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4.6.5 Windows

Multiple factors influence the windows efficiency, making certain types of window frames or window glass much more suitable than others, depending on the area in which they will be applied and the climatic conditions to which they will be subjected.

Regarding the chosen openings, the first guidelines were dictated by the climate analysis, which highlighted, given the prevalence of low temperatures during the year, the need of using high-performance windows, which would allow to have low heat losses. At the same time the windows should have had an adequate g value, to capture the highest possible percentage of sunlight during the coldest months, so as to reduce the heat load due to heating, while at the same time increasing the light transmission as much as possible.

The analysis started from the window types used in local buildings: historic buildings use single glazing, which therefore guarantees very low performance, while the construction standard of new homes now requires the installation of double-glazed windows.

Therefore the provisions of the Italian legislation were evaluated, which with the DM 11/01/201710, defines the transmittance limits of the windows for new

4.10 Italian ministerial decree 11 January 2017, Adoption of minimum environmental criteria for interior furnishings, construction and textile products.

construction interventions depending on the climate zone.

The area of Pingyao, as defined in the climatic analysis, corresponds to the climate zone F: in this case there is a limit of U = 1.60 W/m2K.

To connect the initial indications, with the normative standards, the choice was directed towards the use of low-emissivity glass windows, which appeared to be the most suitable for the design requirements.

Today their application field foresees the use in two types of windows: those with low-emissivity double-glazed glass, coated on one side and filled with argon or air inside; those with three-sheet low-emissivity double-glazed glass, equipped with two film-coated plates filled with argon or air. However, triple glazing has a higher average cost than double glazing, in the order of 10-20% more, is much heavier, weighing on the frame structure and the glass is less transparent, greatly reducing light transmittance; referring to the latter, during the mid-seasons, when the heating is not used, they have an excessive isolation, not allowing the sunrays to pass and heat the internal environment. As a result, triple glazing is suitable for dwellings exposed to harsh temperatures, such as mountain homes, or for houses constantly exposed to cold winds.

Therefore the choice fell on the use of double glazed windows, which reach all the necessary values, both thermal, acoustic; those with triple panels would be excessive for the project carried out in this thesis, both in terms of cost and performance.

Regarding the gap between the glasses, it is filled with argon gas, whose properties allow the heat losses to be reduced.

The chosen glasses have the following values:

Talking about the window frames instead, mainly three materials are used nowadays: wood, PVC and aluminium. Therefore it is necessary to understand which is the best application in terms of thermal and acoustic insulation, to evaluate the component cost, which clearly depends on the economic availability, and to understand the atmospheric conditions that the fixtures want to resist over time.

The wooden frames provide good thermal insulation, and for this reason they are often applied in cold places. Furthermore, wood gives the possibility to be cut, finished and personalized according to the project requirements. On the other hand, wood suffers prolonged exposure to the sun and to the weather in general, requiring some maintenance in order to last for years.

Regarding the wood type to use, in colder climates the best choice are the conifers, or trees with needle-like leaves. On the other hand, the trees with

Window composition (from in to out) 4 - 20 - 4 [mm]

Thickness 28 mm

Ug value 1.1 W/m2K

g value 65 %

Light transmittance value 82 %

Tab. 4.1.4

Assembly, thickness and thermal transmittance of the internal floor of the designed buildings.

broad-leaves are indicated for a milder climate. A valid alternative is the teak, a wood whose characteristics give it high resistance to the elements and a good durability.

The aluminium frames are a valid alternative since they offer some substantial advantages: the main one is that they do not deteriorate over time, resulting in a long-term investment; in addition, they weigh little and are ideal if large-sized frames are needed, or whose shape should be moldable. On the other hand aluminium is a conductive material therefore it does not provide good thermal insulation, or even acoustic; furthermore, condensation is often formed due to the difference in temperature between the inside and outside of the building. But nowadays to compensate for these problems, aluminium can be used very effectively in combination with wood, with considerably higher costs.

The PVC (polyvinylchloride) frames instead, are the cheapest on the market and they can be modelled according to needs, in terms of shape or colour;

in addition, unlike aluminium, they guarantee good thermal and acoustic insulation without creating any condensation problems.

The main contraindication is the potential deterioration of PVC itself when exposed to certain conditions: the prolonged exposure to the sun, for example, strongly undermines the solidity of the PVC frames and, consequently, their effectiveness in insulating the house acoustically and thermally. Another negative aspect is the inability to restore the original state of PVC in case it deteriorates.

Since each of the three materials has advantages and defects, which make them more or less suitable depending on the project, the last option is to evaluate fixtures that take advantage of a combination of two of the materials above mentioned, combining the best features of each.

Fig. 4.67 Graphic rendering of window (on the left), and glazed door (on the right) used.

Source: Author’s elaboration

In the aluminium-wood fixtures, for example, the external part is made of aluminium, but on the inside they have a wooden cover that increases the thermal performance. This combination can be the best one since it limits defects and maximizes advantages of both materials. As a result, the frame becomes resistant over time, a characteristic of aluminium, and exploits the thermal and acoustic insulation that wood guarantees over the years. The main negative aspect concerns the costs of aluminium-wood fixtures, as processing requires both materials to be processed, thus increasing the costs of the frames themselves.

For this project, finding a compromise between costs and fixture performance, it has been chosen to use the wooden frames, thanks to all their favourable characteristics related to insulation.

The total width of the frames is 15 cm, and its U value is 1.327 W/m2K. It is then possible to make an example calculating the total transmittance of a window designed for the private house, that is one of the windows facing north in the bedrooms, of 50x150 cm.

The formula is:

Uw = AfUf + AgUg + lgψg where

Af = frame surface Ag = glass surface lg = glass perimeter Uf = frame transmittance Ug = glass transmittance

ψg = distance correction value of the glass

Uw = 0.533*1.327 + 0.434*1.1 + 3.3*0.048 = 1.38 W/m2K

In document Design of energy efficient pilot buildings in the Hou Ji Village, Pingyao, China (Page 164-167)