The heat loss through a glazed roof will be horrendous even if double glazed if our old conservatory roof was anything to go by. We replaced it with a lightweight insulated roof and so far through the coldest parts of winter the temperature has stayed about 1C at most below the temperature of the lounge where the thermostat is situated this is despite a considerable amount of glazing remaining. If you feel you need some light coming through the roof then I would limit it to smaller roof lights.

Richard