Heat loss through windows

      Comments Off on Heat loss through windows

Around 60% of the heat energy you lose through single glazed windows is through radiated heat (energy) passing through the glass. This makes windows a real focal point where energy savings can be made.

With double or triple glazing, the inside pane absorbs the heat from the room and it is transmitted by convection and conduction to the outer pane which is in contact with the outside World. This level of transfer is known as the ‘U’ value, and is calculated in Watts per square metre per degree of temperature difference, or W/M2C.


With double or triple glazing only a small amount of heat is lost via convection between the panes. Warm air between the panes rises and is replaced by cooler air and so a convection loop transfers heat from the inner pane through to the outer pane, but this loss is negligible.

Aside from the glass, heat energy can also be lost through the window frame. The ‘U’ value here depends on the material used for the frame. Metal transfers more easliy than wood or plastic.

The choice of opening style also has a bearing on heat loss. Casement opening windows generally fare better than sash style.

It’s also possible to lose heat around the frame, especially if the body of the window has been poorly installed, or there is insufficient filler material around the frame within the wall cavity.

The final place where heat can be lost is due to the spacer bars within double glazing. As these are made from aluminium they do present a very small contribution to the overall heat loss.

All this talk of energy loss might seem like bad news?

Your windows are not a one way street… in warmer weather there is going to be some ‘solar gain’ directly from the sun and also some heat absorbed by the glass which is transferred inwards by radiation and convection. This gain is referred to by a ‘G’ Value which actually refers to the degree that your glazing blocks heat from sunlight, and is the amount of energy from the sun or the surrounding air that enters through your windows.

The ‘G’ value is a number between zero and one. A low ‘G’ Value means that less solar heat is transmitted. ‘G’ Value depends on the type of glass making up your glazing unit, and in most cases manufacturers aim to allow solar energy in, whilst preventing energy from escaping. However, this can make rooms with a lot of window space quite warm during the summer months.

To counter this you can plan your garden accordingly, with deciduous trees placed between the sun and your windows. This means that the leaves on the trees will block most of the direct sunlight in the late spring and summer months, and then when the tree sheds it’s leaves in autumn, you can benefit from direct sunlight in the winter.

Even if you have modern glazing, do give us a call or send us an email to make sure you’re saving on your fuel bills…