Loft conversion - window advice

I'm finishing my plans for a loft conversion.

The back of the house is SOUTH facing and will have a dorma extension.

I'm now considering whether to have Velux windows on the NORTH (front) side too and would welcome people's opinions about the following :

1) they should increase head-height, perhaps as much as 3 inches (advantage)
2) they should make the room lighter, obviously (advantage)
3) what will the heat insulation impact be ??
I can find no information on the Velux website about heat retention
Can one compare double-glazing to loft roof insulation somehow ??
4) they will cost more - I can estimate how much from the Velux website.

Thanks in advance ... Mike

I don't see the point if your extension house is south facing, I have no problem with light on my velux windows on the east side of my property. Although there's nothing stopping you to put a velux window on the north side after the loft conversion. I can't tell you what the heat insulation impact will be but you will have to put in 50mm insulation slabs on a 4"x 2" rafters anyway. Also have a look at velux sunlight tunnel which I have just fitted in my utility room for extra light and now I need sunglasses