Windows also have a G value. This is the amount of heat they absorb from sunlight. When manufacturers make a window with a U value of 1.4 it’s also very likely that the coatings used on the glass to produce this low U value have blocked most of the sunlight, giving a low G value. A window with a U value of 1.4 might have a G value of 0.45 (45% of sunlight passes through it) and a normal window with a U value of 1.80 might have a G value of 0.63 (63% of 0f sunlight passes through it.) Using the high spec window with a low U value didn’t really make your house any warmer. Less heat from the house will escape due to the low U value but heat will be lost because the coating in the window blocks out the light of the sun. In some situations this is an advantage. Lets say you want a highly glazed conservatory which is heated in the winter. On a dark cloudy day in winter your greatest need is to keep the heat from the central heating system inside the conservatory so you want a low U value, perhaps as low as 1.1. On a bright summer day your greatest worry is overheating so you want a low G value, perhaps as low as 0.25. Conservatory glass is often made with those sort of values. This is solar cooling glass.
High energy glass has a low U value and a high G value. It is possible to produce windows with U values as low as 1.30 and G values as high as 0.75. This sort of glass is likely to be more expensive but it achieves a very high U value without any of the loss in heat from the sun that occurs in normal high spec double glazing. Is it likely to cause any harm by having such a high G value? Would it damage wallpaper or books that are behind the window? It isn’t especially dangerous in this respect as it has the same G value as ordinary single glazing. Is it likely to make the house too warm on a hot sunny day? This could happen and it’s important to have a common sense approach to your design. In a large ground floor room it’s a good idea to have doors that can be opened wide to cool the building down if necessary. In bedrooms and bathrooms it’s a good idea to make sure that the windows can fully open. If you’re worried about young children climbing out of the windows of the upstairs bedrooms fit a lock on the lower window and have a sufficiently long window at the top that can still be opened.
When would I recommend this? High energy glass is useful when you’re trying to achieve a very high spec for planning regulations, for example, if the CO2 has to be 10% lower than SAP 2012 it can be a cheaper alternative to fitting a solar panel. You also won’t need the huge hot water tanks of a solar hot water system or to worry about whether your solar PV panels should generate your own electricity or discharge into the mains. Unless you’re really convinced that you want solar panels fit high energy glass in a high spec new build house.
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