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General Information on Heat Loss in Glazed Timber Sash & Case Windows

Single glazed timber sash and case windows are very poor at conserving energy. The heat loss through single glazing which has a U-Value of 5.8 is around 70%.

This is caused by the single pane of glass which will be at approximately the same temperature as it is outside. In colder conditions in a room at around 20 degrees centigrade, the warm air will contact the cold single glazing and drop downwards at a rate exceeding two metres per second, sometimes mistakenly considered as a draught through window construction joints.

This causes a constant convection in a room where the air is being heated and then cooled by the cold single glazing, resulting in an expensive, continual 70% heat loss, through the glass.

Low E Double Glazing such as Slimlite reduces this heat loss by at least 50%, due in part to the Low E glass which reflects the long wave radiation or heat back into the room, combined with the insulating inert gases contained in the cavity of Slimlite, Krypton and Xenon, which are the most effective inert gas insulators.

The insulating effect keeps the inside pane, normally the Low E glass much warmer than the outside temperature, thereby considerably slowing down the convection mentioned above and reducing heat loss by around 50%.

Recent figures estimate that Low E double glazing such as Slimlite because of the escalating costs of energy will provide a pay back term of 3 to 5 years, depending on the insulation value.

Replacement of one square meter of single glazing by Low E double glazing will provide a saving of approximately 90Kg of carbon dioxide emissions per year.

The very design of sash and case windows permit the ingress of air which does not affect the thermal performance of Slimlite Self Cleaning Double Glazing. However a good quality draught proof system should reduce the draughts by around 80%.

Document L England - Section 6 Scotland
These new insulation requirements for windows are a result of the Kyoto Agreement to reduce carbon emissions and bring to an end the poor insulation of single glazed windows except for Listed Buildings. However Edinburgh have recently made a major policy change allowing replacement double glazing such as Slimlite to A and B Listed Buildings. As Edinburgh has more Listed Buildings than any other City in the UK except London, other City Authorities may well consider their current policies.