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What to know before you start replacing windows?

The windows are energy terms a key element in building envelope and the heat losses are windows (relative to their surface) greater than the losses through the peripheral wall. The windows are quite complicated part of the envelope of the house and quality windows are also quite expensive. It's the fact that we put on the window very contradictory requirements.

As a rule, we want windows to be opened, but when closed, must be sealed. Must transmit light and solar heat inside, but at the same time avoiding heat leaks out. They not transmit noise inside, but it must be reasonably light and thin.

The resulting window solutions is therefore always a compromise. If we want to reduce heat loss windows, it is good to know something about how to actually heat through the windows of the house leaks. The heat loss windows takes place in principle in four ways:

Infiltration: It is the penetration of air leakage between the door and the jamb (fixed frame embedded in a wall), or even between the jamb and the wall. We say that the window "pulls". In cold windy weather, or poor, or poorly fitted windows can recharge represent the largest portion of total heat loss of the window.

Convection: This is the movement of air around the glass. What thermally insulating, it is glass, but the (stationary) thin layer of air near the glass. As a result of the temperature difference but the air around the glass begins to flow and thus transmits heat.

Management: The air is fortunately a relatively poor conductor of heat, and therefore this method appreciably occur only where the glass close to each other (say, less than 1 cm).

Radiation: Every heated body radiates heat in the form of longwave infrared radiation (thermal radiation). This heat transfer only slightly less than the transfer by convection.

Choosing a new window

When choosing new windows or improving old and we want to make the most thermally efficient, so there are three important technical parameters:

U - heat transfer coefficient of the entire window, which indicates how much heat passes through one square meter, a difference of 1 ° C per 1 sec.
Usually manufacturers indicate Uw, which is the value for the entire window frame with Ug, which pays for itself glazing.
Currently produced triple glazing have a heat transmission coefficient Ug in the range of approximately 0.5 to 0.8 Wm-2.K-1

g - This value indicates the proportion of energy from the sun passes through 1 m2 of glazing. Higher g-value of glazing means that penetrate inside more solar heat and thus, saving more energy for heating. Conventional triple glazing having g = 0.50 (50%), insulating glazing 0,60 (60%).

ILV - coefficient feeler air permeability, which expresses the volumetric air flow in m3 / s to 1 m length of joints of windows and doors, at a pressure difference of 1 Pa.

We strive to make the U-value is minimized (low heat dissipation), while the highest value of g (greater heat gain from the sun). Unfortunately, the lower the U-value is accompanied by a lower value of g. Air permeability is a characteristic that shows the total "leakage" of the window and the value iLV should therefore be minimized.

Author: Karel Murtinger, adviser on energy savings.

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