Double glazing incorporating low-emissivity glass such as Pilkington K Glass™ does not lead to significantly greater overheating problems in summer than does single glazing, or ordinary double glazing. With any type of clear glass in a conservatory it is essential to have adequate means of ventilation to reduce the possibility of high temperatures.
People sometimes think that Pilkington K Glass™, because it keeps heat in so much better, will cause extra overheating problems in Summer. However, this is not the case.
In Winter, with cold outside temperatures, the prime concern is to retain heat within the building, in order to keep the interior temperature at a comfortable level without excessive heating cost. The advantage of Pilkington K Glass™ in this respect can be explained technically by looking at the U value. This indicates the amount of heat flowing out through it.
With a U value of 1.9 W/m2K, Pilkington K Glass™ double glazing lets out only about ⅓ of the heat that escapes through single glass (U value = 5.6 W/m2K) and about ⅔ of that which escapes through ordinary double glazing (U value = 2.8 W/m2K). Any of the sun's heat which enters through a window is a bonus, reducing the amount of heating required in the building.
In Summer, the situation is somewhat more complicated. Double glazing lets in less of the sun's heat than does single glazing, due to the extra pane of glass, and Pilkington K Glass™ double glazing lets in less than ordinary double glazing (although it is not a solar control glass) due to the presence of the Pilkington K Glass™ coating.
In technical terms:
Single clear glass 86% of the solar radiation is transmitted
Clear double glazing 76% of the solar radiation is transmitted
Pilkington K Glass™ double glazing 72% of the solar radiation is transmitted
The reduction in solar heat transmission, through windows facing the sun, largely offsets the ability of each type of double glazing to retain this heat. The net result is that the type of clear glass in the windows - whether single glazing, ordinary double glazing or Pilkington K Glass™ double glazing - has a limited effect on the temperature rise in an unventilated room or conservatory.
If the proportion of window area is high in relation to the size of the room, for instance as it is in a conservatory, all three types can give rise to unacceptably high temperatures, unless there is adequate ventilation.