Eco-building Club members

Project “Eco-Building International Club for advanced European sustainable energy technology dissemination in Europe and China ”

is funded by the EC DG TREN within ENERGIE Programme

Advanced Glazing Systems

The increased use of glass in architecture today makes it imperative to consider the comfort of a building's occupants.

Windows are often one of the largest sources of heat loss in winter due to their low insulating ability and high air leakage rates. Windows are also generally the major source of unwanted heat gain in the summer. As a result, windows are typically net energy losers, and can be responsible for 25 to 50 percent of the energy used to heat and cool homes and commercial buildings. However, improved windows, combined with proper consideration of their placement and other details, can result in windows that provide a net energy gain.

MAIN SOURCES OF ENERGY LOSSES WHITHIN WINDOWS USE

Air leakage

Cracks around windows are usually the major source of the air leakage that affects so significantly the performance of buildings. Air infiltration increases the heating load in winter and may limit comfort conditions adjacent to windows; in air conditioned buildings it increases the cooling load in summer. On the other hand, it provides some or all of the outside air needed to control odours and relative humidity in buildings without mechanical ventilation, and may supply as well the air required for the combustion of fuel for heating.

Air leakage around windows also affects the performance of the windows themselves. The configuration of the air flow path is important with respect to their resistance to rain leakage and dust penetration, and with double windows largely determines the conditions under which condensation between panes will occur.

Heat losses and condensation risk

Due to their high thermal transmittance value(U), old windows represent energetic cracks in the buildings, especially in cold climates.

The transmittance is the rate at which any body, in particular windows, conducts non-solar heat flow. It's usually expressed in units of W/m 2 K. For windows, skylights, and glass doors, a Uw factor refer to the entire window performance, including frame and spacer material. The lower the Uw factor, the more energy-efficient the window, door, or skylight.

U-values are important in colder climates, wherever 24-hour comfort is important, and where condensation must be avoided. The U-value (or R-value, the inverse of the U-value) is most important where heating costs are a concern, since the difference between indoor and outdoor temperatures is usually highest in winter. It is also critical for occupant comfort. In cold weather, anyone near a single-pane window with a high U-Value will feel chilled, regardless of room temperature, due to the body's heat loss by radiation to that cold glass surface. Additionally, room air chilled by contact with the cold glass falls along the window, creating a cold draft.

The condensation

Moisture vapour in the air tends to condense on cold surfaces in a home where the air reaches its dew point. This is common when there is great difference between inside and outside temperatures and when the relative humidity of the warm side is high. Typically, the coldest surfaces in a home are on the windows - most often at the edges where conduction is greatest. In extreme cases, when indoor humidity is very high, chronic condensation at the edges of the glass can create a significant moisture problem that leads first to peeling paint, then to mildew (a type of mould), and eventually to rot. Even for this, the lower the U-Value, the lower the condensation possibilities.

Thermal bridges

A thermal bridge is created when materials that are poor insulators come in contact, allowing heat to flow through the path created. The picture below shows an infrared picture.

The greenhouse effect and the overheating risk

Buildings with large glazed facades receive a lot of energy from the sun mostly in the form of visible light. The bulk of this energy is not absorbed by the intern air since this is transparent to visible light. 50% of this solar energy reaches walls and the floors, absorbed as heat. Because of their higher temperature, those surfaces radiate energy in infrared range. Air is able to absorb infrared radiation as it is not transparent to infrared radiation. Infrared radiation is absorbed from all directions and is passed as heat to all the room.

This effect may be or not be a positive fact or not depending from the local conditions. In cold climates the greenhouse effect allows to get an important solar gain that let possible reduce the fuel consumption for heating. In hot climates the environment will need extra power for cooling the exceeding temperatures. This is worst especially for commercial buildings that in general are built with a large use of glass.

EXAMPLES OF SOLUTIONS OF DEFECTS

Double-triple glazing

The other key component to the overall performance of the window is the profile, having even 30% of the total impact, in terms of exposed surface area.

PVC profiles designed with closed rooms posed in series are particularly suited to obtain excellent thermal transmittance values. Moreover the PVC has a relatively low thermal conductivity. This allows greatly improved performance compared to materials such as aluminium, where the influence of thermal bridges is predominant.

Installing double-triple glazing is one of the best ways of making homes more energy efficient. The use of different glass panes reduces the transfer of heat between two glass layers which individually has poor thermal insulation characteristics. The space between glasses panes, which usually is filled with air or gases with more efficient thermal insulation characteristics (like krypton or argon), acts as higher thermal resistance layer that bring the whole conductivity U w to lower values.

Over a space of 16 mm in thickness, however, internal convective movements of the gas not allow more performance improvements.

Low Emissivity

Emissivity measures how strongly a product emits or radiates absorbed heat. The lower the number, the more efficiently the glass reduces conductive heat gain or heat loss, which means a lower U-Factor and better insulation. For comparison, uncoated glass has an emissivity of 0.84 while an advanced low emission glass has this value around 0.15, this means only 15 percent of heat absorbed is re-emitted from the coated side. This feature is useful as it reflects energy back towards where it came from. If a solar control glass is used, then adding a lite of Low-E on the room side acts as a barrier to the absorbed heat in the glass passing to the inside of the building. For buildings that require passive heat gains, a low emissivity coating with clear glass allows direct solar radiation to pass through the glass and then traps it inside.

The correct installation

It is possible to tighten air leaks in the building envelope. In many cases it is difficult, e.g., electric installations are not designed to be airtight. By careful planning of the placement of the plane of tightness in the construction it will be possible to reduce the penetrations of the building envelope. The drawback is that the building might be too airtight leading to a high relative humidity indoors.

Selective film

Shading systems

A good way to take advantage of solar gains during cold periods, avoiding overheating risks during the summer is a smart shading system design. This solution could seem too simple but represent the best rate between efficiency and a low cost solution. The base principle to keep in mind is that while during the winter the solar orbit is lower, that becomes higher during the summer. In consideration of that, it is possible to set external shading architectural elements in the right length and height proportions; that allows to the sun rays to reach the glazed surfaces when outside is cold and the sun is lower, and don't let it possible during the summer.

Benefits

•  The environmental comfort

•  Improve comfort

•  Inside temperature of the glass is closer to room temperature

•  Drafts are reduced or eliminated

•  Eliminate condensation

•  Allow installation of smaller capacity heating and cooling equipment

•  Reduce fading of carpets and furniture fabrics

( document prepared by ISNOVA )