On average 25 to 40% of heat escapes through the windows. To limit this occurrence, windows used should be characterized by good parameters. Pay attention to heat transfer coefficient U, and thermal energy transmission coefficient g. However, keep in mind that loss of the heat is also determined by window’s construction as a whole and the way of installation. There should be used windows with low heat transfer coefficients in order to reduce heat loss, lower than standard 1.6W/ (m2*K) for a whole window.
The guidelines for houses with low energy demand say, that woodwork cannot have Uw higher than 1.3 W/(m2*K). This value can be found as a maximum in many projects where cost evaluation is the cheapest option, which is understandable because the warmer woodwork the higher price. Very often manufacturers offer windows with better parameters, even with U equal to 0.5W/(m2*K) which allow for significant reduction in energy loss.
At the same time the designers try to layout the rooms in the building that will provide functionality and allow for maximum use of solar energy getting into the home through the windows. In this manner expenses spent on purchase of the windows can be compensated by reduced usage of other sources like: gas, oil, electricity. The highest profits are given by the windows where the glass have high transmission coefficient for solar energy g. The higher it is the more radiation goes through the window into the inside.
The lowest energy loss at the highest profits are achieved by so-called active windows that have Uw S 0.9 W(m2/K), and g > 45% or less.