New loft, old grow. Design advice needed!

Hi all, I have just moved and need to setup again. In the old place I had a spare room with a 1.2x1.2 tent and a little DR90 for veg. I have moved and have a small loft at my disposal, after chopping and bracing a couple of trusses I have made a space about 1.75m x 1.15 x 1.4 (wxdxh)
I am trying to re-use my old kit (apart from tents) so I have a lumatech 600w ballast, a standard 600 bulb in a 1 meter parabolic, isomax 6" fan and slim CF (plus contacters, ducting etc)
My plan (currently) is just over 1m square flowering room with a 40 odd cm (width) veg next to it, height is an issue for flowering as parabolic is about 25cm high so after construction I will be lucky to get 100+ cm.
Construction plans are T and G loft floor then 25mm polystyrene Sheet then 9mm OSB for a 'floating' floor (I am mid terrace so noise is an issue) the floor then has a cheap tarpaulin over it, the frame will be made of 3"x2" stud, exterior clad in 6mm hard board then 50mm celotex then 12-15mm air gap (1st question is will that be enough gap?) then inside is 9mm plasterboard hopefully covered in wickes aluminium insulation, fan and filter will fit above parabolics outer edge, now I am venting out through old shower roof vent 4" (2nd question, should I reduce from 6 to 4" duct inside or outside tent? I know bigger is better but I have no choice) I was looking to use a couple of 90 degree 6" poly pipe bends to passively feed from side veg room into flower with 6" pc fan filter for dust and actively feed the veg room with a 4" inline pulling air from the bathroom extractor via a Y connection to current ducting.
That much I am roughly sure of, Q3 what's the best way to do the doors, I will need 2 and they will be insulated same as the sides/roof! I have no idea how to do them, stepped in, set back? No clue so help!
Sorry about the essay!
Thanks for looking.
Cheers
g

your from across the pond .......

as for your spacing for air no not big enough
passive intakes need to be double the size of the exhaust 6 inch exhaust u need a 12x12 opening for the air to be drawl in

as for where the reducer goes .........put it at the end of all the 90 degree bends right where the 6 inch needs to go down to 4 inch to attach to the pipe going out

doors are easy just buy IR blocker and cut it to the door size
http://www.amazon.com/Thermal-Blocker-Shield-Hydroponic-Plant/dp/B0058JCSAS

the other method takes more work ..........u need to get a inside hollow door then basically gut the inside of the door so u can fill it with spray foam .......it will do the same thing as IR blocker ........just rem IR blocker only works as long as u keep those air bubbles unpopped that is what keeps it from transferring the heat to the other side