Heat losses through windows
A building loses good heat to the outside when the heat is emitted through the floors, walls and roof. Heat losses also occur via the ventilation or porosities in the building envelope. However, the question is whether porous and poorly insulated windows are not perhaps the greatest villain in terms of heat loss. In some houses, windows have been estimated as accounting for as much as 35% of heat losses. The heat losses are due to room heat being emitted through the glazing, frame and sash (radiation losses), through movements of air between the glazing (convection losses), via fittings and spacers (conductive losses), and through porosities between the frame and sash or poor sealing strips (leakage). Energy-rated quality windows are designed to minimise heat losses via stringent requirements governing sealing characteristics and insulating capacity.
EQ imposes requirements on air tightness
All windows sold within the EU are covered by the harmonised product standard EN 14351-1 and must carry the CE marking. This also means that the window’s essential characteristics must be set out in a declaration of performance. The purpose of this is for the customer to be able to compare windows from different manufacturers and arrive at a relevant assessment as to whether the window meets national requirements.
CE marking does not impose requirements on the value/class of the declared performance, but only on how characteristics are to be determined and reported. If a window is lacking the stipulated value/class for a characteristic, the manufacturer may choose to enter “NPD”, which is an abbreviation for “No performance determined”.
If air tightness is to be reported in the CE marking for a window, air permeability in accordance with EN 12207 may not exceed 50 m3 of air per hour per m2 of window area. If the window does not achieve this or if the characteristic has not been tested, the window may still carry the CE marking if “NPD” is entered (see above).
Energy rating (and labelling) imposes a requirement that the window carry the CE marking and requirements relating to the value/class of selected characteristics in the CE marking. The requirement on an energy-rated window in energy class A is that air permeability should not exceed 1 m3, i.e. only one fiftieth of the lowest class in the CE marking.
You get a guaranteed return
The energy-rating requirements in terms of air tightness are extremely strict and ensure the window is energy-efficient and also impermeable to moisture. You also get better soundproofing, as where air gets in, so will noise. By investing in new energy-efficient windows you get a lot for your money. In addition, you are making an important contribution to the environment, as each kWh saved reduces carbon dioxide emissions by up to a kilo.
Good windows mean greater comfort
A window with a low U-value is warmer on the inside, including when temperatures drop below freezing outdoors. You can therefore sit next to the window without feeling a draught. If you do feel a draught, it is usually because you are losing heat because your body heat radiates against a cold surface, e.g. the window pane. Cold floors are usually due to downdraughts from the window. With well-insulated windows, you can sit right next to the window without feeling cold, and the entire room will feel warmer too.
Reduce the indoor temperature
Well-insulated windows that are warm on the inside do not result in downdraughts or radiation and contribute to a more pleasant indoor climate. You can therefore reduce the indoor temperature by one or more degrees without compromising on comfort. Each degree by which the temperature is reduced saves approximately 5% in heating costs.