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Conservatory roofs – G values and U values explained


The G-value, expressed either as a percentage or as a decimal fraction of 1, shows how much of the sun’s radiant energy is allowed to pass into the room. The lower the G-value, the less the conservatory will overheat during the summer months. Two 4mm sheets of basic clear glass might have a G-value of 0.75 (75%) whereas replacing the outer sheet with Pilkington Perfect Day Super-Blue brings the G-value down to 0.4 (40%).

The U-value is the overall heat-transfer coefficient; a numerical expression of the calculation (W/m2 K) Watts per square meter times temperature (Kelvin being the scale of absolute temperature). Simply put for our purpose, it is the measure of how well a material is able to retain heat. The lower the U-value, the warmer the conservatory will be in the winter months. The two 4mm sheets of clear glass in the previous example will give a U-value of 2.8, whereas a top-sheet of Pilkington Perfect Day Super Blue brings this down to a U-value of 1.2. These values are logarithmic, e.g. a U-value of 1.2 gives much more than double the heat-retention of U value 2.4.

Polycarbonate roofs versus glass roofs – which option gives better insulation?

A roof panelled with 32mm seven-wall tinted polycarbonate will give U-value of 1.4:- a much better performance than is achieved by two sheets of clear glass but not as good as the U value of 1.2 that is achieved by the purpose-made Pilkington Super Blue roof glass – and a tinted polycarbonate roof will allow in considerably less light than will a tinted glass roof.

In many cases, we are able to replace an existing polycarbonate roof with a solar control glass roof – which is much less expensive than replacing the whole conservatory! We offer a free no-obligation visit to your property, where we will assess and discuss with you the potential to replace your existing conservatory roof. To book an appointment, call us on 01865 788333.

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