Loft boards and insulation

[scottylaa]

Just wondering if anyone could help me with something thats been rattling my brain for a few days? There's a few issues so i apologize for the novel im about to write!
In the summer i am going to empty all the junk from my loft so that i can completely board the whole thing. Joists, eaves and gable wall ect. I am also having a combi boiler fitted and the water tank in the loft removed. This means that the airing cupboard will be used as access to the loft via ladder up through where the tank used to be.
Heres my plan so if there is anything im doing wrong please put me right before i wreck my house.

Im going to tongue and groove board the floor first with all the extra insulation and cross battens ect. I was wondering tho if i could completely enclose the whole loft with foil insulation beforehand leaving an air gap all the way round the outside for airflow?. So if you imaging a sort of suspended room within the loft with 270mm of wool then a layer of foil under the floor. The only air entering the inside of the room will be thru the hatch.

would condensation form in the floor if i put foil under it ontop of the wool? Bare in mind there will be a 50mm gap round the perimiter of the floor.

I plan on stapling the foil to the eaves then putting batten over it to fix the plaster boards to.
Im only going to use the loft for storage but wanted to mate a feature of it at the same time. Thanks

 

[PrenticeBoyofDerry]

and what is your reasoning for using the foil?
If your plan is to enclose the rafters with it, it is a pointless exercise unless you intend to make the loft a warm area, then you would need to consider a full insulation job and ventilation between insulation of rafters and roof.
If this is not going to be a warm area, just insulate the ceiling void with 270mm of insulation, you can raise your joist levels be running new joists at right angle to the existing. then board out the floor and leave the ventilation around the edges, you could also insulate the loft hatch.

 

[scottylaa]

I was originally going to use kingspan but its just too expensive. Im a dry liner working for a company at the min and use kingspan all the time but didnt realize how much it cost per sheet until pricing it up myself haha.

Im kinda trying to create a draught free storage room with with air flowing around the outside of it thru the eaves and out the top

Within the eaves there would be an air gap on both sides of the foil. One between the foil and roof tiles, and on the other side a gap created by the batten between the foil and plasterboard.
The main issue i have tho is the condensation under the chipboard floor. So there would be the joists with 100mm of existing wool, then new cross battens on top at 90 degrees to the joists with a further 170mm between them. If i were to put reflective foil insulation over the top of all that then board over it would condensation form on the underside of the foil?

 

[PrenticeBoyofDerry]

If it's a full fill and you offer no ventilation to floor/ceiling void, condensation should not be an issue.