Aluminum windows: warm and cold

Climatic impacts on windows

The main climatic influences, which must resist the windows of buildings - and aluminum windows too, are:

• rain load - heavy, especially scythe, rain;
• wind load;
• excessive penetration of outside air;
• large temperature difference outside and inside and the associated heat loss through the window;
• big changes in outdoor temperature during the year.

These climatic effects, of course, They are very different in different parts of the globe. National standards of different countries, usually, take into account local climatic conditions, when set technical requirements for windows - aluminum, wood, plastic, steel. Therefore, the windows requirements, for example, in Finland or Norway, will differ significantly from the requirements of the Mediterranean countries.

Water-resistant aluminum windows

Torrential rain is the main cause of damage, which arise due to increased moisture. And not the most aluminum window frames, a wall adjacent to a window. If aluminum window Properly designed and installed, it protects from rain, not only himself, but also the adjacent wall. Aluminum profiles, unlike other window profiles allow you to organize effective drainage of water, that somehow gets inside the box.

Figure 1 – Designing optimal drainage [1]

Figure 2 – Mutual arrangement of glass unit linings and drainage holes [1]

Wind load on the window

Strong winds can cause damage to buildings, including, windows. Therefore window aluminum profiles must have a sufficiently high stiffness and strength, to resist any wind load deflection, like elastic, and irreversible. Known calculation methods and computer programs, which allows to determine the maximum window size - height and width - for different types of profiles for different wind loads, those, which are typical for the area.

Breathability aluminum windows

Wind creates a pressure difference between the air inside and outside the building. This leads to the penetration of outside air box and, Consequently, heat loss, especially during the cold season. For each new design of this feature of the window - the resistance of air permeability - are determined experimentally in a test lab.

Aluminum window profiles securely hold their shape during the entire service life, including, straightness and flatness. This ensures a tight contiguity of the various elements of the window, for example, frame and sash, to each other, and, Consequently, reliable window sealing.

Figure 3 – Sealing between glass, frame and glazing bead [1]

Thermal resistance of aluminum windows

Window Frame of purely aluminum profiles - without termorazvyazki - will pass through itself in the cold considerable amount of heat, the. Such pure aluminum frame is called a "cold" aluminum frame, in contrast to the "warm" aluminum frame.

Figure 4 – Three ways of heat loss in an aluminum profile [1]

Each structural member "warm" aluminum window consists of an outer and inner aluminum profiles, which are interconnected termorazvyazkoy of a material with low thermal conductivity. Termorazvyazki material should not only have low thermal conductivity, but also sufficient strength, to provide a "warm" aluminum profiles the necessary rigidity and strength.

Glass-filled is most often used as a material for thermal decoupling of aluminum profiles (25 %) polyamide and, rarely, polyurethane.

Changes in ambient temperature

The thermal expansion of aluminum and its alloys is extremely large. it is 0,024 mm in each meter with temperature change 1 Celsius degree. Temperature drop of aluminum profile from winter to summer can be 100 oC, it is quite possible, especially for aluminum profiles with a dark surface - painted or anodized. It means, the length of each meter of the outer box element will try to change a little bit, a lot on 2,4 mm.

In tropical countries, especially for large aluminum structures, thermal expansion of the outer elements of the windows can be a serious problem - they can lead to distortion – buckling. To prevent warpage of aluminum windows apply various special methods sliding compounds, provide gaps between adjacent aluminum profiles and apply a special sealing joints, which allows displacement of the joint surfaces.

Aluminum alloys for window profiles

Aluminum windows - the frame and sash - is made almost exclusively from two related and similar alloys - 6060 and 6063. International Aluminum Alloy 6063 coincides almost completely with the domestic alloy AD31.

Figure 4 – Chemical composition of the aluminum alloys 6060, 6063 and other alloys of the 6xxx series [1]

Aluminum window profiles

These aluminum alloys - alloys 6060, 6063 and AD31 good press on extrusion press - have a good compressibility. It allows to make of these profiles with complex cross-sections with the, to provide them with maximum functionality as window profiles.

Moreover, profiles made of these alloys lend themselves well to machining: they are easy to cut, drill, mill, punching holes in them. In svezhezakalennom, unaged state, and after natural aging of these alloys respond well to bending. When alloys need 6060, 6063 AD31 and easily welded.

Aluminum window profiles have the following advantages compared to other joinery, for example, polyvinylchloride (PVC) and wooden:

• the cross-section of aluminum profiles can take on the most complex shapes;
• aluminum profiles have high chemical resistance under the influence of the environment;
• window aluminum profiles have a low weight per running meter;
• aluminum window frame has high corrosion resistance;
• the aluminum window frame is practically maintenance-free;
• the aluminum frame does not emit poisonous gases in the event of a fire;
• the surface of the aluminum frame can have almost any color;
• Aluminum window frames easily and metallurgical processing served reapply after their useful life.

Termorazvyazka aluminum profiles

The only serious "weakness" of aluminum, as a material for window frame - is its high thermal conductivity. This problem is solved by the use of "warm" aluminum profiles - aluminum profiles with termorazvyazkoy.

Figure 5 - Aluminum window without termorazvyazki

In the world use two basic methods of manufacture of "warm" aluminum profiles.

One of them is, that between the outer and the inner aluminum profiles are inserted and mechanically secured so-called insulating or thermo-insertion insertion. Sometimes they are also called thermal bridges. The material of these thermoinserts often a polyamide with the addition of 25 % fiberglass (drawing 6).

Figure 6 - Aluminum window termorazvyazkoy polyamide thermoinserts.

Another way, distributed mainly in North America, a method, which is called «pour and cut», something like "pour and cut" (picture 7). Aluminum profile consists of two halves, outer and inner, which are interconnected by thin bridges. The half-cavity aluminum poured special liquid plastic mass, usually, polyurethane. After that, as polyurethane hardens, webs between the outer and inner parts of the aluminum profile are cut. Thus, polyurethane forms a thermally insulated insert between two aluminum profiles.

Figure 7 - Aluminum window termorazvyazkoy «pour and cut»
(или «fill and de-bridge»)

Figure 8 – Three methods of manufacturing combined profiles for better thermal insulation [1]

Thermal resistance of aluminum windows

The figure 9 shows the result of a numerical comparative calculation of the temperature distribution over the window cross-section for winter conditions: a "cold" aluminum window (without thermal isolation) and a "warm" aluminum window (with thermal isolation).

Figure 9 - Temperature distribution over the window section in winter time:
and - aluminum window without termorazvyazki,
b - aluminum window termorazvyazkoy

It is seen, that in an aluminum box without termorazvyazki cold outside easily extends into the window and the inner window frame surface which is substantially below the inner surface temperature of the glazing unit.

The "warm" aluminum window termorazvyazka stops cold and on the inner surface temperature of the aluminum frame is not lower than, than on the inner surface of the glazing unit. Front negative temperature passes through the middle of the glass and bridging.

Effect of structure on the thermal resistance of the window

Examples of design features of the influence on thermal resistance of the window shown in Figures 10-13.

Figure 10 – Influence of design features on heat transfer [1]

Figure 11 – Installation of double-glazed windows of various thicknesses in windows [1]

Figure 12 – Corner connection angle 45 degrees [1]

Figure 13 – Corner connection angle 90 degrees [1]

See. also Aluminum window: heat transfer