
Thermal resistance of a window system can be improved by choosing insulating materials like uPVC or timber and/or frames with insulated cavities and the thermal bridges eliminated. Highly thermally conductive frames have become virtually extinct in Europe, yet remain a standard offering in Australia.

Installation

To improve thermal performance, windows need to be installed airtight to the building. This can be achieved by using expanding foams or tapes, which together with good sealing systems can dramatically improve airtightness. Under the Building Code, all windows have to meet Australian Standard air leakage rates. The UK and the USA, for example, go further by mandating physical air tightness testing of new buildings.

There is no doubt that windows have a hugely important role to play in improving the energy efficiency of residential buildings. What we install today, whether for new builds or upgrades, will potentially lock in greenhouse gas emission impacts for decades.
A valid concern is the initial purchasing cost. High performance windows are more expensive in Australia than the incumbent standard, single glazed, thermally bridged window. And no doubt a function of supply-demand, they are costlier here than in major overseas markets where they are mainstream.
However, an Australian Window Association capacity/capability study found there was capacity locally to switch to majority fabrication of IGUs within 12 months. Based on overseas experience, the cost of IGUs can be expected to drop substantially when codes and policies shift the market to higher performing windows. With the right policy transition pathways and market pull, there is no reason not to expect a move to efficiently insulating windows similar to that of the US or European window markets.
So can windows be as thermally efficient as a wall? In theory, yes, evidenced by the emergence of 'Super Windows' with U-values less than 1.0 W/m2K in overseas markets. But here, we must first acknowledge that window systems are an important element for achieving residential energy efficiency.
 

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