Energy-efficient windows

"Variant Ltd." starts production of new energy-efficient wooden wondow, heat-insulating properties of which are twice as high as the ones of ordinary wooden windows with the profile thickness of 68 mm.

Modified coefficient of resistance to heat transfer of the new construction is 1,28 m² °С/W for the double-glass pane and 1,11 m² °С/W for the whole window. The latter is achieved due to the production of the frame from four-layered glued timber and, as a result, broadening of the window profile up to 86 mm. The additional glued connection affects the total heat conductivity of the whole comstruction, since the applied polyurethane adhesive PU-501 is perfect heat insulator. To explain haw we have succeeded in reducing heat transfer of the double-glass pane let us see how heat leakage through the window occurs.

Nowadays it is well-known that heat leakage through the window makes up 40%. The main reasons are the following:

First of all, glass possesses high heat conductivity rate. To reduce this rate you can resort to a two-, three- or four-chambered double-glass pane.

Secondly, heat leakage occurs through cracks between the frame and the pane in the windows of low quality. To prevent this problem you should seal the joints between the double-glass pane and the window timber thouroughly.

Nevertheless, the main reason for heat leakage is infrared radiation due to which heat leakage makes up 70%. To prevent this problem special energy-efficient glass should be applied. One of the surfaces of such glass has a low-emission coating.

Glass (as any non-conductor) possesses great emissive capacity E (common glass has E = approximately 0,83) in comparison with metals. The less E glass has, the better its energy-efficient properties are, as well as its capacity to reflect heat radiation back into the house. E of metals = 0,02-0,04.

Taking into account this information, specialists have become skilled at spreading by means of magnetron deposition over the glass surface one-layer or multi-layer covering which ensures heat radiation up to 0,04. This covering is a semitransparent layer of metals (mainly silver) with the system of antireflection coatings of various oxides (BiO₂, TiO₂, SiO₂). Glass with such an optically low-emission composition reflects back into the house over 90% of heat energy which is lost through the window. Thus, the warmth from electrical appliances which penetrates into the mass of energy-efficient glass cannot go out due to low radiating capacity of the covering and it has to come back into the house. Solar short-wave radiation in the visible range (wavelength is from 380 to 780 nm) free penetrate into the house from the inside. Thus, solar energy easily penetrates into the house.

Taking into account the indisputable advantages of the use of low-emission glass, we have conducted a trial and found out that minimal heat loss can be achieved by setting двухкамерного double-glass pane with 2 energy-efficient pieces of I-glass. Such glass must be installed from the outer side and the covering must be turned inward the double-glass pane. The width of such a double-glazed window should be 48 mm, compulsory condition is different width of the spacers (I4-20-4-16-I4).

If all these specifications are followed such a window construction is twice as warm as a usual двухкамерный double-glazed window and helps to reduce power inputs on the average by 40%. During the heating season the effect of the wooden energy-efficient double-glazed window by "Variant Ltd" is equivalent to fluid combustion in the amount of 120 kg.

The extended wooden profile makes up 86 mm

Nowadays Belarusian market proposes windows produced from three-layered glued timber the profile thickness of which is 68 or 78 mm. The constructions with less broad profiles are too cold for our climate. Even the windows with standard breadth (68х78 and 78х78) possess low heat-efficient properties. The resistance coefficient of heat transfer of window constructions should make up at least 1. Unfortunately, the resistance coefficient of heat transfer of the windows which Belarusian market proposes to customers today varies from 0,3 to 0,6 m² °С/W.

That is why "Variant Ltd." Is developing the production of new energy-efficient wooden windows, heat-insulating properties of which are twice as high as the ones of ordinary wooden windows with the profile thickness of 68 mm. Modified coefficient of resistance to heat transfer of the new construction is 1,28 m² °С/W for the double-glass pane and1,11 m² °С/W for the whole window .

How did we manage to achieve such a result?

1. The window profile is broadened up to 86 mm due to gluing four layers of timber instead of three layers.

Our experience and operation of glued wooden structures shows that the thickness of the glued parts should not exceed 35 mm, otherwise internal stresses in adhesive joints may increase. At the same time, the thickness of the layer for the glued window bars should be not less than 15 mm. The reason is that the thickness of the outer fold, which protects the conjugation pf the window leaves from rain, is 8-12 mm, and the adhesive joint should be hidden from direct atmospheric effects. Four-layer construction is the most balanced for the window profile with the thickness of 86 mm. It should be noted that the requirement of symmetry of glued timber must be strictly observed: the thickness of the outer layers should be the same. The middle two layers can be of any thickness, but not more than 35 mm. Another important factor is that the four-layered structure eliminates the influence of heterogeneity of wood on shape stability of the product.

The use of well-dried material of high quality.

Lamellas which are used for window timber gluing, match the angle of deflection of the annual rings towards the timber face. We use only radial logging whose angle is 45-900. The point is that the radial shrinkage of wood is twice as little as that tangential. This means that the thickness of the lamellae and beams, and hence the thickness of the details of window frames is almost unchanged during shrinkage and swelling. Thus, the density is guaranteed contiguity of window elements and as a result, high thermal insulation characteristics of the product.

3. The use of polyurethane adhesive for lamella gluing.

We use moisture-hardening single-component reagent polyurethane adhesive PU-501 which possesses high water and heat resistance. Among all the advantages of such adhesive the main property is heat insulation. Due to profile broadening up to 86 mm and use of four-layered timber there is an extra adhesive joint, which increases the total thermal conductivity of the whole structure.