Windows and doors

An average house with a floor area of 144 square metres uses around 15 000 kWh annually for heating. But one third of the heat is lost through ordinary double glazed windows.

The U-value is a measure of the heat loss

The U-value specifies the heat loss. The U-value of a window is a measure of the quality of the insulation provided by the glass, frame and casing. The lower the U-value, the better the insulation provided by the window.

Cold down-draughts

Ordinary double-glazed windows give rise to cold down-draughts. The low temperature of the inner glass pane may be perceived as “draught” from the windows. The reason is the heat loss from your body to the cooler glass pane.

When building a new house, you should specify windows with a U-value of 1.0, and when you rebuild the house, specify a U-value not exceeding 1.3.

Energy-efficient windows have two or three glass panes assembled into an insulating unit. The spaces between the glass panes in the insulating unit are hermetically sealed, and often filled with a noble gas in order to reduce the heat loss. The window admits solar heat, but makes it difficult for the heat from the room to escape. Frames and casings are designed to minimize the heat losses.

Daylight and solar heat are important

It is also important for the windows to admit daylight (daylight transmittance) and solar heat (solar energy transmittance). The window should therefore admit at least 63 percent of daylight and 52 percent of the incident solar energy. Ask your salesman for the U-value and transmittance values of the window.

Replacing sound windows by new, energy efficient windows as the only measure is seldom economically justifiable. But if you intend to do something to the heating system or frontage of the house, for instance, the investment may be more profitable if you change the windows at the same time. Are the windows poor and do they need replacing? If so, choose energy-efficient windows.

Energy-efficient windows

Since the heat demand of the house decreases if the windows are more energy efficient, you will be able to choose a smaller boiler or install fewer radiators. Energy-efficient windows thus save money in several ways, while also greatly improving the indoor comfort. You may not need to change your windows to make them energy efficient. Changing the windows may make a major impact on the appearance and aesthetics of the house. A better solution may instead be to supplement the original windows with energy-efficient glass units or replace the inner glass panes with energy-efficient panes.

Entrance door

The entrance door is an important part of the climate shell of the house, and must be able to keep out the cold, rain, snow and burglars.

If you buy a new door, ask for its U-value. The lower the U-value, the better the insulation it provides. The U-value must cover the whole structure comprising the door and frame.

Veranda door

The veranda door is often a real energy thief – a forgotten part of the climate shell of the house. You can often relatively easily insulate the veranda door to improve its properties. It is also advisable to check the draught excluder strips. If you intend to change the veranda door, find out its U-value. Also bear in mind that the veranda door is among the most common places through which burglars break in.

You can read more about how you keep the heat indoors here.