About 30% of the energy consumed in a building can be wasted by the windows, if they are not thermally efficient.
Efficient windows are synonymous with energy conservation, high standards of thermal and acoustic comfort and better health and well-being.


The windows, as an element of the envelope of a building that separates the useful interior space from the outside environment (usually at different temperatures), are subject to different modes of heat transmission from the hot to the cold zone, namely:

  • Thermal conduction, which reflects the ability of the window to transmit heat from the inside to the outside of a home and which can be translated into a parameter called the thermal transmission coefficient (Uw). The lower the value of this parameter, the lower the thermal losses and the better the window's insulation capacity;
  • Radiation, which results from the relationship between the energy of solar radiation that passes into the space and the exterior energy that falls on the window, translated by the solar factor of the glass (g). The lower the “g” value, the better the protection against overheating.

The performance levels of the windows vary depending on the region of the country.
Portugal is divided into three winter and summer climatic zones, for which thermal transmission coefficient (Uw; W / m².K) and solar factor (g) requirements are defined, which are applicable both in the construction of new buildings, or in interventions in existing buildings (for example, in the replacement of windows).



The windows are also decisive for the renovation of the air (ventilation) of the spaces they serve.

Ventilation is the substitution of indoor air for outdoor air, preferably in a controlled manner, and the legislation in force requires that at least 40% of the air be renewed every hour to promote better health conditions for homes.


An orientation of the windows in relation to the sun allows to define some technical conditions so that they do not occur excessive in the winter, nor overheat in the summer.

► In the case of windows facing north (without sun exposure) and to minimize thermal losses to the outside, it is recommended to install windows with thermal cut or with a low thermal transmission coefficient (Uw) value and with colorless glasses without need special low-emissivity requirements.

► In the case of windows facing south, with strong sun exposure, windows with low-emission glass combined with sun protection (blinds, shutters, curtains, etc.) or with glasses with a lower solar factor (gv) should be chosen . Note that, in winter, windows in this orientation can help to have better thermal comfort through thermal gains in space resulting from incident solar radiation.


The windows can be sliding, casement, oscillating, tilting, projecting, fixed, among others.

The most functional option for the intended use and aesthetics should be chosen, considering that, in terms of energy and as a general rule, how sliding windows can perform poorly because they have greater air permeability.


Most existing and older buildings still have simple glass windows (with lower performance), which represents an enormous potential for reducing energy consumption and improving the thermal and acoustic comfort of homes.

The use of double glazing in windows is currently the most adopted solution in new windows. Double glazing was created to increase the thermal insulation of the window, consisting of two glass sheets separated by a space of air or noble gas (eg argon, krypton, xenon).

The thickness of the air gap may vary, and a 16 mm air box is the most common. Triple glazing, like double glazing, appeared to further increase the thermal and acoustic insulation of the windows.

To improve safety (against fires and break-ins) and prevent the falling of fragments, you can opt for laminated glass (with plastic film between glass) and / or tempered glass (with more mechanical resistance and which break if broken). These solutions also allow to improve the sound insulation.

In cases where it is intended to privilege the protection of children, it is recommended that the windows be equipped with:

  • a) tempered safety double glazing, so that, in case of breakage, children are not hurt;
  • b) Inversion of the opening direction of the swing-stop windows so that children cannot intuitively open them.

The use of colored glass, films or films of solar control and the incorporation of noble gases in the air chamber (e.g. argon 90%), allow to obtain an improvement in the thermal performance and control of the situations of overheating of the spaces.


Spacers for double or triple glazing are the structural connecting element that guarantees the stability and physical resistance of the glass. The most frequent are aluminum, which have a good mechanical capacity but conduct heat or cold more quickly, compromising thermal performance. There is another metal that can be used with better thermal performance: stainless steel.

The choice of non-metal (non-thermally conductive) materials for the spacer, such as thermoplastics, fiberglass, silicone, foams or hybrid systems, promotes greater energy efficiency for double or triple glass as a whole.


The natural renovation of the air in existing housing buildings is often carried out by opening windows and by uncontrolled air infiltrations through elements of the environment such as doors, windows and shutters.

To maximize the well-being of the occupants of the spaces, ventilation must be controlled, ensuring the renewal of the indoor air with clean fresh air (from the outside). The windows can promote a correct ventilation of the room, as long as they do not allow air infiltrations that cause discomfort.

When replacing old windows with efficient windows, it is essential to ensure that they guarantee conditions of permanent and controlled ventilation.

The windows have an air permeability rating (from 1 to 4) that measures the amount of air that passes through the window slots when closed. A Class 1 window is more permeable and a Class 4 window less permeable to air. It is recommended to install preferably self-regulating devices (intake grilles) to promote controlled ventilation in Class 3 or 4 windows.

Ventilation ensures the proper functioning of combustion devices (fireplaces, stoves, etc.) and the removal of smoke resulting from an accidental fire (smoke evacuation).

The installation of windows with self-regulating louvers to control ventilation and renew indoor air allows to improve humidity control, dilute and remove pollutants and odors, as well as minimize the proliferation of fungi and molds.

A self-regulating grid works on the principle of variation of the air passage section depending on the pressure variation, and can be installed on the window profiles, on the walls and blinds.


Noise pollution can disturb sleep, rest, relaxation and concentration, with effects on health and well-being. Its prevention may involve reducing the transmission of noise from sources abroad.

The acoustic transmission is divided into:

  • Direct Transmission - carried out through the separation element;
  • Marginal Transmission - that occurs through other constructive or support elements;
  • Parasite Transmission - promoted through openings in the elements.


The sound insulation level of a window (which reflects its ability to resist noise sources from outside) can be measured using the sound reduction index (Rw) expressed in decibels (dB). The higher the value, the better the respective acoustic performance, that is, the greater the reduction of outside noise to the interior of your home.

The installation of double or triple glazing and with glass of different thicknesses is essential to guarantee an adequate acoustic attenuation.

Acoustic well-being can also be promoted by the application of sealing joints between the walls and the frame, in order to eliminate marginal noise transmission. Another solution is the installation of acoustic absorbent materials (styrofoam, rock wool, etc.) in the roller blinds to reduce parasitic noise transmission.


The windows have a water tight rating (from Class 1 to Class 9) which represents the window's ability to prevent water from entering outside into the interior.

In the installation, the correct water tightness must be ensured so that there is no infiltration into the interior. Adequate insulation of the connections between the profiles, the fittings and the seals must be guaranteed.

The application of leaks to ensure the drainage of water accumulated in the frames will have a positive impact on the state of conservation of the window.

The sealing in the casement windows, composed of rubber (EPDM), PVC or mastic, must seal the frames and, whenever possible, be reinforced with double or triple seals. The selected material must also have flexibility and resistance to ultraviolet radiation.

The sliding windows are sealed with plush to allow the sliding of the leaves, and may not be enough to give the frame a complete seal.


Accessories represent a significant part of the cost of windows and are subject to intense use. For this reason, these components are more susceptible to wear, so a correct selection of the hardware solution and maintenance is essential.

The main materials used in the production of hardware are aluminum, brass and stainless steel, the selection of which depends on the type of material used in the frames and the intensity of use.

359/5000 The selection of hardware must be a function of the type of window openings: sliding, stop, swing-stop, tilting, projecting, or other type. When anti-intrusion security is a concern (for example if you live in a house or ground floor), reinforced hardware with frames and safety glass (laminated and tempered) should be used.


Solar protection promote the control of heat and light available in the room.
In the winter it must be ensured that the protections can be opened to maximize the entry of solar gains and in the summer that they are able to control the excessive entry of solar radiation, thus improving comfort and avoiding overheating of the spaces.

The available systems can be divided into exterior, interior and integrated sunscreens (between glass or between windows). The most used protections in Portugal are blinds or shutters and exterior shutters. Interior shutters are more common in old buildings.

The outer protections have a better thermal performance compared to solutions with protections from the inside, as they reduce the solar radiation that falls on the glass and whose heat is transmitted to the interior of the space.



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