CENTRAL HEATING IN ROOF - HEAT LOSS THROUGH VENTS

I'd like to know if anybody has some figures on how much heat can be lost through an open central heating vent in the ceiling.

We have a very well insulated and draft-proof home with an efficient and powerful space heater that heats most of our home up very well. We hardly use the central heating system at all - probably on average about an hour a week, or two hours in the depths of winter. The problem is that the vents are not that easy to open and close and I can't expect my wife to go round the house and open them all up when she wants to use central heating.

I've been told that the open vents in the central heating system are a major cause of heat loss as they form a very effective thermal cooling current, especially at night. They are not as well insulated as the rest of the house and so heat escapes from the rooms into the ducting and then out into the roof. It makes perfect sense to me, and I am also aware that these small breaks in insulation have a major impact on the insulating quality of my insulation 'blanket'.

My ideal solution to this is simple - turn the thing off, plug up the vents with insulation, and save more heat than we gain from the system. But this is not a suitable solution for the boss.

Does anybody know exactly how much heat loss occurs through an open central heating vent, and are there any simple solutions?

In VIC all installs on new homes need to use R1 min insulated ducts, existing homes R0.6 is the min.

I have the R1 ducts and they seem to be pretty insulated(~2" polyester) so I doubt that much heat would be escaping if the fan is not on.

I'm not sure about that. Our roof is insulated with R5.5 except for the 'holes' where the vents are. So if the ducting is insulated with R1, then I have a significant amount of space in my ceiling that is insulated far less effectively than the rest of the ceiling.
It is estimated that a 5% uninsulated area in a roof caused about 50% decreased in total insulation cover and the reason for this is that heat rises to the ceiling and will naturally go to where it can escape (to where the gaps are). So if a roof has open holes leading to a large 'less insulated' area, you are effectively providing an easy escape route for heat, which undoes all the work of putting the large amount of insulation in the rest of the ceiling.
I would say that R1 is not enough for ducting - one of my options is actually to increase this - probably by wrapping it in a foil reflective blanket.

Foil won't do much as the heat escaping is via conduction and not reflection (i.e. sun rays). The ducts are already wrapped in foil, inside and outside layers.

Also any un-insulated area is effectively equivalent to 10 times that area uninsulated. This obviously doesn't count where there is some insulation there.